Pathology Flashcards
What is the definition of hyperplasia, hypertrophy, atrophy and metaplasia?
Hyperplasia - increase in the number of cells
Hypertrophy - increase in the size of cells
Atrophy - decrease in the size of a cell that has at one time been of normal size
Metaplasia - change of epithelial cell type from one to another at a specific site or location (the epithelium is normal in appearance in an abnormal location)
What are the two types of hyperplasia? Give an example for each
Physiological hyperplasia - occurs due to a normal stressor (breasts in pregnancy, liver growth after resection
Pathological hyperplasia - occurs due to an abnormal stressor (adrenal gland growth due to production of ACTH by a pituitary adenoma)
Hyperplasia only happens in cells that divide (not the heart or neuron)
What are the two types of hypertrophy? Give an example for each
Physiological hypertrophy - occurs due to a normal stressor (enlargement of skeletal muscle after exercise)
Pathological hypertrophy - occurs due to an abnormal stressor (increase in heart size due to aortic stenosis
What are the two types of atrophy? Give an example for each
Physiological atrophy - occurs due to a natural stressor (decrease in size of uterus post-pregnancy)
Pathological atrophy - occurs due to an abnormal stressor (usually due to a loss of stilumus to that organ - blood supply, innervation, decreased workload, aging)
What is the mechanism of metaplasia? Give an example
The epithelium normally present at a site cannot handle the new environment so it converts to a type of epithelium that can adapt (Barrett’s oesophagus - squamous to glandular)
What are some causes of cell injury?
- Trauma
- Thermal injury, hot or cold
- Poisons
- Infectious organisms
- Ionising radiation
- Drugs
What are the six mechanisms of cellular injury? Give examples for each
Mechanical disruption - trauma, osmotic pressure
Failure of membrane function integrity - damage to ion pumps, complement-mediated cytolysis, specific blockage of ion channels, alteration of membrane lipids
Membrane damage - free radicals
Deficiency of metabolites - oxygen, glucose, hormones
Blockage of metabolic pathways - interruption of protein synthesis, respiratory poisins
DNA damage or loss - ioinising radiation, chemotherapy, free radicals
What are the two ways in which cell damage can occur?
Reversibly and irreversibly
What are the four things that the effect of cellular injury on a tissue depend on?
- the duration of injury
- the nature of the injurious agent
- the proportion and type of cells that are affected
- the ability of tissues to regenerate
What is the definition of ischaemia and infarction?
Ischaemia is the pathological process resulting from a lack of oxygen due to impaired blood supply
Infarction is the specific process of necrosis resulting from lack of blood supply
What is reperfusion injury?
Under some conditions it is possible for the onset of cell death to be delayed until blood flow has been restored.
It may be due to the generation of reactive oxygen species, free radicals and damage to calcium pups.
In reperfusion injury, are cells damaged via apoptosis or necrosis
Cells damaged in this way probably go through apoptosis rather than necrosis
What is a free radical?
Atoms or groups of atoms with an unpaired electron, as such they may enter into chemical bond formation
What are some of the clinicopathological events involving free radicals?
- toxicity of some poisons
- oxygen toxicity
- tissue damage in inflammation
- intracellular killing of bacteria
What is necrosis and what causes it?
Death of tissues, causes include ichaemia, metabolic and truma. It is a pathological process
What are the six different types of necrosis? Give examples of each
- Coagulative necrosis in most tissues, firm pale area with ghost outlines on microscopy (most common)
- Colliquative necrosis is seen in the brain, the dead area is liquefied
- Causeous necrosis is seen in TB, there is a pale yellow, semi-solid material
- Gangrene is necrosis with putrefaction: it follows vascular occlusion or certain infections and is black
- Fibrinoid necrosis is a microscopic feature in arterioles in malignant hypertension
- Fat necrosis may follow trauma and cause a mass (often in the breast), or may follow pancreatitis visible as multiple white spots
What are the different types of gangrene?
- Wet - in the bowel (incarcarated hernia) or appendicitis
- Dry - usually seen in the toes (gradual arterial or small vessel obstruction in diabetes)
- Gas - result of infection by Clostridium perfringens
- Synergistic - infection by a combination of organisms
What are the two types of cell appearances following injury? Describe them
- Hydropic change - cells when the cytoplasm becomes pale and swollen due to the accumulation of fluid (hypoxia or chemical poisoning) - often reversible
- Fatty change - vacuolation of cells often due to the accumulation of lipid droplets as a result of a disturbance to ribosomal function and uncoupling of lipid from protein metabolism. (often in the liver) - moderate changes are reversible but severe fatty change is irreversible
What is the process of coagulative necrosis?
Following devitalisation, the cells retain their outline as their proteins coagulate and metabolic activity ceases.
Later, it may become soft as a result of digestion by the macrophages
What is the morphology of caseous necrosis?
A pattern of necrosis in which the dead tissue lacks any structure
What is the process of fibrinoid necrosis?
Arterioles are under such pressure that there is necrosis of the smooth muscle wall. This allows seepage of plasma into the media with consequent deposition of fibrin.
How does trauma cause fat necrosis?
The release of intra-cellular fat elicits a brisk inflammatory response, the polymorphs and macrophages phagocytosing the fat, proceeding eventually to fibrosis
What is the process of fat necrosis in pancreatitis?
There is release of pancreatic lipase. As a result, fat cells have their stored fat split into fatty acids, which then combine with calcium to precipitate out as white soaps
What is apoptosis?
Individual cell deletion in physiological growth control and in disease
What does reduced apoptosis cause?
Cell accumulation (neoplasia)
What does increased apoptosis cause?
Extensive cell loss (atrophy)
What are some of the roles of apoptosis?
- Continuining control of organ size
- Unwanted or defective cells undergo apoptosis
What factors control apoptosis?
Inhibitors include growth factors, cell matrix, sex steroids and some viral proteins
Inducers include growth factor withdrawal, loss of matrix attachment, glucocorticoids, some viruses, free radicals, ioinising radiation, DNA damage
How do apoptosis inducers and inhibitors work?
They act via the bcl-2 protein family, inhibiting or activating the death pathway, resulting in activation of caspases
OR
Activation of plasma membrane receptor Fas (CD95) by its ligand bypasses bcl-2 to activate other caspases
What is the process of apoptosis?
Degradation of the cytoskeletal framework
Fragmentation of DNA and loss of mitochondrial function
Nucleus shrinks (pykonosis) and fragments (karyorrhexis)
Cell shrinks, retaining an intact plasma membrane
Alteration of this membrane rapidly induces phagocytosis
What happens to dead cells not phagocytosed?
They break into smaller membrane-bound fragments called apoptotic bodies.
What gene checks the integrity of the genome before mitosis? What happens next?
p-53 gene
Defective cells are swtiched to apoptosis instead
What diseases are associated with increased apoptosis?
AIDs (activate CD4, inducing apoptosis
Neurodegenerative disorders
What is the difference in induction of apoptosis vs necrosis?
Apoptosis - may be induced by physiological or pathological stimuli
Necrosis - invariably due to pathological injury
What is the difference in biochemical events of apoptosis vs necrosis?
Apoptosis - energy-dependent fragmentation of DNA by endogenous endonucleases - lysosomes intact
Necrosis - impairment or cessation of ion homeostasis - lysosomes leak lytic enzymes
What is the difference in extent of apoptosis vs necrosis?
Apoptosis - single cell
Necrosis - cell groups
What is the difference in cell membrane integrity of apoptosis vs necrosis?
Apoptosis - maintained
Necrosis - lost
What is the difference in morphology of apoptosis vs necrosis?
Apoptosis - cell shrinkage and fragmentation to form apoptotic bodies with dense chromatin
Necrosis - cell swelling and lysis
What is the difference in inflammatory response of apoptosis vs necrosis?
Apoptosis - none
Necrosis - usual
What is the difference in fate of dead cells of apoptosis vs necrosis?
Apoptosis - ingested (phagocytosed) by neighbouring cells
Necrosis - ingested (phagocytosed) by neutrophil polymorphs and macrophages
What is the definition of inflammation?
A response of vascularised tissues to infection and damaged tissues that brings cells and molecules of host defence from the circulation to the sites where they are needed, in order to eliminate the offending agents
In a broad sense
What are the key mediators of inflammation?
Phagocytic leukocytes, antibodies and complement proteins
What are some examples of key components of the innate immune system?
Natural killer cells, dendritic cells, and epithelial cells, as well as soluble factors such as the proteins of the complement system.
What are the steps of the typical inflammatory reaction?
- The offending agent, often in extrasvascular tissues, is recognised by host cells and molecules
- Leukocytes and plasma proteins are recruited from the circulation to the site where the offending agents are located
- The leukocytes and proteins are activated and work together to destroy and eliminate the offending substance
- The reaction is controlled and terminated
- The damaged tissue is repaired
How do blood vessels help in inflammation?
- The blood vessels dilate to slow down blood flow, and by increasing their permeability, they enable certain selected proteins to enter the site of infection of tissue damage
- The blood vessel epithelium characteristics change, such that circulating leukocytes first stop then enter the site of infection
What do leukocytes do once they are recruited?
They are activated and acquire the ability to ingest and destroy microbes and dead cells, as well as foreign bodies
What are some of the harmful effects of inflammation?
- Acute temporary things such as pain and functional impairment
- There are many diseases in which the inflammatory reaction is misdirected (against self tissues in autoimmune diseases), occurs against normally harmless substances (allergies) or is inadequately controlled
What cells are involved in these acute diseases:
1. ARDS
2. Asthma
3. Glomerulonephritis
4. Septic shock
- ARDS - neutrophils
- Asthma - IgE antibodies; eosinophils
- Glomerulonephritis - antibodies and complement, neutrophils, monocytes
- Septic shock - cytokines
What is the role of inflammatory mediators such as plasma proteins?
They initiate and amplify the inflammatory response and determine it’s pattern, severity, and clinical and pathological manifestations
Tell me about acute inflammation e.g. duration, mechanisms, and resolution
- Typically develops within minutes or hours and is of short duration, lasting for several hours or a few days
- Main characteristics are the exudation of fluid and plasma proteins (oedema), and the emmigration of leukocytes, particularly neutrophils,
- When it reaches its desired goal, the reaction subsides, but if the response fails to clear the stimulus, the reaction can develop into chronic inflammation
Tell me about chronic inflammation
- It is of longer duration and is associated with more tissue destruction
- Associated with the presence of lymphocytes and macrophages, the proliferation of blood vessels, and the deposition of connective tissue
What is the onset of acute vs chronic inflammation?
Acute - minutes to hours
Chronic - days
What are the cellular infiltrates in acute vs chronic inflammation?
Acute - neutrophils
Chronic - lymphocytes and macrophages
How are local and systemic signs present in acute vs chronic inflammation?
Acute - prominent
Chronic - less
How does the termination of inflammation occur?
It is terminated when the offending agent is eliminated
The reaction resolved because the mediators are broken down and dissipated, the leukocytes have a short life span in tissues
In addition, anti-inflammatory mechanisms are activated, serving to control the host and prevent it from causing excessive damage
What are some examples of things that trigger inflammation?
- Infections - bacterial, viral, fungal, parasitic
- Tissue necrosis elicits inflammation regardless of the cause of cell death, which may include ischaema, trauma, physical and chemical injury
- Foreign bodies - splinters, dirt, sutures - or endogenous molecules depositing in tissues e.g. urate in gout, cholesterol in atherosclerosis
- Immune reactions - autoimmune diseases, allergies
What are some cells and receptors that recognise microbes and damaged cells?
- Cellular receptors for microbes in the plasma membrane (extracellular), the endosomes (ingested), and the cytosol (intracellular) - e.g. Toll-like receptors TLRs
- Sensors of cell damage - all cells have cytosolic receptors that recognise a diverse set of molecules that are liberted of altered as a consequence of cell damage
- Other cellular receptors involved in inflammation - many leukocytes express receptors for the Fc tails of antibodies and for complement proteins
- Circulating proteins - the complement system reacts against microbes and produces inflammatory mediators, mannose-binding lectin recognises microbial sugars and promotes ingestion of the microbes and activation of the complement system
Where are TLRs expressed?
What do they do?
They are expressed on many cell types, including epithelial cells, dendritic cells, macrophages and other leukocytes
Engagement of these receptors triggers production of molecules involved in inflammation including adhesion molecules on endothelial cells, cytokines and other mediators
What are some examples of cytosolic sensors of cell damage?
- Uric acid - a product of DNA breakdown
- ATP - released from damaged mitochondria
- Reduced intracellular K+ concentrations - reflecting loss of ions because of membrane injury
- DNA in the cytoplasm because it should normally be in the nucleus
What do cytosolic sensors of cell damage do?
- They activate a multiprotein cytosolic complex called inflammasome, which induces the production of IL-1
- IL-1 recruits leukocytes and thus induces inflammation
What do the leukocyte receptors for the Fc tails do?
They recognise microbes coated with antibodies and complement (opsonisation) and promote ingestion and destruction of the microbes as well as triggering inflammation
What are the three major components of acute inflammation?
- Dilation of small vessels, leading to an increased blood flow
- Increased permability of the microvasculature enabling plasma proteins and leukocytes to leave the circulation
- Emigration of the leukocytes from the microcirculation, their accumulation in the focus on injury, and their activation to eliminate the offending agent
What is an exudate vs a transudate?
Exudate is an extravascular fluid that has a high protein concentration and contains cellular debris. Its presence implies an ongoing inflammatory response
Transudate is a fluid with low protein content, little or no cellular material, and low specific gravity. It is essentially an ultrafiltrate of blood plasma that is produced as a result of imbalance between hydrostatic and osmotic pressure without an increase in vascular permability
What is edema?
An excess of fluid in the interstital tissue or serous cavities, it can be either exudate or transudate
What is pus?
A purulent inflammatory exudate rich in leukocytes (mostly neutrophils), the debris of dead cells, and in many cases, microbes
What are the changes in vascular flow and caliber after injury?
- Vasodilation is induced by the action of many mediators, notably histamine, on vascular smooth muscle. First the arterioles then capillaries, causing increased blood flow and erythema
- Increased permeability of the microvasculature, with the outpouring of protein-rich fluid into extravascular tissues
- These changes lead to engorgement of small vessels, which slowly moving red cells, a condition termed stasis (again causing erythema)
- As stasis develops, blood leukocytes, principally neutrophils, accumulate along the vascular endothelium, simultaneously, endothelial cells are activated by inflammatory mediators and express increased levels of adhesion molecules, adhering to leukocytes, which then migrate through the vascular wall
What are the different mechanisms of increased vascular permability in short? (we’ll go into more detail after this)
- Contraction of endothelial cells resulting in increased endothelial spaces is the most common mechanism of vascular leakage
- Endothelial injury, resulting in endothelial cell necrosis and detachment
- Increased transport of fluid and proteins called transcytosis, through the endothelial cell
What chemical mediators trigger endothelial cell contraction?
Histamine, bradykinin, leukotrienes, and other chemical mediators
When does endothelial cell contraction happen and how long does it last for?
It is called the immediate transient response because it happens occurs rapidly after exposure to the mediator and is often short-lived (15 to 30 minutes)
What is delayed prolonged leakage?
- In some forms of mild injury (e.g. sunburn), vascular leakage begins after a delay of 2-12 hours and lasts for several hours or even days
- It may be caused by contraction of endothelial cells or mild endothelial damage
What is the mechanism behind endothelial injury causing increased vascular permeability?
Direct damage to the endothelium is encountered in severe injuries and is induced by the action of microbes and microbial toxins that target endothelial cells
Neutrophils that adhere to the endothelium during inflammation may also injure the endothelial cells and thus amplify the reaction
What does leakage occur due to endothelial injury, necrosis and detatchment after injury?
In most instances, leakage starts immediately after injury and is sustained for several hours until the damaged vessels are thrombosed or repaired
How do lymphatic vessels respond to inflammation?
In inflammation, lymphatic flow increases and helps drain oedema fluid. In addition to fluid, leukocytes and cell debris, as well as microbes, may find their way into the lymph.
Lymphatic vessels proliferate during inflammatory reactions to handle the increased load.
What are the most important leukocytes in inflammation?
Neutrophils and macrophages
What is the function of leukoytes in the inflammatory response?
- Ingest and destroy bacteria and other microbes, as well as necrotic tissue and foreign substances
- Produce growth factors that aid in repair
- When strongly activated, they may induce tissue damage and prolong inflammation
The journey of leukocytes from the vessel lumen to the tissue is a multi-step process that is mediated and controlled by what?
Adhesion molecules and cytokines called chemokines
What are the three phases of leukocyte migration?
- In the lumen: margination, rolling, and adhesion to the endothelium
- Migration across the endothelium and vessel wall
- Migration in the tissue toward a chemotactic stimulus
How does margination occur?
In normal flowing venules, red cells are confined to a central axial column, displacing the leukocytes towards the wall of the vessel
Because of stasis (blood flow slowing) in early inflammation, haemodynamic conditions change, and more white cells assume a peripheral position along the enothelial surface
At what stage of the inflammatory process do neutrophils predominate?
6 - 24 hours
At what stage of the inflammatory process do monocytes predominate?
24 - 48 hours
What are the mediators that trigger the following:
1. Leukocyte rolling on endothelium
2. Firm attachment to the endothelium
- Selectins (P-selectin (platelets), L-selectin (leukocytes), E-selectin (endothelium)) trigger leukocyte rolling
- Integrins trigger firm attachment to the endothelium
Which cytokines promote expression of selectins and integrins on endothelium?
TNF
IL-1
Which cytokines increases the avidity of integrins for their ligands and promote directional migration of the leukocytes?
Chemokines
What is transmigration or diapedesis?
It is migration of the leukocytes through the endothelium.
It often happens mainly in the postcapillary venules
After exiting the circulation, the leukocytes move in the tissues toward the site of injury. What is this process called?
Chemotaxis
What are some endogenous chemoattractants? (things released in an injury that attract leukocytes in inflammation)
- Cytokines, particularly those of the chemokine family e.g. IL-8
- Components of the complement system (mainly C5a)
- Arachidonic acid (AA) metabolites, mainly leukotriene B4
What are the three steps of phagocytosis?
- Recognition and attachment of the particle to be ingested by the leukocyte
- Engulfment, with subsequent formation of a phagocytic vacuole
- Killing or degradation of the ingested molecule
What phagocytic receptors bind and ingest molecules?
- Mannose receptors - a lectin that binds to terminal mannose and fucose (which are only on microbes) therefore recognises microbes and not host cells
- Scavenger receptors - bind to LDLs
- Receptors for various opsonins - main ones are IgG antibodies, C3b breakdown product and certain lectins (mannose-binding lectins)
How does engulfment take place in phagocytosis?
- After a particle is bound to the phagocyte receptors, extensions of the cytoplasm (psuedopods) flow around it
- The plasma membrane pinches off to form a vesicle (phagosome) that engulfs the particle
- The phagosome fuses with a lysosome granule, resulting in release of the granules contents into the phagosome
What molecules are used to perform intracellular destruction of microbes and debris once in the phagosome and how?
- Reactive oxygen series are produced by the oxidisation of NADPH by NADPH oxidase - it accompanies phagocytosis - produced by lysosome - may be released extracellularly, causing tissue damage
- Reactive nitrogen series
- Lysosomal enzymes
What are neutrophil extracellular traps?
Extracellular fibrillar networks that provide a high concentration of antimicrobial substances at sites of infection and prevent the spread of the microbes by trapping them in the fibrils
What are mediators of inflammation?
Substances that initiate and regulate inflammatory reactions
What are the most important mediators of acute inflammation?
Vasoactive amines
Lipid products (prostaglandins and leukotrienes)
Cytokines (including chemokines)
Products of complement activation
How are inflammatory mediators usually created?
They are either secreted by cells or generated by plasma proteins.
Cell-derived mediators are usually sequestered by intracellular granules and can be rapidly secreted by granule exocytosis (e.g. histamine in mast cell granules) or are synthesised de novo (e.g. prostagladins, leukotrienes, cytokines) in response to a stimulus
What are the main cells that create inflammatory mediators?
The major cell types that produce mediators of inflammation are the sentinels that detect invaders and damage in tissues, such as macrophages, dendritic cells, and mast cells.
How long do inflammatory mediators last for?
They are usually short-lived.
They quickly decay or are inactivated by enzymes, or they are otherwise scavenged or inhibited.
Can one inflammatory mediator stimulate another one?
Yes!
For instance, products of complement activation can stimulate the release of histamine. the cytokine TNF acts on endothelial cells to stimulate the production of another cytokine IL-1.
Where is histamine created and what does it do?
It is created in mast cells, basophils and platelets.
It causes vasodilation, increased vascular permeability and endothelial activation
Where are prostaglandins created and what do they do?
It is created in mast cells, macrophages, endothelial cells and leukocytes
It causes vasodilation, pain and fever
Where are leukotrienes created and what do they do?
They are created in mast cells and leukocytes.
They cause increased vascular permability, chemotaxis, leukocyte adhesion, and activation
Where are cytokines (TNF, IL-1 and IL-6) created and what do they do?
They are created in macrophages, endothelial cells and mast cells.
Locally, they cause endothelial activation (expression of adhesion molecules).
Systemically, they cause fever, metabolic abnormalities, hypotension (shock)
Where is complement created and what does it do?
It is in the plasma, created by the liver.
It causes leukocyte chemotaxis and activation, direct target killing (membrane attack complex), vasodilation (mast cell stimulation)
Where are kinins created and what do they do?
They are found in the plasma, created in the liver.
They cause increase vascular permeability, smooth muscle contraction, vasodilation and pain
How does histamine increase vascular permeability?
It binds to H1 receptors on microvascular endothelial cells, creating interendothelial gaps in venules and causing endothelial contraction
What things stimulate the release of histamine?
- physical injury, such as trauma, cold or heat
- binding of antibodies to mast cells (immediate hypersensitivity reactions)
- products of complement called anaphylatoxins (C3a and C5a)
- Neuropeptides (substance P) and cytokines (IL-1 and IL-8) may also trigger histamine release
What are some examples of arachidonic acid metabolites?
The lipid mediators prostaglandins and leukotrienes are produced from arachidonic acid (AA) present in membrane phospholipids, and stimulate vascular and cellular reactions in acute inflammation.
Arachidonic acid (AA) is usually found esterified in the membrane phospholipids. How is it released?
Through the action of cellular phospholipases (mainly phospholipase A2).
An increase in cytoplasmic Ca2+ and activation of various kinases in response to external stimuli, activates phospholipase A2
What is created when AA is released from the membrane phospholipids?
From the AA, two major enzymes: cyclooxygenases and lipoxygenases synthesise many AA-derived inflammatory mediators - called eicosanoids
Cyclooxygenases create prostaglandins
Lipooxygenases create leukotrienes and lipoxins
How are prostaglandins generated?
By the actions of cyclooxygenase 1 and 2.
What is the difference between COX-1 and COX-2?
COX-1 is produced in response to an inflammatory stimuli and is also constitutively expressed in most tissues, where it may serve a homeostatic function
COX-2 is induced by inflammatory stimuli and thus generates prostaglandins, but it is low or absent in normal tissues
What is the function of prostacyclin?
It is a vasodilator and a potent inhibitor of platelet aggregation, and also markedly potentiates the permeability-increasing and chemotactic effects of other mediators
What are the most important cytokines in inflammation? What do they do?
TNF & IL-1 serve critical roles in leukocyte recruitment by promoting adhesion of leukocytes to endothelium and their migration through vessels.
* They increase expression of adhesion molecules (E- and P- selectin)
* They increase the production of other cytokines, growth factors and eicosanoids
* They produce fever and can cause sepsis
What are chemokines and what is their role?
A family of small proteins that act as chemoattractants for specific types of leukocytes
What are the main morphological features of acute inflammation?
Dilation of small bloods vessels, accumulation of leukocytes and fluid in the extravascular tissue
What are the different morphological types of acute inflammation?
- Serous inflammation is marked by exudation of cell-poor fluid into spaces created by cell injury or into body cavities lined by the peritoneum, pleura or pericardium
- Fibrinous inflammation is where a fibrinous exudate develops when the vascular leaks are large or there is a local procoagulant stimulus
- Purulent (suppurative) inflammation is characterised by the production of pus, an exudate containing neutrophils, the liquefied debris of necrotic cells and edema fluid
- Ulcers are a local defect, or excavation, of the surface of an organ or tissue that is produced by the sloughing of inflamed necrotic tissue
What is (or isn’t) in the fluid of serous inflammation?
Typically, the fluid is not infected by destructive organisms and does not contain large numbers of leukocytes.
The fluid may be derived from plasma or from the secretions of mesothial cells
What is the mechanism of fibrinous inflammation?
With greater increase in vascular permeability, large molecules such as fibrinogen pass out of the blood, and fibrin is formed and deposited in the extracellular space.
A fibrous exudate is characteristic of inflammation of the body cavities, such as the meninges, pleura and pericardium.
How does fibrinous inflammation present histologically?
Fibrin appears as an eosinophilic meshwork of threads, or sometimes as an amorphuos coagulum
How does fibrinous inflammation resolve?
Fibrinous exudates may be dissolved by fibrinolysis and cleared by macrophages.
If the fibrin is not removed, over time it may stimulate the growth of fibroblasts and blood vessels and thus lead to scarring.
What are abscesses?
They are localised collections of purulent inflammatory tissue caused by a suppuration buried in a tissue, an organ or a confined space
What components make up an abscess?
Abscesses have a central region that appears as a mass of nectrotic leukocytes and tissue cells. There is usually a zone of preserved neutrophils around this necrotic focus, and outside this region, there may be vascular dilation and parenchymal and fibroblastic proliferation, indicating chronic inflammation and repair.
How does an ulcer develop?
During the acute stage, there is intense polymorphonuclear infiltration and vascular dilation in the margins of the defect.
With chronicity, the margins and the base of the ucler develop fibroblastic proliferation, scarring and the accumulation of lymphocytes, macrophages and plasma cells
What are the outcomes of acute inflammation?
- Complete resolution - the injury is short-lived, there is little tissue destruction and the damaged parenchymal cells can regenerate
- Healing by connective tissue replacement (scarring or fibrosis) - there is substantial tissue destruction, abundant fibrin exudation that can’t be cleared, connective tissue grows into the area of damage, converting it into a mass of fibrous tissue, a process called organisation
- Chronic inflammation occurs when the acute inflammatory response cannot be resolved, either due to the microbe or an inadequate immune response
What is chronic inflammation?
A response of prolonged duration (weeks to months) in which inflammation, tissue injury and attempts of repair co-exist, in varying combinations
What are the causes of chronic inflammation?
- Persistent infections by infections that are difficult to eradicate (e.g.mycobacteria, funghi, parasites) - these may be a delayed hypersensitivity reaction or granulomatous reactions
- Hypersensitivity diseases such as autoimmune diseases or allergic diseases
- Prolonged exposure to potentially toxic agents, either endogenous or exogenous for example silicosis (exogenous) or atherosclerosis (endogenous)
What are the morphological characteristics of chronic inflammation?
- Infiltration with mononuclear cells, which include macrophages, lymphocytes and plasma cells
- Tissue destruction, induced by the persistent offending agent, or by the inflammatory cells
- Attempts at healing by connective tissue replacement of damaged tissue, accomplished by angiogenesis and fibrosis
What is the most dominant cell in chronic inflammation and what does it do?
Macrophages contribute to the reaction by secreting cytokines and growth factors that act on various cells, by destryoing foreign invaders and tissues, and by activating other cells, notably T lymphocytes
What are the stages in the life cycle of a macrophage?
- They are tissue cells derived from haematopoietic stem cells in the bone marrow and from progenitors in the embryonic yolk sac and fetal liver during early development. They circulate as monocytes
- Monocytes enter the blood, migrate into various tissues and differentiate into macrophages
- The life span of a monocyte in 1 day, whereas a tissue macrophage can last for several months or years
What are the names of macrophages in the following organs:
1. Liver
2. Spleen and lymph nodes
3. Central nervous system
4. Lungs
What do they form together?
- Liver - Kuppfer cells
- Spleen and lymph nodes - Sinus histiocytes
- CNS - microglial cells
- Lungs - alveolar macrophages
Together they form the mononuclear phagocyte system
There are two major pathways of macrophage activation. What are they?
- Classical macrophage activation may be induced by microbial products such as endotoxins, which engage TLRs and other cells by T cell-drived signals (cytokine IFN-gamma). Classically activate macrophages (M1) produce NO and ROS and upregulate lysosomal enzymes. These things help them kill ingested organisms, trigger inflammation via cytokines.
- Alternate macrophage activation is induced by cytokines other than IFN-gamma (IL-4 and IL-13), produced by T lymphocytes and other cells. The function of alternately activated macrophages (M2) is tissue repair. They secrete GF that stimulates angiogenesis, activate fibroblasts and stimulate collagen synthesis.
What are the main functions of macrophages in chronic inflammation?
- Ingest and eliminate microbes and dead tissue
- Initiate the process of tissue repair
- Secrete mediators of inflammation, such as cytokines (TNF, IL-1, chemokines) and eicosanoids
- Display antigens to T-lymphocytes and respond to signals from T cells
What are the roles of lymphocytes in chronic inflammation?
Microbes and other environmental antigens activate T and B lymphocytes, which amplify and propogate chronic inflammation.
Lymphocytes may be the dominant population in the chronic inflammation seen in autoimmune and other hypersensitivity diseases
There are three subsets of CD4+ T cells that secrete different cytokines and ellicit different types of inflammation. Tell me them…
- TH1 cells produce the cytokine IFN-gamma, which activates macrophages via the classical pathway
- TH2 cells secrete IL-4, IL-5 and IL-13, which recruit and activate eosinophils and are reponsible for the alternative pathway of macrophage activation.
- TH17 cells secrete IL-17 and other cytokines, which induce the secretion of chemokines, reponsible for the recruitment of neutrophils (and monocytes) to the reaction
Lymphocytes and macrophages interact in a bidirectional way, and these interactions play an important role in propogating chronic inflammation. Tell me about them…
- Macrophages display antigens to T cells, express membrane molecules (called costimulators), and produce cytokines (IL-12) that stimulate T cell responses.
- Activated T cells produce cytokines (IFN-gamma), which recruit and activate macrophages, promoting more antigen presentation and cytokine secretion
What is a tertiary lymphoid organ? What is the term used for it’s formation?
In some chronic inflammatory reactions, the accumulated lymphocytes, antigen-presenting cells, and plasma cells cluster together to form lymphoid tissues resembling - this is a tertiary lymphoid organ.
It’s formation is called lymphoid organogenesis
Apart from lymphocytes and macrophages, what other cells are involved in chronic inflammation?
- Eosinophils are abundant in immune reactions mediated by IgE and parasitic infections. Their recruitment is similar to neutrophils, via the chemokine eotaxin. They contain major basic protein, which is toxic to parasites but also mammalian epithelial cells
- Mast cells are present in chronic inflammatory reactions, and because they secrete a plethora of cytokines, they may promote inflammatory reactions in different situations.
- Although neutrophils are characteristic of acute inflammation, many forms of chronic inflammation still show large numbers of neutrophils
What is granulomatous inflammation?
A form of chronic inflammation characterised by collections of activated macrophages, often with T lymphocytes, and sometimes associated with central necrosis
There are two types of granulomatous inflammation, which differ in pathogenesis. What are they and what is their pathogeneses?
- Foreign body granulomas are incited by relatively inert foreign bodies, in the abscense of T cell-mediated immune responses. Typically occur around talc (IVDU) or sutures, things large enough to not be phagocytosed. Epitheliod cells and giant cells are opposed to the foreign body surface.
- Immune granulomas are caused by agents capable of inducing a persistent T-cell mediated immune response. The agent is often difficult to erradicate. In such responses, macrophages keep activatign T cells and T cells keep activating macrophages until giant cells epitheliod cells form.
What are epithelioid and giant cells?
- The activated macrophages may develop abundant cytoplasm and begin to resemble epithelial cells, these are called epithelioid cells
- Some activated macrophages may fuse, forming multinucleate giant cells
What are the two types of reaction that could occur in repair of damaged tissue?
- Regeneration by proliferation of residual (uninjured) cells and maturation of tissue stem cells
- The deposition of scar tissue to form a scar
How does regeneration occur in tissue repair?
Some tissues are able to replace the damaged components and essentially return to a normal state - this is called regeneration
It occurs by proliferation of cells that survive the injury and retain the capacity to proliferate
In what organs does regeneration often occur in tissue repair?
The rapidly dividing epithelia of the skin and intestines, and in some parenchymal organs, notably the liver.
In other cases, tissue stem cells may contribute to the restoration of the damaged tissues
When would scar tissue form in tissue healing?
If the injured tissues are incapable of complete restitution, or if the supporting structures of the tissue are severely damaged, repair occurs by the laying down of connective (fibrous) tissue.
Although the fibrous scar is not normal, it provides enough structural stability that the injured tissue can continue to function
What is fibrosis? and how it this linked with organisation?
The term fibrosis is often used to describe the extensive depostion of collagen that occurs in the lungs, liver, kidney and other organs as a consequence of chronic inflammation or in the myocardium after extensive ischaemic necrosis.
If fibrosis occurs in a tissue space occupied by an inflammatory exudate, it is called an organisation
The regeneration of injured cells and tissues involves cell proliferation. What is the driven by and dependent on?
It is driven by growth factors and is critically dependent on the integrity of the extracellular matrix, and by the development of mature cells from the stem cells.
Several cell types proliferate during tissue repair. What are they and why do they proliferate?
- Remnants of the injured tissue - which attempt to restore normal structure
- Vascular endothelial cells - to create new vessels that provide the nutrients required for tissue repair
- Fibroblasts - the source of the fibrous tissue that forms the scar to fill defects that cannot be corrected by regeneration
The ability of tissues to repair themselves is determined, in part, by their intrinsic proliferative capacity. Based on this, tissues are divided into three groups. What are the groups and tell me about them?
Not examples yet, that will be next …
- Labile (continuously dividing) tissues - cells of these tissues are constantly being lost and replaced by maturation from tissue stem cells and by proliferation of mature cells. They can readily regenerate after injury, as long as the stem cells aren’t damaged
- Stable tissues - cells of these tissues are quiescent (in the G0 stage of the cell cycle) and have only minimal proliferative activity in their normal state. However, they are capable of dividing in response to injury but have a limited capacity for regeneration
- Permanent tissues - The cells of these tissues are considered to be terminally differentiated and nonproliferative in post-natal life. Repair is normally dominated by scar formation
Give me some examples of labile, stable and permanent tissues…
- Labile - haematopoeitic cells in the bone marrow, stratified squamous epithelium of the skin, oral cavity, vagina, and cervix ; the cuboidal epithelia of the ducts draining exocrine organs (salivary glands, pancreas, biliary tract); columnar epithelium of GI tract, uterus, fallopian tubes and the transitional epithelium of the urinary tract
- Stable - the parenchyma of most solid tissues, such as liver, kidney and pancreas. They also include endothelial cells, fibroblasts, smooth muscle cells.
- Permanent - cardiac myocytes and neurons
Where and by what cells are growth factors released that drive tissue repair?
- They are typically produced by cells near the site of damage.
- The most important sources are macrophages activated by the tissue injury, but epithlial cells and stromal cells also produce some of them.
How do growth factors drive tissue repair?
- Several growth factors bind to ECM proteins and are displayed at high concentrations
- All growth factors activate signaling pathways that ultimately induce the production of proteins that are involved in driving cells through the cell cycle and other proteins that release blocks on the cell cycle.
In addition to growth factors, what else can stimulate cell proliferation in tissue repair?
Cells use integrins to bind to ECM proteins, and signals from the integrins can stimulate cell proliferation
What are the most important stem cells for regeneration after injury? Where are they and what do they do?
Tissue stem cells live in specialised niches, and it is believed that injury triggers signals in these niches that activate quiescent stem cells to proliferate and differentiate into mature cells that repopulate the injured tissue
How is the loss of blood cells corrected and what triggers this?
Loss of blood cells is corrected by the proliferation of haematopoietic stem cells in the bone marrow and other tissues, driven by growth factors called colony-stimulating factors, which are produced as the response to the reduced number of blood cells
What are the steps in scar formation that follow tissue injury and the inflammation response?
- Angiogenesis to supply nutrients and oxygen needed to support the repair process. Newly formed vessels are leaky because of incomplete interendothelial junctions and because VEGF, the growth factor that drives angiogenesis, increases vasular permeability
- Formulation of granulation tissue - migration and proliferation of fibroblasts and the deposition of loose connective tissue, together with the vessels and interspersed leukocytes, form granulation tissue. It progressively invades the site of injury
- Remodelling of the connective tissue - maturation and reorganising of the connective tissue produces the stable fibrous scar. The amount of connective tissue increases in the granulation tissue, eventually resulting in a scar
What is the histological appearance of granulation tissue?
Proliferation of fibroblasts and new thin-walled delicate capillaries (angiogenesis) in a loose extracellular matrix, often with admixed inflammatory cells, especially macrophages
How soon after injury do the steps of tissue repair take place?
- Repair begins within 24 hours of injury by the emmigration of fibroblasts and the induction of fibroblast and endothelial cell proliferation.
- By 3-5 days, the specialised granulation tissue that is characteristic of healing is apparent.
What is angiogenesis and when does it happen?
Angiogenesis is the process of new blood vessel development from existing vessels
It is critical:
* in healing at sites of injury
* in the development of collateral circulations at sites of ischaemia
* in allowing tumours to increase in size beyond the constraints of their original blood supply
What are the steps of angiogenesis?
- Vasodilation in response to NO and increased permeability induced by VEGF
- Seperation of pericytes from the abluminal surface and breakdown of the basement membrane to allow formation of a vessel sprout
- Migration of endothelial cells towards the site of tissue injury
- Proliferation of endothelial cells just behind the leading front (‘tip’) of migrating cells
- Remodelling into capillary tubes
- Recruitment of periendothelial cells (pericytes for small vessels, smooth muscle for large) to form the mature vessel
- Suppression of endothelial proliferation and migration and deposition of the basement membrane
The process of angiogenesis involves several signalling pathways, cell-cell interactions, ECM proteins and tissue enzymes.
What growth factors are required (name 5) and what do they do?
- Vascular endothelial growth factors (VEGF), mainly VEGF-A, stimulates both migration and proliferation of endothelial cells, initiating the process of capillary sprouting. It promotes vasodilation by stimulating the production of NO and contributes to the formation of the vascular lumen
- Fibroblast growth factors, mainly FGF-2, stimulates the proliferation of endothelial cells. It also promotes the migration of macrophages and fibroblasts to the area, and stimulates epithelial cell migration to cover epidermal wounds
- Angiopoietins 1 and 2 (Ang 1 & 2) are growth factors that play a role in angiogenesis and the structural maturation of new vessels. Ang1 interacts with an endothelial tyrosine kinase receptor called Tie1
- PGDF recruits smooth muscle cells
- TGF-beta suppresses endothelial cell proliferation and migration, and enhances the production of ECM proteins
What is Notch signalling?
Through ‘cross-talking’ with VEGF, the Notch signalling pathway regulates the sprouting and branching of new vessels and thus ensures that the new vessels that are formed have the proper spacing to effectively supply the healing tissue with blood.
The process of angiogenesis involves several signalling pathways, cell-cell interactions, ECM proteins and tissue enzymes.
What is the function of ECM proteins?
ECM proteins participate in the process of vessel sprouting, largely with interactions with integrin receptors in endothelial cells and providing the scaffold for vessel growth
The process of angiogenesis involves several signalling pathways, cell-cell interactions, ECM proteins and tissue enzymes.
What is the function of the tissue enzymes?
Enzymes in the ECM, notably the matrix metalloproteinases (MMPs), degrade the ECM to permit remodelling and extension of the vascular tube
What are the two steps in the laying down of connective tissue in tissue repair?
- Migration and proliferation of fibroblasts to the site of injury
- The deposition of ECM proteins produced by these cells
The process of laying down connective tissue is orchestrated from what? and where are these produced?
The proceses are orchestrated by locally produced cytokines and growth factors, including PDGF, FGF-2 and TGF-beta.
The major sources for these are inflammatory cells, particularly macrophages activated by the alternative pathway (M2). Mast cells and lymphocytes can also release them
What is the most important cytokine for the synthesis and deposition of the connective tissue proteins?
Transforming growth factor - beta
TGF-beta
Where is TGF-beta produced and what does it do?
- It is produced by most of the granulation tissue, including alternatively activated macrophages.
- It stimulates fibroblast migration and proliferation, increased synthesis of collagen and fibronectin, and decreased degradation of the ECM due to inhibition of metalloproteinases
What factors regulate TGF-beta production?
The levels in tissues are primary regulated not by the transcription of the gene but by the posttranscriptional activation of latent TGF-beta, the rate of secretion of the active molecule, and factors in the ECM (notably integrins), that enhance or diminish TGF-beta activity
What are the processes in which TGF-beta is involved in?
- Scar formation after injury
- Development in fibrosis in the lung, liver and kidney that follow chronic inflammation
- It serves as an anti-inflammatory cytokine that limits and terminates the inflammatory response
How does TGF-beta limit and terminate the inflammatory response?
It inhibits leukocyte production and the activity of other leukocytes
How do fibroblasts change as healing progresses and what is the outcome of this?
As healing progresses, the number of proliferating fibroblasts and new vessels decreases; however, the fibroblasts progressively assume a more synthetic phenotype, and hence there is an increased deposition of ECM.
When does collagen synthesis by fibroblasts begin after injury and how long does it last for?
It begins early in wound healing (3-5 days) and continues for several weeks, depending on the size of the wound.
What is a scar tissue composed of?
Largely inactive, spindle-shaped fibroblasts, dense collagen, fragments of elastic tissue and other ECM components
What happens to the scar as it matures?
There is progressive vascular regression, which eventually transforms the highly vascularised granulation tissue into a pale, largely asvascular scar.
Some of the fibroblasts also acquire features of smooth muscle cells, including the presence of actin filaments, and are called myofibroblasts.
How do myofibrils change a scar over time?
The contribute to the contraction of a scar over time
What affects the outcome of the repair process?
It is influenced by a balance between the synthesis and degradation of the ECM proteins
What family degrades the ECM proteins in scar formation?
The degradation of collagens and other ECM components is accomplished by a family of matrix metalloproteinases (MMPs), so called because they are dependent on metal ions (zinc) for their activity.
What are some examples of some matrix metalloproteinases (MMPs)?
- Interstitial collagenases which cleave fibrillar collagen (MMP-1, -2 and -3)
- Gelatinases which degrade amorphous collagen and firbonectin (MMP-2 and -9)
- Stromelysins which degrade a variety of ECM constituents, including proteoglycans, laminin, fibronectin and amorphous collagen (MMP -3, -10 and -11)
What cells produce MMPs and what regulates them?
- MMPs are produced by a variety of cell types (fibroblasts, macrophages, neutrophils, synovial cells, and some endotheliali cells)
- Their synthesis and secretion is regulated by growth factors, cytokines and other agents
MMPs have to be tightly controlled. What processes control this?
- They are produced as inactive precursors (zymogens) that must first be activated by proteases (e.g.plasmin) likely to be present only at the site of injury
- In addition, activated collagenases can be rapidly inhibited by specific tissue inhibitors of metalloproteinases (TIMPs), produced by most mesenchymal cells.
What are the factors that affect tissue repair?
- Infection prolongs inflammation
- Diabetes prolongs wound healing
- Nutritional status particularly protein deficiency and vitamin C deficiency
- Glucocorticoids (steroids) inhibit TGF-beta production and diminish fibrosis
- Mechanical factors such as increased local pressure or torsion
- Poor perfusion due to either atherosclerosis and diabetes or to obstructed venous damage
- Foreign bodies
- The type and extent of tissue injury - the type of cells, as explained earlier (you know this gurl)
- The location of the injury depends on the type of acute inflammation that happens and some of those repair better than others
What is healing by first intention vs second intention?
