Week 1 - Cell injury Flashcards
What is pathology?
The scientific study of disease
- Investigates the changes in cells, tissues and organs that are seen in disease
What does the degree of cell injury depend on?
- Type of injury
- Severity of injury
- Type of tissue
What effects do severe changes in environment have on cells?
Can leads to:
- Cell adaptation
- Cell injury
- Cell death
What are the causes of cell death and cell injury?
- Hypoxia
- Toxins (chemical agents and drugs, e.g. poisons, pollutants, insecticides, herbicides, asbestos)
- Immune mechanisms
- Physical agents (e.g. direct trauma, extremes of temperature, sudden changes in atmospheric pressure, radiation)
- Micro-organisms (e.g. viruses, bacteria, fungi, other parasites)
- Radiation
- Dietary insufficiency and deficiencies, + dietary excess
- Genetic abnormalities
How does hypoxia cause cell injury?
- Causes increased anaerobic oxidative respiration
- Causes decreased aerobic oxidative respiration, which, if persistent, will cause cell adaptation, cell injury or cell death
What is hypoxia?
Oxygen deprivation
How does hypoxia affect different cells differently?
The length of time that a cell can tolerate hypoxia varies
- Some neurones can only tolerate a few minutes
- Dermal fibroblasts can tolerate a number of hours
What is ischaemia?
Loss of blood supply due to reduced arterial drainage
- Causes a reduced supply of oxygen and metabolic substances
- Resultant injury occurs more rapidly and is more severe than that with hypoxia
What are the causes of hypoxia?
- Hypoxaemic hypoxia: arterial content of O2 is low
- Anaemic hypoxia: decreased ability of haemoglobin to carry oxygen
- Ischaemic hypoxia: interruption to blood supply
- Histiocytic hypoxia: inability to utilise oxygen in cells due to disabled oxidative phosphorylation enzymes
How do immune mechanisms cause cell injury?
- Hypersensitivity reactions where the host tissue is injured secondary to an overly vigorous immune reaction
- Autoimmune reactions where the immune system fails to distinguish self from non-self
What are the targets for cell damage?
- Cell membranes
- Nucleus
- Proteins
- Mitochondria
What happens in reversible hypoxic injury?
- As the cell becomes deprived of oxygen there is decreased production of ATP by oxidative phosphorylation in mitochondria
- When the levels of ATP drop to less than 5-10% of normal concentrations, vital cellular functions become compromised
- There is loss of activity of the Na+/K+ plasma membrane pump
- Cell switches to the glycolytic (anaerobic) pathway of ATP production
- Ribosomes detach from the RER (require energy to stay attached; disrupts protein synthesis)
What is the effect of loss of activity of the Na+/K+ plasma membrane pump in reversible hypoxic injury?
- As the intracellular concentration of Na+ rises, water enters the cell
- The cell and its organelles swell up (oncosis)
- Ca2+ also enters the cell and this results in damage to cell components
What is the effect of switching to the glycolytic (anaerobic) pathway of ATP production in reversible hypoxic injury?
- Results in the accumulation of lactic acid
- Reduces the pH in the cell
- Low pH affects enzyme activity within the cell
- Chromatin clumping is seen
What happens in irreversible hypoxic injury?
Not clear what actually kills the cell
But a key event = the development of profound disturbances in membrane integrity
- Leads to an increase in membrane permeability
- Massive influx of Ca2+ into the cytoplasm
— Activates ATPase, phospholipase, protease, endonuclease
— This causes decreased ATP, decreased phospholipids, disruption of membrane and cytoskeletal proteins and nuclear chromatin damage
- Intracellular substances leak out into the circulation:
— Can be detected in blood samples
— Used to determine where the cellular damage is occurring and how severe the injury is
Describe what happens in hypoxic cell injury (summary)
- Cell is deprived of oxygen
- Mitochondria ATP production stops
- The ATP-driven membrane ionic pumps run down
- Sodium and water seep into the cell
- The cell swells and the plasma membrane is stretched
- Glycolysis enables the cell to limp on for a while
- The cell initiates a heat-shock response, which will probably not be able to cope if the hypoxia persists
- The pH drops as cells produce energy by glycolysis and lactic acid accumulates
- Calcium enters the cell, activating phospholipase, proteases, ATPase and endonucleases
- The ER and other organelles swell
- Enzymes leak out of lysosomes and attack cytoplasmic components
- All cell membranes are damaged and start to show blebbing
- At some point the cell dies (necrosis)
What is ischaemia-reperfusion injury?
If blood flow is returned to a tissue which has been subject to ischaemia but isn’t yet necrotic, sometimes the injury that is then sustained is worse than if blood flow was not restored
May be due to:
- Increased production of oxygen free radicals with reoxygenation (since more O2 is brought in)
- Increased number of neutrophils resulting in more inflammation and increased tissue injury
- Delivery of complement proteins and activation of the complement pathway
What is chemical injury?