- First intention is when the injury only involves the epithelial layer, the principal mechanism of repair is epithelial regeneration
- Second intention is when cell of tissue loss is more extensive, such as in large wounds, abscesses, ulceration and ischaemic necrosis in parenchymal organs, the repair process involves a combination of regeneration and scarring
What are the steps of first intention healing?
- Inflammation
- Proliferation of epithelial and other cells
- Maturation of the connective tissue scar
What happens immediately during healing of first intention?
- Wounding causes the rapid activation of coagulation pathways, which results in the formation of a blood clot on the wound surface.
- In addition to trapped red cells, the clot contains fibrin, fibronectin, and complement proteins.
- The clot serves to stop bleeding and act as a scaffold for migrating cells, which are attracted by growth factors, cytokines and chemokines.
- Release of VEGF leads to increased vascular permeability and oedema.
- As dehydration occurs at the external surface of the clot, a scab covering the wound is formed.
What happens within 24 hours of healing with first intention?
- Neutrophils are seen at the incision margin, migrating towards the fibrin clot. They release proteolytic enzymes that begin to clear the debris.
- Basal cells at the cut edge of the epidermis begin to show increased mitotic activity.
What happens between 24-48 hours of healing with first intention?
- Epithelial cells from both edges have begun to migrate and proliferate along the dermis, depositing basement membrane components as they progress.
- The cells meet in the midline beneath the surface scab, yielding a thin but continuous epithelial layer that closes the wound
What happens by day 3 of healing with first intention?
- Neutrophils have been largely replaced by macrophages, and granulation tissue progressively invades the incision space.
- As you know, macrophages clear extracellular debris, fibrin and other foreign material, promoting angiogenesis and ECM deposition.
- Collagen fibres are now evident at the incision margins.
- Epithelial cells continue to proliferate, forming a covering approaching the thickness of the epidermis
What happens by day 5 of healing with first intention?
- Neovascularisation reaches its peak as granulation tissue fills the incisional space. These new vessels are leaky, allowing the passage of plasma proteins and fluid into the extravascular space. Thus, new granulation tissue is often oedematous.
- Migration of fibroblasts to the site on injury is driven by chemokines, TNF, PDGF, TGF-beta and FGF. Their subsequent proliferation is triggered by multiple growths factors, including PDGF, EGF, TGF-beta and FGF, and the cytokines IL-1 and TNF.
- The fibroblasts produce ECM proteins, and collagen fibrils become more abundant and begin to bridge the incision.
- The epidermis recovers it’s normal thickness as differentiation of surface cells yields a mature epiderminal architechture with surface keratinisation.
What happens during the second week of healing with first intention?
- There is continued collagen accumulation and fibroblast proliferation.
- The leukocyte infiltrate, oedema, and increased vascularity are substantially diminished.
- The process of ‘blanching’ begings, accomplished by increasing collagen deposition within the incisional scar and the regression of the vascular channels
What happens by the end of the first month of healing with first intention?
- The scar comprises a cellular connective tissue largely devoid from inflammatory cells and covered by an essentially normally epidermis
- However, the dermal appendages destroyed in the line of incision are permanently destroyed.
- The tensile strength of the wound increases with time
How does secondary intention differ from primary intention?
- The fibrin clot is larger, there is more exudate and necrotic debris, therefore the inflammatory response is larger and may cause inflammation-mediated injury
- Much larger amounts of granulation tissue, which generally results in a greater mass of scar tissue
- At first, a provisional matrix containing fibrin, plasma fibronectin, and type 3 collagen is formed, but in about 2 weeks this is replaced by a matrix composed primarily by type 1 collagen
- Wound contraction generally plays a larger role in secondary intention by myofibrils. Within 6 weeks, large skin defects may be reduced by 5-10% of their original size
What are the phases of cell cycle?
- G1 - presynthetic growth
- S - DNA synthesis
- G2 - premitotic growth
- M - mitotic
What are quiescent cells? What phase of the cycle are they in?
Cells that are not actively cycling. They are said to be in the G0 phase.
Where can cells enter G1 from?
Either from G0 or after completing a round of mitosis, as for continuously replicating cells.
What regulates the cell cycle?
It is regulated by activators and inhibitors.
- Cell cycle progression is driven by proteins called cyclins and cyclin-associated enzymes called cyclic-dependant kinases (CDKs)
How are cyclins and cyclic-dependant kinases linked?
- CDKs acquire the ability to phosphorylate protein substrates by forming complexes with the relevant cyclins.
- Transiently increased synthesis of a particular cyclin leads to increased kinase activity of the appropriate CDK binding partner.
- As the CDK completes its round of phosphorylation, the associated cyclin is degraded and the CDK activity abades.
- Thus, as cyclin levels rise and fall, the activity of associates CDKs likewise wax and wane.
- The different CDKs happen sequentially, like a relay race
Where are the cell cycle checkpoints and what do they check for?
- The G1-S checkpoint monitors the integrity of the DNA before irreversibly commiting cellular resources to DNA replication.
- The G2-M restriction point ensures that there has been accurate gene replication before the cell actually divides.
What happens if the checkpoints in the cell cycle detect an error in DNA replication?
- When cells do have DNA irregularities, checkpoint activation delays cell cycle progression and triggers DNA repair mechanisms
- If the genetic derangement is too severe to be repaired, the cells will undergo apoptosis.
- Alternatively, they may enter a non-replicative state called senescence - primarily throught the p53-dependant mechanisms
What enzyme ensures the checkpoints in the cell cycle do their job? How does it do that?
It is the job of the CDK inhibitors: they accomplish this by modulating the CDK-cyclin complex activity.
Some examples are p21, p27, and p57
What is the Warburg effect? Why is it important?
- It is the effect to ensure that there are enough resources to perform DNA replication
- It involves increased cellular uptake of glucose and glutamine, increased glycolysis, and (counter-intuitively) decreased oxidate phosphorylation
What are stem cells?
During development, stem cells give rise to all the various differentiated tissues.
In the adult organism, stem cells replace damaged cells and maintain tissue populations as individual cells within them undergo replicative senescence due to attirition of telemeres.
What are two important characteristics of stem cells?
- Self-renewal, which permits stem cells to maintain their numbers
- Asymmetric division, in which one daughter cell enters a differentiation pathway and gives rise to mature cells, while the other remains undifferentiated and retains its self-renewal capacity
What are the two varieties of stem cells?
- Embryonic stem cells are the most undifferentiated. They are present in the inner cell mass of the blastocyst, have virtually limitless cell renewal capacity, and can give rise to every cell in the body. They are totipotent.
- Tissue stem cells are found in intimate association with differential cells a given tissue. They are normally protected within specialised tissue microenvironments called stem cell niches. They have a limited repertoire of differentiated cells they can generate.
Where are mesenchymal stem cells found and what can they differentiate to?
Mesenchymal stem cells are found in bone marrow.
They can differentiate into a variety of stromal cells including chondrocytes (cartilage), osteocytes, adipocytes, and myocytes.
What is a bone fracture?
A fracture is defined as the loss of bone integrity due to mechanical injury and/or diminished bone strength
How can you describe fractures?
- Simple - the overlying skin is intact
- Compound - the bone communicates with the skin surface
- Comminuted - the bone is fragmented
- Displaced - the ends of the bone at the fracture site are not aligned
- Stress - a slowly developing fracture that follows a period of increased physical activity in which the bone is subjected to repetitive loads
- “Greenstick” - extending only partially through the bone, common in infants, where the bone is soft
- Pathological - involving bone weakened by an underlying disease process, such as tumour
What happens in the bone immediately after a fracture?
- Rupture of the blood vessels results in a haematoma, which fills the fracture gap and surround the area of bone injury.
- The clotted blood provides a fibrin mesh, sealing off the fracture site and at the same time creates a framework for the influx of inflammatory cells and ingrowth of fibroblasts and new capillaries.
- Simultaneously, degranulated platelets and migrating inflammatory cells release PDGF, TGF-beta, FGF and other factors, which activate osteoprogenitor cells in the periosteum, medullary cavity, and surrounding soft tissues and stimulate osteoclastic and osteoblastic activity.
What happens in the bone by the end of the first week after a fracture?
- Organisation of the haematoma, matrix production in adjacent tissues, and remodeling of the fractured ends of the bone
- This fusiform and predominantly uncalcified tissue - called soft tissue callus - provides some anchorage between the ends of the fractured bones but not structural rigidity for weight bearing
What happens in the bone by two weeks after a fracture?
- The soft tissue callus is transformed into a bony callus.
- The activated osteoprogenitor cells deposit subperiosteal trabeculae of woven bone that are orientated perpendicular to the cortical axis and within the medullary cortex.
- In some cases, the activated mesenchymal cells also form chondrocytes that make fibrocartilage and hyaline cartilage.
When does the bony cartilage after a bone fracture reach it’s maximal girth?
At the end of the second or third week. It helps stabilise the fracture site.
What happens to the newly formed cartilage after a bone fracture?
The newly formed cartilage along the fracture line undergoes endochondral ossification, forming a contigunous network of bone with newly deposited bone trabeculae in the medulla and beneath the periosteum.
In this fashion, the fracture ends are bridged, and as it mineralises, the stiffness and strength of the callus increases to the point that controlled weight bearing is tolerated.
How does the bony callus disappear in healed bone?
As the callus matures and is subject to weight-bearing forces, the callus portions that are not physically stressed are resorbed.
In this manner, the callus is reduced in size and the shape and the outline of the fractured bone are re-established as lamellar bone
The sequence of events in the healing of a fracture can be easily impeded or blocked. Give some examples of what by…
- Inadequate immobilisation - causes nonunion
- Infection
- Malnutrition and skeletal dysplasia
What is pathological calcification?
The abnormal tissue deposition of calcium salts, together with smaller amounts of iron, magnesium and other mineral salts
What are the two types of calcification?
- Dystrophic calcification occurs despite normal serum levels of calcium and in the absence of derangements of calcium metabolism
- Metastatic calcification - the deposition of calcium in otherwise normal tissues, almost always results from hypercalcaemia secondary to some disturbance in calcium metabolism
In what conditions would dystrophic calcification take place?
It is encountered in areas of necrosis, whether they are of coagulative, caseous or liquefactive type, and in foci of enzymatic necrosis of fat.
Commonly happens in atherosclerosis and heart valves
How does dystrophic calcification present macroscopically and histologically?
- Macroscopically, calcium salts appear as fine, white granules or clumps, often felt as gritty deposits
- Histologically, they have a basophilic, amorphous, granular, sometimes clumped appearance. You can see psammoma bodies, which are mineral deposits around necrotic cells
Are dystrophic and metastatic calcifications causes of organ dysfunction?
- Dystrophic calcification is often a cause of organ dysfunction, such as in calcific valvular disease
- Metastatic calcification if not often a cause of organ dysfunction, but on occasion massive involvement of the lungs produces some respiratory compromise
What are some common causes of transudates?
Heart failure
Liver failure
Kidney failure
Severe nutritional deficiency
What are some causes of increased hydrostatic pressure?
Increases in hydrostatic pressure are mainly caused by disorders that impair venous return. For example:
Congestive heart failure
Constrictive pericarditis
Ascites
Venous obstruction or compression (thrombosis, external pressures e.g. mass, lower extremity inactivity)
It is also sometimes caused by arteriolar dilation. For example in heat or neurohumoral dysregulation
What causes reduced plasma osmotic pressure?
Under normal circumstances albumin accounts for almost half of the total plasma protein; it follows that conditions leading to inadequate synthesis or increased loss of albumin from the circulation commonly cause reduced plasma oncotic pressure.
* Reduced synthesis occurs mainly in severe liver disease and protein malnutrition
* Increased loss occurs in nephrotic syndrome
How does sodium and water retention occur in oedema?
Increased salt retention (with obligate retention of water) causes both increased hydrostatic pressure and diminshes osmotic pressure (due to dilution).
Salt retention occurs whenever renal function is compromised, such as in primary kidney diseases and in cardiovascular diseases that cause hypoperfusion, resulting in the activation of the renin-angiotension-aldosterone axis.
How does lymphatic obstruction cause lymph-oedema?
Trauma, fibrosis, invasive tumours, and infection agents can all disrupt lymphatic vessels and impair the clearance of interstitial fluid, resulting in lymphoedema in the affected part of the body.
What are the different places you can get oedema?
Subcutaneous oedema
Pulmonary oedema
Pulmonary effusions
Peritoneal effusions (ascites)
Brain oedema
How is oedema recognised microscopically?
It is appreciated as clearing and separation of the extracellular matrix and subtle cell swelling
What is hyperemia vs congestion?
Hyperemia and congestion both stem from increased blood volumes within tissues.
Hyperemia is an active process in which arteriolar dilation (inflammation or exercising muscles) leads to increased blood flow. Affected tissues turn red (erythema) because of increased delivery of oxygenated blood
Congestion is a passive process resulting from reduced outflow of blood from a tissue. It can be systemic (heart failure) or localised (isolated venous obstruction)
What does chronic congestion lead to and why?
As a result of increased hydrostatic pressures, congestion commonly leads to edema.
The associated chronic hypoxia may lead to ischaemic tissue injury and scarring.
Capillary rupture can also produce small haemorrhagic foci; subsequent catabolism of extravasated red cells can leave clusters of haemosiderin-laden macrophages
Microscopically, what will you see in acute pulmonary congestion vs chronic pulmonary congestion?
- Acute pulmonary congestion exhibits engorged alveolar capillaries, alveolar septal emboli, and focal intraalveolar haemorrhage
- In chronic pulmonary congestion (often caused by congested heart failure) the septa are thickened and fibrotic, and the alveoli often contain numerous haemosiderin-laden macrophages called heart failure cells
Microscopically, what will you see in acute hepatic congestion vs chronic hepatic congestion?
- In acute hepatic congestion, the central vein and sinusoids are distended. Because the centrilobular area is distal end of the hepatic blood supply, centrilobular hepatocytes may undergo ischaemic necrosis with the periportal hepatocytes may only develop fatty change (better oxygenated)
- In chronic passive hepatic congestion, the centrilobular regions are grossly red-brown and slightly depressed and are accentuated against the surrounding zones of uncongested tan liver (nutmeg liver).
What is haemostasis?
A precisely organised process involving platelets, clotting factors, and endothelium that occurs at the site of the vascular injury and culminates in the formation of a blood clot, which serves to prevent or limit the extent of bleeding
What two groups are abnormal haemostasis divided into?
- Haemorrhagic disorders - characterised by excessive bleeding, haemostatic mechanisms are either blunted or insufficient to prevent abnormal blood loss.
- Thrombotic disorders - blood clots (thrombi) form within intact blood vessels or within the chambers of the heart.
Are thrombotic and haemorrhagic disorders always separate entities?
No, sometimes in generalised activation of clotting paradoxically produces bleeding due to the consumption of coagulation factors, as in DIC.
What are the four steps in haemostasis?
1) Arteriolar vasoconstriction
2) Primary haemostasis - the formation of the platelet plug
3) Secondary haemostasis - deposition of the fibrin
4) Clot stabilisation and resorption
What happens in arteriolar vasoconstriction during haemostasis?
- It occurs immediately and markedly reduced blood flow to the injured area.
- It is mediated by reflex neurogenic mechanisms and augmented by the local secretion of factors such as endothelin, a potent endothelium-derived vasoconstrictor.
- This effect is transient and bleeding would continue if it wasn’t for the next steps
What happens in primary haemostasis - formation of the platelet plug?
- Disruption of the endothelium exposes subendothelium vWF and collagen, which promote platelet adherence and activation.
- Activation of platelets results in a dramatic shape change (from small rounded discs to flat plates with spiky protrusions that markedly increase surface area), as well as the release of secretory granules (ADP, TxA2)
- Within minutes, the secreted products recruit additional platelets, which undergo aggregation to form a primary haemostatic plug
What happens in secondary haemostasis - deposition of fibrin?
- Tissue factor is exposed at the site of injury. Tissue factor is a membrane-bound procoagulant glycoprotein that is normally expressed by subendothelial cells in the vessel wall, such as smooth muscle cells or fibroblasts.
- Tissue factor binds and activates factor VII, setting in motion a cascade of reactions that generates thrombin.
- Thrombin cleaves circulating fibrinogen into insoluble fibrin, creating a fibrin mesh-work, and is also a potent activator of platelets, leading to additional platelet aggregation at the site of injury.
What happens during clot stabilisation and resorption during haemostasis?
- Polymerised fibrin and platelet aggregates undergo contraction to form a solid, permanent plug that prevents further haemorrhage.
- At this stage, counteregulatory mechanisms (e.g. tissue plasminogen factor t-PA) are set into motion that limit clotting to the side of injury and eventually lead to clot resorption and tissue repair.
What are platelets?
Disc-shaped anucleate cell fragments that are shed from megakaryocytes in the bone marrow into the bloodstream.
What do platelets need to function?
Their function depends on several glycoprotein receptors, a contractile cytoskeleton, and two types of cytoplasmic granules.
What cytoplasmic granules do platelets rely on to function?
- α-granules have the adhesion molecule P-selectin on their membranes and contain proteins involved in coagulation, such as fibrinogen, coagulation factor V, and vWF, as well as protein factors that may be involved in wound healing, such as fibronectin, platelet factor 4 (a heperin-binding chemokine), platelet-derived growth factor (PDGF), and transforming growth factor-β.
- Dense (or δ) granules contain ADP and ATP, ionised calcium, serotonin, and adrenaline
After a traumatic vascular injury, platelets encounter consistuents of the subendothelial connective tissue, such as vWF and collagen. On contact with these proteins, platelets undergo a sequence of reactions the culminate the formation of a platelet plug. What are they?
- Platelet adhesion
- Platelets rapidly change shape
- Secretion of granules content
- Platelet aggregation
How does platelet adhesion take place in plug formation?
- Platelet adhesion is mediated largely by interactions with vWF, which acts as a bridge between the platelet surface receptor glycoprotein 1b (Gp1B) and exposed collagen.
- Notably, the genetic deficiencies of vWF or Gp1B (Bernard-Soulier syndrome) result in bleeding disorders.
What happens when the platelets change shape in plug formation?
- The platelets change shape following adhesion, being converted from smooth discs to ‘sea urchins’ with greatly increased surface area.
- This change is accompanied by alterations in glycoprotein IIb/IIIa that increase it’s affinity for fibrinogen, and by the translocation of the negatively charged phospholipids (particularly phosphatidylserine) to the platelet surface.
- These phospholipids bind calcium and serve as nucleation sites for the assembly of coagulation factor complexes.
What is platelet activation in the plug formation? What triggers it?
Secretion of the granule contents occurs along with the changes in shape; these two events together are known as activation.
- Platelet activation is triggered by a number of factors, including the coagulation factor thrombin and ADP.
- Thrombin activates platelets through a special type of G-protein coupled receptor, protease-activated receptor (PAR), which is switched on by a proteolytic cleavage by thrombin.
- ADP is a component of dense body granules; thus, platelet activation and ADP release begets additional rounds of platelet activation, a phenomenon known as recruitment.
- Activated platelets also produce the prostaglandin thromboxane A2 (TxA2), a potent inducer of platelet aggregation.
What happens during platelet aggregation?
- Platelet aggregation follows their activation.
- The conformational change in glycoprotein 2b/3a that occurs with platelet activation allows a binding of fibrinogen, a large bivalent plasma polypeptide that forms bridges between adjacent platelets, leading to their aggregation
- The inital wave of aggregation is reversible, but concurrent activation of thrombin stabilises the platelet plug by causing further platelet activation and aggregation, and by promoting irreversible platelet contraction
- Platelet contraction is dependent of the cytoskeleton and consolidates the aggregated platelets.
- In parallel, thrombin also converts fibrinogen into insoluble fibrin, cementing platelets in place and creating the definitely secondary hemostatic plug.
- Entrapped red cells and leukocytes are also found in haemostatic plugs, in part due to adherence of leukocytes to P-selectin expressed on activated platelets
What is the coagulation cascade?
It is a series of amplifying enzymatic reactions that leads to the deposition of an insoluble fibrin clot.
What does each step in the coagulation cascade have in common? Where do they happen?
Each reaction step involves an enzyme (an activated coagulation factor), a substrate (an inactive proenzyme form of a coagulaton factor), and a cofactor (a reaction accelerator).
These components are assembled on a negatively charged phospholipid surface, provided by activated platelets.
Which ion and vitamin are important in the coagulation cascade?
- Calcium ions - assembly of reaction complexes depend on calcium, which binds to γ-carboxylated glutamic acid residues that are present in factors
- The enzymatic reactions that produce γ-carboxylated glutamic acid use Vitamin K as a cofounder and are antagonised by many drugs.
What are the two pathways in the coagulation cascade? What do we use to assess them?
Extrinsic and Intrinsic Pathways
* The prothrombin time (PT) assay assess the function of the proteins in the extrinsic pathway (factors VII, X, V, II and fibrinogen). In brief, tissue factor, phospholipids, and calcium are added to plasma and the time for a fibrin clot to form is recorded
* The partial thromboplastin time (PTT) assay screens the function of the proteins in the intrinsic pathway (factors XII, XI, IX, VIII, X, V, II, and fibrinogen). In this assay, clotting of plasma is initiated by addition of negative-charged particles (e.g. ground glass) that activate factor XII (Hageman factor) together with the phospholipids and calcium, and the time to fibrin clot formation is recorded
What do the deficiencys in the different clottin factors lead to? What does this show?
- Deficiencies in factors V, VII, VIII, IX and X are associated with moderate to severe bleeding disorders
- Prothrombin deficiency is incompatible with life
- Factor XI deficiency is only associated with mild bleeding
- Individuals with factor XII deficiency do not bleed and are in fact susceptible to thrombosis
This shows that the process in vivo and in the lab are different!
Which is the most important coagulation factor?
Thrombin, because its various enzymatic activities control divers aspects of haemostasis and link clotting to inflammation and repair
What are thrombin’s most important roles?
- Conversion of fibrinogen into crosslinked fibres - Thrombin directly converts soluble fibrinogen into fibrin monomers that polymerise into an insoluble clot, and also amplifies the coagulation process, not only by activating factor XI, but also by activating factors V and VIII.
- Stabilises the secondary haemostatic plug by activating factor XIII, which covalently cross-links fibrin.
- Platelet activation - thrombin in a potent inducer of platelet activation, and aggregation through its ability to activate PARs, therefore linking platelet function to coagulation
- Pro-inflammatory effects - PARs are also expressed on inflammatory cells, endothelium, and other cell types, and activation of these receptors by thrombin is believed to mediate proinflammatory effects that contribute to tissue repair and angiogenesis
- Anticoagulant effects - upon encountering normal endothelium, thrombin changes from a procoagulant to an anticoagulant. This reversal in function prevents clotting from extending beyond the site of vascular injury
Once initiated, coagulation must be restricted to the site of vascular injury. What are the factors that limit coagulation?
- Simple dilution - blood flowing past the site of injury washes out activated coagulation factors, which are rapidly removed by the liver
- The requirement for negatively charged phospholipids - which are mainly provided by platelets that have been activated by contact with the subendothelial matrix at sites of vascular injury.
- Factors that are expressed by intact endothelium adjacent to the injury
Activation of the coagulation cascade also sets into motion the fibrinolytic cascade that limits the size of the clot and contributes to it’s later dissolution. Tell me about this process…
- Fibrinolysis is largely accomplished through the enzymatic activity of plasmin, which breaks down fibrin and interferes with its polymerisation.
- There is an elevated level of breakdown products of fibrinogen (often called fibrin split products) e.g. d-dimer.
- Plasmin is generated by enzymatic catabolism of the inactive circulating precursor plasminogen, either by a factor XII-derived pathway or by plasminogen activators e.g. tPA.
- Once activated, plasmin is tightly controlled by counterregulatory factors such as α2-plasmin inhibitor, a plasma protein that binds and rapidly inhibits plasmin.
Normal endothelial cells express a multitude of factors that inhibit the procoagulant activities of platelets and coagulation factors and that augment fibrinolysis. What are the different antithrombotic properties of endothelium?
- Platelet inhibitory effects - Intact epithelium acts as a barrier that shields platelets from subendothelial vWF and collagen. It also releases NO, prostacyclin (PGI2), and ADP which inhibit platelet activation and aggregation. They also bind and alter the activity of thrombin, which is a potent activators of platelets.
- Anticoagulant effects - Intact epithelium shields coagulation factors from tissue factor. It also expresses multiple factors that actively oppose coagulation, such as thrombomodulin, endothelial protein C receptor, heparin-like molecules, and tissue factor pathway inhibitor.
- Fibrinolytic effects - normal endothelial cells synthesise t-PA, a key component of the fibrinolytic pathway
How do thrombomodulin and endothelial protein C receptor have anticoagulant effects?
They bind thrombin and protein C, respectively, in a complex on the endothelial cell surface. When bound in this complex, thrombin loses it’s ability to activate coagulation factors and platelets, and instead cleaves and activated protein C, a vitamin K-dependent protease that requires a cofactor, protein S. Activated protein C/protein S complex is a potent inhibitor of coagulation factors Va and VIIIa
How do Heparin-like molecules have anticoagulant properties?
Heparin-like molecules on the surface of endothelium bind and activate anti-thrombin III, which then inhibits thrombin and factors IXa, Xa, XIa, and XIIa.
The clinical utility of heparin and related drugs is based on their ability to stimulate antithrombin III activity.
How does tissue factor pathway inhibitor have anticoagulant properties?
TFPI, like Protein C, requires Protein S as a cofactor, and binds and inhibits tissue factor/factor VIIa complexes
What are the very broad causes of haemorrhagic disorders?
- Primary or secondary defects in vessel walls e.g. aortic dissection or arterial ruptures
- Platelet disorders
- Coagulation factor disorders e.g. haemophilia
How do defects of primary haemostasis present? Give an example
- Platelet defects or von Willebrand disease
- Often present with small bleeds in skin or mucosal membranes, which take the form of petechiae or purpura
- The capillaries of the mucosa and skin are more prone to rupture following minor trauma and under normal circumstances platelets seal these defects virtually immediately.
- Mucosal bleeding associated with disorders of primary haemostasis may present with epistaxis, GI bleeding, or menorrhagia. A fatal complication is intracerebral haemorrhage
What are petechiae, purpura and ecchymoses?
- Petechiae - minute 1-2mm haemorrhages
- Purpura - slightly larger (>3mm) haemorrhages
- Ecchymoses - haemorrhages of 1-2 cm in size
How do defects of secondary haemostasis present? Give an example
- Coagulation factor defects e.g. haemophilia
- Often present with bleeding into soft tissue (muscles) and joints
- Haemarthrosis after minor trauma is particularly characteristic of haemophilia
What does the clinical significance of haemorrhage depend on?
- Volume of the bleed
- Rate at which it occurs
- Location
What percentage of blood loss may lead to haemorrhagic shock?
Over 20%
What are the primary abnormalities that lead to thrombosis? (Virchow’s triad)
- Endothelial injury
- Stasis or turbulent blood flow
- Hypercoagulability of the blood
Where in the body is endothelial injury the most common cause of thrombosis and why?
Endothlial injury leading to platelet activation almost inevitably underlies thrombus formation in the heart and the arterial circulation, where the high rates of blood flow impede clot formation.
These clots are typically rich in platelets, which is why aspirin is good.
What is endothelial activation or dysfunction?
Severe endothelial injury may trigger thrombosis by exposing vWF and tissue factor. However, inflammation and other noxious stimuli also promote thrombosis by shifting the pattern of gene expression in endothelium to one that is ‘prothrombotic’. This is called endothelial activation or dysfunction.
It can be caused by physical injury, infection, abnormal blood flow, inflammatory mediators, metabolic abnormalities and toxins
What are the main endothelial changes that occur during endothelial activation?
- Procoagulant changes - endothelial cells activated by cytokines downregulate the expression of thrombomodulin, which results is sustained action of thrombin, which stimulates platelets and augments inflammation through PARs expressed on platelets and inflammatory cells. Endothelial cells also downregulate the expression of other anticoagulants, such as protein C and tissue factor pathway inhibitor
- Antifibrinolytic effects - activated endothelial cells secrete plasminogen activator inhibitors (PAIs), which limit fibrinolysis, and downregulate the expession of t-PA
What is laminar flow?
Normal blood flow is laminar such that the platelets (and other blood cellular elements) flow centrally in the vessel lumen, separated from endothelium by a slower moving layer of plasma
How do stasis and turbulent flow cause endothelial injury and thrombosis?
- Promote endothelial activation, enhancing procoagulant activity and leukocyte adhesion, in part through flow-induced changes in the expression of adhesion molecules and pro-inflammatory factors
- Disrupt laminar flow and bring platelets into contact with the endothelium
- Prevent washout and dilution of activated clotting factors by fresh flowing blood and the inflow of clotting factor inhibitors
What are some of the causes of stasis and turbulent blood flow in vessels?
- Ulcerated atherosclerotic plaques not only expose subendothelial vWF and tissue factor but also cause turbulence
- Aortic and arterial dilatations (aneurysms) result in local stasis
- Acute MIs are associated with areas of noncontractile myocardium and cardiac aneurysms, both are associated with stasis and flow abnormalities
- Hyperviscosity such as in polycythaemia vera causes increased resistance to flow and stasis
- Deformed red cells in sickle cell anaemia impede blood flow through small vessels, resulting in stasis
What is hypercoagulability? What groups can it be split into
Also called thrombophilia, hypercoagulability can be loosely defined as any disorder of the blood that predisposes to thrombosis.
It can be divided into primary (genetic) and secondary (acquired) disorders
What are some primary causes of hypercoagulability?
Common
* Factor V mutation (Arg to Glu substitution in amino acid residue 506 leading to resistance to factor C; Factor V Leiden
* Prothrombin mutation (G20210A noncoding sequence variant leading to increased prothrombin levels)
* Increased levels of factors VIII, IX, XI or fibrinogen
Rare
* Antithrombin III deficiency
* Protein C deficiency
* Protein S deficiency
Very rare
* Fibrinolysis defects
* Homozygous homocystinuria
How do the following secondary causes of hypercoagulability cause it?
* oral contraceptive
* disseminated cancers
* old age
- Oral contraceptive - increased hepatic synthesis of coagulation factors and reduced anticoagulant synthesis
- Disseminated cancers - release of various pro-coagulants from tumours
- Older age - reduced endothelial PGI2 production
What are some secondary causes of hypercoagulability?
High risk for thrombosis
* prolonged bed rest or immobilisation
* MI
* AF
* Tissue injury (fracture, burn, surgery)
* Cancer
* Prosthetic cardiac valves
* DIC
* Heparin-induced thrombocytopaenia
* Antiphospholipid syndrome
Lower risk for thrombosis
* Cardiomyopathy
* Nephrotic syndrome
* Hyperestrogenic states (pregnancy, oral contraceptive)
* Sickle cell anaemia
* Smoking
What is Heparin-Induced Thrombocytopaenic Syndrome?
- HIT occurs following the administration of unfractionated heparin, which may induce the appearance of antibodies that recognise complexes of heparin and platelet factor 4 on the surface of platelets, as well as complexes of heparin-like molecules and platelet factor 4-like molecules on endothelial cells.
- Binding of these antibodies to platelets results in their activation, aggregation and consumption. This produces a pro-thrombotic state.
- It happens less in LMWH
How may antiphospholipid antibody syndrome present?
- Recurrent thromboses
- repeated miscarriages
- cardiac valve vegetations
- thrombocytopaenia
- pulmonary hypertension from recurrent subclinical PEs
- stroke
- bowel infarction
- renovascular hypertension
What are the two types of antiphospholipid antibody syndrome?
- Secondary antiphospholipid antibody syndrome is when it occurs with individual with a well-defined autoimmune disease, such as SLE
- In primary antiphospholipid antibody syndrome, patients exhibit only the manifestations of a hypercoagulable state and lack evidence of other autoimmune disorders; occasionally it comes following drugs or an infection
Which way to arterial and venous thrombi occur and which ways do they grow?
- Arterial or cardiac thrombi usually begin at sites of turbulence and tend to grow retrograde
- Venous thrombi often begin at sites of stasis and extend in the direction of blood flow
What is a line of Zahn? Where does it happen and what does it mean?
- Thrombi often have grossly and microscopically apparent laminations called lines of Zahn, which are pale platelet and fibrin deposits alternating with darker red cell-rich layers
- They signify that a thrombus formed in flowing blood; they can therefore distinguish post-mortem to ante-mortem clots
What are mural thrombi?
Thrombi that form in heart chambers or in the aortic lumen.
What is different in the content or arterial vs venous clots?
Arterial thrombi often consist of a friable meshwork of platelets, fibrin, red cells, and degenerating leukocytes
Venous thrombi contain more enmeshed red cells and relatively few platelets - they are therefore known as red or stasis thrombi
If a patient survive the initial thrombus, in the ensuing days to weeks, thrombi can have one of four processes happen. What are they?
- Propagation - thrombi accumulate additional platelets and fibrin
- Embolisation - thrombi dislodge and travel to other sites in the vasculature
- Dissolution - the result of fibrinolysis, which can lead to shrinkage and total disappearance of the recent thrombi - only happens in the first few hours
- Organisation and recanalisation - older thrombi become organised by the ingrowth of endothelial cells, smooth muscle cells and fibroblasts. Capillary channels eventually form that reestablish the continuity of the original lumen, albiet to a variable degree. Eventually, with remodeling and contraction of the mesenchymal elements, only a fibrous lump may remain
What veins do superficial and deep venous thrombosis affect?
Superficial - in the saphenous veins - rarely embolise but same symptoms as DVT
Deep - in the popliteal, femoral or iliac veins - often embolise
What happens in DIC?
Widespread formation of thrombi in the microcirculation. These microvascular thrombi can cause diffuse circulatory insufficiency and organ dysfunction, particularly of the brain, lungs, heart and kidneys.
The runaway thrombosis ‘uses up’ platelets and coagulation factors and often activates fibrinolytic mechanisms . Thus, symptoms initially related to thrombosis can evolve into a bleeding catastrophe, such as haemorrhagic stroke or hypovolaemic shock.
What is an embolism?
An embolus is a detached intravascular solid, liquid, or gaseous mass that is carried by the blood from it’s point of origin to a distant site, where it often causes tissue dysfunction or infarction.
What is the most common thromboembolic disease?
Pulmonary emboli from the DVT
Where do pulmonary emboli travel?
Fragmented thrombi from DVTs are carried through progressively larger veins and the right side of the heart before slamming into the pulmonary arterial vasculature.
Depending on the size of the embolus, it can occlude the main pulmonary artery, straddle the pulmonary artery bifurcation (staddle embolus), or pass out into the smaller branching arteries.
Once you have one PE, what happens to your risk?
It increases significantly.
Frequently there are multiple emboli, occuring either sequentially or simultaneously as a shower of small emboli from a single large mass
What is a paradoxical embolism?
Rarely, a venous embolus passes through an interatrial or interventricular defect and gains access to the systemic arterial circulation.
At what percentage occlusion does a PE cause sudden death or Cor Pulmonale?
When emboli obstruct 60% or more of the pulmonary circulation
Why does embolic obstruction of medium-sized pulmonary arteries with subsequent vascular rupture not often cause an infarct? Why is this different in small end-arteriolar pulmonary branches
Because the lung is supplied by both the pulmonary arteries and the bronchial arteries, and an intact bronchial circulation is usually sufficient to perfuse the attached area.
This is not the case in small end-arteriolar pulmonary branches, so they often produce haemorrhage or infarction
Where do most systemic thromboembolisms come from?
80% arise from intracardiac mural thrombi, two thirds of which are associated with left ventricular wall infarcts and one quarter with left atrial dilatation and fibrillation.
The remainder originates from aortic aneurysms, atherosclerotic plaques, valvular vegetations, or venous thrombi
Where do systemic thromboembolisms often end up?
75% in the lower extremities
10% in the brain
May also involve the intestines, kidneys, spleen, and upper extremities
How do fat and marrow emboli occur?
Microscopic fat globules - sometimes associated with marrow- can by found in the pulmonary vasculature after fractures of long bones, soft tissue trauma and burns
Presumably these injuries rupture vascular sinusoids in the marrow or small venules, allowing marrow and adipose tissue to herniate into the vascular space and travel to the lung.
Do fat embolisms often give symptoms?
Fat embolism occurs in 90% of invidiuals with severe skeletal injuries, but less than 10% have any clinical findings.
What is fat embolism syndrome?
The term applied to the minority of patients who become symptomatic.
Characterised by pulmonary insufficiency, neurologic symptoms, anaemia and thrombocytopaenia, and is fatal in about 5 to 15% of cases
What are the symptoms of fat embolism syndrome?
Typically, 1 to 3 days after injury, there is a sudden onset of tachypnoea, dyspnoea, and tachycardia; irritability and restlessness can progress into delirium or come
Why does thrombocytopaenia occur in fat embolism syndrome?
It is attributed to platelet adhesion to fat globules and subsequent aggregation or splenic sequestration; anaemia can result from similar red cell aggregation and/or haemolysis.
A diffuse petechial rash is related to rapid onset of thrombocytopaenia and can be a useful diagnostic feature.
What is the pathogenesis of fat embolism syndrome?
- Fat microemboli and associated red cell and platelet aggregates can occlude the pulmonary and cerebral microvasculature.
- Release of fatty acids from the fat globules exacerbated the situation by causing local toxic injury to endothelium, and platelet activation and granulocyte recruitment (with free radical, protease, and eicosanoid release) complete the vascular assault.
What is an air embolism?
Gas bubbles within the circulation can coalesce to form frothy masses that obstruct vascular flow and cause distal ischaemic injury.
What volume of air is required to have an effect on the pulmonary circulation?
Generally more than 100cc
What is decompression sickness? Why does it happen?
It occurs when an individual experiences sudden decrease in atmospheric pressure.
When air is breathed at high pressure, increased amounts of gas (particularly nitrogen) are dissolved in the blood and tissues. If the diver then ascends too rapidly, the nitrogen comes out of the solution in the tissues and blood
What happens if you get an air embolus in your lungs?
Gas bubbles in the vasculature cause oedema, haemorrhage and focal atelectasis or emphysema, leading to a form of respiratory distress called the chokes
What happens in caisson disease? Who does it happen to?
Persistence of gas emboli in the skeletal system causes multiple foci of ischaemic necrosis; common in the femoral head, tibia and humeri
Happens in chronic decompression sickness, like workers in pressurised vehicles
What is an amniotic fluid embolis? What is it’s mortality?
It is the infusion of amniotic fluid or fetal tissue into the maternal circulation via a tear in the placental membranes or rupture of uterine veins.
It accounts for roughly 10% of maternal deaths in the US, the mortality rate is up to 80% and 85% of survivors are left with permanent neurological damage
What are the symptoms of an amniotic fluid embolus?
The onset is characterised by sudden severe dyspnoea, cyanosis and shock, followed by neurological impairment ranging from headache to seizures.
If the patient survives the initial crisis, pulmonary oedema typically develops, frequently accompanied by DIC.
How does an amniotic fluid embolus vary from a PE?
Much of the morbidity and mortality of an amniotic fluid embolism may stem from the biochemical activation of coagulation factors and components of the innate immune system by substances in the amniotic fluid, rather than the mechanical obstruction of a PE
What is an infarct?
An infarct is an area of ischaemic necrosis caused by occlusion or either the arterial supply or venous drainage.
Arterial thrombosis or arterial embolism underlies the vast majority of infarctions
How can infarcts be classified? Tell me about both groups
-
Red infarcts occur
1) with venous occlusions (e.g. testicular torsion)
2) in loose, spongy tissues (e.g. lung), where blood can collect in the infarcted zone
3) in tissues with dual circulations (e.g. lung and intestine) that allow to blood to flow from an unobstructed parallel supply in a necrotic zone
4) in tissues previously congested by sluggish venous outflow
5) when flow is reestablished to a site of previous arterial occlusion and necrosis - White infarcts occur with arterial occlusions in solid organs with end-arterial circulation (e.g. heart, spleen and kidney), and where tissue density limits the seepage of blood from adjoining capillary beds into the necrotic area.
What shape are infarcts?
Infarcts tend to be wedge-shaped, with the occluded vessel at the apex and the periphery of the organ forming the base.
How does the morphology differ over the first few days and based on the location of infarcts?
- When the base is a serousal surface, there may be an overlying fibrinous exudate resulting from an acute inflammatory response to mediators release from injured and necrotic cells
- Fresh infarcts are poorly defined and slightly haemorrhagic, but over a few days their margins become better defined by a narrow rim of congestion attributable to inflammation
Why are some infarcts red and some white?
- Over time, infarcts resulting from arterial occlusion without a dual blood supply typically become progressively paler and more sharply defined - white infarcts
- Extravasated red cells in haemorrhagic infarcts are phagocytosed by macrophages, which convert heme iron to haemosiderin, extensive haemorrhage can leave a firm, brown, haemosiderin-rich residuum.
What are septic infarctions?
They occur when infected cardiac valve vegetations embolise or when microbes seed necrotic tissue.
In these cases, the infarct is converted into an abscess, with a corresponding greater inflammatory response
What are the factors that influence development of an infarct?
- Anatomy of the vascular supply - is there an alternative blood supply?
- Rate of occlusion - slowly developing occlusions are less likely to cause infarction, because they provide time for collateral pathways of perfusion to form.
- Tissue vulnerability to hypoxia - neurons undergo irreversible damage within 3-4 minutes, myocardial cells last 20-30 minutes, fibroblasts within myocardium can last hours
- Hypoxaemia - the lower the oxygen, the more likely the infarct
What is shock?
Shock is a state in which diminished cardiac output or reduced effective circulating blood volume impairs tissue perfusion and leads to cellular hypoxia
What are the three main types of shock? Why do they happen? What are the other types of shock?
1) Cardiogenic shock results from low cardiac output due to myocardial pump failure. This can be due to intrinsic myocardial damage (infarction), ventricular arrythmias, extrinsic compression (tamponade), or outflow obstruction (PE)
2) Hypovolaemic shock results from low cardiac output due to low blood volume, such as can occur with massive haemorrhage or fluid loss from severe burns
3) Shock associated with systemic inflammation may be triggered by microbial infections, burns, trauma or pancreatitis. The common pathogenic feature is a massive outpouring of inflammatory mediators from innate and adaptive immune cells that produce vasodilation, vascular leaking and venous blood pooling. These result in tissue hypoperfusion, cellular hypoxia and metabolic derangements
4) Less commonly, shock can be neurogenic or anaphylactic, where acute vasodilation leads to hypotension and tissue hypoperfusion
What are most common triggers for septic shock?
Gram-positive bacterial infections, followed by gram-negative bacterial infections and then funghi
In broad terms, what are the factors believed to play a major role in the pathophysiology of septic shock?
- Inflammatory and counter-inflammatory responses
- Endothelial activation and injury
- Induction of a procoagulant state
- Metabolic abnormalities
- Organ dysfunction
What inflammatory responses have a play in the pathophysiology of septic shock?
- Microbial cell wall consituents engage receptors on innate immune cells.
- Likely initiators of inflammation are signalling pathways that lie downsteam of TLRs (which recognise PAMPs), and G-protein coupled receptors that detect bacterial pathogens.
- Upon activation, innate immune cells produce TNF,IL-1, IFN-γ, IL-12 and IL-18
- ROS, platelet activating factor and prostaglandins are also elaborated
- These effector molecules induce endothelial cell activation to upregulate adhesion molecule expression and further stimulate chemokines and cytokines production.
How is the complement cascade involved in the pathophysiology of septic shock?
- The complement cascade is also activated by microbial components, directly and indirectly by the proteolytic activity of plasmin
- This results in the production of anaphylotoxins (C3a, C5a), chemotactic fragments (C5a) and opsonins (C3b), all of which contribute to the pro-inflammatory state
How does the hyperinflammatory state initiated by sepsis also activate counter-regulatory immunosuppressive mechanisms? What does this mean?
- Septic patients may oscillate between hyperinflammatory and immunosuppressed states during their clinical course.
- Proposed mechanisms for the immune suppression include a shift from pro-inflammatory TH1 cells to anti-inflammatory TH2 cells, production of anti-inflammatory mediators (e.g. soluble TNF receptor, IL-1 receptor antagonist, and IL-10), lymphocyte apoptosis, the immunosuppressive act of apoptotic cells, and the induction of cellular energy
How does endothelial activation and injury contribute to the pathophysiology of septic shock?
- It leads to widespread vascular leakage and tissue oedema, which have deleterious effects on both nutrient delivery and waste removal.
- The oedema created impedes tissue perfusion
- Activated endothelium also upregulates production of NO and other vasoactive inflammatory mediators (e.g. C3a, C5a, and PAF), which may contribute to vascular smooth muscle relaxation and systemic hypotension
How does the induction of a procoagulant state contribute to the pathophysiology of septic shock?
- Pro-inflammatory cytokines increase tissue factor production by monocytes and possibly endothelial cells as well, and decrease the production of endothelial anti-coagulant factors (e.g. TFPI, thrombomodulin and protein C)
- They also dampen fibrinolysis by increasing plasminogen activator inhibitor 1 expression.
- The vascular leak and vasodilation causes stasis.
- All of these things together lead to systemic activation of thrombin and the deposition of fibrin-rich thrombi in small vessels throughout the body, often compromising tissue perfusion
How do metabolic abnormalities contribute to the pathophysiology of septic shock?
- Septic patients exhibit insulin resistance and hyperglycaemia.
- Cytokines such as TNF and IL-1, and stress hormones (glucagon, GH, and glucocorticoids) and catecholamines all drive gluconeogenesis.
- Pro-inflammatory cytokines suppress insulin release whilst promoting insulin resistance in the liver and other tissues, likely by impairing the expression of GLUT-4.
- Hyperglycaemia reduced neutrophil function - and causes increased endothelial cell adhesion molecule expression
- Cellular hypoxia and diminished oxidative phosphorylation leads to increased lactate production and lactic acidosis
How does organ dysfuction contribute to the pathophysiology of septic shock?
- Systemic hypotension, interstitial oedema and small vessel thrombosis all decrease the delivery of oxygen and nutrients to the tissue, which fail to properly utilise those nutrients that are delivered due to cellular hypoxia.
- High levels of cytokines and secondary mediators diminish myocardial contractility and cardiac output, and increased vascular permeability and endothelial injury can lead to the acute respiratory distress syndrome.
What affects the outcomes and severity of septic shock?
- The extent and virulence of the infection
- the immune status of the host
- the presence of other co-morbid conditions
- the pattern and level of mediation production
What are the three general phases of hypovolaemic and cardiogenic shock?
- An initial nonprogressive phase during which reflex compensatory mechanisms are activated and perfusion of vital organs is maintained
- A progressive stage characterised by tissue hypoperfusion and onset of worsening circulatory and metabolic imbalances, including a lactic acidosis
- An irreversible stage that sets in after the body has incurred cellular and tissue injury so severe that even if the haemodynamic defects are corrected, survival is not possible
What are the neurohumeral mechanisms that occur in the early non-progressive stage of shock that help to maintain cardiac output and blood pressure?
- Baroreceptor reflexes, catecholamine release, activation of the renin-angiotensin axis, ADH release, and generalised sympathetic stimulation.
- The net effect is tachycardia, peripheral vasoconstriction, and renal conservation of fluid
- Coronary and cerebral vessels are less sensitive to the sympathetic response and therefore maintain relatively normal caliber, blood flow and oxygen delivery
What happens during the progressive phase of shock?
- There is widespread tissue hypoxia
- In the setting of persistent oxygen deficit, intracellular aerobic respiration is replaced by anaerobic glycolysis with excessive production of lactic acid
- The lactic acidosis lowers the tissue pH and blunts the vasomotor response, arterioles dilate, and blood begins to pool in the microcirculation
- Peripheral pooling not only worsens the cardiac output, but also put the endothelial cells at risk of developing anoxic injury with subsequent DIC.
What happens during the irreversible stage of shock?
- Widespread cell injury is reflected in lysosomal enzyme leakage, further aggrevating the shock state.
- If ischaemic bowel allows intestinal flora into the circulation, bacterial septic shock may be superimposed
- At this point, the patient may develop anuria as a result of acute tubular necrosis and acute renal failure
- They may develop a coagulopathy
What are the morphological adrenal changes during shock?
There is cortical cell depletion. This does not reflect adrenal exhaustion but rather the conversion of the relatively inactive vacuolated cells to metabolically active cells that utilise stored lipids for the synthesis of steroids
Immunity is protection from infectious pathogens. The mechanisms of defense against microbes fall into two broad categories. What are they?
- Innate immunity refers to the mechanisms that are ready to react to infections even before the occur, and that have evolved to specifically recognise and combat microbes
- Adaptive immunity consists of mechanisms that are stimulated by microbes and are capable of recognising microbial and non-microbial substances.
When do innate immunity and adaptive immunity take place?
- Innate immunity is the first line of defence. It is mediated by cells and molecules that recognise products of microbes and dead cells and induce rapid protective host reactions.
- Adaptive immunity develops later, after exposure to microbes and other foreign substances, and is even more powerful that innate immunity in combating infections
What are the stages of innate immunity?
Innate immunity is always present, ready to provide defense against microbes and to eliminate damaged cells.
The three stages are: recognition of microbes and damaged cells, activation of various mechanisms, and elimination of unwanted substances
What are the major components of innate immunity?
Epithelial barriers that block entry of microbes, phagocytic cells (mainly neutrophils and macrophages), dendritic cells, natural killer cells, and several plasma proteins, including the proteins of the complement system.
What are the epithelial barriers in innate immunity?
- Epithelia of the skin and GI and resp tracts provide mechanical barriers to the entry of microbes from the external environment
- Epithalial cells also produce antimicrobial molecules, such as defensins, and lymphocytes located in the epithelia combat microbes at these sites
How do monocytes and neutrophils contribute towards innate immunity?
They are phagocytes in the blood that can rapidly be recruited to any site of infection; monocytes nter the tissues and mature to macrophages.
All tissues contain resident macrophages, which not only sense the presence of microbes and other offeding agents, but also ingest (phagocytose) these invaders and destroy them
How do dendritic cells contribute to innate immunity?
- They are a specialised cell population present in epithelia, lymphoid organs and most tissues.
- They capture protein antigens and display peptides for recognition by T lymphocytes.
- They are endowed with a rich collection of receptors that sense microbes and cell damage and stimulate the secretion of cytokines, mediators that play critical roles in inflammation and anti-viral defense
- Thus, dendritic cells are involved in the initiation of innate immune responses, but not destruction of microbes and other offending agents.
What are the roles of natural killer cells in innate immunity?
They provide early protection against many viruses and intracellular bacteria
What are pathogen-associated molecular patterns (PAMPs)?
Cells that participate in innate immunity are capable of recognising certain microbial components that are shared among related microbes and are often essential for infectivity (and thus cannot be mutated). These are called PAMPs
What are damage-associated molecular patterns?
Leukocytes recognise molecules released by injured and necrotic cells, which are called damage-associated molecular patterns (DAMPs).
What and where are pattern recognition receptors?
The cellular receptors that recognise PAMPs and DAMPs are often called pattern recognition receptors. They are located in all the cellular compartments where microbes may be present: plasma membrane receptors detect extracelluls microbes, endosomal receptors detect ingested microbes, and cytosolic receptors detect microbes in the cytoplasm
What are the different classes of pattern recognition receptors?
- Toll-like receptors
- NOD-like receptors
- C-type lectin receptors
- RIG-like receptors
- G protein-coupled recetprs
- Mannose receptors
Where are Toll-like Receptors (TLRs) and what do they do?
They are present in the plasma membrane and endosomal vesicles.
All these receptors signal by a common pathway that culminates in the activation of two sets of transcription factors:
1) NF-κB, which stimulates the synthesis and secretion of cytokines and the expression of adhesion molecules
2) Interferon regulatory factors (IRFs), which stimulate the production of antiviral cytokines, type I interferons
Where are NOD-like receptors (NLRs) and what do they recognise?
They are cytosolic receptors that recognise a wide variety of substances, including products of necrotic cells (uric acid and released ATP), ion disturbances (loss of K+), and some microbial products.
What is the inflammasome and how is it related to NLRs? What does it do?
Several of the NLRs signal via a cytosolic multiprotein complex - the inflammasome, which activates an enzyme (caspase-1) that cleaves a precursor form of IL-1 to generate the biologically active form.
Where are C-type lectin receptors (CLRs)? What do they do?
- They are expressed on the plasma membrane of macrophages and dentritic cells.
- They detect fungal glycans and ellicit inflammatory reactions to funghi.
Where are RIG-like receptors (RLRs)? What do they do?
- They are located in the cytosol of most cell types
- They detect nucliec acids of viruses that replicate in the cytoplasm of infected cells.
- They stimulate the production of antiviral cytokines
Where are G protein-coupled receptors? What do they do?
- They are on neutrophils, macrophages and most other types of leukocytes
- They recognise short bacterial peptides containing N-formylmethionyl residues, which is produced by all bacteral proteins and few mammalian proteins, which enables neutrophils to detect bacterial proteins and stimulate chemotactic responses of the cells
What do mannose receptors do?
They recognise microbial sugars and induce phagocytosis of the microbes
The innate immune system provides host defense by two main reactions. What are they?
- Inflammation - cytokines and products of complement activation are produced during innate immune reactions and trigger the vascular and cellular components of inflammation. The recruited leukocytes destroy microbes and ingest and eliminate damaged cells
- Antiviral defense - type I inferferons produced in response to viruses act on infected and uninfected cells and activate enzymes that degrade viral nucleic acids and inhibit viral replication.
How many different receptors does the innate immune system use compared to the adaptive immune system and why?
Innate immunity does not have memory or fine antigen specificity. It is estimated that innate immunity uses about 100 different receptors to recognise 1,000 molecular patterns.
In contrast, adaptive immunity uses two types of receptors (antibodies and T-cell receptors), each with millions or variations
What does the adaptive immune system consist of?
Lymphocytes and their products, inlduing antibodies.
There are two types of adaptive immunity. What are they and what are they mediated by?
- Humoral immunity, which protects against extracellular microbes and their toxins, is mediated by B lymphocytes and their secreted products, antibodies (AKA immunoglobulins).
- Cell-mediated immunity, which is responsible for defence against intracellular microbes, is mediated by T-lymphocytes.
Where are lymphocytes and why are they there?
They are not fixed in particular tissues but constantly circulate among lymphoid and other tissues via the vlood and the lymphatic circulation.
This promotes immune surveillance by allowing lymphocytes to home to any site of infection.
What are the different types of lymphocytes (5) and what do they do?
What are naive lymphocytes?
Mature lymphocytes that have not encountered the antigen for which they are specific are said to be naive (immunologically inexperienced).
After lymphocytes are activated by recognition of antigens, they differentiate into what? What do these cells do?
- Effector cells, which perform the function of eliminated microbes
- Memory cells, which live in a state of heightened awareness and are able to react rapidly and strongly to combat the microbe in case it returns
What is clonal selection?
Lymphocytes specific for a large number of antigens exist before exposure to antigen, and when an antigen enters, it selectively activates the antigen-specific cells. This fundamental concept is called clonal selection.
Antigen receptor diversity is generated by somatic recombination of the genes that encode the receptor proteins. How does this process happen?
All cells of the body contain antigen receptors genes in the germline configuration, in which the genes encoding these receptors consist of spatially separated segments that cannot be expressed as proteins. During lymphocyte maturation, these gene segments recombine in random sets and variations are introduced at the sites of recombination, forming many different geners that can be transcribed and translated into functional antigen receptors
What is the enzyme in developing lymphocytes that mediates recombination of the gene segments?
It is the product of RAG-1 and RAG-2 (recombination activating genes)
What cells contain recombined antigen receptor genes? How does this influence tumour identification?
Germline receptor genes are present in all cells in the body, but only T and B cells contain recombined antigen receptor genes (T-cell receptors in T cells and immunoglobulin in B cells).
Hence the presence of recombined T-cell receptors or Ig genes, is a marker of T- or B-lineage cells.
There are three major populations of T cells, which serve distinct functions. What are they and what do they do?
- Helper T lymphocytes stimulate B lymphocytes to make antibodies and activate other leukocytes (e.g. phagocytes) to destroy microbes
- Cytotoxic T lymphocytes kill infected cells
- Regulatory T lymphocytes limit immune responses and prevent reactions against self-antigens
Where do T cells develop and from what?
They develop in the thymus from precursors that arise from haematopoetic stem cells.
Where are mature T cells found and what percentage of lymphocytes do that constistute?
They are found in the blood, where they constitute 60-70% of lymphocytes, and in T-cell zones of peripheral lymphoid organs.
How do T cells recognise antigens and what is this thing made up of?
Each T cell recognises a specific cell-bound antigen by means of an antigen-specific TCR.
In approximately 95% of T cells, the TCR consists of a disulfide-linked heterodimer made up of an α and a β polypeptide chain, each having a variable (antigen-binding) region and a constant region.
The αβ TCR recognises peptide antigens that presented by what?
Major histocompatibility complex (MHC) molecules on the surfaces of antigen-presenting cells.
What is MHC restriction? What is its purpose?
By limiting the specificity of T cells for peptides displayed by cell surface MHC molecules, called MHC restriction, the immune system ensures that T cells see only cell-associated antigens (e.g. those derived from microbes in cells or from proteins ingested by cells
What forms the TCR complex?
Each TCR is noncovelently linked to six polypeptide chains, which form the CD3 complex and the ζ chain dimer. The CD3 and ζ proteins are invarient (identical) in all T cells. They are involved in the transduction of signals into the T cell that are triggered by binding of antigen to the TCR. Together, with the TCR, these proteins form the TCR complexes
In addition to CD3 and ζ proteins, T cells express several other proteins that assist the TCR complex in functional responses. These include CD4, CD8, CD28 and integrins. CD4 and CD8 are expressed on two mutually exclusive subsets of αβ T cells. What percentage are CD4 and what percentage are CD8. What do they do?
- Approximately 60% of mature T cells are CD4+ and about 30% are CD8+.
- Most CD4+ T cells function as cytokine-secreting helper cells that assist macrophages and B lymphocytes to combat infections.
- Most CD8+cells function as cytotoxic (killer) T lymphocytes to destroy host cells harboring microbes
What MHC molecules do CD4+ and CD8+ bind to? What happens after that?
- During antigen recognition, CD4 molecules bind to class II MCH molecules that are displaying antigen.
- CD8 molecules bind to class I MHC molecules
- The CD4 or CD8 coreceptor initiates signals that are necessary for activation of the T cells.
What cells are the only cells in the body capable of producing antibody molecules?
B lymphocytes
Where are B lymphocytes produced and where are they found?
B lymphocytes develop from precursors in the bone marrow.
Mature B cells consistute 10% to 20% of the circulating peripheral lymphocyte population and are also present in peripheral lymphoid tissues such as lymph nodes, spleen and mucose-associated lymphoid tissues.
How do B cells recognise antigens? How do they bind to them?
- B cells recognise antigen via the B-cell antigen receptor complex.
- Membrane bound antibodies of the IgM and IgD isotypes, present on the surface of all mature, naive B cells, are the antigen-binding component of the B-cell receptor complex.
What happens in B cells after they have bound to an antigen?
After stimulation by antigen and other signals, B cells develop into plasma cells, vertiable protein factories for antibodies.
How many antibody molecules can plasma cells secrete per second?
A single plasma cell can secrete hundreds to thousands of antibody molecules per second.
What are plasmablasts?
Antibody-secreting cells also detected in human peripheral blood.
What does the B-cell antigen receptor complex consist of?
It contains a heterodimer of Igα (CD79a) and Igβ (CD79b), which are essential for signal transduction through the antigen receptor
Apart from CD79a and CD79b, what are some other molecules that B cells also express that are essential for their reponses and what do they do?
- The type 2 complement receptor (CD2 or CD21), which recognises complement products generated during innate immune responses to microbes
- CD40, which receives signals from helper T cells
- CD21 is also used by the EBV as a receptor to enter and infect B cells
What is the purpose of dendritic cells?
They are the most important antigen-presenting cells for initiating T-cell responses against protein antigens.
Several features of dendritic cells account for their key role in antigen presentation. What are they?
- They are located at the right place to capture antigens - under epithelia, the common site of entry of microbes and foreign antigens, and in the interstitia of all tissues, where antigens may be produced
- They express many receptors for capturing and responsing to microbes (and other antigens), including TLRs and lectins
- In response to microbes, dendritic cells are recruited to the T-cell zones of lymphoid organs, where they are ideally located to present antigens to T cells.
- Dendritic cells express high levels of MHC and other molecules needed for presenting antigens to and activating T cells
What is a follicular dendritic cell? What do they do?
A second type of cell with dendritic morphologi present in the germinal centers of lypmhoid follicles in the spleen and lymph nodes and is called the follicular dendritic cell.
These cells bear Fc receptors for IgG and receptors for C3b and can trap antigen bound to antibodies or complement proteins. Such cells play a role in humoral immune responses by presenting antigens to B cells and selecting the B cells that have the highest affinity for the antigen, thus improving the quality of the antibody produced.
What is the function of natural killer cells?
The function of NK cells is to destory irreversibly stressed and abnormal cells, such as virus-infected cells and tumour cells
What is the morphology of natural killer cells?
They are somewhat larger than small lymphocytes, and they contain abundant azurophilic granules
What ability of natural killer cells makes then an early line of defense against viral infections and, perhaps, some tumours?
Natural killer cells are endowed with the ability to kill a variety of virus-infected cells and tumour cells, without prior exposure to or activation by these microbes or tumours.
What are the two cell surface molecules on natural killer cells? What do they do?
Two cell surface molecules, CD16 and CD56, are commonly used to identify NK cells.
CD16 is an Fc receptor for IgG, and it confers on NK cells the ability to lyse IgG-coated target cells. This process is known as antibody-dependent cell-mediated cytotoxicity.
How is the functional activity of NK cells regulated?
- By a balance between signals from activating and inhibitory receptors.
- There are many types of activating receptors, of which the NKG2D family is the best characterised - they recognise surface molecules that are induced by various kinds of stress, such as infections and DNA damage.
- NK cell inhibitory receptors recognise self class I MHC molecules, which are expressed on all healthy cells. The inhibitory receptors prevent NK cells from killing normal cells.
How does viral infection or neoplastic transformation trigger natural killer cells?
It often enhances expression of ligands for activating receptors and at the same time reduces the expression of class I MHC molecules.
As a result, the balance is tilted towards activation, and the infected or tumour cell is killed.
What cytokines are associated with natural killer cells?
Which ones do they secrete? Which ones stimulate proliferation and activate killing?
- Natural killer cells secrete cytokines such as interferon-γ, which activates macrophages to destroy ingested microbes, and thus NK cells provide early defense against intracellular microbial infections.
- The activity of NK cells is regulated by many cytokines, including the interleukins (IL-2, IL-15, and IL-12)
- IL-2 and IL-15 stimulate proliferation of NK cells
- IL-12 activated killing and secretion of IFN-γ
What are the peripheral lymphoid organs and what do they do?
- Lymph nodes - antigen-presenting cells in the nodes are able to sample the antigens of microbes that may enter through epithelia into tissues and are carried in the lymph. Dendritic cells transport antigens of microbes from epithelia and tissues via lymph vessels to the nodes. Thus, the antigens become concentrated in lymph nodes
- Spleen - Blood entering the spleen flows through a network of sinusoids. Bloodborne antigens are trapped by dendritic cells and macrophages in the spleen
- The cutaneous and mucosal lymphoid systems are located under the epithelia of the skin and the GI and resp tracts, respectively. Pharyngeal tonsils and Peyer’s patches of the intestine are two anatomically defined mucosal lymphoid tissues
Within the peripheral lymphoid organs, T lymphocytes and B lymphocytes are segregated into different regions. What are they?
- In lymph nodes the B cells are concentrated in discrete structures, called follicles, located around the periphery, or cortex, of each node. If the B cells in a follicle have recently responded to an antigen, this follicle may contain a central region called a germinal center.
- The T lymphoctyes are concentrated in the paracortex, adjacent to the follicles.
- The follicles contain the follicular dendritic cells, involved in B cell activation, and the paracortex contains the dendritic cells that present antigens to T lymphocytes
What is lymphocyte recirculation and why is it important?
Lymphocytes constantly recirculate between tissues and home to particular sites; naive lymphocytes traverse the peripher lympoid organs where immune responses are initiated and antigens are concentrated and effector T cells migrate to sites of infection and inflammation, to locate and eliminate microbes at any site of infection.
What is the purpose of MHC molecules?
To display peptide fragments of protein antigens for recognition by antigen-specific T cells.
What are human leukocyte antigens (HLA)?
In humans, the MHC molecules are called HLA because they were initially detected on leukocytes by the binding of antibodies.
On the basis of their structure, cellular distibution and function, MHC gene products are classified into two major classes. What are these classes and where are they found?
- Class I MHC molecules are expressed on all nucleated cells and platelets. They are heterodimers made of a polymorphic α heavy chain linked convalently to a smaller non-polymorphic β2-microglobulin.
- Class II MHC molecules are mainly expressed on cells that present ingested antigens and respond to T-cell help (macrophages, B lymphocytes, and dendritic cells).
What is the role of Class I and Class II MHC molecules?
- Class I MHC molecules display peptides that are derived from proteins, such as viral and tumour antigens, that are located in the cytoplasm and usually produced in the cell, and class I-associated peptides are recognised by CD8+ T lymphocytes
- Class II MHC molecules present antigens that are internalised into vesicles, and are typically derived from extracellular microbes and soluble proteins, class II-peptide complex is recognised by CD4+ T cells, which function as helper cells
MHC molecules play several key roles in regulating T cell-mediated immune reponses. What are they?
1) Because diferent antigenic peptides bind to different MHC molecules, it follows that an individual mounts an immune response against a protein antigen only if they inherit the genes for those MHC molecules that can bind peptides derived from the antigen and present it to T cells
2) By segregating cytoplasmic and internalised antigens, MHC molecules ensure that the correct immune reponse is mounted against different microbes - cytotoxic lymphocyte mediated killing of cells harboring cytoplasmic microbes, and helper T cell-mediated antibody and macrophage activation to combat extracellular microbes.
Cytokines contribute to different types of immune responses. How do they contribute to innate vs adaptive immune responses? Give examples for each type
- In innate immune responses, cytokines are produced rapidly after encounter with microbes and other stimuli, and function to include infammaltion and inhibit virus replication - TNF, IL-1, IL-2, type 1 IFNs, IFN-γ, and chemokines, produced mainly by macrophages, dendritic cells and natural killer cells
- In adaptive immune responses, cytokines are produced principally by CD4+ T lymphocytes activated by antigen and other signals, and function to promote lymphocyte proliferation and differentiation and to activate effector cells. - IL-2, IL-4, IL-5, IL-17, IFN-γ.
What cells secrete IL-2 and what does it do?
One of the earliest responses of CD4+ helper T cells is secretion of the cytokine IL-2 and expression of high-affinity receptors for IL-2. IL-2 is a growth factor that acts on these T lymphocytes and stimulates their proliferation, leading to an increase in the number of antigen-specific lymphocytes.
Some of the activated CD4+ T cells differentiate into effector cells that secrete different sets of cytokine and perform different functions. What are these sets and their functions?
- T helper 1 cells secrete the cytokine IFN-γ, which is a potent macrophage activator. The combination of CD40- ad IFN-γ mediated activation results in “classical” macrophage activation, leading to the induction of microbicidal substances in macrophages and the destruction of ingested microbes
- T helper 2 cells produce IL-4, which stimulates B cells to differntiate into IgE-secreting plasma cells, and IL-5, which activates eosinophils. Eosinophils and mast cells bind to IgE-coated microbes such as helminthic parasites, and function to eliminate helminths. They also induce the ‘alternative’ activation pathway of macrophages
- T helper 17 cells, which produce IL-17, recruit neutrophils and monocytes, which destroy some extracellular bacteria and funghi and are involved in some inflammatory diseases
What do activated CD8+ T lymphocytes differetiate into? What do they do?
They differentiate into cytotoxic T lymphocytes that kill cells harboring microbes in the cytoplasm. By destroying the infected cells, CTLs eliminate the reservoirs of infection
Antibody responses to most protein antigens require T cell help and are said to be T-dependent. In what way is this the case?
- B cells ingest protein antigens into vesicles, degrade them, and display peptides bound to class II MHC molecules for recognition by helper T cells.
- The helper T cells are activated and express CD40L and secrete cytokines, which work together to stimulate the B cells.
What antigens are T-independent? What happens here?
Many polysaccharide and lipid antigens cannot be recognised by T cells but have multiple identical antigenic determinants (epitopes) that are able to engage many antigen receptor molecules on each B cell and initate the process of B-cell activation; these responses are said to be T-independent
Each plasma cell is derived from an antigen-stimulated B cell and secretes antibodies that recognise the same antigen that initiated the reponse. What type of antibodies do different molecules stimulate the secretion of?
- Polysaccarides and lipids stimulate secretion mainly of IgM antibody
- Protein antigens, by virtue of CD40L- and cytokine-mediated helper T-cell actons, induce the production of antibodies of different classes, or isotypes (IgG, IgA, IgE)
What is affinity maturation and isotype switching? Where do they occur and what stimulates them?
- Isotype switching is induced by cytokines including IFN-γ and IL-4.
- Helper T cells also stimulate the production of antibodies with high affinities for the antigen. This process, affinity maturation, improved the quality of the humoral immune response
- They occur mainly in germinal centers, which are formed by proliferating B cells, especially in helper T cell-dependent response to protein antigens
What are follicular helper T cells? What do they do?
Some activated B cells migrate into follicles and form germinal centers, which are the major sites of isotype switching and affinity maturation. The helper T-cells that stimulate these process in B lymphocytes migrate to and reside in the germinal centers and are called follicular helper T cells.
The humoral immune response combates microbes in many ways. What are they?
- Antibodies bind to microbes and prevent then from infecting cells, thus neutralising the microbes
- IgG antibodies coat (opsonise) microbes and target them for phagocytosis as phagocytes express receptors for the Fc tails of IgG.
- IgG and IgM activate the complement system by the classical pathway, and complement products promote phagocytosis and destruction of microbes
- IgA is secreted from mucoseal epithelia and neutralises microbes inthe lumens of the respiratory and GI tracts
- IgG is actively transported across the placenta and protects the newborn until the immune system becomes mature
- IgE and eosinophils cooperate to kill parasites, mainly by release of eosinphil granules contents that are toxic to the worms.
- TH2 cytokines stimulate the production of IgE and activeate eosinophils, and thus the response to helminths is orchestrated by TH2 cells
There are several important general features of hypersensitivity disorders. What are they?
- Hypersensitivity reactions can be eilicited by exogenous environmental antigens (microbial and non-microbial) or endogenous self antigens
- Hypersensitivity usually results from an imbalance between the effector mechanisms of immune responses and the control mechanisms that serve to normally limit such responses
- The development of hypersensitivity diseases is often associated with the inheritance of a particular susceptibility genes (HLA genes)
- The mechanisms of tissue injury in hypersensitivity reactions are the same as the effector mechanisms of defense against infectious pathogens
What are the different types of hypersensitivity reactions?
1) Immediate hypersensitivity - caused by TH2 cells, IgE antibodies, and mast cells and other leukocytes. Mast cells release mediators that act on vessels and smooth msucle and proinflammatory cytokiens that recruit inflammatory cells
2) Antibody-mediated disorders - secreted IgG and IgM antibodies injure cells by promoting their phagocytosis or lysis and injure tissues by inducing inflammation
3) Immune complex-mediated disorders - IgG and IgM antibodies bind antigens usually in the circulation, and the antigen-antibody complexes deposit in tissues and induce inflammation. The leukocytes that are recruited produce tissue damage by release of lysosomal enzymes and generation of toxic free radicals
4) Cell-mediated immune disorders - sensitised T lymphocytes (Th1 and Th17 cells and CTLs) are the cause of the tissue injury. Th2 cells indue lesions that are part of immediate hypersensitivity reactions and are not considered a form of type IV hypersentivity
How is an immediate or type 1 hypersensitivity reaction triggered?
It is a rapid immunological reaction occurring in a previously sensitised individual that is triggered by the binding of an antigen to IgE antibody on the surface of mast cells
Local reactions are diverse and vary depending on the portal of entry of the allergen in type 1 hypersensitivity reaction?
Localised cutaneous rash or blisters (shin allergy, hives), nasal and conjunctival discharge (allergic rhinitis and conjunctivitis), hay fever, bronchial asthma, or allergic gastroenteritis
Many local type 1 hypersensitivity reactions have two well-defined phases. What are they and what happens during them?
1) The immediate reaction is characterised by vasodilation, vascular leakage, and depending on the location, smooth muscle spasm or glandular secretions. These changes usually become evident within minutes after exposure to an allergen and tend to subside in a few hours.
2) A second, late-phase reaction sets in 2-24 hours later without additiona exposure to antigen and may last for several days. This late-phase reaction is characterised by infiltration of tissues with eosinophils, neutrophils, basophils, monocytes and CD4+ T cells, as well as tissue destruction, typically in the form of mucosal epithelial cell damage
Most immediate hypersensitivity disorders are caused by excessive responses from what cells? What do they do?
Most are caused by excessive Th2 responses and these cells play a central role by stimulating IgE production and promoting inflammation.
The first step in the generation of Th2 cells is the presentation of the antigen (likely by dendritic cells) to naive CD4+ helper T cells. How do naive CD4+ helper cells respond to this?
- In response to antigen and other stimuli (IL-4), the T cells differentiate into Th2 cells.
- The newly minted Th2 cells produce a number of cytokines upon consequence encounter with the antigen (IL-4, IL-5, IL-13).
- IL-4 acts on B cells to stimulate class swtiching to IgE and promotes the development of Th2 cells
- IL-5 is involved in the development and activation of eosinophils.
- IL-13 enhances IgE production and acts on epithelial cells to stimualte mucus secretion
- In addition, Th2 cells produce chemokines that attract more Th2 cells, as well as other leukocytes to the reaction site
Where are mast cells? What do they contain?
- They are bone marrow-derived cells that are widely distributed in the tissues, being abundant near blood vessels and nerves and in subepithelial tissues
- Mast cells have cytoplasmic membrane-bound granules that contain a variety of biologically active mediators
How are mast cells activated in type 1 hypersensitivity reactions?
- They are activated by the cross-linking of high-affinity IgE Fc receptors
- They may also be triggered by complement components C5a and C3a (anaphylatoxins), both of which act by binding to receptors on the mast cell membrane.
- Other mast cell secretagogues include some chemokines (e.g. IL-8), drugs such as codeine, morphine, adenosine, bee stings and physical stimuli (heat, cold, sunlight)
How are basophils and mast cells similar and what makes them different?
They are similar in many ways, including the presence of cell surface IgE Fc receptors as well as cytoplasmic granules.
In contrast to mast cells, however, basophuls are not normally present in tissues but rather circulate in the blood in extremely small numbers.
Mast cells and basophils express a high-affinity receptor, called FcεRI. What is this specific for and what does this mean?
FcεRI is specific for Fc portion of IgE and therefore avidly binds IgE antibodies.
When are masts cells sensitised?
IgE-coated mast cells are said to eb sensitised, because they are sensitive to a subsequent encounter with the specific antigen.
When a sensitised mast cell is exposed to the same antigen, what happens?
The cell is activated, leading eventually to the release of an arsenal of powerful mediators responsible for the clinical features of immediate hypersensitivity reactions.
What are the steps of mast cell activation?
- The antigen binds to the IgE antibodies precious attached to the amst cells
- Multivalent antigens bind to and cross-link adjacent IgE antibodies.
- The underlying Fcε receptors are brought together, and this activates signal transudction pathways from the cytoplasmic portion of the receptors
- These signals lead to the production of mediators that are responsible for the initial, sometimes explosive, symptoms of immediate hypersensitivity, and they also set into motion the events that lead to the late-phase reaction
Mast cell activation leads to degranulation, with the discharge of preformed (primary) mediators that are stored in the granules, and de novo synthesis and release of secondary mediators, including lipid products and cytokines. Name some preformed mediators and what they do?
- Vasoactive amines - most important is histamine, causing smooth muscle contraction, increased vascular permeabilityk and increased mucus secretion
- Enzymes - contained in the granule matrix, include neutral proteases (chymase, tryptase) and several acid hydrolases. The enzymes cause tissue damage and lead to the generation of kinins and activated components of complement (C3a) by acting on their precursor proteins
- Proteoglycans - these include heparin and chondroitin sulfate, they seve to packes and store the amines in the granules
Mast cell activation leads to degranulation, with the discharge of preformed (primary) mediators that are stored in the granules, and de novo synthesis and release of secondary mediators, including lipid products and cytokines. Name some lipid mediators and what they do?
The major lipid mediators are arachidonic acid-derived products. Reactions in the mast cell membranes lead to activation of phospholipase A2.
* Leukotrienes C4 and D4 are the most potent vasoactive and spasmogenic agents known, they are several thousant times more active than histamine in increaseing vascular permeability and causing bronchial smooth muscle contraction. Leukotriene B4 is highly chemotactic for neutrophils, eosinophils and monocyes
* Prostaglandin D2 is the most abundant mediator produced in mast cells by the COX pathway, it causes intense bronchospasm and increased mucus secretion
* Platelet-activating factor is a lipid mediated produced by some mast cell populations but is not derived from AA. It causes platelet aggregation, release of histmine, bronchospasm, increased vascular permeability, and vasodilation.
Mast cell activation leads to degranulation, with the discharge of preformed (primary) mediators that are stored in the granules, and de novo synthesis and release of secondary mediators, including lipid products and cytokines. Name some cytokines and their roles.
- TNF, IL-1 and chemokines, which promote leukocytes recruitment
- IL-4, which amplified the Th2 response
On a morphological level, what happens during the late-phase reaction during type 1 hypersensitivity reactions?
In the late-phase reaction, leukocytes are recruited that amplify and sustain the inflammatory response without additional exposure to the tiggering antigen.
* Eosinophils are often an abundant leukocyte population in these reactions
How are eosinophils recruited to sites of immediate hypersensitivity?
By chemokines, such as eotaxin, and others that may be produced by epithelial cells, Th2 cells, and mast cells
The Th2 cytokine IL-5 is the most potent eosinophil-activating cytokine known.
Upon activation in late-phase immediate hypersensitivity reactions, what do eosinophils do?
They liberate proteolytic enzymes, as well as two unique proteins called major basic protein and eosinophil cationic protein, which damage tissues.
An increase propensity to develop immediate hypersensitivity reactions is called atopy. What are the morphological changes that atopic individuals tend to have?
They tend to have higher serum IgE levels and more IL-4 producing Th2 cells than does the general population.
Studies in patients with asthma reveal linkage to polymorphisms in several genes. What chromosome are these genes encoded, what are they and what do they do?
- Some of these genes are located in the chromosome 5q31 region
- These include genes encoding the cytokines IL-3, IL-4, IL-5, IL-9, IL-13 and GM-CSF.
What is nonatopic allergy?
It is estimated that 20-30% of immediate hypersensitivity reactions are triggered by non-antigenic stimuli, such as temperature extremes and exercsie, and do not invovle Th2 cells or IgE; such reactions are sometimes called nonatopic allergy
Give me an overview of what happens during antibody-mediated (type II) hypersensitivity reactions
Anibodies that react with antigens present on cell surfaces or in the extracellular matrix cause disease by destroying these cells, triggering inflammation, or interfering with normal functions.
What are the three steps that occur during antibody mediated (type II hypersensitivity) reactions?
1) Opsonisation and phagocytosis
2) Complement and Fc receptor-mediated inflammation
3) Antibody-mediated cellular dysfunction
What happens during opsonisation and phagocytosis in antibody mediated (type 2) hypersensitivity reaction?
- Cells opsonised by IgG antibodies are recognised by phagocyte Fc receptors.
- When IgM or IgG antibodies are deposited on the surfaces of cells, they may activated the complement system and C3b and C4b are deposited on the surfaces of the cells and recognised by phagocytes that express receptors for these proteins.
The net result is phagocytsis of the opsonised cells and their destruction.
Clinically, in what situations does antibody-mediated cell destruction and phagocytosis occur?
1) Transfusion reactions, in which cells from an incompatible donor react with and are opsonised by preformed antibody in the host
2) Heamolytic disease of the newborn, in which there is an antigenic difference between the mother and the fetus, and IgG antierythrocyte antibodies from the mother cross the placenta and cause destruction of fetal red cells
3) Autoimmune haemolytic anaemia, agranulocytosis, and thrombocytopaenia, in which individuals produce antibodies to their own blood cells, which are then destroyed
4) Certain drug reactions, in which a drug acts as a “hapten”, by attaching to plasma membrane proteins of red cells and antibodies are produced against the drug protein complex
What happens during the inflammation stage of antibody mediated (type II) hypersensitivity?
- When antibodies deposit in fixed tissues, such as basement membranes and extracellular matrix, the resultant injury is due to inflammation.
- The deposited antibodies activate complement, generating by-products, including chemotactic agents (C5a), which direct the migration of polymorphonuclear leukocytes and monocytes and anaphylatoxins (C3a and C5a), which increase vascular permeability
- The leukocytes are activated by engagement of their C3b and Fc receptors.
- This results in the production of other substances, that damage tissues, such as lysosomal enzymes, including proteases capable of digesting basement membrane, collagen, elastin, and cartilage, and ROS
Clinically, in what situations does antibody-mediated inflammation occur as the mechanism responsible for tissue injury?
In some forms of glomerulonephritis, vascular rejection in organ grafts
How does cellular dysfunction happen during antibody-mediated (type II) hypersensitivity reactions? Give examples
In some cases, antibodies directed against cell surface receptors impair or dysregulate function without causing cell injury or inflammation.
* In myasthenia gravis, antibodies reactive with acetylcholine receptors in the motor end plates of skeletal muscles block neuromuscular transmission and therefore cause muscle weakness
* In Graves disease, antibodies against the thyroid-stimulating hormone receptor on thyroid epithelial cells stimulate the cells, resulting in hyperthyroidism
Give a general overview of what happens in immune complex-mediated (type III) hypersensitivity?
Antigen-antibody complexes produce tissue damage mainly by eliciting inflammation at the sites of deposition
How is the pathological reaction usually initiated in immune complex-mediated (type III) hypersensitivity?
- It is usually initiated when antigen combines with antibody in the cirulation, creating immune complexes that typically deposit in vessel walls.
- Less frequently, the complexes may be formed at sites where antigen has been “planted” previously (called in situ immune complexes).
- The antigens that form immune complexes may be exogenous or endogenous
What organs do immune complex-mediated (type III) hypersensitivity reactions often take place in?
They tend to be systemic, but often preferentially involve the kidney (glomerulonephritis), joints (arthritis), and small blood vessels (vasculitis), all of which are common sites of immune complex deposition
The pathogenesis of systemic immune complex disease can be divided into three phases. What are they and what happens in each one?
1) Formation of immune complexes - the introduction of a protein antigen triggers an immune response that results in the formation of antibodies, typcically about a week after injection. These antibodies are secreted into the blood, where they react with the antigen still present and form antigen-antibody complexes
2) Deposition of immune complexes - the circulating antigen-antibody complexes are deposited in various tissues, often in the glomeruli and joints
3) Inflammation and tissue injury - once the complexes are deposited, they initiate an acute inflammatory reaction. The resultant inflammatory lesion is termed vasculitis if it occurs in blood vesels, glomerulonephritis if it occurs in renal glomeruli, arthritis if it occurs in the joints
What are the symptoms of immune complex-mediated (type III) reactions? When do thy happen?
During the inflammation and tissue injury phae (approx 10 days after antigen administration), clinical features such as fever, urticaria, joint pains, lymph node enlargement and proteinuria appear.
What antibodies are involved in immune complex-mediated (type III) hypersensitivity?
Complement-fixing antibodies (IgG and IgM) and antibodies that bind to leukocyte Fc receptors (some subclasses of IgG) induce the pathological lesions of immune complex disorders.
Give some examples of immune complex-mediated diseases
SLE
Post-strep glomerulonephritis
Polyarteritis nodosa
Reactive arthritis
Serum sickness
What is an Arthus reaction?
The Arthus reaction is localised area of tissue necrosis resulting from acute immune complex vasculitis, usually elicited in the skin.
These complexes precipitate in the vessel walls and cause fibrinoid necrosis, and superimposed thrombosis worsens the ischaemic injury.
Give a general overview of the process of T Cell-mediated (type IV) hypersensitivity?
The cell mediated type of hypersensitivity is caused by inflammation resulting from cytokines produced by CD4+ T cells and cell killing by CD8+ T cells
Give a general overview of what happens in CD4+ T cell-mediated hypersensitivity
In CD4+ T cell-mediated hypersensitivity reactions, cytokines produced by the T cells induce inflammation that may be chronic and destrucive.
What is the prototype of T cell-mediated inflammation? What happens during this?
Give an example
Delayed-type hypersensitivity, a tissue reaction to antigens give to immune individuals. An antigen administered into the skin of a previously immunised individual results in a detectable cutaneous reaction within 24-48 hours
An example of this is the tuberculin reaction
The inflammatory reactions stimulated by CD4+ T cells can be divided into sequential stages. What are these?
1) Activation of CD4+ T cells - naive CD4+ T cells recognise peptides displayed by dendritic cells and secrete IL-2, which functions as an autocrine growth factor to stimulate proliferation of the antigen-responsive T cells. The T cells differentiate into either Th1 (due to IFN-γ) or Th17 (via IL-1, IL-6 or IL-12).
2) Responses of differentiated effector T cells - upon repeat exposure to an antigen, Th1 cells secrete IFN-γ, which activates macrophages and they do their thing; if the activation is sustained, continued inflammation and tissue injury result
This produces granulomatous inflammation, which you know from before
Give some examples of T cell-mediated (type IV) diseases…
RA
MS
T1DM
TBD
Psoriasis
Contact sensitivity
What happens during CD8+ T Cell-mediated cytoxocity in T cell-mediated (type IV) hypersensitivity?
CD8+ CTLs kill antigen-expressing target cells. Tissue destruction by CTLs is important in T cell-mediated diseases, such as type 1 diabetes. The killing of infected cells leads to the elimination of the infection, but in some cases it is responsible for cell damage that accompanies the infection.
What is the principal mechanism of T cell-mediated killing of targets involves what?
Perforins and granzymes, preformed mediators contained in the lysosome-like granules of CTLs.
* CTLs that recognise the target cells secrete a complex consisting of perforin, granzymes, and other proteins which enters target cells by endocytosis.
* In the target cell cytoplasm, perforin facilitates the release of the granzymes from the complex. Granzymes are proteases that cleave and activate caspases, which induce apoptosis of teh target cells
* Activated CTLs also express Fas ligand, which can bind to Fas expressed on target cells and trigger apoptosis
What is the complement system?
It is a collection of soluble proteins and membrane receptors that function mainly in host defence against microbes and in pathological inflammatory reactions.
What does the complement system consist of?
More than 20 proteins, some of which are numbered C1 through C9
When does the complement system function?
In both innate and adative immunity for defense against microbial pathogens.
Broadly, what are the effects of complement activation?
Severeal cleavage products of complement proteins are elaborated that cause increased vascular permeability, chemotaxis, and opsonisation
Complement proteins are present in inactive forms in the plasma, what is the critical step in complement activation?
The proteolysis of the third (and most abundant) component, C3.
Cleavage of C3 in the complement pathway can occur by one of three pathways. What are these pathways?
- The classical pathway, which is triggered by fixation of C1 to antibody (IgM or IgG) that has combined with antigen
- The alternative pathway, which can be triggered by microbial surface molecules (e.g. endotoxin or LPS), complex polysaccharides, cobra venom, and other substances, in the absence of antibody
- The lectin pathway, in which plasma mannose-binding lectin binds to carbohydrates on microbes and directly activates C1
All three complement pathways converge lead to the same outcome. What is this and what happens next?
- They all lead to the formation of an active enzyme called the C3 convertase, which splits C3 into two functionally distinct fragments, C3a and C3b.
- C3a is released, and C3b becomes convalently attached to the cell or molecules where complement is being activated.
- More C3b then binds to the previously generated fragments to form C5 convertase, which cleaves C5 to release C5a and leave C5b attached to the cell surface.
- C5b binds the late components (C6-C9), culminating in the formation of the membrane attack complex (MAC)
The complement system has three main functions. What are they and how do they occur?
- Inflammation - C3a, C5a (anaphylatoxins) and, to a lesser extent, C4a stimulate histamine release from mast cells and thereby increase vascular permeability and cause vasodilation. C5a is also a chemotactic agent and activates the lipooxygenase pathway of AA, releasing leukotrienes.
- Opsonisation and phagocytosis - C3b and iC3b (inactive), when fixed to a microbial cell wall, act as opsonins and promote phagocytosis
- Cell lysis - the deposition of the MAC on cells makes these cells permaeblt ot water and ions and results in lysis of the cells. - this is particularly importnat in microbes with thin walls such as Neissaria bacteria
The activation of complement is tightly controlled by cell-associated and circulatin regulatory proteins. What are these and what do they do?
- C1 inhibitor (C1 INH) blocks the activation of C1
- Decay accelerating factor (DAF) and CD59 are two proteins that are linked to plasma memrbanes by a glycophosphatidyl anchor. DAF presents formation of C3 convertases and CD59 inhibits formation of the MAC
What are oncogenes and proto-oncogenes?
Proto-oncogenes - normal cellular genes whose products promote cell proliferation
Oncogenes - mutated or overexpressed versions of proto-oncogenes that function autonomously, having lost dependence on normal growth promoting signals
How are oncogenes created?
By mutations in proto-oncogenes and encode proteins called oncoproteins that have the ability to promote cell growth in the absence of normal growth-producing signals
What are the roles of proto-oncogenes?
They have multiple roles, but all participate at some level in signaling pathways that drive proliferation. Thus, pro-growth proto-oncogenes may encode growth factors, growth factor receptors, signal transducers, transcription factors or cell cycle components.
When discussing tumours, what does differentiation and anaplasia mean?
Differentiation refers to the extent to which neoplastic parenchymal cells resemble the corresponding normal parenchymal cells, both morphologically and functionally
Lack of differentiation is called anaplasia
Are benign or malignant cells well differentiated or not?
- In general, benign tumours are well differentiated, mitoses are usually rare and of normal configuration.
- Malignant neoplasms exhibit a wide range of parenchymal cell differentiation, most exhibit morphological alterations that betray their malignant nature.
Malignant neoplasms that are composed of poorly differentiated cells are said to be anaplastic. This is often associated with many other morphological changes. What are these?
- Pleomorphism - variety in size and shape, cells within the same tumour are not uniform
- Abnormal nuclear morphology - the nuclei are often disproportionately large for the cell
- Mitoses - many cells are in mitosis, reflecting the high proliferative activity of the parenchymal cells.
- Loss of polarity - the orientation of anaplastic cells is markedly disturbed. Sheets or large amsses of tumour cells grow in an anarchic, disorganised fashion
- Large central area of ischaemic necrosis
What are the ranges of cells that occur with malignant pleomorphisms?
Cells within the same tumour are not uniform, they range from small cells with an undifferentiated appearance, to tumour giant cells many time larger than their neighbours.
Some tumor giant cells possess only a single huge polymorphic nucleus, whilst others may have two or more large hyperchromatic nuclei.
What is abnormal about the nuclear morphology of malignant cells?
They have a nuclear-to-cytoplasm ratio that may approach 1:1 instead of the normal 1:4 or 1:6.
The nuclear shape is variable and often irregular, and the chromatin is often coarsely clumped and distributed along the nuclear membrane, or more darkly stained than normal (hyperchromatic)
The better the differentiation of the transformed cell, the more completely it retains the functional capabilities of its normal counterpart. What does this mean for endocrine tumours?
Benign neoplasms and well-differentiated carcinomas of endocrine glands frequently secrete hormones characteristic of their origin. Increased levels of these hormones in the blood are used clinically to detect and follow such tumours.
In some instances of highly anaplastic undifferentiated cells, new and unanticipated functions emerge. What does this mean? Give some examples
Some tumours express fetal proteins that are not produced by comparable cells in the adult, while others express proteins that are normally only found in other types of adult cells. e.g. bronchogenic carcinomas may produce corticotropin, PTH, insulin, glucagon and other hormones, giving rise to paraneoplastic syndromes
What is dysplasia?
It is a term that literally means ‘disordered growth’. It is encountered principally in epithelia and is characterised by a constellation of changes that include the loss of uniformity of the individual cells as well as loss in their architectural orientation.
Morphologically, what are some characteristics of dysplastic cells?
They may exhibit considerable pleomorphism and often contain large hyperchromatic nuclei with a high nuclear-to-cytoplasmic ratio.
In addition, mitotic figures are more abundant that in the normal tissue and rather than being confined to the basal layer may instead be seen at all levels, including surface cells.
What is a carcinoma in situ?
When dysplastic changes are marked and involve the full thickness of the epithelium, but the lesion does not penetrate the basement membrane, it is considered a pre-invasive neoplasm and is referred to as carcinoma in situ
What is an invasive tumour?
Once the tumor cells breach the basement membrane, the tumour is said to be invasive
Does dysplasia always progress to cancer?
Dysplastic changes are often found adjacent to foci of invasive carcinoma, and in some situations, frequently antedates the appearance of cancer.
Some mutations associated with cancers may be present in even ‘mild’ dysplasia.
Therefore, although dysplasia may be a pre-cursor to malignant transformation, it does not always progress to cancer.
How does the process of growth vary between malignant and benign tumours?
- The growth of cancers is accompanied by progressive infiltration, invasion, and destruction of the surrounding tissue
- Nearly all benign tumours grow and cohesive expansile masses that remain localised to their site of origin and lack the capacity to infiltrate, invade or metastasize to distant sites
What is a capsule associated with benign tumours and what does it consist of?
Because benign tumours grow and expand slowly, they usually develop a rim of compressed fibrous tissue called a capsule that separates them from the host tissue.
This capsule consists largely of extracellular matrix deposited by stromal cells such as fibroblasts, which are activated by hypoxic damage resulting from the pressure of the expanding tumour
How does the capsule of a benign tumour affect how it feels clinically?
Such encapsulation does not prevent tumour growth, but it creates a tissue plane that makes the tumour discrete, readily palpable, moveable (non-fixed), and easily excisable by surgical enucleation
How do malignant tumors vary from benign tumours re: the capsule and demarcation from normal tissue?
Malignant tumours are, in general, poorly demarcated from the surrounding normal tissue, and a well-defined cleavage plane in lacking.
Slowly explanding malignant tumours, however, may develop an apparently enclosing fibrous capsule and may push along a broad fron into adjacent normal structures.
Histologically, what would you see in ‘pseudoencapsulated’ malignant masses?
They almost always show rows of cells penetrating the margin and infiltrating the adjacent structures, a crablike pattern of growth that consitutes the popular image of cancer.
Next to the development of metastases, invasiveness is the most reliable feature that differentiates cancers from benign tumours. How?
Most malignant tumours do not recognise normal anatomic boundaries and can be expected to penetrate the wall of the colon or uterus for example.
Some cancers seem to evolve from a preinvasive stage - carcinoma in situ. Name some examples.
Carcinomas of the skin, breatst and uterine cervix
What is metastasis?
Metastasis is defined by the spread of a tumour to sites that are physically discontinuours with the primary tumour, and unequivocally marks a tumour as malignant, as by definition noeplasms do not metastasise.
In general, the likelihood of a primary tumour to metastasise is what?
It correlates with lack of differentiation, aggressive local invasion, rapid growth, and large size.
What percentage of newly diagnosed solid tumours present with metastases?
30%
Dissemination of cancers may occur through one of three pathways. What are these pathways?
1) Direct seeding of body cavities or surfaces
2) Lymphatic spread
3) Haematogenous spread
Where does seeding of body cavity and surfaces take place?
Seeding of body cavities and surfaces may occur whenever a malignant neoplasm penetrated into a natural ‘open field’ lacking physical barriers.
Most often involved is the peritoneal cavity, but any other cavity - pleural, pericardial, subarachnoid, and joint spaces - may be affected.
What is pseudomyxoma peritonei?
Sometimes mucus-secreting appendiceal carcinomas or ovarian carcinomas fill the peritoneal cavity with a gelatinous neoplastic mass referred to as pseudomyxoma peritonei
What is the most common pathway for the initiall dissemination of carcinomas?
Lymphatic spread
How do tumours spread through lymphatics?
Tumours do not contain functional lymphatics, but lymphatic vessels located at the tumor margins are apparently sufficien for the lymphatic spread of tumour cells
What cancers often spread through seeding of body cavities?
It is particularly characteristic of carcinomas arising in the ovaries, which, not infrequently, spread to peritoneal surfaces, which become coated with a heavy cancerous glaze.
What cancers often spread via lymphatic spread? and via which nodes?
- Because carcinomas of the breast usually arise in the upper outer quadrants, they generally disseminate first to the axillary lymph nodes. Cancers of the inner quadrants drain to the nodes along the internal mammary arteries. Thus, the infraclavicular and supraclavicular nodes may become involved
- Carcinomas of the lyng arising in the major respiratory passages metastasis first to the perihilar tracheobronchial and mediastinal nodes.
What are skip metastases regarding lymph nodes? Why do they happen?
Local lymph nodes, however, may be bypassed - so called skip metastasis - because of venous-lymphatic anastomoses or because inflammation or radiation has obliterated lymphatic channels
What is a sentinal lymph node?
A senitel lymph node is defined as ‘the first node in a regional lymphatic basin that receives lymph flow from the primary tumour’
Does nodal enlargement in proximity to a cancer always equate with dissemination of the primary lesion?
Drainage of tumour cell debris or tumour antigens, or both, also induces reactive changes within nodes.
Thus, enlargement of nodes may be caused by the spread and growth of cancer cells or reactive hyperplasia
Are arteries or veins more or less readily penetrated by haematogenous spread of metastases?
Arteries, with their thicker walls, are less readily penetrated than the veins.
In what situation does arterial spread of metastases occur?
Arterial spread may occur, however when tumour cells pass through the pulmonary capillary beds or pulmonary arteriovenous shunts or when pulmonary metastases themselves give rise to additional tumour emboli.
In what situation does venous invasion of metastases occur?
With venous invasion, the bloodborne cells follow the venous flow draining the site of the neoplasm, and the tumour cells often come to rest in the first capillary bed they encounter. Understandably, the liver and lungs ar the most frequently involved in such haematogenous dissemination.
Certain cancers have a propensity for invasion of veins. What are they and where do they spread?
- Renal cell carcinoma often invades the branches of the renal vein and then the renal vein itself, from where it may grow in a snakelike fashion up the inferior vena cava, sometimes reaching the right side of the heart
- HCCs often penetrate portal and hepatic radicles to grow within them into the main venous channels.
What are the different modes of activation in tumours that proto-oncogenes can do?
Overexpression
Amplification
Point mutation
Translocation
Why is angiogenesis essentional in tumour growth?
Even if a solid tumour possesses all of the genetic aberrations that are required for malignant transofrmation, it cannot enlarge beyond 1 to 2mm in diameter unless it has the capacity to induce angiogenesis
What happens due to neovascularisation?
Perfusion supplied needed nutrients and oxygen, and newly formed endothelial cells stimulate the growth of adjacent tumour cells by secreting growth factors, such as insulin-like growth factos (IGFs) and PDGF.
Is tumour neovascularisation normal?
No, it is effective at delivering nutrients and removing wastes but it is not entirely normal; the vessels and leaky and dilated, and have a haphazard pattern of connection, features that can be appreciated on angiograms.
What is an angiogenic switch?
Early in their development, most human tumours do not induce angiogenesis. Starved of nutrients, these tumours remain small or in situ, possible for years, until an angiogenic switch terminates the stage of vascular quiescence.
What is the molecular basis of the angiogenic switch?
It involves increased production of angiogenic factors and/or loss of angiogenic inhibitors. These factors may be produced by the tumour cells themselves or by inflammatory cells or other stromal cells associated with the tumours.
What are the two phases of the metastatic cascade?
1) invasion of the extracellular matrix
2) vascular dissemination, homing of tumour cells and colonisation
Tumour cells must interact with the extracellular matrix at several stages in the metastatic cascade. What are they?
- A carcinoma must first breach the underlying basement membrane, then traverse the interstitial connective tissue, and ultimately gain access to the circulation by penetrating the vascular basement membrane.
- This process is repeated in revers when tumour cell emboli extravasate to a distant site.
Invastion of the ECM initiates the metastatic cascade and is an active process that can be resolved into several steps. What are they?
- “Loosening up” of tumour cell-tumour interactions
- Degradation of ECM
- Attachment to novel ECM components
- Migration and invasion of tumour cells
Dissociation of cancer cells from one another is often the result of alterations in intercellular adhesion molecules and is the first step in the process of invasion. How is the function of E-cadherins related to this?
Normal epithelial cells are tightly glued to each other and the ECM by a variety of adhesion molecules. E-cadherins mediate the homotypic adhesion of epithelial cells, serving to both hold the cells toegterh and to relay signals between the cells.
In several epithelial tumours, including adenocarcinomas of the colon, stomach and breast, E-cadherin function is lost.
Degradation of the basement membrane and interstitial connective tissue is the second step in invasion. How does it occur?
Tumour cells may accomplish this by either secreting proteolytic enzymes themselves or by inducing stromal cells (e.g. fibroblasts and inflammatory cells) to elaborate proteases.
Many different families of proteases have been implicated in tumour cell invasion. Name some and explain what they do?
- Metalloproteinases (MMPs) regulate tumour invasion not only by remodeling insoluble components of the basement membrane and interstitial matrix but also by releasing ECM-sequestered growth factors.
- Cathepsin D
- Urokinase plasminogen activator
The third step of tumour invasion involves changes in attachment of tumour cells to ECM proteins. How does this happen?
- Loss of adhesion in normal cells leads to induction of apoptosis, but tumour cells are resistant to this form of cell death.
- Additionally, the matrix itself is modified in ways that promote invasion and metastasis - e.g. cleavage of the basement membrane proteins collagen IV and laminin by MMP2 or MMP9 generates novel sites that bind to receptors on tumour cells and stimulate migration.
What is measles?
Measles is an acute viral infection that affects multiple organs and causes a wide range of disease, from mild, self-limiting infections to severe systemic manifestations
What type of virus is the measles virus?
It is a single-stranded RNA virus of the paramyxovirus family, which includes mumps, RSV, parainfluenze and human metapneumovirus.
How many serotypes of the measles virus are there? How is it transmitted?
There is only one serotype of measles virus.
Measles virus is transmitted by respiratory droplets.
What are three cell-surface receptors that have been identified for measles? Where are they and what do they bind to?
- CD46 (a complement-regulartory protein that inactivates C3 convertases) is expressed on all nucleated cells
- Signaling lypmhocytic activation molecule (SLAM, a molecule involved in T-cell activation) is expressed on cells of the immune system
- Nectin 4 (adherens junction protein) is expressed on epithelial cells.
All of these receptors bind the viral haemgglutinin protein.
How does the measles virus replicate?
- It can replicate in a variety of cell types, including epithelial cells and leukocytes.
- The virus initially multiplies within the respiratory tract and then spreads to local lymphoid tissues
- Replication of the virus in lymphatic tissue is followed by viremia and systemic dissemination to many tissues, including the conjunctiva, skin, respiratory tract, urinary tract, small blood vessles, lymphatic system, and CNS
How do children develop immunity to measles?
- Most children develop T-cell-mediated immunity to measles virus that helps control the viral infection and produces the measle rash.
- Antibody-mediated immunity to measles virus protects against reinfection.
What does measles cause, that is responsible for much of measles-related morbidity and mortality? How does this happen?
- It can cause transient but profound immunosuppression, resulting in secondary bacterial and viral infections, which are responsible for much of measles-related morbidity and mortality.
- Alterations of both innate and adaptive immune responses occur following measles infection, including defects in dendritic cell and lymphocyte function.
What are some rare late complications of measles?
Subacute sclerosing panencephalitis and measles inclusion body encephalitis (in immunocompromised individuals).
What is the morphology of measles virus?
- The blotchy, reddish brown rash of measles virus infection on the face, trunk and proximal extremities is produced by dilated skin vessels, oedema, and a mononuclear perivascular infiltrate.
- Ulcerated mucosal lesions in the oral cavity near the opening of the Stensen ducts (the pathognomic Koplic spots) are marked by necrosis, neutrophilic exudate, and neovascularisation.
- The lymphoid organs typically have marked follicular hyperplasia, large germinal centers, and randomly distributed multinucleate giant cells, called Warthin-Finkeldey cells, which have eosinophilic nuclear and cytoplasmic inclusion bodies, found in the lung and sputum
What is mumps?
Mumps is an acute systemic viral infection usually associated with pain and swelling of the salivary glands
What type of virus is mumps?
Like measles, mumps is a member of paramyxovirus family.
What are the two types of surface glycoproteins that the mumps virus has?
- One with hemagglutinin and neuraminidase activities
- The other with cell fusion and cytolytic activities
How do mumps viruses replicate and effect cells?
- They enter the upper respiratory tract through inhalation of respiratory droplets, spread to draining lymph nodes where they replicate in lymphocytes (preferentially in activated T cells), and then spread through the blood to the salivary and other glands
- Mumps virus infects salivary gland ductal epithalial cells, resulting in desquamation of involved cells, oedema, and inflammation that leads to the classic salivary gland pain and swelling.
Where can mumps spread to?
Mumps can spread to other sites, including the CNS, testis, ovary and pancreas.
Aseptic meningitis is the most common extrasalivary gland complication of mumps infection, occurring in up to 15% of cases.
What can be used to get a definitive diagnosis of mumps?
Serology, viral culture, or PCR assays.
What is the morphology of mumps parotitis?
- Mumps parotitis is bilateral in 70% of cases
- The affected glands are enlarged, have a doughy consistency, and are moist, glistening, and reddish-brown on cross-section.
- On microscopic examination the gland interstitium is oedematous and diffusely infiltrated by macrophages, lymphocytes, and plasma cells, which compress acini and ducts.
- Neutrophils and nectrotic debris may fill the duct lumen and cause focal damage to the lining epithelium
What is the morphology of mumps orchitis?
- In mumps orchitis testicular swelling may be marked, caused by oedema, mononuclear cell infiltration, and focal haemorrhages.
- Because the testis is tightly contained within the tunica albuginea, parenchymal swelling may compromise the blood supply and cause areas of infarction.
- The testicular damage can lead to scarring, atrophy, and, if severe, sterility.
How does mumps affect the pancreas and the CNS?
- Infection and damage of acinar cells in the pancreas may release digestive enzymes, causing parenchymal and fat necrosis and neutrophil-rich inflammation.
- Mumps encephalitis causes perivenous demyelination and perivascular mononuclear cuffing.
What is poliovirus infection?
Poliovirus causes an acute systemic viral infection, leading to a wide range of manifestations, from mild, slef-limited infections to paralysis of limb muscles and respiratory muscles.
What type of virus is polio?
It is a spherical, unencapsulated ENA virus of the enterovirus genus.
What type of vaccine is polio?
- Salk formalin-fixed (killed) vaccine
- Sabin oral, attenuated (live) vaccine
How is poliovirus transmitted and how does it replicate?
- Polio, like other enteroviruses, is transmitted by the fecal-oral route.
- The virus infects human cells by binding to CD155, an epithelial adhesion molecule.
- The virus is ingested and replicates in the mucosa of the pharynx and gut, including tonsils and Peyer patches in the ilerum.
- It then spreads through lymphatics to lymph nodes and eventually the blood, producing transient viremia and fever.
- Although most polio infections are symptomatic, in 1% of people it invades the CNS and replicates in motor neurons of the spinal cord (spinal poliomyelitis) or brain stem (bulbar poliomyelitis).
What is Viral haemorrhagic fever?
It is a severe life-threatening multisystem syndrome in which there is vascular dysregulation and damage, leading to shock.
What causes viral haemorrhagic fever?
It is caused by enveloped RNA viruses belonging to four different genera: Arenaviridae, Filoviridae, Bunyaviridae, and Flaviviridae.
What are the symptoms of viral haemorrhagic fever?
It can produce a spectrum of illnesses, ranging from a mild acute disease, characterised by fever, headache, myalgia, rash, neutropaenia, and thrombocytopaenia to severe, life-threatening disease in which there is sudden haemodynamic deterioration and shock.
How is viral haemorrhagic fever transmitted?
- All the viruses pass through an animal or insect host during their life cycles and therefore their ranges are restricted to areas in which their hosts reside.
- Humans are incidental hosts who are infected when they come into contact with infected hosts (typically rodents) or insect vectors (mosquitos adn ticks). Some viruses that cause haemorrhagic fever (Ebola) also can spread from person to person
What is the pathogenesis of viral haemorrhagic fever?
- The pathogenesis of the infection and its complications vary among the different viruses but there are some common features
- Damage to blood vessels is often prominent. It may be caused by direct or indirect infection of and damage to endothelial cells, or infection of macrophages and dendritic cells leading to production of inflammatory cytokines.
- There may be haemorrhagic manifestations, including petechiae, caused by a combinations of thrombocytopaenia or platelet dysfunction, endothelial injury, cytokine-induced DIC, and deficiency of clotting factors because of hepatic injury.
- Necrosis of tissues secondary to the vascular lesions and haemorrages may be seen and varies from mild and focal to massive, but the attendant inflammatory response is usually minimal.
What is a latent infection?
Latency is defined as the persistence of viral genomes in cells that do not produce infectious virus.
What happens when a latent infection is reactivated?
Dissemination of the infection and tissue injury stem from reactivation of the latent virus.
What are the viruses that most frequently establish latent infections in humans?
Herpesviruses
What type of virus are herpesviruses?
They are large encapsulated viruses with double-stranded DNA genomes that encode approximately 70 proteins.
What are the patterns of infection caused by herpesvirus?
Herpesviruses cause acute infection followed by latent infection in which the viruses persist in a noninfectious form with periodic reactivation and shedding of infectious virus.
There are eight types of human herpesviruses, belonging to three subgroups that are defined by the type of cell most frequently infected and the site of latency. What are the groups, give some examples and what cells do they infect?
- α-group viruses, including HSV-1, HSV-2 and VZV, which infect epithelial cells and produce latent infection in neurons
- Lymphotropic β-group viruses, including CMV, human herpesvirus-6 (which causes exanthem subitum, also known as roseola infantum and sixth disease, a benign rash of infants), and human herpesvirus-7, which infect and produce latent infection in a variety of cell types
- γ-group viruses EBV and KSHV/HHV-8, the cause of Kaposi sarcoma, which produce latent infection mainly in lymphoid cells.
What are the similarities of HSV-1 and HSV-2?
HSV-1 and HSV-2 differ serologically but are closely related genetically and cause a similar set of primary and recurrent infections
How do HSV-1 and HSV-2 replicate and infect?
- Both viruses replicate in the skin and the mucous membranes at the site of entry of the virus (usually oropharynx or genitals), where they produce infectious virions and cause vesicular lesions of the epidermis.
- The viruses spread to sensory neurons that innervate these primary sites of replication.
- Viral nucleocapsids are transported along axons to the neuronal cell bodies, where the viruses establish latent infection
In immunocompetent hosts, how long does it take for primary HSV infection to resolve?
A few weeks, although the virus remains latent in nerve cells
What happens for the HSV during latency?
- During latency the viral DNA remains within the nucleus of the neuron, and only latency-associated viral RNA transcripts (LATs) are synthesised.
- No viral proteins appear to be produced during latency.
- LATs may contribute to latency by conferring resistance to apoptosis, silencing lytic gene expression through heterochromatin formation, and serving as precursors for microRNAs that downregulate expression of critical HSV lytic genes.
How does reactivation occur?
- Reactivation of HSV-1 and HSV-2 may occur repeatedly with or without symptoms, and results in the spread of virus from the neurons to the skin or to mucous membranes.
- Reactivation can occur in the presence of host immunity, because HSVs have developed ways to avoid immune recognition.
What ways have HSV developed to avoid immune recognition?
- HSVs can evade antiviral CTLs by inhibiting the MHC class I recognition pathway
- They can elude humoral immune defenses by producing receptors for teh Fc domain of immunoglobulin and inhibitors of complement.
HSV-1 is the major infectious cause of corneal blindness in the US. How does it cause it?
- Corneal epithelial disease is thought to be due to direct viral damage, whilst corneal stromal disease appears to be immune-mediated.
HSV-1 is the major cause of fatal sporadic encephalitis in the US. Where in the brain does it involve?
When the infection spreads to the brain, it usually involves the temporal lobes and orbital gyri of the frontal lobes.
How does HSV-2 infection effect HIV transmission?
HSV-2 infection increases the risk of HIV transmission by four-fold and increases the risk of HIV acquisition by two-to-three-fold.
What is the morphology of HSV-infected cells?
They contain large, pink to purple intranuclear inclusions (Cowdry type A) that consist of viral replication proteins and virions at various stage of assembly that push the host cell chromatin out to the edges of the nucleus. Due to cell fusion, HSVs also produc inclusion-bearing multinucleated syncytia.
Where are HSV-1 and HSV-2 lesions?
- They cause lesions ranging from self-limited cold sores and gingivostomatitis to life-threatening disseminated visceral infection and encephalitis.
- Fever blisters or cold sores favor the facial skin around mucosal orifices (lips, nose), where their distribution is frequently bilateral and independent of skin dermatomes.
What is gingivostomatitis? Is it HSV-1 or -2?
- It is usually encountered in children, and is caused by HSV-1
- It is a vesicular eruption extending from the tongue to the retropharynx and causing cervical lymphadenopathy.
What is herpetic whitlow in HSV? Who does it happen to?
Swollen, erythematous HSV lesions of the fingers of palm (herpetic whitlow) occur in infants and, occasionally, in health care workers
What are genital herpes? Is it caused by HSV-1 or -2?
- It is more often caused by HSV-2 than by HSV-1.
- It is characterised by vesicles on the genital mucous membranes as well as on the external genitalia that are rapidly converted into superficial ulcerations, rimmed by an inflammatory infiltrate
How is HSV transmitted to neonates? Is it usually HSV-1 or HSV-2? How does it affect them?
- Usually HSV-2 can be transmitted to neonates during passage through the birth canal of infected mothers.
- Although HSV-2 infection in the neonate may be more mild, more commonly it is fulminating with generalised lymphadenopathy, splenomegaly and necrotic foci throughout the lungs, liver, adrenals and CNS
Two forms of corneal lesions are caused by HSV. What are they? How do they present?
1) Herpes epithelial keratitis shows typical virus-induced cytolysis of the superficial epithelium
2) Herpes stromal keratitis is characterised by infiltrates of mononuclear cells around keratinocytes and endothelial cells, leading to neovascularisation, scarring, opacification of the cornea, and eventual blindness. Here the damage is caused by an immunologic reaction to the HSV infection, rather than the cytopathic effects of the virus itself
What is Varicella-Zoster Virus?
Acute infection with VZV causes chickenpox and reactivation of latent VZV causes shingles.
What is the pathological process of VZV?
- Like, HSV, VZV infects mucous membranes, skin, and neurons and causes a self-limited primary infection in immunocompetent individuals.
- Also like HSV, VZV evades immune responses and establishes a latent infection in sensory ganglia.
- In contrast to HSV, VZV is transmitted by respiratory aerosols, disseminates haemtogenously, and causes widespread vesicular skin lesions
Where is VZV infection seen? How is it reactivated?
- Latent VZV infection is seen in neurons and/or satellite cells around neurons in the dorsal root ganglia.
- Reactivation and clinical recurrences causing shingles are uncommon but may occur many years after the primary infection
Where is localised recurrence of VZV most frequent and painful?
In dermatomes innervated by the trigeminal ganglia, where the virus is more likely to be latent.
How is VZV infection diagnosed?
By viral culture, or detection of viral antigens in cells scraped from superficial lesions.
How long after a respiratory infection does the chickenpox rash present? How does it present?
- The chickenpox rash occurs approx 2 weeks after respiratory infection. Lesions appear in multiple waves centrifugally from the torso to the head and extremities.
- Each lesion progresses rapidly from a macule to a vesicle, which resembles a dewdrop on a rose petal.
- After a few days most chickenpox vesicles rupture, crust over, and heal by regeneration, leaving no scars. However, bacterial superinfection of vesicles that are ruptured by trauma may lead to destruction of the basal epidermal layer and residual scarring.
How does chickenpox appear histologically?
Chickenppox lesions show intraepithelial vesicles with intranuclear inclusions in epithelial cells at the base of the vesicles.
When does shingles occur?
When VZV that has long remained latent in dorsal root ganglia after a previous chickenpox infection is reactivated and infects sensory nerves that carry it to one or more dermatomes.
What cells does VZV infect in shingles and how does it present?
The virus infects keratinocytes and causes vesciular lesions, which, unlike chickpox, are often associated with intense itching, burning, or sharp pain because of concomitant radiculoneuritis. This pain is especially severe when the trigeminal nerves are involved
What is Ramsay Hunt syndrome?
Rarely, the geniculate nucleus is involved in a shingles infection, causing facial paralysis (Ramsay Hunt syndrome).
Histologically, what do the sensory ganglia appear like in shingles?
They contain a dense, predominantly mononuclear infiltrate, with herpetic intranuclear inclusions within neurons and their supporting cells.
What is cytomegalovirus?
CMV, a β-group herpesvirus, can produce a variety of disease manifestations, depending on the age of the host, and, more importantly, on the host’s immune status.
How does CMV infection start?
CMV latently infects monocytes and their bone marrow progenitors and can be reactivated when cellular immunity is depressed
How does CMV vary due to the host’s immune system?
CMV cause an asymptomatic or mononucleosis-like infection in healthy individuals but devastating systemic infections in neonates and in immunocompromised people, in whom the virus may infect many different cell types and tissues.
How do CMV-infected cells appear?
CMV-infected cells exhibit gigantism of both the entire cell and its nucleus, which typically contains a large inclusion surrounded by a clear halo “owl’s eye)
Transmission of CMV can occur be several mechanisms, depending on the age group affected. What are these?
- Transplacental transmission, from a newly acquired or primary infection in a mother who does not have protective antibodies (congenital CMV)
- Neonatal transmission, through cervical or vaginal secretions at birth, or later throough breat milk from a mother who has an active infection (perinatal CMV)
- Transmission through saliva during preschool years, especially in day care centers. Toddlers so infected readily transmit the virus to their parents
- Tramission by the genital route is the dominant mode after about 15 years of age. Spread may also occur via respiratory secreation and the fecel-oral route.
- Iatrogenic transmission, at any age through organ transplants or blood transfusions
How does acute CMV infection affect the immune system?
It induces transient but severe immunosuppression. CMV can infect dendritic cells and impair antigen processing and the ability of dentritic cells to stimulate T lymphocuyes
How does CMV evade the immune system?
- By downmodulating MHC class I and II molecules and by producing homologues of TNF receptor, IL-10, and MHC class I molecules.
- Interestingly, CMV can evade NK cells by producing ligands that block activating receptors and class I-like proteins that engage inhibitory receptors.
- thus, CMV both hides from and actively suppresses immune responses.
How do MCV infected cells appear?
- They are strikingly enlarged, often to a diameter of 40μ, and show cellular and nuclear pleomorphism.
- Prominent intranuclear basophilic inclusions spanning half the nuclear diameter are usually set off from the nuclear membrane by a clear halo.
- Within the cytoplasm of infected cells, smaller basophilic inclusions can also be seen.
- In the glandular organs, the parenchymal epithalial cells are infected; in the brain, the neurons; in the lungs, the alveolar macrophages and epithelial and endothelial cells; and in the kidneys, the tubular epithelial and glomerular endothelial cells.
- Disseminated CMV causes focal necrosis with minimal inflammation in virtually any organ.
How does congential CMV present?
- Infection acquired in utero may take many forms.
- Inapproximately 95% of cases it is asymptomatic.
- However, sometimes when the virus is acquired from a mother with primary infection (who does not have protective antibodies), classic cytomegalic inclusion disease develops.
How does cytomegalic inclusion disease present in utero? How is diagnosis of neonatal CMV made?
- It resembles erythroblastosis fetalis.
- Affected infants may suffer IUGR, and present with jaundice, hepatosplenomegaly, anaemia, bleeding due to thrombocytopaenia, and encephalitis.
- In fatal cases, the brain is often smaller than normal (microcephaly) and may show foci of calcification
- Diagnosis of neonatal CMV is made by viral culture or PCR amplification of viral DNA in urine or saliva
What are the effects for an infant who had cytomegalic inclusion disease?
- The infants who survive usually have permanent deficits, including intellectual disability, hearing loss, and other neurologic impairments.
- The congenital infection is not always devastating, however, and may take the form of interstitial pneumonitis, hepatitis, or a haematologic disoder
- Most infants with this milder form recover, although a few develop intelectual disability later
How does perinatal infection of CMV usually present?
- Infection acquired during passage through the birth canal or from breast milk is usually asymptomatic due to protective maternal anti-CMV antibodies, which are transmitted to the fetus across the placenta.
- Despite the lack of symptoms, many of these infants continue to excrete CMV in their urine or saliva for months to years.
- Subtle effects on hearing and intelligence later in life have been reported in some studies
- Much less commonly, infected infant develop interstitial pneumonitis, failure to thrive, rash or hepatitis
What is the most common clinical manifestation of CMV infection in immunocompetent hosts beyond the neonatal period?
In healthy young children and adults the disease is nearly always asymptomatic, 50-100% of adults have had previous exposure. If there are symptoms, most commonly they are an infectious mononucleosis-like illness, with fever, atypical lymphocytosis, lymphadenopathy and hepatitis, marked by hepatomegaly and abnormal liver function tests.
What organs does CMV primarily affect in immunosuppressed patients? What happens in these organs
- The lungs (pneumonitis) and GI (colitis)
- In the pulmonary infection an interstitial mononuclear infiltrate with foci of necrosis develops, accompanied by the typical enlarged cells with inclusions. The pneumonitis can progress to full-blown ARDS.
- Intestinal necrosis and ulceration can develop and be extensive, leading to the formation of pseudomembranes and debilitating diarrhoea.
What are transforming viral infections? Name some
- Some viruses can transform infected cells into benign or malignant tumour cells.
- Oncogenic viruses can stimulate cell growth and survival by a variety of mechanisms.
- Examples include EBV, HPV, HBV.
What does EBV cause?
EBV causes infectious mononucleosis, a benign, self-limited lymphoproliferative disorder, and is associated with the pahtogenesis of several human tumours, most commonly certain lymphomas and nasopharyngeal carcinomas.
What are the symptoms of infectious mononucleiosis?
It is characterised by fever, sore throat, generalised lymphadenopathy, splenomegaly, and the appearace in the blood of atypical activated T lymphocytes (mononucleiosis cells).
Some people develop hepatitis, meningoencephalitis, and pneumonitis.
In what age group does EBV usually occur?
Infectious mononucleiosis occurs principally in late adolescents or young adults among upper socioeconomic classes in developed nations.
In the rest of the wold, primary infection with EBV occurs in childhood and is usually asymptomatic.
How is EBV transmitted?
By close human contact, frequently through the saliva during kissing.
How does EBV infect the body?
- EBV infects B cells and possibly epithelial cells of the oropharynx.
- An EBV envelope glycoprotein binds CD21 (CR2), the receptor for C3d component of complement, which is present on B cells.
- In a minority of B cells, infection is lytic, leading to viral replication and eventual cell lysis accompanied by release of virions, which may infect other B cells.
- In most B cells however, EBV establishes latent infection, during which the virus persists as an extrachromosomal episome.
A small number of EBV-encoded proteins are believed to be particularly important in the establishment of latency. What are they and what do they do?
- Epstein-Barr nucleur antigen 1 (EBNA1) binds the EBV genome to host cell chromosomes during mitosis, thereby ensuring that viral episomes are partitioned evenly to daughter cells when infected cells divide
- Latent membrane protein 1(LMP1) drives B-cell activation and proliferation. LMP1 does so by mimicking a constitutively active form of CD40, a B cell surface receptor. Like activating CD40, LMP1 binds to TNF receptor-assocated factors, adaptor molecules that trigger downstream events that activate NF-κB and the JAK/STAT signaling pathway. In addition, LMP1 prevents apoptosis by activating Bcl-2.
- EBNA2 also promotes B-cell activation and replication. It turns on the transcription of several host cell genes, inclduing genes that encode proteins that drive cell cycle antry, such as cyclin D.
- EBV produces a homologue of IL-10 (vIL-10), which inhibits its amcrophages and dendritic cells and suppresses anti-viral T cell responses.
What happens as a result of the actions of the EBV proteins?
- B cells that are latently infected with EBV are activated and begin to proliferate and to disseminate.
- This uncontrolled, expanding polyclonal population of EBV-infected B cells secretes antibodies with many specificities, inclduing antibodies that recognise sheep or horse red cells.
- These so-called heterophile antibodies are detected in diagnostic tests for mononucleosis.
Where in the body is EBV shed?
- In the saliva.
- It is not known whether the source of the virus is B cells, oropharyngeal epithelial cells, or both
The symptoms of infectious mononucleosis appear uopn initiation of the host immune response. What happens during this response on a cellular level?
- Cellular immunity mediated by EBV-specific CD8+ cytotoxic T cells and CD16+ NK cells is the most important component of this response.
- The reactive proliferation of T cells is largely centered in lymphoid tissues, which accounts for the lymphadenopathy and splenomegaly.
- Early in the course of the infection, IgM antibodies are formed against viral capsid antigens; later IgG antibodies are formed that persist for life.
How does the response to EBV vary from healthy to immunosupressed people?
- In healthy people, the fully developed humoral and cellular responses to EBV act as brakes on viral shedding, resulting in the elimination of B cells expressing the full complement of EBV latency-associated genes.
- In hosts with acquired defects in cellular immunity, reactivation of EBV can lead to B-cell proliferation, which can progress through a multistep process to EBV-associated B-cell lymphomas.
EBV contributes to the development of which lymphoma? How?
It contributes to the development of some cases of Burkitt lymphoma, in which a chromosomal translocation (most commonly an 8:14 translocation) involving the MYC oncogene is the critical ongongenic event.
What are the morphological changes in EBV?
- The peripheral blood shows absolute lymphocytosis. Many of these are large, atypical lymphocytes, characterised by an abundant cytoplasm containing multiple clear vacuolations, an oval, indented, or folded nucleus, and scattered cytoplasmic azurophilic granules.
- The lymph nodes, histologically, have expansion of paracortical areas due to activation of T cells (immunoblasts). A minor population of EBV-infected B cells can also be detected in the paracortex.
- EBV-infected B cells resembling Reed-Sternberg cells may be found.
What are the physical and histological changes in the spleen in EBV?
- The spleen in enlarged in most cases. It is usually soft and fleshy, which a hyperemic cut surface
- The histological changes are an expansion of white pulp follicles and red pulp sinusoids due to the presence of numerous activated T cells.
- These spleen are especially vulnerable to rupture, possibly in part because the rapid increase in size produces a tense, fragile splenic capsule.
What are the physical and histological changes to the liver in EBV?
- Heptamegaly is at most moderate.
- On histological examination, atypical lymphocytes are seen in the portal areas and sinusoids, and scattered, isolated cells or foci of parenchymal necrosis may be present.
What are the clinical features of EBV?
- In young children is classically presents with fever, sore throat, lymphadenitis, and other features mentioned earlier.
- However, malaise, fatigue and lymphadenopathy are the common presentation in young adults with infectious mononucleiosis and can raise the specter of leukaemia or lymphoma
- EBV can also prsent as a fever of unknown origin without significant lymphadenopathy or other localised findings, hepatitis resembling one of the hepatotropic viral syndromes, or a febrile rash resembling rubella.
What does the diagnosis of infectious mononucleiosis involve?
The diagnosis depends on:
1) lymphocytosis with the characteristic atypical lymphocytes in the peripheral blood
2) a positive heterophile antibody reaction (Monospot test)
3) a rising titer of specific antibodies for EBV antigens (viral capsid antigens, ealry antigens, or Epstein-Barr nuclear angiten)
In most patients, infectious mononucleosis resolves within 4-6 weeks, but sometimes the fatigue lasts longer. One of more complications ocassionally supervene. What are these?
- Most common is marked hepatic dysfunction with jaundice, elevated hepatic enzyme levels, disturbed appetite, and rarely, even liver failure
- Other complications involve the nervous system, kidneys, bone marrow, lungs, eyes, heart and spleen.
- Splenic rupture can occur even with minor trauma, leading to haemorrhage that may be fatal.
What are the common conditions caused by staphylococcal aureus?
- Skin lesions (boils, carbuncles, impetigo, and scalded-skin syndrome)
- Abscesses
- Sepsis
- Osteomyelitis
- Pneumonia
- Endocarditis
- Food poisoning
- Toxic shock syndrome
What type of bacteria are staph aureus?
- Pyogenic, gram-positive cocci that form clusters resembling bunches of grapes
What type of infections does staph. epidermidis cause?
Opportunistic infections in:
* catheterised patients
* patients with prosthetic cardiac valves
* drug addicts
What is the pathogenesis of staph aureus infections?
- It produces a mulitude of virulence factors, which include surface proteins involved in adherence and evasion of the host immune response, secreted by enzymes that degrade host structures, secreted toxins that damage host cells, and proteins that cause antibiotics resistance.
- S. aureus expresses surface receptors for fibrinogen (called clumping factor), fibronectin, and vitronectin and use these molecules to find to host endothelial cells
How do staphylococci infecting prosthetic valves and catheters do that?
They have a polysaccharide capsule that allows them to attach to artificial materials and resist host cell phagocytosis.
Staphylococco have their surface protein A. What does this do?
It binds the Fc portion of immunoglobulins, allowing the organism to escape antibody-mediated killing.
S. aureus produces multiple membrane-damaging (haemolytic) toxins. What are they and what do they do?
- α-toxin, a protein that intercalates into the plasma membrane of host cells, forming pores that allow toxic levels of calcium to leak into cells
- β-toxin, a sphingomyelinase
- δ-toxin, which is a detergent-like peptide.
- Staphylococcal γ-toxin and leukocidin lyse red cells and phagocytes, respectively
How do the exfoliative A and B toxins produced by S.aureus work?
- They are serine proteases that cleave the desmosomal protein desmoglein 1, which holds epidermal cells tightly together.
- This causes keratinocytes to detach from one another and from the underlying basement membrane, resulting in a loss of barrier function that often leads to secondary skin infections
- Exfoliation may occur locally at the site of infection (bullous impetigo) or may result in widespread loss of superficial epidermis (staphycoccal scalded-skin syndrome).
Superantigens produced by S.aureus may cause what?
Food poisoning and toxic shock syndrome.
What is the pathophysiology of toxic shock syndrome?
- It is often caused by hyperabsorbent tampons, which become colonised with S.aureus during use. It can also be caused by strep. pyogenes.
- The most common sites are the vagina and infected surgical sites.
- Bacterial superantigens cause polyclonal T cell proliferation by binding to conserved portions of MHC molecules and to relatively conserved portions of T-cell receptor β chains.
- In this manner, superantigens may stimulate up to 20% of T lymphocytes, leading to release of TNF and IL-1 in such large amounts that they trigger the inflmmatory response.
What are the clinical features of toxic shock syndrome?
- It is characterised by hypotension, renal failure, coagulopathy, liver disease, respiratory distress, a generalised erythematous rash, and soft tissue necrosis at the site of infection.
- If not treated promptly, it can be fatal.
- Superantigens produced by S.aureus also cause vomiting, presumably by affecting the CNS or enteric nervous system
What is the morphology of S.aureus infection?
Where the lesion is located in the skin, lungs, bones or heart valves, S.aureus causes pyogenic inflammation that is distinctive for its local destruction of host tissue.
Where do staphylococcal skin infections begin?
Excluding impetigo, which is restricted to the superficial epidermis, staphylococcal skin infections are centered around the hair follicles where they begin
What is a furuncle or boil in association with S.aureus? Where and how do they happen?
- A focal suppurative inflammation of the skin and subcutaneous tissue.
- They may be soliatry or multiple or recur in successive crops.
- Furuncles are most frequent is moist, hairy areas, such as the face, axillae, groin, legs and submammary folds.
- Beginning in a single hair follicle, a boil develops into a growing and deepening abscess that eventually ‘comes to a head’ by thinning and rupturing the overlying skin.
What is a carbuncle associated with S.aureus? Where and how do they happen?
- A carbuncle is a deeper suppurative infection that spreads laterally beneath the deep subcutaneous fascia and then burrow superficially to erupt in multiple adjacent skin sinuses
- Carbuncles typically appear beneath the skin of the upper back and posterior neck, where fascial planes favor their spread
What is hidradenitis?
It is a chronic suppurative infection of apocrine glands, most often in the axilla. Caused by S.aureus
What is the morphology of S.aureus lung infections? What normally causes them
- They have a polymorphonuclear infiltrate similar to that of S.pneumoniae infections, but cause much more tissue destruction.
- They usually arise from a haematogenous source, such as an infected thrombus, or in the setting of a predisposing condition such as infleunza.
What is staphylococcal scalded-skin syndrome? Who does it happen in and how does it happen?
- It occurs most frequently in children with S.aureus infection of the nasopharynx or skin.
- There is a sunburn-like rash that spreads over the entire body and evolves into fragile bullae that lead to partial or total skin loss.
- The desquamation of the epidermis in staphylococcal scalded-skin syndrome occurs at the level of the granulosa layer, distinguishing it from the toxic epidermal necrolysis, or Lyell disease, which is secondary to drug hypersensitivity and causes desquamation at the level of the epidermal-dermal junction.
What are some species that cause respiratory diseases and what diseases do the often cause?
- Streptococcus pyogenes - pharyngitis
- Corynebacterium diphtheria - diphtheria
- Bordatella pertussis - pertussis
- Streptococcus pnuemonia - lobar pneumonia
- Mycobacterium tuberculosis - TB
- Legionella pneumophila - Legionnaire disease
What are some species that cause gastrointestinal diseases and what diseases do the often cause?
- Helicobacter pylori - peptic ulcers
- Vibrio cholerae, enterotoxigenic E.coli - non-inflammatory gastroenteritis
- Shigella, Salmonella, Campylobacter, enterohaemorrhagic E.coli - Inflammatory gastroenteritis
- Salmonella typhi - enteric (typhoid) fever
- Clostridium difficile - pseudomembranous colitis
What are some species that cause nervous system diseases and what diseases do the often cause?
- Neisseria meningitidis, Streptococcus pneumonia, Haemophilus influenza, Listeria monocytogenes - acute meningitis
- Clostridium tetani, Clostridium botulinum - paralytic intoxications, tetanus and botulism
What are some species that cause urogenital diseases and what diseases do the often cause?
- Escherichia coli, Pseudomonas aeruginosa, Entercoccus species - UTIs
- Neisseria gonorrhoeae - gonorrhea
- Chlamydia trachomatis - chlamydia
- Treponema pallidum - syphilis
What are some species that cause skin and adjacent soft tissue diseases and what diseases do the often cause?
- Staphylococcus aureus - abscess, cellulitis
- Streptococcus pyogenes - impetigo, erysipela, necrotising fasciitis
- Clostridium perfringens - gas gangrene
- Bacillus anthracis - cutaneous anthrax
- Pseudomonas aeruginosa - burn infections
- Mycobacterium leprae - leprosy
What are some species that cause disseminated infections and what diseases do the often cause?
- Yersinia pestis - plague
- Borrelia burgdorferi - lyme disease
What are some species that cause disseminated neonatal infection and what diseases do the often cause?
- Streptococcus agalactiae, Listeria monocytogenes - neonatal bacteraemia, meningitis
- Treponema pallidum - congenital syphilis
What infections do Streptococci cause?
- Suppurative infections of the skin, oropharynx, lungs and heart valves
- Postinfectious syndromes, including rheumatic fever, immune complex glomerulonephritis, and erythema nodosum.
What type of bacteria are Streptococci?
They are gram-positive cocci that grow in pairs or chains.
β-haemolytic streptococci are typed according to their surface carbohydrate (Lancefield) antigens. What are the different types of β-haemolytic streptococci and what do they cause?
- Strep. pyogenes (group A) causes pharyngitis, scarlet fever, erysipelas, impetigo, rheumatic fever, toxic shock syndrome, and glomerulonephritis.
- Strep. agalactiae (group B) colonises the female genital tract and causes sepsis and meningitis in neonates and chorioamnionitis in pregnancy.
What is the most important α-haemolytic stretococcus? What does it cause?
Strep. pneumoniae is a common cause of CAP in older adults and meningitis in children and adults.
What does Strep. mutans cause?
Dental caries
What type of bacteria are enterococci? What do they cause
- Enterococci are gram-positive cocci that grow in chains.
- They are often resistant to commonly used antibiotics
- They are a significant cause of endocarditis and urinary tract infections.
What is the pathogenesis of Strep. pyogenes?
- Expresses M protein, a surface protein that prevents bacteria from being phagocytosed
- Expresses a complement 5a peptidase that degrades degrades this chemotactic peptide
- It secretes a phage-encoded pyrogenic exotoxin that causes fever and rash in scarlet fever
What do S.pyogenes, S.agalactiae, and S.pneumoniae all have that allows them to resist phagocytosis?
They have capsules that resist phagocytosis
How is post-streptococcal acute rheumatic fever cause?
By antistreptococcal M protein antibodies and T cells that cross-react with cardiac proteins
Although the antiphagocytic capsule is the most important virulence factor of S.pneumoniae, what other virulence factor does it produce?
Pneumolysin, a toxin that inserts into host cell membranes and lyses cells, greatly increasing tissue damage
How does S.mutans produce caries?
By metabolising sucrose to lactic acid (which causes demineralisation of tooth enamel) and by secreting high-molecular-weight glucans that promote aggregation of bacteria and plaque formation.
Enterococci are low-virulence bacteria, although they do have an antiphagocytic capsule and produce enzymes that injur host cells, what is the primary cause of the emergence of enterococci?
Their resistance to antibiotics
What is the morphology of streptococcal infections?
They are characterised by diffuse interstitial neutrophilic infiltrates with minimal destruction of host tissues.
What causes Erysipelas? How does it present both clincically and histologically?
- It is caused by exotoxins from superficial infection with S.pyogenes
- Characterised by rapidly spreading erythematous cutaneous swelling that may begin on the face, or less frequently, on the body or an extremity. The rash has a sharp, well-demarcated, serpiginous border and may form a ‘butterfly’ distribution on the face.
- Histologically, there is a diffuse, oedematous, neutrophilic inflammatory reaction in the dermis and epidermis extending into the subcutaneous tissues. Microabscesses may be formed, but tissue necrosis is usually minor.
What are the symptoms of streptococcal pharyngitis?
- Oedema, epiglottic swelling, and punctate abscesses of the tonsillar crypts, sometimes accompanied by cervical lymphadenopathy.
- Swelling associated with severe pharyngeal infection may encroach on the airways, especially if there is peritonsillar or retropharyngeal abscess formation.
What bacteria causes scarlet fever? How does it present?
- Associated with pharyngitis cause by S.pyogenes.
- Most common between the ages of 3 and 15 years
- Manifested y a punctate erythematous rash that is most prominent over the trunk and inner aspects of the arms and legs.
- The face is also involved, but usually a small area about the mouth remains relatively unaffected, producing circumoral pallor.
- They skin usually becomes hyperkeratotic and scaly during defervescence.
What is the bacteria that causes Diphtheria? How is it spread?
It is caused by Corynebacterium diphtheriae, a slender gram-positive rod with clubbed ends that spreads from person to person in respiratory droplets or skin exudate.
What symptoms does diphtheria infection cause?
- Respiratory diphtheria causes pharyngeal, or less often, nasal or laryngeal infection.
- There is toxin-mediated formation of a gray pharyngeal membrane, and damage to the heart, nerves, and other organs.
- Cutaneous diphtheria causes chronic ulcers with a dirty gray membrane, but does not cause systemic damage.
What is the pathogenesis of diphtheria?
- C. diphtheriae produces a phage-encoded A-B toxin that blocks host cell protein synthesis.
- The A fragment does this by catalysing the covalent transfer of ADP–ribose to elongation factor-2 (EF-2).
- This inhibits EF-2 function, which is required for the translation of mRNA into protein.
- A single molecule of diphtheria toxin can kill a cell by ADP-ribosylating, and thereby inactivating, more than a million EF-2 molecules.
How does immunisation with diphtheria toxoid work?
Immunisation with diphtheria toxoid (formalin-fixed toxin) stimualtes production of toxin-neutralising antibodies that protect people from the lethal effects of the toxin
What is the morphology of C.diphtheriae?
- Inhaled C.diphtheriae carried in respiratory droplets proliferate at the side of attachment on the mucosa of the nasopharynx, oropharynx, larynx, or trachea. The bacteria also form satellite lesions in the oesophagus or lower airways.
- Release of exotoxin causes necrosis of the epithelium, accompanied by an outpouring of a dense fibrinosuppurative exudate. The coagulaton of this exudate on the ulcerated necrotic surface created a tough, dirty gray to black, superficial membrane, sometimes called pseudo-membrane because it is not formed by viable tissue.
- There is an intense neutrophilic infiltration with marked vascular congestion, interstitial edema and fribin exudation.
- When the membrane sloughs off its inflamed and vascularised bed, bleeding and asphyxiation may occur.
- With control of the infection, the membrane is coughed up or removed by enzymatic digestion, and the inflammatory reaction subsides.
Although the bacterial invasion of diphtheriae remains localised, what happens with entry of exotoxin into the blood adn its systemic distribution?
There may be a fatty change in the myocardium with isolated myofiber necrosis, polyneuritis with degeneration of the myelin sheaths and axis cylinders, and (less commonly) fatty change and focal necroses of parenchymal cells in the liver, kidneys and adrenals.
What type of bacteria causes cholera?
Vibrio cholerae are comma-shaped, gram-negative bacteria, that cause cholera, a disease that has been endemic in the Ganges Valley of India and Bangladesh for almost all of recorded history
There is a marked seasonal variation in the incidence of cholera in most climates. Why?
Due to radpi growth of Vibrio bacteria at warm temperatures.
Where do people get cholera from?
While the bacteria can be present in food, the infection is primarily transmitted by contaminated drinking water.
Thus, cholera can become rampant in areas devastated by natural or man-made disasters, such as earthquakes or war, that threaten sewage systems and drinking water supplied.
What is the pathogenesis of cholera?
- Cholera toxin is composed of five B subunits and a single A subunit.
- The B subunit binds GM1 ganglioside on the surface of intestinal epithelial cells, and is carried by endocytosis to the endoplasmic reticulum, a process called retrograde transport.
- Here, the A subunit is reduced by protein disulfide isomerase, and a fragment of the A subunit is unfolded and released.
- This peptide fragment is transported into the cytosol using host cell machinery that moves misfolded proteins from the endoplasmic reticulum to the cytosol. The A subnit refolds to void degradation.
- The refolded A subunit peptide then interacts with cytosolic ADP ribosylation factors (ARFs) to ribosylate and activate the stimulatory G protein Gsα, which stimulates adenylate cyclase and the resulting cAMP opens the cystic fibrosis transmembrane conductance regulator, CFTR, which releases chloride ions into the lumen.
- Chloride and sodium absoprtion are also inhibited by cAMP.
- The resulting accumulation accumulation of chloride, bicarbonate, and sodium within the lumen creates an osmotic driving force that draws water into the lumen and causes massive diarrhoea
What are the clinical features of cholera?
- Most individuals exposed are asymptomatic or develop only mild diarrhoea.
- In those with severe disease there is an abrupt onset of watery diarrhoea and vomiting following an incubation period of 1 to 5 days.
- The voluminous stools resemble rice water and are sometimes described as having a fishy odor.
- The rate of diarrhoea may reach 1L per hour, leading to dehydration, hypotension, muscular cramping, anuria, shock, LOC and death.
What is the most common bacterial enteric pathogen in developed countries? What do you get it from?
Campylobacteri jejuni is an important cause of traveler’s diarrhoea
* Most infections are associated with ingestion of improperly cooked chicken, but outbreaks can also be caused by unpasteurised mild or contaminated water
The pathogenesis of Campylobacter infection remains poorly defined, but what are the four major properties that contribute to virulence?
- Motility - flagella allow campylobacter to be motile
- Adherence - the motility facilitates adherence and colonisation, which are necessary for mucosal invasion.
- Cytotoxins that cause epithelial damage and a cholera toxin-like enterotoxin are also released by some C.jejuni isolates.
With Campylobacter infection, when does dysentry and enteric fever occur?
- Dysentry, i.e. blood diarrhoea, is generally associated with invasion and only occurs with a small minority of Campylobacter strains.
- Enteric fever occurs when bacteria proliferate within the lamina propria and mesenteric lymph nodes.
What are some extra-intestinal complications of Campylobacter?
- Campylobacter infection can result in reactive arthritis, primarily in patients with HLA-B27.
- Erythema nodosum and Guillain-Barre syndome.
What is the morphology of Campylobacter and how is it diagnosed?
- Diagnosis is primarily by stool culture, since biopsy findings are nonspecific, and reveal acute self-limited colitis with features ocmmon to many forms of infectious colitis.
- Mucosal and intraepithelial neutrophil infiltrates are prominent, particularly within the superficial mucosa; cryptitis (neutrophil infiltration of the crypts) and crypt abscesses may also be present.
- Importantly, crypt architecture is preserved, although this can be difficult to assess in cases with severe mucosal damage
What are the clinical features and inbucation period of campylobacter?
- Incubation period is up to 8 days
- Watery diarrhoea, either acute of following an influenza-like prodrome, is the primary symptom, but dysentery develops in 15% of adults and more than 50% of children.
- Patients may shed bacteria for 1 month or more after clinical resolution.
What type of bacteria are shigella?
Shigella are gram-negative unencapsulated, nonmotile, facultative anaerobes that belong to teh enterobacteriaceae family and are closely related to enteroinvasive E.coli.
How is shigella transmitted? What populations are most affected?
- By the faecal-oral route or via contaminated water and food.
- In the US and Europe, children in daycare, migrant workers, travelers, and those in nursing homes are more commonly affected.
What is the pathogenesis of Shigella?
- Shigella are resistant to the harsh acidic environment of the stomach, thereby explaining teh extremely low infective dose
- Once in the intestine, organisms are taken up by M, or microfold cells. There are epithelial cells, which are specialised for sampling and presentation of luminal antigens.
- Shigella proliferate intracellularly, escape into the lamina propria, and are phagocytosed by macrophages, in which they induce apoptosis.
- The ensuing inflammatory response damages surface epithelia and allows shigella within the intestinal lumen to gain access to the basolateral membranes of colonic epithelial cells, which is the preferred domain for invasion.
- All Shigella species carry virulence plasmids, some of which encode a type III secretion system capable of directly injecting bacterial proteins into the host cytoplasm.
- Shigella. dysenteriae serotype 1 also release the Shiga toxin Stx, which inhibits eukaryotic protein synthesis, resulting in host cell damage and death.
Where in the colon are Shigella infections most prominent and why?
In the left colon, but the ileum may also be involved, perhaps reflecting the abundance of M cells in the dome epithelium over the Peyer patches.
What is the histology of Shigella?
The mucose is haemorrhagic and ulcerated, and pseudomembranes may be present
* The histology of early cases is similar to other acute self-limiting colitides, such as Campylobacter, but because of the tropism for M cells, apthous-appearing ulcers similar to those seen in Crohn disease may occur
What are the clinical features of Shigella, including the incubation period?
- After an incubation period of up to 1 week, Shigella causes self-limiting disease characterised by about 1 week fo diarrhoea, fever and abdominal pain
- The initially watery diarrhoea progresses to a dysenteric phase in approx 50% of patients, and constitutional symptoms can persist for as long as 1 month.
- The subacute presentation that develops in a minority of patients is characterised by several weeks of waxing and waning diarrhoea that can mimic new-onset ulcerative colitis.
What are the extra-intestinal complications of shigella?
- They are uncommon and include a triad of sterilr reactive arthritis, urethritis and conjunctivitis that preferentially affects HLA-B27 positive men between 20 and 40 years of age.
- Haemolytic-uraemic syndrome, may also occur after infection with serotype 1 infection that secretes Shiga toxin.
- Toxic megacolon and intestinal obstruction are uncommon complications
How do antibiotics affect Shigella and Campylobacter?
- No effect in Campylobacter
- Shortens the clinical course and reduces the duration of organism shedding in stools in Shigella
What type of bacteria is Salmonella? How are they divided
Salmonella, which are classified within then Entero-bacteriaceae family of gram-negative bacilli, are divided into Salmonella typhi, the causative agent of typhoid fever and the nontyphoid Salmonella, of which S.enteritidis is the most common.
What causes Salmonella?
Ingestion of contaminated food, particularly raw or udnercooked meat, poutlry, eggs, and milk.
Centralised food processing can lead to large outbreaks
Can you get a vaccine for Salmonella?
Yes, and so can farm animals e.g. egg-laying hens
What increases your risk of Salmonella?
Very few viable Salmonella are necessary to cause infection, and the absence of gastric acid, in individuals with atrophic gastritis or those on acid-suppressive therapy, further reduces the required inoculum.
What is the pathogenesis of Salmonella?
- Salmonella possess virulence genes that encode a type III secretion system capable of transferring bacterial proteins into M cells and enterocytes.
- The transferred proteins activate host Rho GTPases, thereby triggering actin rearrangement and bacterial endocytosis which, in turn, allows bacterial growth within endosomes.
- In addition, flagellin, the core protein of bacterial flagellae, activates TLR5 on host cells and icnreases the local inflammatory response.
- Similarly, bacterial lipopolysaccharide activates TLR4, although some Salmonella strains express a virulence factor that prevents TLR4 activation.
- Salmonella also secrete a molecule that induces epithelial cell release of the eicosanoid hepoxilin A3, thereby drawing neutrophils into the intestinal lumen and potentiaing mucosal damage
What are the clinical features on Salmonella?
- They are clinically indistinguishable from other enteric pathogens, and symptoms range from loose stools to cholera-like profuse diarrhoea to dysentery.
- Fever often resolves within 2 days, but diarrhoea can persist for a week and organisms can be shed in the stool for several weeks after resolution.
Are antibiotics recommended in Salmonella?
Antibiotics therapy is not recommended in uncomplicated cases because it can prolong the carrier state or even cause relapse and does not typically shorten the duration of diarrhoea.
Most salmonella infections are self-limited, but deaths do occur.
What bacteria causes typhoid fever?
- It is caused by Salmonella enterica and its two subtypes, typhi and paratyphi.
- The majority of cases in endemic countries are due to S.typhi, while infection by S.paratyphi is more common among travelers, perhaps because travelers tend to be vaccinated against S.typhi.
What countries are associated with typhoid?
India, Mexico, the Philippins, Pakistan, El Salvador, and Haiti.
How is S.typhi and S.paratyphi transmitted?
Humans are the sole reservoir for S.typhi and S. paratyphi and transmission occurs from person to person or via food or contaminated water.
What is the pathogenesis of S.typhi?
- S.typhi are able to survive in gastric acid and, once in the small intestine, they are taken up by and invade M cells.
- Bacteria are then engulfed by mononuclear cells in the underlying lymphoid tissue.
- Unlike S.enteritidis, S.typhi can then disseminate via lymphatic and blood vessels.
- This causes reactive hyperplasia of phagocytes and lymphoid tissues throughout the body.
What is the morphology of typhoid fever?
- Infection causes Peyer patches in the terminal ileum to enlarge into sharp, delineated, plateau-like elevations up to 8cm in diameter.
- Draining mesenteric lymph noces are also enlarged.
- Neutrophils accumulate within the superficial lamina propria, and macrophages containing bacteria, red cells, and nuclear debris mix with lymphocytes and plasma cells in the lamina propria.
- Mucosal damage creates oval ulcers, oriented along the axis of the ileum, that may perforate.
- The draining lymph nodes also harbor organisms and are enlarged due to phagocyte accumulation
How does typhoid affect the liver and spleen morphologically?
- The spleen is enlarged and soft, with uniformly pale red pulp, obliterated follicular markings, and prominent phagocyte hyperplasia.
- The liver shows small, randomly scattered foci of parenchmal necrosis in which hepatocytes are replaced by macrophage aggregates, called typhoid nodules; such nodules may also develop in the bone marrow and lymph nodes.
What are the clinical features of typhoid?
- Patients experience anorexia, abdominal pain, bloating, nausea, vomiting, and blood diarrhoea followed by a short asymptomatic phase that gives way to bacteraemia and fever with flu-like symptoms.
- Patients have sustained high fevers and abdominal tenderness, that may mimic appendicitis
- Rose spots, small erythematous maculopapular lesions, are seen on the chest and abdomen.
- Symptoms abate after several weeks, although relapse can occur
Do antibiotics help with typhoid?
- Blood cultures are positive in more than 90% of affected individuals during the febrile phase.
- Antibiotic treatment can prevent further disease progression.
- In patients who do not receive antibiotics, the initial febrile phase continues for up to 2 weeks.
What is AIDS?
Acquired immunodeficiency syndrome is a disease causes by the retrovirus human immunodeficiency virus (HIV) and characterised by profound immunosuppression that leads to opportunistic infections, secondary neoplasms, and neurologic manifestations.
Epidemiological studies in the US have identified five groups of adults at high risk for developing AIDS. What are they?
- Homosexual or bisexual men constitute the largest group, accounting for more than 50% of the reported cases.
- IVDUs with no previous history of homosexuality are the next largest group, representing about 20% of infected indiviuals
- Haemophiliacs, especially those who receive large amounts of factor VIII or factor IX concentrated before 1985
- Recepients of blood and blood components who are not haemophiliacs but who received transufusions of HIV-infected whole blood or components account for 1% of patients
- Heterosexual contacts of members of other high-risk groups (chiefly IVDUs), globally heterosexual transmission is most common mode by which HIV is spread - mainly female partners of male IVDUs.
The transmission of HIV occus under conditions that facilitate exchange of blood or bodily fluids containing the virus or virus-infected cells. What are the three major routes and tell me a bit about them?
- Sexual transmission is clearly the dominant mode of infection worldwide, accounting for >75% of cases. Viral tramission occurs in two ways 1) direct inoculation into the blood vessels breached by trauma and 2) infection of dendritic cells or CD4+ cells within the mucosa.
- Parenteral transmission occurs by sharing of needles and syringes with HIV-containing blood.
- Mother-to-infant transmission is the major cause of paediatric AIDS. It can occur by three routes: 1) in utero by transplacental spread. 2) during delivery through an infected birth canal, and 3) after birth by ingestion of breast milk. Transmission during birth and in the immediate period after is the most common.
How do STIs affect HIV transmission? Why?
Sexual transmission of HIV is enhanced by coexisting STDs, especially those associated with genital ulceration - syphillips, herpes. Other STDs are also co-factors for transmission, perhaps because in these genital inflammatory states there is a greater concentration of the virus and virus-containing cells in genital fluids, as a result of increased numbers of inflammatory cells in the semen
Transmission of HIV by transfusion of blood of blood products has been virtually eliminated by three public health measures. What are they?
1) Screening of donated blood and plasma for antibody to HIV
2) Stringent purity criteria for factor VIII and factor IX preperations
3) Screening of donors on the basis of history
What type of virus is HIV?
A non-transforming human retrovirus belonging to the lentivirus family.
What is the structure of HIV?
- The HIV-1 virion is spherical and contains an electron-dense, cone-shaped core surrounded by a lipid envelope derived from the host cell membrane
- The virus core contains 1) the major capsid protein 24; 2) nucleocapsid protein p7/p9; 3) two copies of viral genomic RNA; and 4) the three viral enzymes (protease, reverse transcriptase, and integrase).
- The viral core is surrounded by a matrix protein called p17, which lies underneath the virion envelope.
- Studding the viral envelope are two viral glycoproteins, gp120 and gp41, which are critical for HIV infection of cells.
What does the HIV-1 RNA genoma contain?
- The gag, pol and env genes, which are typical of retroviruses.
- The products of the gag and pol genes are large precursor proteins that are cleaved by the viral protease to yield the mature proteins.
- HIV contains several other accessory genes, including tat, rev, vif, nef, vpr and vpu, which regulate the synthesis and assembly of infectious viral particles and the pathogenicity of the virus
Two genetically different but related forms of HIV have been isolated from patients with AIDS. What are these and where are they most common?
- HIV-1 is the most common type associated with AIDS in the US, Europe and Central Africa
- HIV-2 causes a similar disease principalle in West Africa and India
HIV-1 can be divided into three subgroups. What are they? Tell me more about the most common one
- Group M (major) viruses are the most common worldwide, and they are further divided into several subtypes designated A through K. Subtype B is the most common in Western Europe and the US.
- Group O (outlier)
- Group N (neither M nor O)
What is the broad pathogenesis of HIV and AIDs?
- The two major targets of HIV infection are the immune system and the CNS.
- Profound immune deficiency results chiefly from infection and subsequent loss of CD4+ T cells as well as the impairment in the function of surviving helper T cells. Macrophages and dendritic cells are also targets of HIV infection.
- HIV enters thebody through mucosal tissues and blood and first infects T cells as well as dendritic cells and macrophages
- The infection becomes established in lymphoid tissues, where the virus may remain latent for long periods.
- Active viral replication is associated with more infection of cells and profression to AIDS.
What does the life cycle of HIV consist of?
Infection of cells, integration of the provirus into the host cell genome, activation of viral replication, and production and release of infectious virus.
How does the infection of cells by HIV occur?
- HIV infects cells by using the CD4 molecule as a receptor and various chemokine receptors as corectpors.
- The initial step of infection is the binding of gp120 envelope glycoprotein to CD4 molecules, which leads to a conformation change that results in the formation of a new recognition site on gp120 for the coreceptors CCR5 and CXCR4.
- The conformational changes in gp41 result in the exposure of a hydrophobic region called the fusion peptide at the tip of gp41.
- This peptide inserts into the cells membrane of the target cells, leading to fusion of the virus with the host cell.
- After fusion, the virus core containing the HIV genome enters the cytoplasm of the cell
How do chemokines effect HIV infection of cells in culture?
They sterically hinder it by occupying their receptors, and therefore, the level of chemokines in the tissues may influence the efficiency of viral infection in vivo
Once HIV is internalised into the cells, the RNA genome of the virus undergoes reverse trascription, leading to the synthesis of double-stranded complementary DNA (cDNA; proviral DNA). What are the options of what can happen next?
- In quiescent T cells, HIV cDNA may remain in the cytoplasm in a linear episomal form.
- In dividing T cells, the cDNA circularises, enters the nucleus, and is then integrated into the host genome.
- After this integration, the provirus may be silent for months of years, a form a latent infection.
- Alternatively, proviral DNA may be transcribed, with the formation of complete viral particles that bud from the cell membrane. Such productive infection, when associated with extensive viral budding, leads to death of infected cells.
What type of T cells does HIV infect?
HIV infects memory and activated T cells but is ineffecient at productively infecting naive (unactivatived T cells).
Why can HIV not productively infect naive T cells? How does this change in activated T cells?
- Naive T cells contain an active form of an enzyme that introduces mutations in the HIV genome.
- This enzyme is called APOBEC3G. It is a cytidine deaminase that introduces cytosine-to-uracil mutations in the viral DNA that is produced by reverse transcription.
- These mutations inhiti further DNA replication.
- Activation of T cells converts cellular APOBEC3G into an inative, high-molecular-mass complex, which explains why the virus can replicate in previously activated (memory) T cells and T-cell lines.
How is HIV linked to NF-κB?
- Activation of T cells upregulates several transcription factors, including NF-κB, which stimulate transcription of genes encoding cytokines such as IL-1
- The long-terminal-repeat sequences that flank the HIV genome also contain NF-κB-binding sites that can be triggered by the same transcription factors.
- Induction of NF-κB-in latently infected CD4+ cells activates the transcription of HIV proviral DNA and leads ultimately to the production of virions and to cell lysis.
- It seems that HIV thrives when the host T cells and macrophages are physiologically activates, an act that can best be described as ‘subversion from within’
How does infection effect HIV infection?
- HIV-infected people are at increased risk for recurrent exposure to other infections, which lead to increased lymphocyte activation and production of proinflammatory cytokines.
- These, in turn, stimulate more HIV production, loss of additional CD4+ T cells, and more infection.
- A vicious cycle may be set up that cluminates in inexorable destruction of the immune system.
What is the main mechanism of T-cell depletion in HIV infection?
- Loss of CD4+ T cells is mainly because of infection of the cells and direct cytopathic effects of the replicating virus.
- Many infected cells may be in mucosal and other peripheral lymphoid organs and death of these cells is a major cause of the relentless, and eventually profound, cell loss
In addition to firect killing of cells by the virus, other mechanisms may contribute to the loss of T cells. What are they?
- Chronic activation of uninfected cells, responding to HIV itself or to infections that are common in individuals with AIDS, leads to apoptosis of these cells by the process of activation-induced cell death.
- Non-cytopathic (abortive) HIV infection activates the inflammasome pathway and leads to a form of cell death called pyroptosis - inflammatory cytokines and cellular contents are released.
- HIV infects cells in lymphoid organs (spleen, lymph nodes and tonsils) and may cause progressive destruction of the architecture and cellular composition of lymphoid tissues.
- Loss of immature precursors of CD4+ T cells, either by direct infection of thymus progenitor cells or by infection of accessory cells that secrete cytokines.
- Fusion of infected and uninfected cells with formation of giant cells
- Defects in Th1-type responses relative to the Th2 type, defects in intracellular signaling.
- Selective loss of memory CD4+ helper T cells.
In addition to infection and loss of CD4+ T cells, infection of macrophages and dendritic cells is also important in the pathogenesis of HIV infection. What are some aspects of HIV infection of macrophages?
- HIV-1 can infect and multiply in terminally differentiated nondividing macrophages. THe Vpr protein allows nuclear targeting of the HIV preintegration complex through the nuclear pore.
- Infected macrophages bud relatively small amounts of virus from the cell surface, but these cells contain large numbers of virus particles often located in intracellular vacuoles.
- Macrophages are quire resistant to the cytopathic effects of HIV, so in late stages, when CD4+ T cell numbers are low, macrophages are an important site of continued viral replication.
- They have functional defects such as impaired microbicidal activity, decreased chemotaxis, decreased secretion of IL-1, inappropriate secretion of TNF, and poor capacity to present antigens.
In addition to macrophages, two types of dendritic cells are also important targets for the initiation and maintenance of HIV infection. What are they and what do they do?
- Mucosal dendritic cells are infected by the virus and may transport it to regional lymph nodes, where the virus is transmitted to CD4+ T cells
- Dentritic cells also express a lectin-like receptor that specifically binds HIV and displays it in an intact, infectious form to T cells, thus promoting infection of T cells.
- Follicular dendritic cells in the germinal centers of lymph nodes are potential reservoirs of HIV.They have receptors for the Fc portion of immunoglobulins and hence they trap HIV virions coated with anti-HIV antibodies. These virions retain their ability to infect CD4+ T cells as they traverse the intricate meshwork formed by the dendritic processes of the follicular dendritic cells.
How is the function of B cells affected by HIV infection?
- There is polyclonal activation of B cells, resulting in germinal center B-cell hyperplasia, bone marrow plasmacytosis, hypergammaglobulinaemia, and formation of circulating immune complexes.
- This activation may result from: reactivation of or reinfection with CMV and/or EBV, both of which are polyclonal B-cell activators; gp41 itself can promote B-cells growth and differentiation; HIV-infected macorphages produce increased amounts of IL-6, which stimulates proliferation of B cells.
What is the pathogenesis of CNS involvement in HIV infection?
- Macrophages and microglia are the predominant cell types in the brain that are infected with HIV
- The neurological deficit is likely caused indirectly by viral products and by soluble factors produced by infected microglia, e.g. IL-1, TNF and IL-6
What is the natural history of HIV infection?
- Virus typically enters through mucosal epithelia
- An acute retroviral syndrome
- A middle, chronic phase, in which most individuals are asymptomatic
- Clinical AIDS
What happens during the acute infection of HIV?
- Acute infection is characterised by infection of memory CD4+ T cells (which express CCR5) in mucosal lymphoid tissues, and death of many infected cells.
- Mucosal infection is often associated with damage to the epithelium, defects in mucosal barrier functions, and translocation of microbes across the epithelium.
Mucosal infection is followed by dissemination of the virus and the development of host immune responses. How does this happen?
- Dendritic cells in epithelia at sites of virus entry capture the virus and then migrate into the lymph nodes.
- Once in lymphoid tissue, dentritic cells may pass HIV on to CD4+ T cells through direct cell-t-cell contact.
- Within days after the first exposure to HIV, viral replication can be detected in the lymph nodes
- This replication leads to viremia. The virus disseminates throughout the body and infects helper T cells, macrophages, and dendritic cells in peripheral lymphoid tissues.
- As the infection. spreads, the individual mounts antiviral humoral and cell-mediated immune responses evidenced by seroconversion and by the development of virus-specific CD8+ cytotoxic T cells.
What immune responses to HIV infection occur at what time?
- A cytotoxic T lymphocyte response to HIV is detectable by 2-3 weeks after the initial ifnection, and it peaks by 9-12 weeks.
- Marked expansion of virus-specific CD8+ T cell clones occurs during this time, and up to 1-% of patients CTLs may be specific at 12 weeks.
- The humoral immune response to HIV peaks at about 12 weeks.
What is the acute retroviral syndrome in HIV infection?
- It is the clinical presentation of the initial spread of the virus and the host response.
- 40-90% if individuals who acquire a primary infection develop this syndrome
- It typically occurs 3-6 weeks after infection, and resolves spontaneously in 2-4 weeks.
- Clinically, this phase is associated with a self-limited acute illness with nonspecific symptoms, including sore throat, myalgias, fever, weight loss and fatigue, resembling a flulike syndrome.
What does the viral set point in HIV mean and what does it predict?
- The viral load at the end of the acute phase reflects the equilibrium reached between the virus and the host response, and in a given patient it may remain fairly stable for several years.
- The level, called the viral set point, is a predictor of the rate of decline of CD4+ T cells, and, therefore, profression of HIV disease.
Because the loss of HIV immune containment is associated with declining CD4+ T cell counts, the CDC classification of HIV infection stratifies three categories on the basis of CD4+ cell counts. What are they and what levels are there?
What happens during the chronic phase of HIV?
- Lymph nodes and the spleen are sites of continuous HIV replication and cell destruction
- During this period, few or no clinical manifestations of the HIV infection are present.
- Although the majority of peripheral blood T cells do not harbor the virus, destruction of CD4+ T cells within lymphoid tissues continues and the numer of circulating blood CD4+ T cells steadily declines.
- Host defenses begin to wane, and the proportion of the surviving CD4+ cells infected with HIV increases, as does the viral burden per CD4+ cell.
The final phase of HIV infection is AIDS. What is this characterised by?
- A breakdown of host defence, a dramatic increase in plasma virus, and severe, life-threatening clinical disease.
- Long-lasting fever (>1 month), fatigue, weight loss and diarrhoea.
- After a variable period, serious opportunistic infections, secondary neoplasms, or clinical neurologic disease emerge, and the patient is said to have developed AIDS.
In the absence of treatment, most patients with HIV infection progress to AIDS after how long?
- After a chronic phase lasting 7-10 years
- Exceptions to this typical course are exemplified by rapid progressors and long-tern nonprogressors
- In rapid progressors the midd,e chronic phase in telescoped to 2-3 years after primary infection.
- About 5-15% of infected individuals are long-term nonprogressors, defined as untreated HIV-1-infected individuals who remain asymptomatic for 10 years or more, with stable CD4+ T-cell counts and low levels of plasma viremia.
What are the clinical features of AIDS?
- They range from a mild acte illness ro severe disease.
- In the US, the typical adult patient presents with fever, weight loss, diarrhoea, generalised lymphadenopathy, multiple opportunistic infections, neurologic disease, and , in many cases, secondary neoplasms
What are the AIDS-defining opportunistic infections?
Protozoal and Helminthic infections
Cryptosporidiosis or isosporidiosis (enteritis)
Pneumocystis jireoveci (pneumonia or disseminated infection)
Toxoplasmosis (pneumonia or CNS infection)
Fungal infections
Candidiasis (oesophageal, tracheal, or pulmonary)
Cryptococcosis (CNS infection)
Coccidiodomycosis (disseminated)
Histoplasmosis (disseminated)
Bacterial infections
Mycobacteriosis tuberculosis and mycobacterium avium-intracellulare
Nocardiosis (pneumonia, meningitis, disseminated)
Salmonella infections, disseminated
Viral infections
CMV (pulmonary, intestinal, retinitis, or CNS infections)
HSV (localised or disseminated infection)
VZV (localised or disseminated infection)
Progressive multifocal leukoencephalopathy
What are some AIDS-defining neoplasms?
Kaposi sarcoma
Primary lymphoma of brain
Invasive cancer of uterine cervix - likely secondary to HPV
What is Kaposi sarcoma?
- A vascular tumour
- The lesions of KS are characterised by the proliferation of spindle-shaped cells that express markers of both endothelial cells (vascular or lymphatic) and smooth muscle cells.
- The tumour is usually widespread in AIDS, affecting the skin, mucous membranes, GI tract, lymph nodes and lungs.
- There is also a profusion of slitlike vascular spaces, suggesting that the lesions may arise from primitive mesenchymal precursors of vascular channels.
- In addition, KS lesions display chronic inflammatory cell infiltrates.
What is the pathogenesis of KS?
- The spindle cells produce proinflammatory and angiogenic factors, which recruit the inflammatory and neovascular components of the lesion, and the latter components supply signals that aid in spindle cell survival and growth
What causes Kaposi sarcoma? How?
- It is caused by the KS herpesvirus, also called human herpesvirus 8 (HHV8).
- HHV8 establishes latent infection, during which several proteins are produced with potential roles in stimulating spindle cell proliferation and preventing apoptosis. These include a viral homologue of cyclin D and several inhibitors of p53
Why lymphoma is linked with Kaposi Sarcoma? why?
- KSHV infection is not restricted to endothelial cells.
- This virus is related phylogenetically to the lymphotropic subfamily of herpesvirus (γ-herpesvirus). Its genome is found in B cells of infected subjects
- It is linked to rare B-cell lymphomas in AIDS patients (primary effusion lymphoma) and to multicentric Castleman disease, a B-cell lymphoproliferative disorder
What percentage of people with AIDS develop lymphoma?
Roughly 5% present with lymphoma, and approx another 5% develop lymphoma
What are the mechanisms of lymphoma development in AIDS?
- Unchecked proliferation of B cells infected with oncogenic herpesvirus in the setting of profound T cell depletion (AIDS). T cell immunity is required to restrain the proliferation of B cells infected with oncogenic viruses such as VBD and KSHV. AIDS patients are at high risk of developing aggressive B cell lymphomas composed of tumour cells infected by oncogenic viruses, particularly EBV.
- Germinal center B-cell hyperplasia in the setting of early HIV infection. In germinal centers, B cells diversify their immunoglobulin genes via lesions introduced into their DNA by the enzyme activation-induced deaminase (AID). AID can cause mutations in oncogenes implcated in B-cell lymphomagenesis.
How does EBV in HIV cause lymphomas?
- Once EBV immunity is established, it persists as a latent infection in memory B cells.
- Activation of such cells, by antigen or by cytokines, reawakens an EBV-encoded program of gene expression that drives B-cell proliferation.
- Patients with AIDS have high levels of several cytokines, some of which, including IL-6 are growth factors for B cells.
- In the absence of T-cell immunit, these activated, EBV infected clones proliferate and eventually acquire additional somatic mutations, leading to their outgrowth as full-blown EBV positive B-cell lymphomas.
Apart from B-cell lymphoma, what are some other EBV-related proliferations that are common in AIDS?
- Hodgkin lymphoma - all instances of HIV-associated Hodgkin lymphoma, the characteristic tumour cells (Reed-Sternberg cell) are infected with EBV.
- Oral hair leukoplakia (white projections on the tongue), which results from EBV-driven sqaumour cell proliferation of the oral mucosa.
What are some CNS diseases in AIDS?
- Self-limited meningo-encephalitis occuring at the time of seroconversion
- Aseptic meningitis
- Vaculoar myelpathy
- Peripheral neuropathies
- Most commonly, a progressive encephalopathy
In very broad terms, what do the antiretroviral drugs do for HIV?
- They target the viral reverse transcriptase, protease, and integrase.
- They are given in combination to reduce the emergence of mutants that develop resistance to any one
- Treatment regimens are commonly called highly active antiretroviral therapy (HAART) or combination antiretrovoral therapy.
What is the immune reconstitution inflammatory syndrome that may occur with antiretrovirals? Why does it happen?
- Some patients with advanced disease who are given antiretroviral therapy develop a paradoxical clinical deterioration during the period of recovery of the immune system. This occurs despite increasing CD4+ T cell counts and decreasing viral load.
- It is postulated to be a poorly regulated host response to the high antigenic burden of persistent microbes.
What are the adverse effects of HAART?
- Lipoatrophy (loss of facial fat)
- Lipoaccumulation (excess fat deposition centrally)
- Elevated lipids
- Insulin resistance
- Peripheral neuropathy
- Premature cardiovascular kidney and liver disease
What is the morphology of early stage HIV infected lymph nodes?
- A marked hyperplasia of B cell follicles. The follicles are enlarged and often take on unusual, serpiginous shapes
- The mantle zones that surroun the follicles are attenuated, and teh germinal centers impinge on interfollicular T cell areas.
- This hyperplasia of B cells is the morphologic reflection of the polyclonal C-cell activation and hypergammaglobulinaemia seen in HIV-infected individuals.
How does the morphology of lymph nodes change throughout HIV infection?
- With disease progression, the frenzy of B-cell proliferation subsides and gives way to a pattern of severe lymphoid involution.
- The lymph nodes are depleted of lymphocytes, and the organised network of follicular dendritic cells is disrupted. The germinal centers may even become hyalinised.
- These ‘burn-out’ lymph nodes are atrophic and small and may harbor numerous opportunistic pathogens, often with macrophages.
What are some modifiable and non-modifiable risk factors for atherosclerosis?
NON-MODIFIABLE
* Genetics - familial hypercholesterolaemia, diabetes risk
* Age - between ages 40-60, the incidence of MI increases five-fold. Death rates from IHD rise with each decade
* Gender - very low risk in premenopausal women. After menopause however, the incidence increases in women and at older ages actually exceeds that of men
MODIFIABLE
* Hyperlipidaemia - specificially LDL levels
* HTN - both systolic and diastolic levels - chronic HTN is the most common cause of LV hypertrophy
* Smoking
* Diabetes mellitus induces hypercholesterolemaeia and markedly increases the risk of atherosclerosis
What are some additional risk factors for atherosclerosis?
- Inflammation is present during all stages of atherogenesis and is intimately linked with atherosclerotic plaque formation and rupture. CRP correlates with IHD risk.
- Hyperhomocystinemia is associated with premature vascular disease. Although low folate and vit G12 levels can ingrease homocystiene, supplements don’t affect the CVD risk
- Metabolic syndrome associated with central obesity, is characterised by insulin resistance, HTN, dyslipidaemia, hypercoagulability, and a proinflammatory state
- Lipoprotein a is an altered form of LDL and is associated with coronary and cerebrovascular disease risk, independent of total cholesterol or LDL levels.
- Factors affecting hemostasis (plasminogen activator inhibitor 1, thrombin)
What is CRP? How does it work?
CRP is an acute phase reactant synthesisted primarily by the liver.
Its expression is increased by a number of inflammatory mediators, particularly IL-6, and it augmetns the innate immune response by binding to bacteria and activating the classical complement cascade.
What is the pathogenesis pathway of atherosclerosis?
- Endothelial injury and dysfunction, causing increased vascular permeability, leukocyte adhesion and thrombosis
- Accumulation of lipoproteins (mainly LDL and its oxidised forms) in the vessel wall
- Monocyte adhesion to the endothelium, followed by migration into the intima and transformation into macrophages and foam cells
- Platelet adhesion
- Factor release from activated platelets, macrophages and vascular wall cells, inducing smooth muscle cell recruitment, either from the media or from circulating precursors
- Smooth muscle cell proliferation, extrcellular matrix production and recruitment of T cells
- Lipid accumulation both extracellularly and within cells (macrophages and smooth muscle cells)
How does endothelial injury precipitate atherosclerosis?
- Early human lesions begin at sites of morphologically intact endothelium. Thus, endothelial dysfunction underlies most atherosclerosis.
- The intact but dysfunctional endothelial cells exhibit increased endothelial permeability, enhanced leukocyte adhesion, and altered gene expression.
Endothelial injury can be caused by toxins from cigarette smoke, infectious agent and inflammatory cytokines. But what are the two most important causes of endothelial dysfunction?
- Haemodynamic disturbances such as stasis or flow turbulence
- Hypercholesterolaemia
What are the different types of hypercholesterolaemia?
- Lipids are transported in the bloodstream bound to specific apoproteins (forming lipoprotein complexes)
- Dyslipoproteinemias include 1) increased LDL cholesterol levels, 2) decreased HDL cholesterol levels, and 3) increased levels of the abnormal lipoprotein (a)
- Familial hypercholesterolaemia, caused by defective LDL receptors and inadequate hepatic LDL uptake, can precipitate MIs before age 20.
What are the mechanisms by which hyperlipidaemia contributes to atherogenesis?
- Chronic hyperlipidaemia can directly impair endothelial cell function by increasing local reactive oxygen species production; besides causing membrane and mitochondrial damage, oxygen free radicals accelerate nitric oxide decay, dampening its vasodilator activity.
- Lipoproteins accumualte within the intima, where they may aggregate or become oxidised by free radicals produced by inflammatory cells. Such modified LFL is then accumulated by macrophages. Because the modified lipoproteins cannot be completely degraded, chronic ingestion leads to the formation of lipid-filled macrophages ‘foam cells; smooth muscle cells can similarly transform into lipid-laden foam cells by ingesting modified lipids through LDL-receptor related proteins.
- Not only are the modified lipoproteins toxic to endothelial cells, smooth muscle cells, and macrophages, but their binding and uptake also stimulates the release of growth factors, cytokines, and chemokines that create a vicious cycle of monocyte recruitment and activation
How does inflammation cause atherosclerosis?
- It causes endothelial damage (you know how).
- Activated macrophages produce ROS that enhance LDL oxidation, and elaborate growth factors that drive smooth muscle cell proliferation.
- Activated T cells in the growing intimal lesions elaborate inflammatory cytokines, e.g. interferon-γ, which , in turn, can activate macrophages as well as endothelial cells and smooth muscle cells
- These leukocytes and vascular wall cells release growth factors that promote smooth muscle cell prolfieration and syntehsis of extracellular matrix proteins. Thus, many of the lesions of atherosclerosis are atrributable to the chronic inflammatory reaction in the vessel wall.
How does smooth muscle proliferation and matrix synthesis take place in atherosclerosis?
- Intimal smooth muscle cell proliferation and extracellular matrix deposition convert a fatty streak into a mature atheroma and contribute to the progressive growth of atherosclerotic lesions.
- Intimal smooth muscle cells have a proliferative and synthetic phenotype distinct from the underlying medial smooth muscle cells.
- Several growth factors are implicated in smooth muscle cell proliferation, including platelet-derived growth factor (PDGF), FGF and TGF-α. These factors also stimulate smooth muscle cells to syntehsis extracellular matrix (notably collagen), which stabilised atherosclerotic plaques.
- In contract, activated inflammatory cells in atheromas may increase breakdown of extracellular matrix components, resulting in unstabel plaques
What are atheromas?
They are dynamic lesions consisting of dysfunctional endothelial cells, proliferating smooth muscle cells, and admixed T lymphocytes and macrophages.
How does the make-up of atheromas change over time?
- At early stages, intimal plaques are little more than aggregates of smooth muscle cells, macrophages, and foam cells; death of these cells releases lipids and necrotic debris
- With progression, the atheroma is modified by extracellular matrix synthesised by smooth muscle cells; connective tissue is particularly prominent on the intimal aspect forming a fibrous cap, although lesions typically retain a central core of lipid-laden cells and fatty debris that can become calcified.
What is the outcome of an atheroma?
The intimal plaque may progressively encroach on the vessel lumen, or may compress the underlying media, leading to its degeneration this in turn may expose thrombogenic factors, resulting in thrombus formation and acute vascular occlusion
What are fatty streaks in atherosclerosis?
- Fatty streaks are composed of lipid-filled foamy macrophages.
- Beginning as multiple minute flat yellow spots, they eventially coalesce into elongated streaks 1cm long or longer
- These lesions are no sufficiently raised to cause significant flow disturbances.
- Although fatty streaks can evolve into plaques, not all are destined to become advanced lesions
What are the key processes in atherosclerosis?
Intimal thickening and lipid accumulation, which together form plaques
How do atherosclerotic plaques and lesions present morphologically?
- Atheromatous plaques are white-yellow and encroach on the lumen of the artery; superimposed thrombu over ulcerated plaques is red-brown.
- Atherosclerotic lesions are patchy, usually involving only a portion of any given arterial wall adn are rarely circumferential; on cross-section, the lesions therefore appear “eccentric”. The focality of atherosclerotic lesions is attributable to the vagaries of vascular haemodynamics.
What are the most common vessels prone to atherosclerosis?
The most extensively involved vessels are the lower abdominal aorta, the coronary arteries, the popliteal arteries, the internal carotid arteries, and the vessels of the circle of Willis.
Atherosclerotic plaques have three principal components. What are they?
1) Smooth muscle cells, macrophages and T cells
2) Extracellular matrix, including collagen, elastic fibers, and proteoglycans
3) Intracellular and extracellular lipids
How are the components of an atherosclerotic plaque lay out?
- Typically, there is a superficial fibrous cap composed of smooth muscle cells and relatively dense collagen
- Beneath and to the side of the cap (the “shoulder”) is a more cellular area containing macrophages, T cells, and smooth muscle cells.
- Deep to the fibrous cap is a necrotic core, containing lipid (primarily cholesterol and cholesterol esters), debris from dead cells, foam cells (lipid-laden macrophages and smooth muscle cells), fibrin, variably organised thrombus, and other plasma proteins; the cholesterol content is frequently present as crystalline aggregates that are washed out during routine tissue processin and leave behind only empty “clefts”.
Atherosclerotic plaques generally continue to change and progress over time, what are the processes that allow this?
Through cell death and degradation, synthesis and degradation (remodeling) of ECM, and organisation of any superoimproved thrombus.
Moreover, atheromas often undergo calcification.
Atherosclerotic plaques are susceptible to which clinically important pathologic changes?
- Rupture, ulceration, or erosion of the surface of atheromatous plaques exposes highly thrombogenic substances and leads to thrombosis, which may partially or completely occlude the vessel lumen.
- Haemorrhage into a plaque - rupture of the overlying fibrous cap, or of the thin-walled vessels in the areas of neovascularisation, can cause intraplaque haemorrhage; a contained haemtoma may expand the plaque or induce plaque rupture
- Atheroembolism - plaque rupture can discharge atherosclerotic debris into the blood stream, producing microemboli
- Aneurysm formation - atherosclerosis-induced pressure or ischaemic atrophy of the underlying media, with loss of elastic tissue, causes weakness and potential rupture
What is atherosclerotic stenosis?
- In small arteries, the atherosclerotic plaques can gradually occlude vessel lumina, compromising blood flow and causing ischaemic injury.
- At early stages of stenosis, outward remodeling of the vessel media tends to preserve the size of the lumen. However, there are limits of the extent of remodeling, and eventually the expanding atheroma impinges on the lumen to such a dgeree that blood flow is compromised.
What is critical atherosclerotic stenosis?
- The stage at which the occlusion is sufficiently severe to produce tissue ischaemia.
- In the coronary circulations, this typically occurs at when the occlusion produces a 70% decrease in luminal cross-scetional area.
In what conditions does a plaque rupture?
When they are unable to withstand mechanical stresses generated by vascular shear forces.
The events that trigger abrupt changes in plaques and subsequent thrombosis are complex and include both intrinsic factors (e.g. plaque structure and composition) and extrinsic elements (e.g. blood pressure, platelet reactivity, vessel spasm)
The composition of atherosclerotic plaques is dynamic and can contribute to risk of rupture. What compositions increase the risk of rupture?
Plaques that contain large areas of foam cells and extracellular lipid, and those in which the fibrous caps are thin or contain few smooth muscle cells of have clusters of inflammatory cells, are more likely to rupture; there are referred to as “vulnerable plaques”
What is the major structural component of the fibrous cap in atherosclerotic caps? What affects its integrity?
Collagen accounts for its mechnical strength and stability.
Thus, the balance of collagen synthesis versus degradation affects cap integrity.
Collagen in atherosclerotic plaque is produced primarily by smooth muscle cells so that loss of these cells results in a less sturdy cap
What controls collagen turnover in atherosclerotic plaques?
- Collagen turnover is controlled by metalloproteinases (MMPs), enzymes elaborated largely by macrophages and smooth muscle cells within the atheromatous plaque
- Conversely, tissue inhibitors of metalloproteinases (TIMPs) produced by endothelial cells, smooth muscle cells and macrophages modulate MMP activity.
How does cholesterol deposits affect atherosclerotic plaques?
- In general, plaque inflammation results in a net increase in collagen degradation and reduced collagen synthesis, thereby destabilising the mechanical integrity of the fibrous cap.
- The inflammation induced by cholesterol deposits themselves may contribute to plaque destabilisation.
Influencs extrinsic to plaques also contribute to acute plaque changes. Like what?
- Adrenergic simulation can increase systemic BP or induce local vasoconstriction, thereby increasing the physical stresses on a given plaque.
- The adrenergic stimulation associated with wakening can cause BP spikes that have causally inked to the pronounced circadian periodicity for onset of acute MI.
Vasoconstriction compromises lumen size, and, by increasing the local mechanical forces, can potentiate plaque disruption. What may stimualte vasoconstriction at sites of atheroma?
- Circulating adrenergic agonist
- Locally released platelet contents
- Endothelial cell dysfunction with impaired secretion of endothelial-derived relaxing factors (NO) relative to contracting factors (endothelin)
- Mediators released from peri-vascular inflammatory cells
What is an aneurysm?
A localised abnormal dilation of a blood vessle or the heart that may be congenital or acquried
What are the different types of aneurysm? Give examples
- A ‘true’ aneurysm is when it involves an attenuated but intact arterial wall or thinned ventricular wall of the heart - atheroscleroric, syphilitic and congenital vascular aneurysms, ventricular aneurysms that follow MIs
- A false aneurysm is a defect in the vascular wall leading to extravascular haematoma that freely communicates with the intravascular space “pulsating haematoma” - ventricular rupture after MI or a leak at the sutured junction of a vascular graft with a natural artery
- An arterial dissection arises when blood enters a defect in the arterial wall and tunnels between its layers. Dissections are often but always aneurysmal.
All of these can rupture, often with catastrophic consequences
Descriptively, aneurysms are classified by macroscopic shape and size. How would you do this?
- Saccular aneurysms are spherical outpouchings involving only a portion of the vessel wall; they may vary from 5 to 20 cm in diameter and often contain thrombus
- Fusiform aneurysms are diffuse, circumferential dilations of a long vascular segment; they vary in diameter (up to 20cm) and in length and can involve extensive portions of the aortic arch, abdominal aorta or even the iliacs.
When do aneurysms occur?
- To maintain their structural and functional integrity, arterial walls constantly remodel by synthesising, degrading, and repairing damage to their ECM constituents.
- Aneurysms can occur when the structure or function of the connective tissue within the vascular wall is compromised.
What are some inherited defects in connective tissues that increase the risk of aneurysm and dissection? How?
- In Marfan syndrome, defective synthesis of the scaffolding protein fibrillin leads to aberrant TGF-β activity and weakening of elastic tissue; in the aorta, this may result in progressive dilation.
- In Loeys-Dietz syndrome mutations in TGF-β receptors lead to defective synthesis of elastin and collagens I and III. Aneurysms in such individuals can rupture fairly easily (even at small size) and are thus considered to follow an “aggresive” course
- Weak vessel walls due to defective type III collagen synthesis are also a hallmark of the vascular forms of Ehlers-Danlos syndrome
- Altered collagen cross-linking associated with vitamin C deficiency (scurvy) is an example of a nutritional basis for aneurysm formation
How is aneurysm formation linked to collagen synthesis and degradation? What enzymes are also related?
- Increased matrix metalloprotease (MMP) expression, particularly by macrophages in atherosclerotic plaque or vasculitis, likely contributes to aneurysm development; these enzymes have the capacity to degrade virtually all components of the ECM in the arterial wall (collagens, elastin, proteoglycans, laminin, fibronectin).
- Decreased expression of tissue inhibitors of metalloproteases (TIMPs) can also contribute to the ECM degradation.
- The risk of aneurysm formation in the setting of inflammatory lesions (atherosclerosis) may be associated with MMP and/or TIMP polymorphisms
In development of aneurysms, the vascular wall is weakened through loss of smooth muscle cells or the synthesis of non-collagenous or nonelastic extracellular matrix. How does this happen?
- Ischaemia of the inner media occurs when there is atherosclerotic thickening of the intima, which increases the distance that oxygen and nutrients must diffuse.
- Systemic hypertension can also cause significant narrowing of arterioles of the vasa vasorum (e.g. in the aorta), which causes outer medial ischaemia
- Medial ischaemia may lead to “degenerative changes” of the aorta, whereby smooth muscle cell loss - or change in the synthetic phenotype - leads to scarring (and loss of elastic fibers), inadequate extracellular matrix synthesis, and production of increasing amoutns of amorphous ground substance (glycosaminoglycan).
Histologically, what is cystic medial degeneration?
It is smooth muscle cell loss leading to scarring (and loss of elastic fibers), inadequate ECM synthesis, and production of increasing amounts of amorphous ground substance (glycosaminoglycan) that occurs in Marfan syndrome and scurvy
How does tertiary syphilis cause aortic aneurysms>
The obliterative endarteritis characteristic of late-stage syphilis shows a predilection for small vessels, including those of the vasa vasorum of the thoracic aorta.
This leads to ischaemic injury of the aortic media and aneurysmal dilation, which sometimes involves the aortic valve annulus.
The two most important causes or aortic aneurysms are atheroscleosis and HTN. Where do they often affect the aorta?
Atherosclerosis is a greater factor in AAAs, while HTN is the most common etiology associated with ascending aortic aneurysms.
Other factors that weaken vessel walls and lead to aneurysms include trauma, vasculitis, congenital defects (fibromuscular dysplasia and berry aneurysms) and infections such as mycotic aneurysms. Where can mycotic aneurysms originate?
1) from embolisation of a septic embolus, usually as a complication of infective endocarditis
2) as an extension of an adjacent suppurative process
3) by circulating organisms directly infecting the arterial wall
In what populations do AAAs occur more frequently?
- Men
- Smokers
- Age >50
Where in the aorta is an AAA? How big can they be?
Usually positioned below the renal arteries and above the bifurcation of the aorta. They can occasionally affect the renal and superior or inferior mesenteric arteries, either by direct extensior of by occluding vessel ostia with mural thrombi.
AAA can be saccular or fusiform, up to 15cm in diameter and up to 25cm in length
What is the morphology of a AAA?
There is severe complicated atherosclerosis and destruction and thinning of the underlying aortic media; the aneurysm frequently contains a bland, laminated, poorly organised mural thrombus.
Although they are less common than the usual atherosclerotic anuerysm, what are three AAA variants thet merit special mention because of their unusual features? tell me about them
- Inflammatory AAA account for 5-10% of all AAA; they typically occur in younger patients, who often present with back pain and elevated inflammatory markers (elevated CRP).
- A subset of inflammatory AAA may be a vascular manifestation of an entity called immunoglobulin G4 (IgG4)-related disease. A disorder marked by high plasma levels of IgG4 and tissue fibrosis associated with frequent infiltrating IgG4-expressing plasma cells
- Mycotic AAA are lesions that have become infected by the lodging of circulating microorganisms in the wall. In such cases, suppuration further destroys the media, potentiating rapid dilation and rupture
How are inflammatory AAA characterised morphologically? What causes it?
- Inflamatory aneurysms are characterised by abundant lymphoplasmacytic inflammation with many macrophages (and even giant cells) associated with dense periaortic scarring that can extend into the anterior retroperioneum.
- The cause is a presumed localised immune response to the abdominal aortic wall; remarkably, most cases are not associated with inflammation of other arteries.
What are the clinical features of AAA?
Most cases of AAA are asymptomatic and are discovered incidentally on physical exam as an abdominal mass (often palpably pulsating) that may mimic a tumour. The other clinical manifestations of AAA include:
* rupture into the periotoneal cavity of retroperitoneal tissues with massive, potentially fatal haemorrhage
* obstruction of a vessel branching off from the aorta, resulting in ischaemic injury to the supplied tissue; for example, iliac, renal, mesenteric or vertebral arteries
* embolism from atheroma or mural thrombus
* impingement on an adjacent structure, for example, compression of a ureter or erosion of vertebrae
The risk of rupture is directly related to the size of the aneurysm. Tell me about the risk related to the size:
- 4cm or less - no risk
- 4-5cm - 1% per year
- 5-6 cm - 11% per year
- > 6cm - 25% per year
How quickly do aneurysms expand?
Most expand at a rate of 0.2 to 0.3 cm/year but 20% expand more rapidly.
How are AAA managed?
In general, aneurysms 5cm or larger are managed aggresively, usually by surgical bypass with prosthetic grafts, although treatment via endoluminal approaches using stent grafts rather than surgery is now available for selected patients
Timely surgery for AAA is critical. Why?
Operative mortality for unruptured aneurysms is approx 5%, whereas emergency surgery after rupture carries a mortality rate of >50%
What are the main associations for thoracic aortic aneurysms?
They are most commonly associated with HTN, although other causes such as Marfan syndrome and Loeys-Dietz syndrome are increasingly recognised.
What are the clinical features of thoracic aortic aneurysm?
1) respiratory difficulties due to encroachment on the lungs and airways
2) difficulty in swallowing due to compression of the oesophagus
3) persistent cough due to compression of the recurrent laryngeal nerves
4) pain caused by erosion of bone (i.e. ribs and vertebral bodies)
5) cardiac disease as the aortic aneurysm leads to aortic valve dilation with valvular insufficiency or narrowing of the coronary ostia causing MI
6) rupture
What is an aortic dissection?
Aortic dissection occurs when blood separates the laminar planes of the media to form a blood-filled channel within the aortic wall; this can be catastrophic if the dissection then ruptures through the adventitia and haemorrages into adjacent spaces.
Aortic dissection occurs principally in two groups of patients. What are they? What are the rarer groups of populations
1) men aged 40-60 years with antecedent HTN (>90% of cases)
2) younger adults with systemic or localised abnormalities of connective tissue affecting the aorta (e.g. Marfan syndrome)
It can also be iatrogenic e.g. following arterial cannulations during coronary catheterisation or bypass.
Rarely pregnancy is associated with aortic dissection. What trimester and why?
- This typically occurs during or after third trimester
- It may be related to hormone-induced vascular remodeling and the haemodynamic stresses of the perinatal period.
What is the pathogenesis of aortic dissection?
- HTN is the major risk factor for aortic dissection
- Aortas of hypertensive patients have medial hypertrophy of vasa vasorum associated with degenerative changes such as loss of medial smooth muscle cells and disorganised ECM, suggesting that ischaemic injury (due to diminished flow through the vasa vasorum, possibly exacerbated by high wall pressures) is contributory.
- The trigger for intimal tear and initial intramural aortic haemorrhage is not known in most cases.
- Once a tear has occurred, blood flow under systemic pressure dissects through the media, leading to progression of the haematoma.
- Accordingly, aggressive pressure-reducing therapy may be effective in liminiting an evolving dissection.
- In some cases, disruption of penetrating vessels of the vasa vasorum can give rise to an intramural haematoma without an intimal tear
Morphologically, what changes occur prior to/at the beginning of an aortic dissection?
- The most frequent preexisting histologically detectable lesion is cystic medial degeneration, and inflammation is characteristically absent.
An aortic dissection usually initiates with an intimal tear. Where is the location that the vast majority of spontaneous dissections occur? What shape are they? Where do they go?
- The tear often occurs in the ascending aorta, usually within 10cm of the aortic valve
- Such tears are typically transverse with sharp, jagged edges up to 1-5cm in length.
- The dissection can extend retrograde towards the heart as well as distally, sometimes into the iliac and femoral arteries
Which layers of the aorta does a dissection often affect?
The dissecting haematoma spreads characteristally along the laminor planes of the aorta, usually between the middle and outer thirds.
It can rupture through the adventitia causing massive haemorrhage (e.g. into the thoracic or abdominal cavities) or cardiac tamponade (haemorrhage into the pericardial sac)
What is a double-barreled aorta? What happens to this over time?
In some instances, the dissecting haematoma reenters the lumen of the aorta through a second distal intimal tear, creating a new false vascular channel - double-barreled aorta.
This averts a fatal extraaortic haemorrhage, and over time, such false channels can be endothelialised to become recognisable chronic dissections
What are the classifications of aortic dissections?
- The more common (and dangerous) proximal lesions (called type A dissections), involving either both the ascending and descending aorta (type 1 Debakey) or just the ascending aorta only (type II DeBakey)
- Distal lesions not involving the ascending part and usually beginning distal to subclavian artery (called type B dissections or DeBakey type III)
What are the clinical features of an aortic dissection?
The classic clinical symptoms or aortic dissection are the sudden onset of excruciating pain, usually beginning in the anterior chest, radiating to the back between the scapulae, and moving downward as the dissection progresses; the pain can be confused with that of an MI.
Common clinical manifestations also include cardiac tamponade and aortic insufficiency
What is the most common cause of death from aortic dissection?
Where can aortic dissections extend into? What does it cause?
They can extend into the great arteries of the neck or into the coronary, renal, mesenteric, or iliac arteries, causing vascular obstruction and ischaemic consequences such as MI; involvement of spinal arteries can cause transverse myelitis.
What is the treatment for type A dissections?
Rapid diagnosis and institution of intensive antihypertensive therapy coupled with surgical plication of the aortic intimal tear can save 65-85% of patients.
What is the prognosis of type A dissections?
Mortality approaches 70% in those who present with haemorrhage or symptoms related to distal ischaemia, and the overall 10-year survival is only 40-60%.
What is the prognosis and treatment of type B dissections?
Most type B dissections can be managed conservatively; patients have a 75% survival rate whether they are treated with surgery or antihypertensive medication only/
What is ischaemic heart disease
IHD represents a group of physiologically related syndromes resulting from myocardial ischaemia - an imbalance between myocardial supply (perfusion) and cardiac demand for oxygenated blood.
What is myocardial ischaemia vs hypoxia?
Ischaemia not only limits tissue oxygenation (and thus ATP generation), but also reduced the availability of nutrients and the removal of metabolic wastes.
What are the main causes of myocardial ischaemia?
> 90% cases of myocardial ischaemia results from reduced blood flow due to obstructive atherosclerotic lesions in teh epicardial coronary arteries.
Consequently, IHD is frequently referred to as coronary artery disease
But it can also be caused by coronary emboli, myocardial vessel inflammation, or vascular spasm.
Does IHD have a long or short period of progression?
In most cases there is a long period (up to decades) of silent, slow progression of coronary lesions before the sudden onset of symptoms.
Thus, IHD is often the late manifestatoin of coronary atherosclerosis that began during childhood or adolsecence.
How can IHD present?
As one or more of the following clinical syndromes:
* Myocardial infarction, where ischaemia causes frank cardiac necrosis
* Angina pectoris, where ischaemia is not severe enough to cause infarction, but the symptoms nevertheless portend infarction risk
* Chronic IHD with heart failure
* Sudden cardiac death
The number of deaths from IHD has significantly improved. Why?
- Prevention - modifying important risk factors, such as smoking, blood cholesterol, and HTN, weight loss and increased exercise
- Diagnostic and therapeutic advances such as statins, CCUs, thrombolysis, PCI, stents, CABG, implantable defibrillators
More than 90% of patients with IHD have atherosclerosis involving one or more coronary arteries. What percentage surface area obstruction means significant coronary artery disease?
A fiex lesion obstructing greater than 75% of vascular cross-sectional area defines sigificant coronary artery disease; this is generally the threshold for symptomatic ischaemia precipitated by exercise
Obstruction of what % of cross-sectional area of the coronary lumen can lead to inadequate coronary blood flow even at rest?
90%
Progressive myocardial ischaemia induced by slowly developing occlusions may lead to what?
It may stimulate the formation of collateral vessels over time, which can often protect against myocardial ischaemia and infarction and mitigate the effects of high-grade stenoses.
Clinically significant plaques can be located anywhere along the course of the coronary vessels. Where most commonly?
Particularly the RCA, although they tend to predominate within the first several centimeters of the LAD and LCX. Somtimes the major epicardial bracnhes are also involved (i.e. LAD diagonal bracnhes, LCX obtuse marginal branches, or posterior descending branch of the RCA), but the atherosclerosis of the intramural (penetrating) branches is rare.
The risk of an individual developing clinically important IHD depends on what?
In part on the number, distribution, sturcture, and degree of obstruction of atheromatous plaques. Also the acute change of a plaque
Acute coronary syndromes are typically initiated by what?
An unpredictable and abrupt conversion of a stable atherosclerotic plaque to an unstable and potentially life-threatening atherothrombotic lesion through rupture, superficial erosion, ulceration, fissuring, or deep haemorrhage.
In most instances, plaque changes - typically associated with intralesional inflammation - may precipitate the formation of a superimposed thrombus that partially or completely occludes the artery
What are the consequences of myocardial ischaemia?
- Stable angina results from increases in myocardial oxygen demand that outstrip the ability of stenosed coronary arteries to increase oxygen delivery; it is usually not associated with plaque disruption
- Unstable angina is caused by plaque disruption that results in thrombosis and vasoconstriction, and leads to severe but transient reductions in coronary blood flow.
- Myocardial infarction is often the result of acute plaque change that induces an abrupt thrombotic occlusion, resulting in myocardial necrosis
- Sudden cardiac death may be caused by regional myocardial ischaemia a fatal ventricular arrhythmia.
What is angina pectoris?
It is characterised by payxoysmal and usually recurrent attacks of substernal or precordial chest discomfort caused by transient (15 seconds to 15 minutes) myocardial ischaemia that is insufficient to induce myocyte necrosis.
What causes the pain of angina?
The pain itself is likely a consequence of the ischaemia-induced release of adenosine, bradykinin, and other molecules that stimulate sympathetic and vagal afferent nerves.
There are three overlapping patterns of angina pectoris caused by varying combinations of decreased perfusion, increased demand, and coronary arterial pathology. What are they and in what situations do they happen?
- Stable angina is the most common form of angina; caused by an imbalance in coronary perfusion (due to chronic stenosing coronary atherosclerosis) relative to myocardial demand, such as physical activity, emotion or stress.
- Prinzmetal variant angina is an uncommon form of episodic myocardial ischaemia; caused by coronary artery spasm.
- Unstable or crescendo angina refers to a pattern of increasing frequent, prolonged (>20 mins), or severe angina or chest discomfort described as frank pain. In most patients, is it caused by the disruption of an atherosclerotic plaque with superimposed partial thrombosis and possibly embolisation or vasospasm (or both)
How is stable angina often described and what usually relieves it?
As a deep, poorly localised pressure, squeezing, or burning sensation (like indigestion), but unusually as pain, and is usually relieved by rest (decreasing demand) or administering vasodilators, such as nitroglycerin and calcium channel blockers (increasing perfusion)
What does Prinzmetal angina usually respond well to?
Generally responds promptly to vasodilators
What does increases the incidence and risk factors of MI?
- Age - Mi can occur at virtually any age; nearly 10% occur in people <40. 45% occur in people <65 years old.
- Race Black and white people are equally affected
- Through middle age male gender incrases the relative risk of MI
What is the pathogenesis of coronary arterial occlusion?
- A coronary artery atheromatous plaque undergoes an acute change consisting of intraplaque hemorrhage, erosion or ulceration, or rupture or fissuring
- When exposed to subendothelial collagen and necrotic plaque contents, platelets adhere, become activated, release their granule contents, and aggregate to form microthrombi
- Vasospasm is stimulated by mediators released from platelets
- Tissue factor activates the coagulation pathway, adding to the bulk of the thromus
- Within minutes, the thrombus can expand to completely occlude the vessel lumen
In approximately 10% of cases, transmural MI occurs in the absence of the typical coronary atherothrombosis. In such situations, what happens?
- Vasospasm with or without coronary atherosclerosis, perhaps in association with platelet aggregation or due to drug ingestion (e.g. cocaine or ephedrine)
- Emboli from the left atrium in association with AF, a left-sided mural thrombus, vegetations of infective endocarditis, intracardiac prosthetic material or paradoxical emboli from the right side of the heart or the peripheral veins, traversing a patent foramen ovale and into the coronary rteries
- Ischaemia without detectable or significant coronary atherosclerosis and thrombosis may be caused by disorders of small intramural coronary vessels (vasculitis), haematological abnormalities (sickle cell disease), amyloid deposition in vasular walls, vascular dissection, marked hypertrophy (AS), shock
What is the myocardial response to coronary arterial obstruction?
Coronary arterial obstruction diminishes blood flow to a region of myocardiam, causing ischaemia, rapid myocardial dysfunction, and eventually - with prolonged vascular compromise - myocyte death.
What does the outcome of myocardial infarction depend on?
Predominantly on the severity and duration of flow deprivation
What are the early biological consequences of MI? How does this affect myocardial function?
- The cessation of aerobic metabolism within seconds, leading to inadequate production of high energy phosphates (e.g. creatine phosphate and ATP) and accumulation of potentially noxious metabolites (e.g. lactic acid)
- Because of the exquisite dependence of myocardial function on oxygen and nutrients, myocardial contractility ceases within a minute or so of the onset of severe ischaemia. Such loss of function actually precipitates heart failure long before myocyte death occurs
What are the ultrastructural changes that occur in MI? How long do they take? are they irreversible?
- Ultrastructural changes - myofibrillar relaxation, glycogen depletion, cell and mitochondrial swelling - develop within a few minutes of the onset of ischemia
- There early manifestations of ischaemic injury are potentially reversible
- Only severe ischemia (blood flow <10% of normal) lasting 20-30 minutes or longer leads to irreversible damage (necrosis) of cardiac myocytes.
Where is ischaemia most pronounced in the heart?
- Due to the myocardial perfusion pattern from epicardium to endocardium, ischaemia is most pronounced in the subendocardium; thus, irreversibly injury of ischaemic myocytes occurs first in the subendocardial zone.
- With more extended ischaemia, a wavefront of cell death moves through the myocardium to encompass progressively more of the transmural thickness and breadth of the ischaemic zone.
How long does it take for necrosis to involve half of the thickness of the myocardium and the full way through? What may change this?
- Necrosis involves approximately half of the thickeness of the myocardium in 2-3 hours of the onset of severe MI, and is usually transmural within 6 hours.
- However, in instances where chronic sublethal ischaemia has induced a more well-developed coronary collateral circulation, the progression of necrosis may follow a more protracted course (12 hours or longer)
What does the LAD branch of the coronary artery supply?
Most of the apex of the heart, the anterior wall of the left ventricle, and the anterior two thirds of the ventricular septum.
What is the “dominant” coronary artery?
Either RCA or LCX - that perfuses the posterior third of the septum is called ‘dominant’ even though the LAD and PCX perfuse the majority of the left ventricular myocardium
What proportion of the population are right dominant circulation and what does this mean?
- Present in 80% of individuals
- The RCA supplies the entire right ventricular free wall, the posterobasal wall of the left ventricle, and the posterior third of the ventricular septum, while the LCX generally perfuses only the lateral wall of the left ventricle
- Thus, RCA occlusions can potentially lead to left ventricular damage.
The distribution of myocardial necrosis correlates with the location and cause of the decreased perfusion. How does the location affect myocardial necrosis?
- Transmural infarction. MI caused by an epicardial vessel (in the absence of any therapeutic intervention) are typically transmural - the necrosis involves virtually the full thickness of the ventricular wall in the distribution of the affected coronary.
- Subendocardial (nontransmural) infarction. As the subendocardial zone is normally the least perfused region of myocardium, this area is most vulnerable to any reduction in coronary flow. It typically involves roughly the inner third of the ventricular wall.
- Multifocal microinfarction pattern is seen when there is pathology involving only smaller intramural vessels.
What usually causes the different patterns of myocardial infarction?
- Transmural infarction is usually associated with a combination of chronic coronary atherosclerosis, acute plaque change, and superimposed thrombosis
- Subendocardial (nontransmural) infarction can occur as a result of plaque disruption followed by a coronary thrombus that becomes lysed (therapeutically or spontaneously) before myocardial necrosis extends across the full thicckness of the wall. They can also result from prolonged, severe reduction in systemic blood pressure, as in shock on superimpsoed on chronic, otherwise noncritical, coronary stenoses
- Multifocal infarction may occur in the setting of microembolisation, vasculitis, or vascular spasm, for example, due to endogenous catechols (adrenaline) or drugs (cocaine)
In the subendocardial infarcts that occur as a result of global hypotension rather than an acute plaque, how is the pattern of infarction different?
The myocardial damage is usually circumferential, rather than being limited to the distribution of a singele major coronary artery
What proportion of the artery perfusion zone do transmural myocardial infarcts effect?
Nearly all transmural infarcts involve at least a portion of the left ventricle (comprising the free wall and ventricular septum) and encompass nearly the entire perfusion zone of the occluded coronary artery save for a narrow rim (approx 0.1mm) of preserved subendocardial myocardium that is preserved by diffusion of oxygen and nutrients from the ventricular lumen.
Of MIs caused by a right coronary obstruction, what proportion affect which parts of the heart?
- 15-30% extend from the posterior free wall of the septal portion of the left ventricle into the adjacent right ventricular wall.
- Isolated infarction of the right ventricle is unusual (only 1-3% of cases), as is infarction of the atria.
What are the frequencies of involvement of each of the three main arterial trunks and the correspondint sites of myocardial lesions resulting in infarction (in the typical right dominant heart)?
- Left anterior descending coronary artery (40-50%): infacrts involving the anterior wall of left ventricle near the apex, the anterior portion of the ventricular septum; and the apex circumferentially
- Right coronary artery (30-40%): infarcts involving the inferior/posterior wall of left ventricle; posterior portion of ventricular septum; and the inferior/posterior right ventricular free wall in some cases
- Left circumflex coronary artery (15-20%): infarcts involving the lateral wall of left ventricle except at the apex
Early morphological recognition of acute MI can be difficult. But if the infarct preceded death by 2-3 hours, how is it possible to highlight the area of necrosis? What does this show?
By immersion of tissue slices in a solution of triphenyl-tetrazolium chloride. This stain imparts a brick-red colour to intact, noninfarcted myocardium where LDH activity is preserved. Because dehydrogenases leak out through the damaged membranes of dead cells, an infarct appears as an unstained pale zone
Reperfused MIs are usually hemorrhagic. Why?
Because the vascular is injured during ischaemia and there is bleeding after flow is restored.
What is reperfusion injury?
This term encompasses various forms of damage that can occur after restoration of flow to “vulnerable” myocardium that is ischaemic but not yet irreversibly damaged
The laboratory evaluation of MI is based on measuring the proteins that leak out of irreversibly damaged myocytes. What are these?
- The most useful are cardiac specific troponins T and I, and the MB fraction of CK.
- The diagnosis of MI is established when blood levels of these cardiac biomarkers are elevated.
What does the rate of appearance of cardiac markers in the peripheral circulation following MI depend on?
- their intracellular location and molecular weight
- the blood flow and lymphatic drainage in the area of the infarct
- the rate of elimination of the marker from the blood
Following MI, how does the troponin level change over time?
- Troponins I and T are not normally detectable in the circulation.
- Following an MI, levels of both begin to rise at 3-12 hours
- cTnT levels peak somewhere between 12-46 hours
- cTnI levels are maimal at 24 hours
Creatinine kinase is an enzyme expressed in brain, myocardium and skeletal muscle. It is a dimer composed of two isoforms. What are they and where are they found?
- It is a dimer composed of two isoforms designated “M” and “B”.
- MM homodimers are found predominantly in cardiac and skeletal muscle
- BB homodimers in brain, lung and may other dissues
- MB heterodimers are principally localised to cardiac muscle. Thus, the MB form of CK (CK-MB) is sensitive but not specific, sincis can also be elevated after skeletal muscle injury
When do CK-MB, cTnI and cTnT return to normal following an MI?
- CK-MB returns to normal in 48-72 hours
- cTnI in 5-10 days
- cTnT in 5-14 days
What are the different types of cardiac rupture that occur post-MI?
- Rupture of the ventricular free wal (most common), with haemopericardium and cardiac tamponade (A)
- Rupture of the ventricular septum (less common), leading to an acute VSD and left-to-right shunting (B)
- Papillary muscle rupture (least common), resulting in the acute onset of severe mitral regurgitation (C)
When does pericarditis often occur after MI?
About the second or third day following a transmural infarct as a result of underlying myocardial inflammation (Dressler sydrome)
Vegetations on heart valves are the classic hallmark of IE. How do they present morphologically?
Vegetations on heart valves are the classic hallmark of IE; these are friable, bulky, potentially destructive lesions containing fibrin, inflammatory cells, and bacteria or other organisms.
What valves are the most common sites of infective endocarditis? Is it only one valve they happen in?
The aortic and mitral valves are the most common sites of infection, although the valves of the right heart may also be involved, particularly in IVDUs.
Vegetations can be single or multiple and may involve more than one valve.
What are some of the consequelae of infective endocarditis vegetations?
They can occasionally erode into the underlying myocardium and produce an abscess (ring abscess Fig B).
Vegetations are prone to embolisation; because the embolic fragments often contain virulent organisms, abscesses frequently develop where they lodge, leading to sequelae such as septic infarcts or mycotic aneurysms.
How do the vegetations of subacute endocarditis differ from acute endocarditis? Both in terms of destruction and microscopically
- The vegetations of subacute endocarditis are associated with less valvular destruction that those of acute endocarditis, although the distinction can be subtle.
- Microscopically, the vegetations of subacute IE typically exhibit granulation tissue at their bases indicative of healing. With time, fibrosis, calcification, and a chronic inflammatory infiltrate can develop.
What are the three pathological patterns of cardiomyopathy? and which ones are most/least common?
- Dilated cardiomyopathy (including arrhythmogenic right ventricula cardiomyopathy) - 90% of cases
- Hypertrophic cardiomyopathy
- Restrictive cardiomyopathy - least common
What is the left ventricular ejection fraction for the following cardiomyopathies:
* Dilated
* Hypertrophic
* Restrictive
Dilated < 40%
Hypertrophic - 50-80%
Restrictive - 45-90%
What are the causes of the following cardiomyopathies:
* Dilated
* Hypertrophic
* Restrictive
- Dilated - genetic, alcohol, peripartum, myocarditis; haemochromatosis; chronic anaemia; sarcoidosis; idiopathic
- Hypertrophic - genetic ; Friedrich ataxia; storage disease; infants of diabetic mother
- Restrictive - amyloidosis; radiation-induced fibrosis; idiopathic
What percentage of dilated cardiomyopathy are familial? What genes are specific for this?
- DCM is familial in at least 30-50% of cases, in which it is caused by mutations in a diverse group of more than 20 genes encoding porteins involved in the cytoskeleton, sarcolemma and nuclear envelope.
- In particular, mutations in TTN, a gene that encodes titin, may account for approximately 20% of all cases of dilated cardiomyopathy
What is the gross morphology of dilated cardiomyopathy?
- In dilated cardiomyopathy, the heart is usually enlarged, heavy (often weighing 2-3 times normal), and flabby, due to dilation of all chambers.
- Mural thrombi are common and may be a source of thromboemboli
- There are no primary valvular alterations; if mitral (or tricuspid) regurgitation is present, it results from left (or right) ventricular chamber dilation (functional regurgitation)
- Either the coronary arteries are free of significant narrowing or the obstructions present are insufficient to explain the degree of cardiac dysfunction.
What are the histological changes of dilated cardiomyopathy?
- The histological abnormalities in DCM are nonspecific and usually do not point to a specific etiology
- Most muscle cells are hypertrophied with enlarged nuclei, but some are attenuated, stretched, and irregular
- Interstitial and endocardial fibrosis of variable degree is present, and small subendocardial scars may replace individual cells or groups of cells, probably reflecting healing of previous ischaemic necrosis of myocytes caused by hypertrophy-induced imbalance between perfusion and demand
- Morever, the severity of morphological changes may not reflect either the degree of dysfunction or the patient’s prognosis
What is the morphology of arrhythmogenic right ventricular cardiomyopathy?
- Morphologically, the right ventricular wall is severely thinned due to loss of myocytes, accompanied by extensive fatty infiltration and fibrosis.
- Although myocardial inflammation may be present, ARVC is nor considered an inflammatory cardiomyopathy.
What are the genetic links associated with arrhythmogenic right ventricular cardiomyopathy?
- Classic ARVC has autosomal dominant inheritance with a variable penetrance.
- The disease has been attributed to defective cell adhesion proteins in the desmosomes that link adjacent cardiac myocytes.
- Naxos syndrome is a disorder characterised by arrhythmogenic right ventricular cardiomyopathy and hyperkeratosis of plantar palmar skin surfaces specifically associated with mutations in the gene encoding the desmosome-associated protein plakoglobin.
What are the genetic links associated with hypertrophic cardiomyopathy?
- In most cases, the pattern of transmission is autosomal dominant with variable penetrance.
- HCM is caused by mutations in any one of the several genes that encode sarcomeric proteins; there are more than 400 different known mutations in nine different genes, most being missense mutations
- Mutations causing HCM are found most commonly in the gene encoding β-myosin heavy chains, following by genes coding for cardiac TnT, α-tropomyosin, and myosin-binding protein C.
- The prognosis of HCM varies widely and correlates strongly with specific mutations
What are the most important histological features of the myocardium in hypertrophic cardiomyopathy?
1) massive myocyte hypertrophy, with transverse myocyte diametes frequently greater than 40μm (normal, approx 15μm)
2) haphazard disarray of bundles of myocytes, individual myocytes, and contractile elements in sarcomeres within cells (termed myofiber disarray)
3) interstitial and replacement fibrosis
What is the histological morphology of myocarditis?
- Active myocarditis is characterised by an interstitial inflammatory infiltrate associated with focal myocyte necrosis.
- A diffuse, mononuclear, predominantly lymphocytic infiltrate is most common.
- Although endomyocardial biopsies are diagnostic in some causes, they can be spuriosly negative because inflmmatory involvement of the myocardium may be focal or patchy.
- If the patient survives the acute phase, the inflammatory lesions either resolve, leaving no residual changes, or heal by progressive fibrosis
What is the histological morphology of giant-cell myocarditis? What is its prognosis?
- It is characterised by a widespread inflammatory cellular infiltrate containing multinucleate giant cells (fused macrophages) interspersed with lymphocytes, eosinophils, plasma cells, and macrophages.
- Focal to frequently extensive necrosis is present.
- This variant likely represents the fulminant end of the myocarditis spectrum and carries a poor prognosis
What is the histological morphology of Chagas disease myocarditis?
It is distinctive by virute of the parasitisation of scattered myofibers by trypanosomes accompanied by a mixed inflammatory infiltrate of neutrophils, lymphocytes, macrophages, and occasional eosinophils
What are the gross morphological features or purulent pericarditis?
- The exudate ranges from a thin cloudy fluid to frank pus up to 400-500mL in volume
- The serosal surfaces are reddened, granular, and coated with the exudate.
What is the gross morphology of aortic stenosis?
The gross morphologic hallfmark of non-rheumatic calcific aortic stenosis is mounded calcified masses within the aortic cusps that ultimately protrude through the outflow surfaces into the sinuses of Valvalva and prevent cuspal opening. The free edges of the cusps are usually not involved.
What is the pathology of a congenitally bicuspid aortic valve? What area is a major site for calcific deposits?
- There are only two functional cusps, usually of unequal size, with the larger cusp having a midline raphe resulting from incomplete commissural separation during development; less frequently the cusps are of equal size and the raphe is absent.
- The raphe is frequently a major site of calcific deposits.
Where do calcific deposits usually deposit in the mitral valve? How do they appear?
- As opposed to the predominantly cuspal involvement in aortic valve calcification, degenerative calcific deposits in the mitral valve typically develop in the fibrous annulus.
- Grossly, these appear as irregular, stony hard, occasionally ulcerated nodules (2-5mm in thickness) at the base of the leaflets (fig C and D).
What are the gross morphological changes in mitral valve prolapse?
- The characteristic anatomic change in MVP in interchordal ballooning (hooding) of the mitral leaflets or portions thereof.
- The affected leaflets are often enlarged, redundant, thick and rubbery.
- The associated tendinous cords may be elongated, thinned, or even ruptured, and the annulus may be dilated.
- The tricuspid, aortic or pulmonary valves may also be affected.
What are the key histological changes associated with mitral valve prolapse?
- Marked thickening of the spongiosa layer with deposition of mucoid (myxomatous) material, called myxomatous degeneration
- There is also attenuation of the collagenous fibrosa layer of the valve, on which the structural integrity of the leaflet depends.
In mitral valve prolapse, secondary changes reflect the stresses and tissue injury indicent to the billowing leaflets. What are these changes?
1) fibrous thickening of the valve leaflets, particularly where they rub against each other
2) linear fibrous thickening of the left ventricular endocardial surface where the abnormally long cords snap or rub against it
3) thickening of the mural endocardium of the left ventricle or atrium as a consequence of friction-induced injury induced by the prolapsing, hypermobile leaflets
4) thrombi on the atrial surfaces of the leaflets or the atrial walls (B)
5) focal calcifications at the base of the posterior mitral leaflet (C)
What are the main types of acquired atelectasis, which is encountered principally in adults? What causes each one each?
- Resorption atelectasis stems from complete obstruction of an airway. It is most often caused by excessive secretions (mucus plugs) or exudates within smaller bronchi, as may occur in bronchial asthma, chronic bronchitis, bronchiectasis, and post-op states. Aspiration, foreign bodies and rarely tumours may also lead to airway obstruction
- Compression atelectasis results whenever significant volumes of fluid (transudate, exudate, or blood), tumor or air accumulated within the pleural cavity.
- Contraction atelectasis occurs when focal or generalised pulmonary or pleural fibrosis prevents full lung expansion
How do resorption and compression atelectasis effect the mediastinum?
In resorption atelectasis, the lung volume is diminshed and the mediastinum shifts toward the atelectatic lung
In compression atelectasis, the mediastinum is pushed away from the affected lung
Based on the segments of the respiratory units that involved emphysema is classified into four major types. What are they? Which ones cause significant obstruction? Which is most common?
1) Centriacinar
2) Panacinar
3) Paraseptal
4) Irregular
Of these, only the first two cause clinically significant airflow obstruction.
Centriacinar emphysema is the most common form, consistuting more than 95% of clinically significant cases.
What parts of the acini (the terminal respiratory units) does centriacinar emphysema effect? Where in the lung are the lesions commonly found?
- The central or proximal parts of the acini, formed by respiratory bronchioles, are affected, whereas distal alveoli are spared.
- Thus, both emphysematous and normal arispaces exist within the same acinus and lobule.
- The lesions are more common and usually more severe in the upper lobes, particularly in the apical segements.
- Inflammation around bronchi and bronchioles is common.
What parts of the acini are affected in panacinar emphysema? Where does it tend to happen?
- The acini are uniformly enlarged from the level of the respiratory bronchiole to the terminal blind alveoli.
- In contrast to centriacinar emphysema, panacinar emphysema tends to occur more commonly in the lower zone and in the anterior marghins of the lungs, and it is usually most severe at the bases
What part of the acini does distal acinar (paraseptal) emphysema usually happen? Where in the lungs does it happen?
- The proximal portion of the acinus is normal, and the distal part is predominantly involved.
- The emphysema is mroe striking adjacent to the pleura, along the lobular connective tissue septa, and at the margins of the lobules.
- It occurs adjacent to areas of fibrosis, scarring or atelectasis and is usually more severe in the upper half of the lungs
What diseases are often associated with centriacinar, panacinar and distal acinar emphysema?
- Centriacinar emphysema occurs predominantly in heavy smokers, often in association with chronic bronchitis (COPD)
- Panacinar emphysema is associated with α1-antitrypsin deficiency
- Distal acinar emphysema probably underlies many cases of spontaneous pneumothorax in young adults
In broad terms, what is the pathogenesis of emphysema?
- Inhaled cigarette smoke and other noxious particles cause lung damage and inflammation
- This result in parenchymal destruction (emphysema) and airway disease (bronchiolitis and chronic bronchitis).
What are the factors that influence the development of emphysema?
- Inflammatory mediators and leukocytes including leukotriene B4, IL-8, TNF, are released by resident epithelial cells and macrophages and attract inflamatory cells, amplify the inflammatory process and induce structural changes
- Protease-antiprotease imbalance - several proteases are released from the inflammatory cells and epithelial cells that break down connective tissue components
- Oxidative stress - substances in tobacco smoke, alveolar damage, and ifnlammatory cells all produce oxidants, which may beget more tissue damage and inflammation
- Infection may exacerbate the associated inflammation and chronic bronchitis
What is the gross morphology of advanced emphysema?
- Advanced emphysema produces voluminous lungs, often overlapping the heart and hiding it when the anterior chest wall is removed.
- Generally, the upper two thirds of the lungs are more severely affected.
- Large apical blebs or bullae are more characteristic of irregular emphysema secondary to scrring and ofdistal acinar emphysema
- Large alveoli can easily be seen on the cut surface of fixed lungs
Between predominantly bronchitis and predominantly emphysema, how do the following things differ:
* Age
* Dyspnea
* Cough
* Infections
* Respiratory insufficiency
* Cor pulmonale
* Airways resistance
* Elastic recoil
* CXR
* Appearance
What is bullous emphysema?
This is a descriptive term for large subpleural blebs or bullae (spaces greater than 1cm in diameter in the distended state) that can occur in any form of emphysema
These localised accentuations of emphysema occur near the apex, sometimes near old TB scarring.
On occasion, rupture of the bullae may give rise to PTx
A fundamental abnormality in asthma is an exaggerated TH2 reponse to normally harmless environmental antigens. What happens in this process?
- TH2 cells secrete cytokines that promote inflammation and stimulate B cells to produce IgE and other antibodies.
- These cytokines include IL-4, which stimulates the production of IgE; IL-5, which activates locally recruited eosinophils; and IL-13, which stimulates mucus secretion from bronchial submucosal glands and also promotes IgE production by B cells.
- The T cells and epithelial cells secrete chemokines that recruit more T cells and eosinophils, thus exacerbating the reaction.
- IgE binds to the Fc receptors on submucosal mast cells, and repeat exposure to the allergen triggers the mast cells to release granule contents and produce cytokines and other mediators, which collectively induce the early-phase (immediate hypersnesitivity) reaction and the **late-phase reaction.
What happens during the early reaction in asthma?
- The early reaction is dominated by bronchoconstriction, increased mucus production, variable degrees of vasodilation, and increased vascular permeability
- Bronchoconstriction is triggered by direct stimulation of subepithelial vagal (parasympathetic) receptors through both central and local reflexes trigerred by mediators produced by mast cells and other cells in the reaction.
What happens during the late-phase reaction in asthma?
The late-phase reaction is dominated by recruitment of leukocytes, notably eosinophils, neutrophils, and more T cells.
What happens during “airway remodelling” in asthma?
Over time, repeated bouts of allergen exposure and immune reactions result in structural changes in the bronchial wall, referred to as “airway remodelling”. These changes include:
* Thickening of the airway wall
* Subbasement membrane fibrosis (due to deposition of type I and III collagen)
* Increased vascularity
* An increase in the size of submucosal glands and number of airway goblet cells
* Hypertrophy and/or hyperplasia of the bronchial wall muscles
ALI/ARDS is initiated by injury of pneumocytes and pulmonary endothelium, setting in motion a vicious cycle of increasing inflammation and pulmonary damage. What are the steps involved in this?
Endothelial activation
Adhesion and extravasation of neutrophils
Accumulation of intraalveolar fluid and formation of hyaline membranes
Resolution of injury
What happens in endothelial activation during ARDS?
- In some instances, endothelial activation is secondary to pneumocyte injury, which is sensed by resident alveolar macrophages.
- In response, these immune sentinels secrete mediators such as TNF that act on the neighbouring endothelium.
- Alternatively, circulating inflammatory mediators may activate pulmonary endothelium directly in the setting of severe tissue injury or sepsis.
- Some of these mediators injure endothelial cells, while others (notably cytokines) activate endothelial cells to express increased levels of adhesion molecules, procoagulant proteins and chemokines
What happens during adhesion and extravasation of neutrophils in ARDS?
- Neutrophils adhere to the activated endothelium and migrate into the interstitium and the alveoli, where they degranulate and release inflammatory mediators, including proteases, ROS and cytokines.
- Macrophage migration inhibitory factor (MIF) released into the local milieu also helps to sustain the ongoing pro-inflammatory response.
- The result is increased recruitment and adhesion of leukocytes, causing more endothelial injury, and local thrombosis.
- This cycle of inflammation and endothelial damage lies at the heart of ALI/ARDS.
What happens regarding accumulation of alveolar fluid and formation of hyaline membranes during ARDS?
- Endothelial activation and injury make pulmonary capillaries leaky, allowing interstitial and intraalveolar oedema fluid to form.
- Damage and necrosis of type II alveolar pnuemocytes leads to surfactant abnormalities, further compromising alveolar gas exchange.
- Ultimately, the inspissated protein-rich oedema fluid and debris from dead alveolar epithelial cells organise into hyaline membranes, a characteristic feature of ALI/ARDS.
What happens regarding resolution of injury during ARDS?
- It is impeded in ALI/ARDS due to epithelial necrosis and inflammatory damage that impairs the ability of remaining cells to assist with oedema resorption
- Eventually, however, if the inflammatory stimulus lessens, macrophages remove intraalveolar debris and release fibrogenic cytokines such as transforming growth factor β and platelet-derived growth factor (PDGF).
- These factors stimulate fibroblast growth and collagen deposition, leading to fibrosis of alveolar walls.
- Bronchiolar stem cells proliferate to replace pneumocytes. Endothelial restoration occurs through proliferation of uninjured capillary endothelium
What is the morphology in the acute stage on ARDS?
In the acute stage, the lungs are heavy, firm, red and boggy.
They exhibit congestion, interstitial and intra-alveolar oedema, inflammation, fibrin deposition, and diffuse alveolar damage.
The alveolar walls become lined with waxy hyaline membranes that are morphologically similar to those seen in hyaline membrane disease of neonates.
Alveolar hyaline membranes consist of fibrin-rich oedema fluid mixed with the cytoplasmic and lipid remnants of necrotic epithelial cells.
While the cause of idiopathic pulmonary fibrosis remains unknown, it spread that the fibrosis arises in genetically predisposed individuals who are prone to aberrant repair of recurrent alveolar epithelial cell injuries caused by environmental exposures. What are the implicated factors?
- Environmental factors - cigarette smoking, exposure to metal fumes or wood dust. It is thought that exposure to these things causes recurrent alveolar epithelial cells damage.
- Genetic factors - potentially germline loss-of-function mutates in the TERT and TERC gibers, which encode components of telomerase, making pneumocytes more sensitive to environmental insults, enhancing the consequences of cellular dysfunction and injury
- Age - occurs in older people
What are the genes associated with pulmonary HTN?
- Inactivating germline mutations in the BMPR2 gene are found in 75% of the familial cases of pulmonary hypertension, and 25% of sporadic cases.
- BMPR2 is down-regulated in lungs from some idiopathic pulmonary arterial HTN patients without mutation in its gene.
- A two-hot model has been proposed whereby a genetically susceptible individual with BMPR2 mutation requires additional genetic or environmental insults to develop the disease
What is BMPR2? What is its role?
- A cell surface protein belonging to the TGF-β receptor superfamily, which binds a variety of cytokines, including TGF-β, bone morphogenetic protein (BMP), activin, inhibin.
- BMP-BMPR2 signalling is nor know to be important for embryogenesis, apoptosis, and cell proliferation and differentiation.
- Haploinsufficiency for BMPR2 leads to dysfunction and proliferation of endothelial cells and vascular smooth muscle cells.
Bacterial pneumonia has two patterns of anatomic distribution. What are they? What’s the difference?
- Patchy consolidation of the lung is the dominant characteristic of bronchopneumonia. The patchy involvement may become confluent, producing virtually total lobar consolidation; however, effective antibiotic therapy may limit involvement to a subtotal consolidation.
- Consolidation of a large portion of a lobe or of an entire lobe defines lobar pneumonia.
In bacterial lobar pneumonia, there are four stages of the inflammatory response. What are they?
- In the first stage, congestion, the lung is heavy, boggy, and red. It is characterised by vascular engorgement, intra-alveolar fluid with few neutrophils, and often the presence of numerous bacteria.
- The stage of red hepatisation that follows is characterised by massive confluent exudation, as neutrophils, red cells, and fibrin fill the alveolar spaces. On gross examination, the lobe is red, firm and airless, with a liver-like consistency, hence the term hepatisation.
- The stage of **gray hepatisation that follows is marked by progressive disintegration of red cells and the persistence of fibrinosuppurative exudate resulting in a colour change to grayish-brown.
- In the final stage of resolution the exudate within the alveolar spaces is broken down by enzymatic digestion to produce granular, semi-fluid debris is resorbed, ingested by macrophages, expectorated, or organised by fibroblasts growing into it.
Where do lung abscesses generally form?
- Abscesses vary in diameter from a few mm to large 5-6cm cavities.
- They may affect any part of the lung and may be single or multiple.
- Pulmonary abscesses due to aspiration are more common on the right and are most often single
- Abscesses that develop in the course of pneumonia or bronchiectasis are usually multiple, basal, and diffusely scattered.
- Septic emboli and pyemic abscesses are multiple and may affect any region of the lungs
What is the morphology of atypical adenomatous hyperplasia in the lung?
- A small lesion (<5mm) characterised by dysplastic pneumocytes lining alveolar walls that are mildly fibrotic.
- It can be single or multiple and can be in the lung adjacent to invasive tumour or away from it
What is the morphology of lung adenocarcinoma in situ?
- A lesion that is less than 3cm and is composed entirely of dysplastic cells growing along pre-existing alveolar septae
- The cells have more dysplasia than atypical adenomatous hyperplasia and may or may not have intracellular mucin (mucinous and non-mucinous, resectively)
What is the morphology of lung adenocarcinoma?
- It is an invasive malignant epithelial tumour with glandular differentiation or mucin production by the tumour cells
- Adenocarcinomas grow in various patterns, including acinar, lepidic, papillary, micropapillar, and solid with mucin formation.
- Compared with squamous cell cancers, the lesions are more peripherally located and tend to be smaller.
- They vary histologically from well-differentiated tumours with obvious glandular elements (A) to papillary lesions resembling other papillary carcinomas to solid masses with only occasional mucin-producing glands and cells
- At the periphery of the tumour there is often a lepidic pattern of spread, in which the tumour cells “crawl” along normal-appearing alveolar septae
What is the morphology of early lung squamous cell carcinoma development?
- Precursor lesions that give rise to invasive squamous cell carcinoma are well characterised.
- Squamous cell carcinomas are often antedated by squamous metaplasia or dysplasia in the bronchial epithelium, which then transforms to carcinoma in situ, a phase that may last for several years.
- By this time, atypical cells may be identified by cytologic smears of sputum or in bronchial leverage fluids or brushings, although the lesion in asymptomatic and undetectable on CXR.
- Eventually, an invasive squamous cell carcinoma appears
Where in the lung is squamous cell carcinoma most common? What may be seen next for the tumour mass?
- In the past, most squamous cell carcinomas were seen to arise centrally from the segmental or sub-segmental bronchi
- However, the incidence of squamous cell carcinoma of the peripheral lung is increasing
- Squamous metaplasia, epithelial dysplasia, and foci of frank carcinoma in situ may be seen in bronchial epithelium adjacent to the tumour mass.
What is the gross morphology of mesothelioma?
- Malignant mesothelioma is a diffuse lesion arising either from the visceral or parietal pleura, that spreads widely in the pleural space and is usually associated with extensive pleural effusion and direct invasion of thoracic structures.
- The affected lung becomes ensheathed by a thick layer of soft, gelatinous, greyish, pink tumour tissue
What does the glomerulus consist of?
- An anastomosing network of capillaries lined by fenestrated endothelium invested by two layers of endothelial cells
- The visceral epithelial cells (podocytes) are incorporated into and become an intrinsic part of the capillary wall, separated from endothelial cells but a basement membrane
- The parietal epithelium, situated on the Bowman’s capsule, lines the urinary space, the cavity in which the plasma filtrate first collects
The glomerular capillary wall is the filtering membrane in the glomerulus. What are the structures that it consists of?
- There is a thin layer of fenestrated **endothelial cells*, which each fenestra being about 70-100nm in diameter
- A glomerular basement membrane with a thick electron-dense central layer, the lamina densa and thinner electron-lucent peripheral layers, the lamina rara interna and lamina rara externa
- The visceral epithelial cells (podocytes)
- The entire glomerular tuft is supported by mesangial cells lying between the capillaries. Basement membrane-like mesangial matrix forms a meshwork in which the mesangial cells are embedded
What is the structure of the visceral epithelial cells of the glomerular capillary wall?
- The visceral epithelial cells (podocytes) possess interdigitating processes embedded in and adherent to the lamina rara externa of the basement membrane
- Adjacent **foot processes* are separated by 20- to 30-nm-wide filtration slits, which are bridged by a thin diaphragm
What are the proteins located in the slit diaphragm or present in assemblies of molecules within visceral epithelial cells in the glomerulus that are attached to the slit diaphragm?
- Nephrin is a transmembrane protein with a large extracellular portion made up of Ig-like domains.
- Nephrin molecules extend toward each other from neighbouring foot processes and dimerise across the slit diaphragm
- Within the cytoplasm of the foot processes, nephron forms molecular connections with podocin, CD2- associated protein, and ultimately the actin cytoskeleton of the visceral epithelial cells
Two forms of antibody associated glomerular injury have been established. What are these? Which one is more common?
1) injury by antibodies reacting in situ within the glomerulus, either binding to insoluble fixed (intrinsic) glomerular antigens or extrinsic molecules planted within the glomerulus
2) injury results from deposition of circulating antigen-antibody complexes in the glomerulus.
It is clear that the major cause of glomerunlonephrtiis resulting from formation of antigen-antibody complexes is the consequence of in situ immune complex formation, and not deposition of circulating complexes as was once thought
How is glomerular disease caused by in situ formation of immune complexes? What disease is a classic example of this?
- In this form of injury, immune complexes are formed locally by antibodies that react with intrinsic tissue antigen or with extrinsic antigens “planted” in the glomerulus from the circulation
- Membranous nephropathy is the classic example of glomerular injury resulting from local formation of immune complexes
Glomerular disease caused by in situ formation of immune complexes has a well-studied experimental counterpart in the Hermann nephritis rat model, from which much of the underlying pathophysiology of glomerular immune complex-mediated diseases has been deduced. What is the Heymann model of glomerulonephritis?
- It is induced by immunising rats with an antigen, megalin, that is present in epithelial cell foot processes.
- The rats develop antibodies to this antigen, and disease develops from the reaction of antibody with the megalin-containing protein complex located on the basal surface of visceral epithelial cells, leading to localised immune complex formation
- Antibody binds to PLA2R (phospholipase A2 receptor) present in the glomerular epithelial cell membrane is followed by complement activation and then shedding of the immune aggregates from the cell surface to form characteristic deposits of immune complexes along the sub-epithelial aspect of the basement membrane
- These sub-epithelial complexes, with resultant host responses, can result in a thickened basement membrane appearance; hence the term membranous nephropathy
In humans, what causes primary and secondary membranous nephropathy?
- Primary membranous nephropathy is an autoimmune disease, caused by antibodies to endogenous tissue components. What triggers these autoantibodies is unclear
- Secondary forms of membranous nephropathy can be experimentally induced by drugs (eg, mercuric chloride), and graft-versus-host disease. In some of these situations there may be uncontrolled B-cell activation, leading to the production of autoantibodies that react with renal antigens.
Antibodies can react in suit with antigens that are not normally present in the glomerulus but are “planted” there. How does this happen?
- Such antigens may localise in the kidney by interacting with various intrinsic components of the glomerulus
- Planted antigens include cationic molecules that bind to anionic components of the glomerulus; DNA, nucleosomes, and other nuclear proteins, which have an affinity for GBM components; bacterial products, large aggregated proteins (eg, aggregated immunoglobulins), which deposit in the mesangium because of their size; and immune complexes themselves, since they continue to have reactive sites for further interactions with free antibody, free antigen or complement.
An example of antibodies against planted antigens that cause membranous nephropathy is how it develops in a small number of infants fed cow’s milk. How does this happen?
- These children have been found to have antibodies to bovine albumin and their lesions contain bovine milk antigens, which presumably become lodged in the glomerular basement membrane following intestinal absorption, where they serve as the substrate for immune complex formation in situ.
- Antibodies that bind to these planted antigens induce a discrete pattern of Ig deposition as granular staining by immunofluorescence microscopy that is indistinguishable from the pattern of staining observed with immune complexes formed from intrinsic antigens
How is disease caused by antibodies directed against normal components of the glomerular basement membrane? How does this compare with other nephropathies?
- In anti-GBM antibody induced glomerulonephritis, antibodies bind to intrinsic antigens homogenously distributed along the entire length of the GBM, resulting in a diffuse linear pattern of staining for the antibodies by immunofluorescence techniques.
- This contrasts with the granular pattern of immunofluorescence staining corresponding to the discrete immune complexes seen in membranous nephropathy, or other glomerular disease in which large complexes of antigens and antibodies form in situ.
- These intrinsic fixed antigens cannot be mobilised to form large, discrete complexes
Do the anti-GBM antibodies in anti-GBM antibody induced glomerulonephritis only react with the GBM?
- Often the anti-GBM antibodies cross-react with other basement membranes, especially this in the lung alveoli, resulting in simultaneous lung and kidney lesions (Goodpasture syndrome).
- The GBM antigen that is responsible for classic anti-GBM antibody-induced glomerulonephritis and Goodpasture syndrome is a component of the non-collagenous domain (NC1) of the α3 chain of type IV collagen that is critical for maintenance of GBM suprastructure.
- Although anti-GBM antibody-induced glomerulonephritis accounts for fewer than 5% of cases of human glomerulonephritis, it causes severe necrotising and crescentic glomerular damage and the clinical syndrome of rapidly progressive glomerulonephritis
How does glomerulonephritis resulting from deposition of circulating immune complexes happen?
- In this type of nephritis, glomerular injury is caused by the trapping of circulating antigen-antibody complexes within glomeruli
- The antibodies have no immunologic specificity for glomerular constituents, and the complexes localise within the glomeruli because of their physicochemical properties and the haemodynamic factors peculiar to the glomerulus.
What does electron microscopy and immunofluorescence show in glomerular injury following immune complex formation?
- Electron microscopy reveals electron-dense deposits, presumably containing immune complexes, that may lie in the mesangium, between the endothelial cells and the GBM (subendothelial deposits), or between the outer surface of the GBM and the podocytes (subepithelial deposits). Deposits may be located at more than one site in a given case.
- By immunofluorescence microscopy, the immune complexes are seen as granular deposits along the basement membrane, in the mesangium, or in both locations.
Several molecular factors affect glomerular localisation of antigen, antibody or immune complexes in glomerular disease. What are these?
- The molecular charge and size of these reactants are clearly important
- Highly cationic antigens tend to cross the GBM, and the resultant complexes eventually reside in subepithelial location
- Highly anionic macromolecules are excluded from the GBM and are trapped subendothelially or are not nephritogenic at all
- Molecules of neutral charge and immune complexes containing these molecules tend to accumulate in the mesangium
- Large circulating complexes are not usually nephritogenic, because they are cleared by the mononuclear phagocyte system and do not enter the GBM in significant quantities
The pattern of glomerular localisation of antigen, antibody or immune complexes is also affected by organ factors. What are these?
- The pattern of localisation is affected by changes in glomerular haemodynamics, mesangial function, and integrity of the charge-selective barrier in the glomerulus.
- These influences may underlie the variable pattern of immune reactant deposition in various forms of glomerulonephritis
The distinct patterns of glomerular localisation of immune complexes is a key determinant of the injury response and the histologic features that subsequently develop. What locations lead to what features?
- Immune complexes located in subendothelial portions of capillaries and in mesangial regions accessible to the circulation are more likely to be involved in inflammatory processes that require interaction and activation of circulating leukocytes
- Diseases in which immune complexes are confined to the subepithelial locations and for which the capillary basement membranes may be a barrier to interaction with circulating leukocytes, as in the case of membranous nephropathy, typically have a non-inflammatory pathology.
What is the cause of most cases of immune complex mediated glomerulonephritis?
*It is a consequence of deposition of discrete immune complexes, which give rise to granular immunofluorescence staining along the basement membranes or in the mesangium
* However, it may be difficult to determine whether the deposition has occurred in situ, by circulating complexes, or by both mechanisms because trapping of circulating immune complexes can initiate further in situ complex formation.
What factors contribute to the diverse morphologic and functional alterations in glomerulonephritis?
It is best to consider that antigen-antibody deposition in the glomerulus is a major pathway of glomerular injury and that in situ immune reactions, trapping or circulation complexes, interactions between these two events, and local haemodynamic and structural determinants in the glomerulus all contribute to the diverse morphologic and functional alterations in glomerulonephritis
Once immune reactants or sensitised T cells have localised in the glomerulus, what are the cellular mediators that are involved in acute and chronic inflammation? What do they do?
- Neutrophils and monocytes infiltrate the glomerulus in certain types of glomerulonephritis, largely as a result of activation of complement, resulting in generation of chemotactic agents (mainly C5a), but also by Fc-mediated adherence and activation. Neutrophils release proteases, which cause GBM degradation; oxygen-derived free radicals, which cause cell damage; and arachidonic acid metabolites, which contribute to the reductions in GFR
- Macrophages and T lymphocytes, which infiltrate the glomerulus in antibody- and cell-mediated reactions, when activated, release a vast number of biologically active molecules
- Platelets may aggregate in the glomerulus during immune-mediated injury. Their release of eicosanoids, growth factors and other mediators may contribute to vascular injury and proliferation of glomerular cells
- Resident glomerular cells, particular mesangial cells, can be stimulated to produce several inflammatory mediators, including ROS, cytokines, chemokines, growth factors, eicosanoids, NO and endothelin
Podocyte injury is common to many forms of both primary and secondary glomerular diseases, of both immune and non-immune etiologies. The term podocytopathy has been applied to diseases with desperate etiologies whose principal manifestation is injury to podocytes. What are the causes of this? How is it reflected?
- This can be induced by antibodies to podocyte antigens; by toxins, conceivably by certain cytokines; by certain viral infections such as HIV or still by inadequately characterised circulating factors, as in some cases of focal segmental glomerulosclerosis
- Such injury is reflected by stereotypic morphologic changes in the podocytes, which include effacement of foot processes, vacuolisation, and retraction and detachment of cells from the GBM, and functionally by proteinuria
In most forms of glomerular injury, loss of normal slit diaphragm is a key event in the development of proteinuria. How does it happen?
- Functional abnormalities of the slit diaphragms may also result from mutations in its components, such as nephron and podocin, without actual inflammatory damage o the glomerulus.
- Such mutations are the cause of rare hereditary forms of the nephrotic syndrome
What is the sequence of events that it thought to lead to focal segmental glomerulosclerosis?
- It entails endothelial and visceral epithelial cell injury, visceral epithelial cell loss leading to segments of GBM denuded of overlying foot processes and consequently increased glomerular permeability to proteins, and accumulation of proteins in the mesangial matrix
- This is followed by proliferation of mesangial cells, infiltration by macrophages, increased accumulation of extracellular matrix (ECM), and segmental and eventually global sclerosis of glomeruli
- With increasing reductions in nephron mass and ongoing compensatory changes, a vicious cycle of continuing glomerulosclerosis sets in.
- Most of the mediators of chronic inflammation and fibrosis, particularly TGF-β, play a role in the induction of sclerosis
What are the most successful interventions to interrupt the mechanisms of progressive glomerulosclerosis?
They involve treatment with inhibitors of the renin-angiotensin system, which not only reduce intraglomerular hypertension, but also have direct effect on each of the mechanisms identified in the flow chart.
Importantly, these agents have been shown to ameliorate progression of sclerosis in both animal and human studies
What is the classic histologic picture of post-strep glomerulonephritis?
- The classic histologic picture is one of enlarged, hypercellular glomeruli.
- The hypercellularity is caused by 1) infiltration by leukocytes, both neutrophils and monocytes; 2) proliferation of endothelial and mesangial cells; and 3) in severe cases by crescent formation
- The proliferation and leukocyte infiltration are typically global and diffuse, that is involving all lobules of all glomeruli
- There is also swelling of endothelial cells, and the combination of proliferation, swelling, and leukocyte infiltration obliterates the capillary lumens.
- There may be interstitial oedema and inflammation, and the tubules often contain red cell casts.
What do you see on immunofluorescence microscopy and electron microscopic findings of post-strep GN?
- By immunofluorescence microscopy, there are granular deposits of IgG, and C3, and sometimes IgM in the mesangium and along the GBM. Although immune complex deposits are almost universally present, they are often focal and sparse
- The characteristic electron microscopic findings are discrete, amorphous, electron-dense deposits on the epithelial side of the membrane, often having the appearance of “humps”, presumably representing the antigen-antibody complexes at the subepithelial cell surface. Subendothelial complexes are commonly seen, typically early in the disease course, and mesangial and intramembranous deposits may be presents.
What is the histologic picture of rapidly progressive (crescentic) glomerulonephritis?
- The histologic picture is dominated by distinctive crescents.
- Crescents are formed by proliferation of parietal cells and by migration of monocytes and macrophages into the urinary space.
- Neutrophils and lymphocytes may be present
- The crescents may obliterate the urinary space and compress the glomerular tuft
- Fibrin strands are frequently prominent between the cellular layers in the crescents.
Membranous nephropathy is a form of chronic immune complex-mediated disease. In secondary, the inciting antigens can sometimes by identified in the immune complexes. Where do these antigens come from in particular diseases?
- The antigens may be endogenous or exogenous
- The endogenous antigens may be renal or non-renal. For example, membranous nephropathy in SLE is associated with deposition of complexes of self nuclear proteins and autoantibodies. Another example is neutral endopeptidase, a membrane protein recognised by place tally transferred maternal antibodies in cases of neonatal membranous nephropathy
- Exogenous antigens include those derived from hepatitis B and Treponema pallidum in patients infected with these microbes
Primary membranous nephropathy was long thought to be of unknown cause. But what is now known about the pathogenesis?
- It is now considered to be an autoimmune disease linked to certain HLA alleles such as HLA-DQA1.
- In many adult cases the auto antigen is the phospholipase A2 receptor.
- The lesions bear a striking resemblance to those of experimental Heymann nephritis
How does the glomerular capillary wall become leaky in membranous nephropathy? What is the main immunoglobulin deposited?
- There is a paucity of neutrophils, monocytes, or platelets in glomeruli
- It is postulated that C5b-C9 complex activates glomerular epithelial and mesangial cells, inducing them to liberate proteases and oxidants, which cause capillary wall injury and increased protein linkage.
- A subclass of IgG, IgG4, which differs from other IgG subclasses in being a poor activator of the classical complement pathway, is the principal immunoglobulin deposited in cases of primary membranous nephropathy.
What is the light and electron microscopy morphologic findings in membranous nephropathy?
- By light microscopy, the glomeruli either appear normal in the early stages of the disease or exhibit uniform, diffuse thickening of the glomerular capillary wall
- By electron microscopy (B), the thickening is seen to be caused by irregular electron dense also deposits containing immune complexes between the basement membrane and the overlying epithelial cells, with effacement of podocyte foot processes
- Basement membrane material is laid down between these despotic, appearing as irregular spikes protruding from the GBM. These spikes are best seen by silver stains (A), which colour the basement membrane, but not the deposits, black.
- In time, these spikes thicken to produce dome-like protrusions and eventually close over the immune deposits, burying them within a markedly thickened, irregular membrane.
What do you see on immunofluorescence microscopy of membranous nephropathy?
- Immunofluorescence microscopy (C) demonstrates that the granular deposits contain both immunoglobulins and complement
- As the disease advances segmental sclerosis may occur; in the course of time glomeruli may become completely sclerosed
- The epithelial cells of the proximal tubules contain protein reabsorption droplets, and there may be considerable interstitial mononuclear cell inflammation
What is the morphology by light microscopy in focal segmental glomerulosclerosis?
- By light microscopy, the focal and segmental lesions may involve only a minority of the glomeruli and may be missed if the biopsy specimen contains an insufficient number of glomeruli
- In the sclerotic segments there is collapse of capillary loops, increase in matrix and segmental deposition of plasma proteins along the capillary wall (hyalinosis), which may become so pronounced as to occlude capillary lumens
- Lipid droplets and foam cells are often present
- Glomeruli that do not show segmental lesions usually appear normal on light microscopy but may show increased mesangial matrix
There is a morphologic variant of focal segmental glomerulosclerosis called collapsing glomerulopathy. What are the characteristics of this? What disease is it associated with?
- It is characterised by retraction and/or collapse of the entire glomerular tuft, with or without additional normal FSGS lesions.
- A characteristic feature is proliferation and hypertrophy of glomerular visceral epithelial cells. This lesion may be idiopathic, but it also has been associated with some drug toxicities (eg, pamidronate), and it is the most characteristic lesion of HIV-associated neuropathy
- Collapsing glomerulopathy is typically associated with prominent tubular injury with formation of microcysts. It has a particularly poor prognosis
What are the morphological features of HIV-associated nephropathy?
- A high frequency of the collapsing variant of FSGS
- A striking focal cystic dilation of tubule segments, which are filled with proteinaceous material, and inflammation and fibrosis
- The presence of large numbers of tubuloreticular inclusions within endothelial cells, detected by electron microscopy. Such inclusions, also present in SLE, have been shown to be modifications of endoplasmic reticulum induced by circulating interferon-α. They are not usually present in idiopathic FSGS and therefore may have diagnostic value in a biopsy specimen
What is membranoproliferative glomerulonephritis histological characterisation?
It is characterised histologically by alterations of the glomerular basement membrane, proliferation of glomerular cells, leukocyte infiltration, and the presence of deposits in mesangial regions and glomerular capillary walls
What is the pathogenesis of type 1 membranoproliferative glomerulonephritis?
- There is evidence of immune complexes in the glomerulus and activation of both classical and alternative complement pathways
- The antigens involved in MPGN are unknown
- In many cases they are believed to be proteins derived from infectious agents such as hepatitis C and B viruses, which presumably behave either as “planted” antigens after first binding to or becoming trapped within glomerular structures or are contained in preformed immune complexes deposited from the circulation
What is the morphology of membranoproliferative glomerulonephritis?
- The glomeruli are large and hypercellular. The hypercellularity is produced both by proliferation of cells in the mesangium and so-called endocapillary proliferation involving capillary endothelium and infiltrating leukocytes
- The glomeruli have an accentuated “lobular” appearance due to the proliferating mesangial cells and increased mesangial matrix.
- The GBM is thickened, and often shows a “double-contour” or “tram-track” appearance, especially evident in silver or PAS stains. The is caused by “duplication” of the basement membrane, usually as a result of new basement membrane synthesis in response to subendothelial deposits of immune complexes.
- Between the duplicated basement membranes there is inclusion or interposition of cellular elements, which can be of mesangial, endothelial, or leukocytic origin. Such interposition also gives rise to the appearance of “split” basement membranes. Crescents are present in many cases
What morphology is characteristic of type 1 membranoproliferative glomerulonephritis?
- It is characterised by the presence of discrete subendothelial electron-dense deposits
- Mesangial and occasional subepithelial deposits may also be present
- By immunofluorescence, IgG and C3 are deposited in a granular pattern, and early complement components (C1q and C4) are often also present indicative of an immune complex pathogenesis
What are the abnormalities that cause dense deposit disease?
- Most patients with dense-deposit disease (formerly called type 2 membranoproliferative glomerulonephritis) have abnormalities resulting in excessive activation of the alternative complement pathway.
- These patients have a consistently decreased serum C3 but normal C1 and C4, the early components of complement
- They also have diminished serum levels of Factor B and properdin, components of the alternative complement pathway.
How does the diminished complement in dense deposit disease contribute to the pathogenesis?
- In the alternative complement pathway, C3 is directly cleaved to C3b.
- The reaction depends on the initial activation of C3 by such substances as bacterial polysaccharides, endotoxin, and aggregates of IgA via a pathway involving Factors B and D.
- This leads to the generation of C3bBb, the alternative pathway C3 convertase.
- Normally, this C3 converts is labile, but more than 70% of patients with dense-deposit disease have a circulating autoantibody termed C3 nephritic factor (C3NeF) that binds the alternative pathway C3 convertase and protects it from inactivation
- This favours persistent C3 activation and hypocomplementaemia
- There is also decreased C3 synthesis by the liver, further contributing to the profound hypocomplementaemia.
What is the defining feature of dense deposit disease by electron microscopy?
Permeation of the lamina densa of the GBM by a ribbon-like homogenous, extremely electron-dense material of unknown composition
What is IgA nephropathy? How do you diagnose it?
- IgA nephropathy, characterised by the presence of prominent IgA deposits in the mesangial regions and recurrent haematuria, is the most common type of glomerulonephritis worldwide
- The disease can by suspected by light microscopic examination, but the diagnosis is made only by the detection of glomerular IgA deposition.
- Mild proteinuria is usually present, and the nephrotic syndrome may occasionally develop. Rarely, patients may present with crescentic RPGN.
What are the histologic features of IgA nephropathy?
- On histologic examination the lesions vary considerably.
- The glomeruli may be normal or show mesangial widening and endocapillary proliferation (mesangioproliferative glomerulonephritis), segmental proliferation confined to some glomeruli (focal proliferative glomerulonephritis), or rarely, overt crescentic glomerulonephritis
- The presence of leukocytes within glomerular capillaries is a variable feature
- The mesangial widening may be the result of cell proliferation, accumulation of matrix, immune deposits of some combination of these abnormalities
- Healing of the focal proliferative lesion may lead to secondary focal segmental sclerosis
What are the immunofluorescent and electron microscopy findings of IgA nephropathy?
- The characteristic immunofluorescent picture is often of mesangial deposition of IgA, often with Cr and properdin and lesser amounts of IgG and IgM. Early complement components are usually absent
- Electron microscopy confirms the presence of electron-dense deposits predominantly in the mesangium; capillary wall deposits, if present, are usually sparse
What are the electron microscopic features of Alport syndrome?
- The GBM shows irregular foci of thickening alternating with attenuation (thinning), and pronounced splitting and lamination of the lamina dense, often producing a distinctive basket-weave appearance.
- Similar alterations can be found in the tubular basement membrane
What are the morphological features of chronic glomerulonephritis?
- The kidneys are symmetrically contracted and have diffusely granular cortical surfaces
- On section, the cortex is thinned, and there is an increase in peripelvic fat.
- The glomerular histology depends on the stage of the disease. In early cases, the glomeruli may still show evidence of the primary disease.
- However, there eventually ensues obliteration of the glomeruli, transforming them into cellular eosinophilic masses, representing a combination of trapped plasma proteins, increased mesangial matrix, basement membrane-like material, and collagen
- Because HTN is an accompaniment of chronic glomerulonephritis, arterial and arteriolar sclerosis may be conspicuous.
- Marked atrophy of associated tubules, irregular interstitial fibrosis and mononuclear leukocytic infiltration of the interstitium also occurs
What are the critical events in the pathogenesis of both ischaemic and nephrotoxic acute tubular injury?
1) tubular injury
2) persistent and severe disturbance in blood flow
Ischaemia causes numerous structural and functional alterations in epithelial cells. What is an early reversible result of ischaemic in the tubules?
- One early reversible result of ischaemia is loss of cell polarity due to redistribution of membrane proteins (eg, the enzyme Na/K/ATPase) from the basolateral to the luminal surface of the tubular cells, resulting in abnormal ion transport across the cells and increased sodium delivery to distal tubules
- The latter incites vasoconstriction via tubuloglomerular feedback
- In addition, ischaemic tubular cells express cytokines and adhesion molecules, thus recruiting leukocytes that appear to participate in the subsequent injury
Over time , what happens to the tubular cells following ischaemia?
- In time, injured cells detach from the basements membranes and cause luminal obstruction, increased intratubular pressure, and decreased GFR.
- In addition, glomerular filtrate in the lumen of the damaged tubules can leak back into the interstitium, resulting in interstitial oedema, increased interstitial pressure, and further damage to the tubule.
What are the morphological characteristics of acute tubular injury?
- ATI is characterised by focal tubular epithelial necrosis at multiple points along the nephron, with large skip areas in between, often accompanied by rupture of basement membranes (tubulorrhexis) and occlusion of tubular lumens by casts
There are two routes by which bacteria can reach the kidneys. What are they?
1) through the bloodstream (haematogenous infection)
2) from the lower urinary tract (ascending infection)
Pyelonephritis caused by haematogenous spread is less common that from a UTI. How does it happen? In what diseases and what patients?
- It results from seeding of the kidneys by bacteria from distant foci in the course of septicaemia or localised infections such as infective endocarditis
- Haematogenous infections is more likely to occur in the presence of ureteral obstruction, and in debilitated patients
- Typically, in patients receiving immunosuppressive therapy, non-enteric organisms, such as staphylococci and certain fungi and viruses, are involved
How does vesicoureteric reflux aid in the bacterial movement from the bladder to the kidneys in pyelonephritis?
- Although obstruction is an important predisposing factor in ascending infection, it is incompetence of the vesicoureteral valve that allows bacteria to ascend the ureter into the renal pelvis
- The normal ureteral insertion into the bladder in a one-way valve that prevents retrograde flow or urine when the intravesical pressure rises, as in micturition.
- An incompetent vesicoureteral orifice allows the reflux of bladder urine into the ureters (vesicoureteral reflux).
- The effect of vesicoureteral reflux is similar to that of an obstruction in that there is residual urine in the urinary tract after voiding, which favours bacterial growth
What are some of the causes if vesicoureteric reflux?
- Reflux is most often due to congenital absence or shortening of the intravesical portion of the ureter, such that the ureter is not compressed during micturition
- In addition, it may be acquired by bladder infection itself. It is postulated that the bacteria themselves or the associated inflammation can promote reflux by affecting ureteral contractility, particularly in children
- Vesicoureteral reflux is estimated to affect 1-2% of otherwise normal children
- Acquired vesicoureteral reflux in adults can result from persistent bladder atony cause by spinal cord injury
How can you radiologically demonstrate intrarenal and vesicoureteral reflux?
- By a voiding cystourethrogram, in which the bladder is filled with a radiopaque dye and films are taken during micturition.
- Vesicoureteral reflux can be demonstrated by this method in about 30% of infants and children with UTIs
What are the morphologic hallmarks of acute pyelonephritis?
- Patchy interstitial suppurative inflammation, intratubular aggregates of neutrophils, neutrophilic tubulitis and tubular necrosis
- The suppuration may occur as discrete focal abscesses or large wedge-like areas and can involve one or both kidneys
What are the morphologic changes that happen during early glomerulonephritis?
- In the early stages, the neutrophilic infiltration is limited to the tubules.
- The tubular lumens are a conduit for the extension of the infection, and soon, the infection extends to the interstitium and produces abscesses that destroy the involved tubules
- Characteristically, glomeruli are relatively resistant o the infection.
- Extensive disease, however, eventually also destroys the glomeruli, and fungal pyelonephritis (eg, Candida) often affects glomeruli and results in granulomatous interstitial inflammation
Three complications of acute pyelonephritis can be encountered. What are they are how do they present morphologically?
- Papillary necrosis is seen mainly in diabetics, sickle cell disease, and those with urinary tract obstruction. Papillary necrosis is usually bilateral but may be unilateral. One of all of the pyramids of the affected kidney may be involved. On cut section, the tips or distal two thirds of the pyramids have areas of gray-white examination the necrotic tissue shows characteristic ischaemic coagulative necrosis, with preservation of outlines of tubules. The leukocytic response is limited to the junctions between preserved and destroyed tissue
- **Pyonephrosis is seen when there is total or almost complete obstruction, particularly when it is high in the urinary tract. The suppurative exudate is unable to drain and thus fills the renal pelvis, calyces, and ureter with pus
- Perinephric abscess is an extension of suppurative inflammation through the renal capsule into the perinephric tissue
What is chronic pyelonephritis? How do you differentiate it from other chronic tubulointerstitial diseases?
- Chronic pyelonephritis is a disorder in which chronic tubulointerstitial inflammation and scarring involve the calyces and pelvis.
- Although several diseases produce chronic tubulointerstitial alterations, only chronic pyelonephritis and analgesic nephropathy affect the calyces, making pelvocalyceal damage an important diagnostic clue
Chronic pyelonephritis can be divided into two forms. What are they? Which is most common?
Reflux nephropathy and chronic obstructive pyelonephritis
Reflux nephropathy is far more common form of chronic pyelonephritic scarring
How does reflux nephropathy cause chronic pyelonephritis?
- Reflux nephropathy occurs early in childhood as a result of superimposition of a urinary infection on congenital vesicoureteral reflux and intrarenal reflux
- Reflux may be unilateral or bilateral; thus, the continuous renal damage may cause scarring and atrophy of one kidney or involve both, leading to renal insufficiency.
- Vessicoureteral reflux occasionally causes renal damage in the absence of infection (sterile reflux), but only when obstruction is severe
What are the morphological hallmarks of chronic pyelonephritis?
- Course, discrete, corticomedullary scars overlying dilated, blunted or deformed calyces, and flattening of the papillae.
- The scars vary from one to several and most are in the upper and lower poles, consistent with the frequency of reflux in these sites
The microscopic changes in chronic pyelonephritis involve predominantly tubules and interstitium. What are these changes?
- The tubules show atrophy in some areas and hypertrophy in diction in others
- Dilated tubules with flattened epithelium may be filled with casts resembling thyroid colloid (thyroidisation).
- There are varying degrees of chronic interstitial inflammation and fibrosis in the cortex and medulla.
What are the causes of papillary necrosis and their specific characteristics?
The main cause of renal dysfunction in multiple myeloma is Bence-Jones proteinuria and cast nephropathy. Two mechanisms seem to account for the renal toxicity of Bence-Jones proteins. What are they?
- First, some Ig light chains are directly toxic to epithelial cells, apparently becuase of their intrinsic physicochemical properties
- Second, Bence-Jones proteins combine with the urinary glycoprotein (Tamm-Horsall protein) under acidic conditions to form large, histologically distinct tubular casts that obstruct the tubular lumens and induce a characteristic inflammatory reaction (light-chain cast nephropathy)
How does the size of the kidneys changes in nephrosclerosis? Why?
- The kidneys are either normal or moderately reduced in size, with average weights between 110 and 130gm
- The cortical surfaces have a fine, even granularity that resembles grain leather.
- The loss of mass is due mainly to cortical scarring and shrinking
What are the histologic examination findings of nephrosclerosis?
- On histologic examination, there is narrowing of the lumens of arterioles and small arteries, caused by thickening and hyalinisation of the walls (hyaline arteriolosclerosis)
- Corresponding to the finely granular surface are microscopic subcapsular scars with sclerotic glomeruli and tubular dropout, alternating with better preserved parenchyma
- In addition, the interlobular and arcuate arteries show medial hypertrophy, replication of the internal elastic lamina, and increased myofibroblastic tissue in the intake, all of which narrow the lumen.
- This change, called fibroelastic hyperplasia, often accompanies hyaline arteriolosclerosis and increases in severity with age and in the presence of hypertension
Two histologic alterations characterise renal blood vessels in malignant hypertension. What are these?
- Fibrinoid necrosis of arterioles. In this form of necrosis, cytologic detail is lost and the vessel wall takes on a smudgy eosinophilic appearance due to fibrin deposition. Inflammation is usually not seen or is minimal. Sometimes the glomeruli become necrotic and infiltrated with neutrophils, and the glomerular capillaries may thrombose
- In the interlobular arteries and arteries, there is intimal thickening caused by a proliferation of elongated, concentrically arranged smooth muscle cells, together with fine concentric layering of collagen and accumulation of pale-staining material that probably represents deposition of proteoglycans and plasma proteins. This alteration has been referred to as onion-skinning because of its concentric appearance. Th lesion, also called hyperplastic arteriolitis, correlates with renal failure.
What is the second most frequent cause of renal artery stenosis? Who does it happen to?
- The second most frequent cause of stenosis is fibromuscular dysplasia of the renal artery.
- This heterogenous entity is characterised by fibrous or fibromuscular thickening that may involve the intima, the media, or the adventitia of the artery
- The stenoses, as a whole, are more common in omen and tend to occur in younger age groups (30-40s)
Embolisation of fragments of atheromatous plaques from the aorta or renal artery into intrarenal vessels occurs in older adults with severe atherosclerosis, especially after surgery on the abdominal aorta, aortography, or intra-aortic cannulisation. Where can these emboli be seen? What are the clinical consequences of them? and in whom are the consequences more severe?
- These emboli can be recognised in the lumens of arcuate and interlobular arteries by their content of cholesterol crystals, which appear as rhomboid clefts.
- The clinical consequences of atheroemboli vary according to the number of emboli and the pre-existing state of renal function.
- Frequently they are of no significant. However, acute renal injury or failure may develop in older adults in whom renal function in already compromised.
What is diffuse cortical necrosis? Who does it happen to? What is the prognosis?
- This uncommon condition occurs most frequently after an obstetric emergency, such as abruption placentae (premature separation of the placenta), septic shock, or extensive surgery
- The cortical destruction has the features of ischaemic necrosis. Glomerular and arteriolar microthrombi are often found and contribute to the necrosis and renal damage.
- Massive acute cortical necrosis is of grave significance, since its gives rise to sudden anuria, terminating rapidly in uraemic death. Instances of unilateral or patchy involvement are compatible with survival
What are the morphological features of diffuse cortical necrosis?
- The morphologic features considerably overlap with thrombotic microangiopathy and disseminated intravascular coagulation
- The gross alterations of massive ischaemic necrosis are sharply limited to the cortex
- The lesions may be patchy, with areas of coagulative necrosis and apparently better preserved cortex
- Intravascular and intraglomerular thromboses may be prominent but are usually focal, and acute necroses of small arterioles and capillaries may occasionally be present
- Haemorrhages occur into the glomeruli, together with the formation of fibrin plugs in the glomerular capillaries
What are ectopic kidneys? What are the complications?
- The development of the metanephros into the kidneys may occur in ectopic foci.
- These kidneys lie either just above the pelvic brim or sometimes within the pelvis
- They are usually normal or slightly small in size but otherwise are not remarkable
- Because of their abnormal position, kinking or tortuosity of the ureters may cause obstruction to urinary flow, which predisposes to bacterial infections.
What is a horseshoe kidney? How common is it?
- Fusion of the upper (10%) or lower (90%) poles of the kidneys produces a horseshoe-shaped structure that is continuous across the midline anterior to the great vessels.
- This anomaly is found in 1 in 500 to 1000 anomalies.
What are the pathologic features of:
* Adult polycystic kidney disease
* Childhood polycystic kidney disease
* Medullary sponge kidney
* Familial juvenile nephronophthisis
* Adult-onset medullary cystic disease
* Multicystic renal dysplasia
* Acquired renal cystic disease
* Simple cysts
- Adult polycystic kidney disease - large multicystic kidneys, liver cysts, berry aneurysms
- Childhood polycystic kidney disease - enlarged, cystic kidneys at birth
- Medullary sponge kidney - medullary cysts on excretory urography
- Familial juvenile nephronophthisis - corticomedullary cysts, shrunken kidneys
- Adult-onset medullary cystic disease - corticomedullary cysts, shrunken kidneys
- Multicystic renal dysplasia - irregular kidneys with cysts of variable size
- Acquired renal cystic disease - cystic degeneration in end-stage kidney disease
- Simple cysts - single or multiple cysts in normal-sized kidney
What are the clinical features of complications of:
* Adult polycystic kidney disease
* Childhood polycystic kidney disease
* Medullary sponge kidney
* Familial juvenile nephronophthisis
* Adult-onset medullary cystic disease
* Multicystic renal dysplasia
* Acquired renal cystic disease
* Simple cysts
- Adult polycystic kidney disease - haematuria, flank pain, urinary tract infection, renal stones, HTN
- Childhood polycystic kidney disease - hepatic fibrosis
- Medullary sponge kidney - haematuria, URI, recurrent renal stones
- Familial juvenile nephronophthisis - salt wasting, polyuria, growth retardation, anaemia
- Adult-onset medullary cystic disease - salt wasting, polyuria
- Multicystic renal dysplasia - association with other renal anomalies
- Acquired renal cystic disease - haemorrhage, erythrocytosis, neoplasia
- Simple cysts - microscopic haematuria
What are the typical outcomes of:
* Adult polycystic kidney disease
* Childhood polycystic kidney disease
* Medullary sponge kidney
* Familial juvenile nephronophthisis
* Adult-onset medullary cystic disease
* Multicystic renal dysplasia
* Acquired renal cystic disease
* Simple cysts
- Adult polycystic kidney disease - chronic renal failure beginning at ago 40-60 years
- Childhood polycystic kidney disease - variable, death in infancy or childhood
- Medullary sponge kidney - benign
- Familial juvenile nephronophthisis - progressive renal failure beginning in childhood
- Adult-onset medullary cystic disease - chronic renal failure beginning in adulthood
- Multicystic renal dysplasia - renal failure if bilateral, surgically curable if unilateral
- Acquired renal cystic disease - dependence on dialysis
- Simple cysts - benign
What is the inheritance of the following:
* Adult polycystic kidney disease
* Childhood polycystic kidney disease
* Medullary sponge kidney
* Familial juvenile nephronophthisis
* Adult-onset medullary cystic disease
* Multicystic renal dysplasia
* Acquired renal cystic disease
* Simple cysts
- Adult polycystic kidney disease - autosomal dominant
- Childhood polycystic kidney disease - autosomal recessive
- Medullary sponge kidney - none
- Familial juvenile nephronophthisis - autosomal recessive
- Adult-onset medullary cystic disease - autosomal dominant
- Multicystic renal dysplasia - none
- Acquired renal cystic disease - none
- Simple cysts - none
The pathogenesis of autosomal dominant polycystic disease is not established, but the currently favoured hypothesis places the cilia-centrosome complex of tubular epithelial cells the centre of the disorder. What is the cilia-centrosome complex?
- The tubular epithelial cells of the kidney contain a single non-motile primary cilium, a 2- to 3-μm long hairlike organelle that projects into the tubular lumen from the apical surface of the cells
- The cilium is made of microtubules, and arises from and is attached to a basal body derived from the centriole. The cilia are part of a system of organelles and cellular structures that sense mechanical signals.
- The apical cilia function in the kidney tubule as a mechanosensor to moniter changes in fluid flow and shear stress, while intercellular junctional complexes monitor forces between cells, and focal adhesions sense attachment to extracellular matrices.
- In response to external signals, these sensors regulate ion flux (cilia can induce Ca2+ flux in cultured kidney epithelial cells) and cellular behaviour, including cell polarity and proliferation.
The idea that defects in mechanosensing, Ca2+ flux, and signal transaction underlie cyst formation in autosomal dominant PCKD is supported by several observations. What are they?
- Both polycystin-2 and polycystin-2 are localised to the primary cilium
- Other genes that are mutated in cystic diseases encode proteins that are localised to cilia and/or basal bodies
What do polycystin-1 and polycystin-2 do in the cilio-centrosome complex in the tubular epithelium? How is this changed in autosomal dominant PCKD?
- Polycystin-1 and polycystin-2 appear to form a protein complex that regulates intracellular Ca2+ in response to fluid flow, perhaps because fluid moving through the kidney tubules cause ciliary bending than opens Ca2+ channels.
- The consequent disruption of normal polycystin activity results in alterations on intracellular Ca2+, which regulating many downstream signalling events, including pathways thats directly or indirectly impact cellular proliferation, apoptosis, and secretory functions.
- The increase in calcium is thought to stimulate proliferation and secretion from epithelial cells lining the cysts, which together result in progressive cyst formation and enlargement.
- In addition, cyst fluids have been shown to harbour mediators, derived from epithelial cells that enhance fluid secretion and induce inflammation.
- Finally, the calcium-induced signals also alter the interaction of epithelial cells with extracellular matrix, and this too is thought to contribute to the cyst formation and interstitial fibrosis that are characteristic of progressive polycystic kidney disease
What is the gross morphology of autosomal dominant PCKD?
- In gross appearance, the kidneys are bilaterally enlarged and may achieve enormous sizes; weights as much as 4kg for each kidney have been reports
- The external surface appears to be composed solely of a mass of cysts, up to 3-4cm in diameter, with no intervening parenchyma
What is the gross morphology of childhood PCKD?
- The kidneys are enlarged and have a smooth external appearance
- On cut section, numerous small cysts in the cortex and medulla give the kidney a spongelike appearance
- Dilated elongated channels are present at right angles to the cortical surface, completely replacing the medulla and cortex
What are the microscopic findings in childhood PCKD?
- On microscopic examination, there is cylindrical or, less commonly, saccular dilation of all collecting tubules
- The cysts have a uniform lining of cuboidal cells, reflecting their origin from the collection ducts
- In almost all cases, the liver has cysts associated with portal fibrosis and proliferation of portal bile ducts
What is the morphology of nephronophthisis?
- The kidneys are small, have contracted granular surfaces, and show cysts in the medulla, most prominently at the corticomedullary junction. Small cysts are also seen in the cortex
- The cysts are lined by flattened or cuboidal epithelium and are usually surrounded by either inflammatory cells or fibrous tissue
- In the cortex, there is widespread atrophy and thickening of the tubular basement membranes, together with interstitial fibrosis.
- In general, glomerular structure is preserved.
What is multicystic renal dysplasia? What is the gross morphology?
- Dysplasia is a sporadic disorder that can be unilateral or bilateral and is almost always cystic.
- The kidney is usually enlarged, extremely irregular, and multicystic. The cysts vary in size from several millimetres to centimetres in diameter
What are the histologic findings in multicystic renal dysplasia?
- The cysts are lined by flattened epithelium
- Although normal nephrons are present, many have immature collecting ducts
- The characteristic histologic feature is the presence of islands of undifferentiated mesenchyme, often with cartilage, and immature collecting ducts.
What are the different types of urinary tract obstructions? What are the different categories of causes?
- They can be sudden or insidious, partial or complete, unilateral or bilateral; it may occur at any level of urinary tract from the urethra to the renal pelvis.
- It can be caused by intrinsic lesions or the urinary tract or extrinsic lesions that compress the ureter
How does the morphology change when a urinary tract obstruction is sudden and complete opposed to subtotal or intermittent?
- When the obstruction is sudden and complete, it leads to mild dilation of the pelvis and calyces and sometimes to atrophy of the renal parenchyma.
- When the obstruction is subtotal or intermittent, progressive dilation ensues, giving rise to hydronephrosis.
- Depending on the level of urinary block, the dilation may affect the bladder first, or the ureter and then the kidney
In urinary tract obstruction, the kidneys may be slightly to massively enlarged, depending on the degree and the duration of the obstruction. How does the gross morphology change over time with hydronephrosis?
- The earlier features are those of simple dilation of the pelvis and calyces, but in addition there is often significant interstitial inflammation, even in the absence of infection
- In chronic cases the picture is one of cortical tubular atrophy with marked diffuse interstitial fibrosis. Progressive blunting to the apices of the pyramids occurs, and these eventually become cupped.
- In far-advanced cases, the kidney may become transformed into thin-walled cystic structure have a diameter of up to 15-20cm with striking parenchymal atrophy, total obliteration of the pyramids, and thinning of the cortex
Where are stones often found? What is their gross morphologic appearance?
- Stones are unilateral in about 80% of patients.
- The favoured sites for their formation are within the renal calyces and pelves and in the bladder.
- If formed in the renal pelvis they tend to remain small, having an average diameter of 2-3mm. These may have smooth contours or may take the form of an irregular jagged mass of spicules.
- Often many stones are found within one kidney
- On occasion, progressive accretion of salts leads to the development of branching structures known as staghorn calculi, which create a cast of the pelvic and calyceal system.
The classification of renal cell carcinoma is based on correlative cytogenic, genetic, and histologic studies of both familial and sporadic tumours. What are the major types of tumour?
- Clear cell carcinoma (70-80%)
- Papillary carcinoma
- Chromophone carcinoema
- Xp11 translocation carcinoma
- Collecting duct (Bellini duct) carcinoma
What cells are clear cell carcinomas made of? Are they inherited? What gene is associated with them?
- The tumours are made up of cells with clear or granular cytoplasm and are non-papillary
- They can be familial but in most cases (95%) are sporadic.
- In 98% of these tumours, there is loss of sequences on the short arm of chromosome 3. The deleted region harbours the VHL gene, which acts as a tumour suppressor gene in both sporadic and familial cancers.
What does the VHL gene that is often deleted in clear cell carcinomas do? What happens when it is inactive?
- The VHL gene encodes a protein that is part of a ubiquitin ligase complex involved in targeting other proteins for degradation.
- Important among the targets of the VHL protein is the transcription factor hypoxia-inducible factor-1 (HIF1).
- When VHL is inactive, HIF-1 level remains high, even under normoxic conditions, causing inappropriate expression of a number of genenes that are turned on by HIF. These include genes that promote angiogenesis, such as VEGF, and genes that stimulate cell growth, scubas insulin-like growth factor (IGF-1).
- In addition, HIF collaborates in complex ways with the oncogenic factor MYC to “reprogram” cellular metabolism in a way that favour growth.
How common is papillary renal carcinoma? What genes is it associated with? What does this gene do?
- Papillary carcinoma accounts for 10-15% of renal cancers.
- It is characterised by a papillary growth pattern and also occurs in both familial and sporadic forms. * These tumours are NOT associated with 3p deletions. The most common cytogenic abnormalities are trisomies 7 and 18, and loss of Y in male patients in the sporadic form, and trisomy 7 in the familial form.
- The gene on chromosome 7 for the familial form has been mapped to MET, a proton-oncogene that encodes the tyrosine kinase receptor for hepatocyte growth factor. MET is also mutated in a small proportion of sporadic papillary carcinomas.
- Hepatocyte growth factor (also called scatter factor) mediates growth, cell, mobility, invasion and morphogenetic differentiation.
What is the gross morphology of clear cell carcinomas? Where do they arise from?
- They most likely arise from proximal tubular epithelium, and usually occur as solitary unilateral lesions
- They are bright yellow-gray-white spherical masses of variable size distort the renal outline.
- The yellow colour is a consequence of the prominent lipid accumulations in tumour cells.
- There are commonly large areas of grey-white necrosis and foci of haemorrhagic discolouration.
- The margins are usually sharply defined and confined within the renal capsule
- The growth pattern varies from solid to trabecular (cord-like) or tubular (resembling tubules)
What are the microscopic findings in clear cell carcinoma?
- The tumour cells have a rounded or polygonal shape and abundant clear or granule cytoplasm, which contains glycogen and lipids
- The tumours have delicate branching vasculature and may show cystic as well as solid areas.
- Most tumour are well differentiated, but some show nuclear atypia with bizarre nuclei and giant cells.
As RCC tumours enlarge, what do they bulge and invade into ?
- As tumours enlarge that may bulge into the calyces and pelvis and eventually fungate through the walls of the collecting system to extend into the ureter.
- One of the striking characteristics of renal cell carcinoma is its tendency to invade the renal vein, in which is may grow as a solid column of cells that extends up the inferior vena cava, sometimes as far as the right side of the heart
What are the morphological changes in papillary carcinomas? Both gross and microscopic! What do they arise from?
- They are thought to arise from distal convoluted tubules, can be multifocal or bilateral.
- They are typically haemorrhagic and cystic, especially the large
- The tumour is composed of cuboidal or low columnar cells arranged papillary formations.
- Interstitial foam cells are common in the papillary cords.
- Psammoma bodies may be present. The stroma is usually scanty but highly vascularised
What is the morphology of chromophobe renal carcinoma?
It is made up of pale eosinophilic cells, often with a perinuclear halo, arranged in solid sheets with a concentration of the largest cells around the blood vessels
What happens in hepatic necrosis? What diseases is it most common in?
- The cell swells due to defective osmotic regulation at the cell membrane: fluid flows into the cell, which swells and ruptures.
- Even before rupture, membrane blebs form, carrying off cytoplasmic contents (without organelles) into the extracellular compartment.
- Macrophages cluster at such sites of injury and mark the sites of hepatocyte necrosis since the dying cells essentially burst and disappear.
- This form of injury is the predominant mode of death in ischaemic/hypoxic injury and a significant part of the response to oxidative stress
Apoptotic hepatocytes were first described in yellow fever. While apoptosis occurs in many forms of liver disease, by convention apoptotic hepatocytes are called something else in other diseases. What is this? What other diseases does it occur in?
In more frequent settings in which apoptotic hepatocytes are seen (eg, acute and chronic hepatitis), the term acidophil bodies is used, due to their deeply eosinophilic staining characteristics
Proliferation of hepatic stellate cells and their activation into myofibroblasts is initiated by a series of changes. What are these?
- An increase in the expression of platelet-derived growth factor receptor β (PDGFR-β) in the stellate cells
- At the same time, Kupffer cells and lymphocytes release cytokines and chemokines that modulate the expression of genes in stellate cells that are involved in fibrogenesis. The include, transforming growth factor β (TGF-β) and its receptors, metalloproteinase 2 (MMP-2), and tissue inhibitors of metalloproteinases 1 and 2 (TIMP-1 and -2)
Once stellate cells are converted into myofibroblasts. What do they release and what do they do?
- As they are converted to myofibroblasts, the cell release chemotactic and vasoactive factors, cytokines, and growth factors.
- Myofibroblasts are contractile cells; their contraction is stimulated by endothelin-1 (ET-1)
The stimuli for stellate cell activation may originate from several sources. What are these?
1) Chronic inflammation, with production of inflammatory cytokines such as tumour necrosis factor (TNF), lymphotoxin and IL-1β, and lipid peroxidation products
2) Cytokine and chemokine production by Kupffer cells, endothelial cells, hepatocytes, and bile duct epithelial cells
3) In response to disruption of the ECM
4) Direct stimulation of stellate cells by toxins
How are fibrous septa created in hepatic scar formation? What happens following this?
- Zones of parenchymal loss transform into dense fibrous septa through a combination of the collapse of the underlying reticulin where large swaths of hepatocytes have irrevocably disappeared and hepatic stellate cells have been activated.
- Eventually, these fibrous septa encircle surviving, regenerating hepatocytes in the late stages of chronic liver diseases that give rise to diffuse scarring described as cirrhosis
What happens if the chronic injury leading to hepatic scar formation is interrupted (eg, clearance of hepatitis virus infection, cessation of alcohol use)?
- Stellate cell activation ceases, scars condense, becoming more dense and thin, and then, due to metalloproteinases produced by hepatocytes, begin to break apart.
- In this way, scar formation can be reversed.
- It should be kept in mind that in any chronic liver disease there are probably areas of both fibrotic progression and regression, but the balance in active disease favours the former and with remission of disease the latter is favoured.
What is the morphology of acute liver failure?
- Acute liver failure usually displays massive hepatic necrosis, with broad regions of parenchymal loss surrounding islands of regenerating hepatocytes.
- These livers are small and shrunken. The prominence of scar and of ductular reactions in these livers depends on the nature and duration of the insult
How does the morphology change in acute liver failure caused by acetaminophen overdose and hepatitis?
- Toxic injuries, such as acetaminophen overdose, usually take place within hours to days, too brief a period to allow time for scar formation or regeneration
- Acute viral infections may cause failure over weeks to a few months, so that while hepatocyte injury continues to outpace repair, regeneration is often demonstrable. Also this time scale allows for early scarring in areas of parenchymal loss
What does all cirrhosis have morphologically in common and how is it different?
- Cirrhosis occurs diffusely throughout the liver, which is comprised of regenerating parenchymal nodules surrounded by dense bands of scar and variable degrees of vascular shunting
- The size of the nodules, the pattern of scarring (linking portal tracts to each other vs linking portal tracts to central veins), the degree of parenchymal collapse in which no viable liver tissue present, the range of macroscopic vascular thrombosis (particularly of the portal vein) all vary between diseases and even, in some cases, between individuals with the same disease
Although uncommon regression of fibrosis, albeit rarely, in fully established cirrhosis, does occur. What are the morphologic changes you would see?
- Scars can become thinner, more densely compacted, and eventually fragment
- As fibrous senate break apart, adjacent nodules of regenerating parenchyma coalesce into larger islands
- All cirrhotic livers show elements of both progression and regression, the balance determined by the severity and persistence of the underlying disease
About 40% of individuals with cirrhosis are asymptomatic until the most advances stages of the disease. What are the symptoms when they are there? What are the ultimate causes of death?
- When symptomatic, they present with non-specific manifestations: anorexia, weight loss, weakness, and, in advanced disease, symptoms and signs of liver failure
- The ultimate causes of death in chronic liver failure, whether cirrhotic or not, include those seen in acute liver failure and additional outcomes such as development of HCC
- Hepatic encephalopathy, bleeding for oesophageal varices and bacterial infections (resulting from damage to mucosal barrier in the gut and Kupffer cell dysfunction) are often the terminal events.
What are the outcomes of impaired oestrogen metabolism and consequent hyperoestrogenaemia from chronic liver disease?
- It can give rise to palmar erythema (a reflection of local vasodilation) and spider angiomas of the skin
- Each angioma is a central, pulsating, dilated arteriole from which small vessels radiate.
- In men, it also leads to hypogonadism and gynaecomastia.
- Hypogonadism can also occur in women from disruption of hypothalamic-pituitary axis function, either through nutritional deficiencies associaetd with the chronic liver disease or primary hormonal alterations.
What type of fluid does ascites consist of? What cells are normally in it? What cells point to what diagnosis?
- The is generally serous, having less than 3gm/dL of protein (largely albumin), and a serum to ascites albumin gradient of >1.1gm/dL.
- The fluid may contain a scant number of mesothelial cells and mononuclear leukocytes.
- Influx of neutrophils suggests infection, whereas the presence of blood cells points to possible disseminated intra-abdominal cancer
The pathogenesis of ascites is complex. What are the mechanisms that cause it?
- Sinusoidal hypertension, altering Starling’s forces and driving fluid into the space of Disse, from where it is removed by hepatic lymphatics; this movement of fluid is also promoted by hypoalbuminaemia
-
Percolation of hepatic lymph into the peritoneal cavity - normal thoracic duct lymph flow approximates 800-1000mL/day. With cirrhosis, hepatic lymphatic flow may approach 20L/day, exceeding thoracic duct capacity. Hepatic lymph is rich in proteins and low in triglycerides, which explains the presence of protein in the ascitic fluid.
Splanchnic vasodilation and hyperdynamic circulation - arterial vasodilation in the splanchnic circulation tends to reduce arterial blood pressure. With worsening of the vasodilation, the heart rate and cardiac output are unable to maintain the blood pressure. This triggers the activation of vasoconstrictors, including the renin-angiotensin system, and also increases the secretion of ADH. The combination of portal hypertension, vasodilation, and sodium and water retention increases the perfusion pressure of interstitial capillaries, causing extravasation of fluid into the abdominal cavity.
How does portal hypertension cause portosystemic shunts?
- With the rise in portal system pressure, the flow is reversed from portal to systemic circulation by dilation of collateral vessels and development of new vessels
- Venous bypasses develop wherever the systemic and portal circulation share a common capillary beds.
- Principal sites are veins around and within the rectum (manifest as haemorrhoids), the oesophagogastric junction (producing varices), the retroperitonium, and the falciform ligament of the liver (involving periumbilical and abdominal wall collaterals).
What is the complication of portosystemic shunts?
- Although hemorrhoidal bleeding may occur, it is rarely massive or life-threatening.
- Much more important are the oesophagogastric varices that appear in about 40% of individuals with advanced cirrhosis and cause massive haematemesis and death in about half of them. Each episode of bleeding is associated with a 30% mortality.
- Abdominal wall collaterals appear as dilated subcutaneous veins extending from the umbilicus toward the rib margins (caput medusae) and constitute an important clinical hallmark of portal hypertension.
What is the clinical course of Hepatitis A virus? What is the incubation period and prognosis?
- Hepatitis A virus *HAV) is a usually benign, self-limited disease with an incubation period or 2-6 weeks.
- HAV does not cause chronic hepatitis or a carrier state and only uncommonly causes acute hepatic failure, so the fatality rate associated with HAV is only about 0.1-0.3%
How is hepatitis A virus spread? Who is therefore at risk?
- HAV is spread by ingestion of contaminated water and foods and is shed in the stool for 2-3 weeks before and 1 week after the onset of jaundice
- Thus, close personal contact with an infected individual or faecal-oral contamination during this period accounts for most cases and explains the outbreaks in institutional settings such as schools and nurseries, and in the water-borne epidemics in places where people live in overcrowded, unsanitary conditions.
How does the IgM antibody against hepatitis A change over time and compare to faecal shedding and IgG anti-HAV?
- Specific IgM antibody against HAV appears with the onset of symptoms, constituting a reliable marker of acute infection.
- Faecal shedding of the virus ends as the IgM titer rises.
- The IgM response usually begins to decline in a few motnhs and is followed by the appearance of IgG anti-HAV
- IgG persists for years, perhaps conferring lifelong immunity against reinfection by all strains of HAV.
- Since there are no routinely available tests for IgG anti-HAV, the presence of IgG anti-HAV is inferred from the difference between total and IgM anti-HAV.
What are the different outcomes of hepatitis B virus?
1) Acute hepatitis followed by recovery and clearance of the virus
2) Non-progressive chronic hepatitis
3) Progressive chronic disease ending in cirrhosis
4) Acute hepatic failure with massive liver necrosis
5) An asymptomatic, “healthy carrier state
HBV-induced chronic liver disease is also an important precursor for the development of HCC even in the absence of cirrhosis
What is the incubation period for hepatitis B virus? Does it remain in the blood?
- HBV has a prolonged incubation period (2-26 weeks).
- Unlike HAV, HBV remains in the blood until and during active episodes of acute and chronic hepatitis.
What proportion of people with HBV have symptoms? What is the outcome of most cases?
- Approximately 65% of adult newly acquiring HBV have mild or no symptoms and do not develop jaundice.
- The remaining 25% have non-specific constitutional symptoms such and anorexia, fever, jaundice and RUQ pain
- In almost all cases the infection is self-limited and resolves without treatment. Chronic disease occurs in 5-10% of infected individuals. Fulminant hepatitis is rare, occurring in approximately 0.1-0.5% of acutely infected individuals
The genome of HBV is a partially double-stranded circular DNA molecule having 3200 nucleotides with four open reading frames. What do those four open reading frames code for?
- A nucleocapsid “core” protein (HBcAg) and a longer polypeptide transcript with a precore and core region, designated HBeAg. The precore region directs the secretion of the HBeAg polypeptide, whereas HbcAg remains in hepatocytes, where is participates in the assembly of complete virions
- Envelope glycoproteins (HBsAg), which consists of three related proteins: large, middle and small HBsAg. Infected hepatocytes are capable of synthesising and secreting massive quantities of non-infective surface protein.
- A polymerase (Pol) that exhibits both DNA polymerase activity and reverse transcriptase activity. Replication of the viral genome occurs via an intermediate RNA template, through a unique replication cycle: DNA -> RNA -> DNA
- HBx protein, which is necessary for virus replication and may act as a transcriptional transactivator of both viral genes and a subset of host genes. It has been implicated in the pathogenesis of HCC in HBV infection
The natural course of hepatitis B virus can be followed by serum markers. How? When would you expect the different ones?
- HBsAg appears before the onset of symptoms, peaks during overt disease, and then often declines to undetectable levels in 12 weeks, although it may persist in some individuals for as long as 24 weeks.
- Anti-HBs antibodies does not rise until the acute disease is over, concomitant with the disappearance of HBsAg. In some cases, however, Anti_HBs antibody is not deteactable for a few weeks to several months fter the disappearance of HBsAG. During this window, serologic diagnosis can be made by detection of IgM anti-HBc antibodies. Anti-HBs may persist for life, conferring protection
- HBeAg, HBV-DNA and DNA polymerase appear in serum soon after HBsAg, and all signify active viral replication. Persistence of HBeAg is an important indicator of continued viral replication, infectivity and probable progression to chronic hepatitis. The appearance of anti-hue antibodies implies that an acute infection has peaked and is on the wane.
- IgM anti-HBc antibody becomes detectable in serum shortly before he onset of symptoms, concurrent with the onset of elevated serum aminotransferase levels (indicative of hepatocyte destruction). Over a period of months the IgM anti-HBc antibody is replaced by IgG anti-HBc. As in the case of anti-HAV, there is direct assay for IgG anti-HBc; its presence is inferred from decline of IgM anti-HBc in the face of rising total anti-HBc
What are the basic microscopic changes in acute and chronic hepatitis?
- They both evoke a lymphoplasmacytic (mononuclear) infiltrate.
- Portal inflammation in acute hepatitis in acute hepatitis is minimal or absent.
What are the cellular changes in severe acute hepatitis?
- Confluent necrosis of hepatocytes is seen around central veins. In these areas there may be cellular debris, collapsed reticulin fibers, congestion/haemorrhage and variable inflammation.
- With increasing severity, there is central-portal bridging necrosis, followed by, even worse, parenchymal collapse.
- In some cases, massive hepatic necrosis and acute liver failure ensue.
- In occasional cases, the injury is not severe enough to cause death, and the liver survives, although with abundant scarring, usually with replacement of areas of confluent necrosis. In such cases, some patients rapidly develop post-hepatitic cirrhosis
What are the defining histologic features of chronic viral hepatitis?
- The defining histologic feature of chronic viral hepatitis is mononuclear portal infiltration. It may be mild to severe and variable from one portal tract to the next.
- There is often interface hepatitis as well, in addition to lobular hepatitis, distinguished by its location at the interface between hepatocellular parenchyma and portal tract stroma
- The hallmark of progressive chronic liver damage is scarring. At first, only portal tracts exhibit fibrosis, but in some patients, with time, fibrous septa - bands of dense scar - extend between portal tracts. In parallel with increasing scarring there is also increasing ductular reaction, reflecting stem cell activation
- In the most severe cases, continued scarring and nodule formation leads to the development of cirrhosis.
A greater range of histologic features distinguish one viral infection from another in chronic hepatitis. How?
- In chronic hepatitis B, “ground glass” hepatocytes - cells with endoplasmic reticulum swollen by HBsAg - is a diagnostic hallmark. Immunostaining can confirm the presence of viral antigen
- Chronic hepatitis C quite commonly shows lymphoid aggregates or fully formed lymphoid follicles. Often hepatitis C, particularly genotype 3, shows fatty change of scattered hepatocytes, although the infection may also cause systemic alterations leading to metabolic syndrome and, therefore, a superimposed NAFLD. Bile duct injury is also prominent in some individuals with hepatitis C infection.
What are the typical morphologic features of autoimmune hepatitis?
- Severe necroinflammatory activity indicated by extensive interface hepatitis or foci of confluent (perivenular or bridging necrosis) or parenchymal collapse
- plasma cell predominance in the mononuclear inflammatory infiltrates
- Hepatocyte “rosettes” in areas of marked activity
Excessive alcohol consumption is the leading cause of liver disease in most Western countries. There are three distinctive, albeit overlapping forms of alcoholic liver injury. What are they?
1) Hepatocellular steatosis or fatty change
2) Alcoholic (or steato-) hepatitis
3) Steatofibrosis (patterns of scarring typical for all fatty liver diseases including alcohol) up to and including cirrhosis in the late stages of disease
Where in the liver does alcohol liver disease begin and progress?
All changes in alcoholic liver disease begin in acinus zone 3 and extend outward toward the portal tracts with increasing severity of injury
What are the morphologic changes in hepatic steatosis (fatty liver)? Is it reversible?
- After even moderate intake of alcohol, lipid droplets accumulate in hepatocytes increasing with amount and chronicity of alcohol intake
- The lipid begins as small droplets that coalesce into large droplets which distend the hepatocyte and push the nucleus aside.
- Macroscopically, the fatty liver in individuals with chronic alcoholism is a large (as heavy as 4-6kg), soft organ that is yellow and greasy
- Fatty change is completely reversible if there is abstention from further intake of alcohol
What are the three morphologic characteristics that occur in alcoholic (steato-) hepatitis?
1) Hepatocyte swelling and necrosis: single or scattered foci of cells undergo swelling (ballooning) and necrosis. The swelling results from the accumulation of fat and water, as well as proteins that are normally exported.
2) Mallory-Denk bodies: these are usually present as clumped, amorphous, eosinophilic material in ballooned hepatocytes. They are made up of tangled skeins of intermediate filaments such as keratins 8 and 18 in complex with other proteins such as ubiquitin. These inclusions are characteristic but not specific feature of ALD, they are also in NAFLD, Wilsons’s disease and chronic biliary tract diseases
3) Neutrophilic reaction: neutrophils permeate the hepatic lobule and accumulate around degenerating hepatocytes, particularly those having Mallory-Denk bodies. They may be more or less admixed with mononuclear cells
What is the prognosis and outcome of NAFLD?
What is the main regulator of iron absorption? What regulates its levels? What does it do?
- The main regulator of iron absorption is the protein hepcidin, encoded by the HAMP gene and secreted by the liver. It is a hepatocellular protein with bactericidal activities.
- Transcription of hepcidin in increased by inflammatory cytokines and iron, and decreased by iron deficiency, hypoxia, and ineffective erythropoiesis.
- Hepcidin binds to the cellular iron efflux channel ferroportin, causing its internalisation and proteolysis, thereby inhibiting the release of iron from intestinal cells and macrophages.
- Therefore, hepcidin lowers plasma iron levels. Conversely, a deficiency in hepcidin causes iron overload
What is the most common mutation is the adult and the juvenile form of haemochromatosis? What does it do?
- The adult form of haemochromatosis is almost always caused by mutations of HFE.
- The HFE gene encodes an HLA class I-like molecule that governs intestinal absorption of dietary iron by regulating hepcidin synthesis
- The most common HFE mutation is a cysteine-to-tyrosine substitution at amino acid 282.
- Mutations of HAMP and HJV cause severe juvenile haemochromatosis
The metabolism of bilirubin by the liver consists of four separate but interrelated events. What are they?
- Uptake from the circulation
- Intracellular storage
- Conjugation with glucuronic acid
- Biliary excretion
What is bilirubin made from?
- Bilirubin is the end product of heme degradation.
- The majority of daily production (0.2-0.3gm, 85%) is derived from breakdown of senescent red cells by the mononuclear phagocytic system, especially in the spleen, liver, and bone marrow.
- Most of the remainder (15%) of bilirubin is derived from the turnover of hepatic heme or hemoproteins (eg, the P450 cytochromes) and from premature destruction of red cell precursors in the bone marrow.
How is bilirubin formed from heme?
- Intracellular heme oxygenase converts heme to biliverdin (step 1), which is immediately reduced to bilirubin by biliverdin reductase
- Bilirubin thus formed outside the liver is released and bound to serum albumin (step 2).
- Albumin binding is necessary to transport bilirubin because bilirubin is virtually insoluble in aqueous solution at physiologic pH.
- Hepatic processing of bilirubin involves carrier-mediated uptake at the sinusoidal membrane (step 3), conjugation with one or two molecules of glucuronic acid by bilirubin uridine diphosphate (UDP) or glucuronyl transferase (UGT1A1) (step 4) in the endoplasmic reticulum, and excretion of the water-soluble, non-toxic bilirubin bilirubin glucoronides into bile.
- Most bilirubin glucoronides are deconjugated in the gut lumen by bacterial β-glucoronides and degraded to colourless urobilinogens (step 5).
- The urobilinogens and the residue of intact pigment are largely excretes in the faeces
The morphological features of cholestasis depend on its severity, duration, and underlying cause. What are these features?
- Common to both obstructive and non-obstructive cholestasis is the accumulation of bile pigment within the hepatic parenchyma
- Elongated green-brown plugs of bile are visible in dilated bile canaliculi.
- Rupture of canaliculi leads to extravasation of bile, which is quickly phagocytose by Kupffer cells.
- Droplets of bile pigment also accumulated within hepatocytes, which take on a fine, foamy appearance, so called “feathery degeneration”
What are the morphologic changes in acute biliary obstruction and ascending cholangitis?
- Acute biliary obstruction, either intrahepatic or extrahepatic, causes distention of upstream bile ducts, which often become dilated
- In addition, bile ductules proliferate at the portal-parenchymal interface, accompanied by stromal oedema and infiltrating neutrophils.
- These labyrinthine ductules reabsorb secreted bile salts, serving to protect the downstream obstructed bile ducts from their toxic detergent action
- The histologic hallmark of ascending cholangitis is the influx of these periductular neutrophils directly into the bile duct epithelium and lumen