Some chemicals act by combining with a cellular component, blocking activity (usually)
- E.g. cyanide binds to mitochondrial cytochrome oxidase and blocks oxidative phosphorylation
What are free radicals?
Reactive oxygen species
- Have a single unpaired electron in an outer orbit
- Unstable configuration so react with other molecules, often producing further free radicals
How are free radicals produced?
During: - Chemical and radiation injury - Cellular ageing - Ischaemia-reperfusion injury And at high O2 concentrations
What do free radicals do?
- Attack lipids in cell membranes and cause lipid peroxidation
- Damage proteins, carbohydrates and nucleic acids, bending them out of shape, breaking them or cross-linking them
- They are mutagenic
- Involved in many pathologic and physiologic events
- Produced by leucocytes and used for killing bacteria and in cell signalling
- Causes oxidative stress if there isn an imbalance between free radical production and free radical scavenging (build up of free radicals)
How can the OH radical be produced?
- Radiation can directly lyse water
- The Fenton and Haber-Weiss reactions also produce OH- (H2O2 and O2- are substrates)
- Fenton reaction is important in injury where bleeding occurs, since iron is released (used in the reaction) so more free radicals are produced
What are heat shock proteins?
- Also called stress proteins, unfoldases and chaperoning
- Used by the cell for protection against the effects of injury
- A heat shock response is triggered by any form of injury, not just heat
- Also present in lower concentrations in unstressed cells
- All cells turn down their usual protein synthesis and turn up the synthesis of HSPs in response to stress
- Concerned with protein repair
- Recognise proteins that are incorrectly folded and repair them by ensuring they are refolded correctly
- Play a key role in maintaining protein viability and thus maximising cell survival
What is oncosis?
Cell death with swelling
- Typically seen with hypoxia and ischaemia
- As cells undergo oncosis, they increase in weight
Describe the main alterations to cells that can be seen with cell injury under a light microscope
Cytoplasmic changes:
- Reduced pink staining due to accumulation of water (reversible)
- May be followed by increased pink staining due to detachment and loss of ribosomes from the RER and accumulation of denatured proteins (irreversible)
Nuclear changes:
- Chromatin is subtly clumped (reversible)
- May be followed by various combinations of pyknosis (shrinkage), karryohexis (fragmentation) and karyolysis (dissolution) of the nucleus (irreversible)
Abnormal intracellular accumulations
Describe the main alterations to cells that can be seen with reversible cell injury under an electron microscope
- Swelling (due to Na+/K+ pump failure)
- Cytoplasmic blebs (symptomatic of cell swelling)
- Clumped chromatin (due to reduced pH)
- Ribosome separation from the ER
Describe the main alterations to cells that can be seen with irreversible cell injury under an electron microscope
- Increased cell swelling
- Nuclear changes (pyknosis, karyolysis, karyolysis)
- Membrane defects
- Swelling and rupture of lysosomes
- Appearance of myelin figures (damaged membranes)
- Lysis of the ER due to membrane defects
- Amorphous densities in swollen mitochondria
What is apoptosis?
Cell death (of a single cell or small cluster of cells) with shrinkage
- Induced by a regulated intracellular program where a cell activates enzymes that degrade its own nuclear DNA and proteins (a suicide programme)
- Characterised by its morphology and by the type of DNA breakdown that occurs, which is a characteristic, non-random, internuclearsomal cleavage of DNA
- Can impart shape (e.g. fingers/toes)
- Membrane integrity is maintained
- Active process that requires energy
- Quick
- Lysosomal enzymes are not involved
- No leakage of cell contents so no inflammation
What is necrosis?
In a living organism the morphologic changes that occur after a cell has been dead some time (4-24 hours)
- These changes are largely due to the progressive degradative action of enzymes on the lethally injured cell
- It is an appearance, not a process
When is necrosis seen?
When there is damage to cell membranes (plasma and organelle)
What happens in necrosis?
Lysosomal enzymes are released into the cytoplasm and digest the cell
How is necrotic tissue removed?
By enzymatic degradation and phagocytosis by white cells
- if some remains, it may calcify (dystrophic calcification)
What are the 2 types of necrosis?
Coagulative and liquefactive
- There is a balance between the 2 processes which determines which of the 2 main patterns are seen
- When protein denaturation is the dominant feature, the proteins tend to ‘clump’ leading to solidity of the dead cells and consequently of the dead tissue (coagulative)
- When release of active enzymes is the dominant feature, the dead cells, and consequently the dead tissue, tends to liquefy (liquefactive)
What happens in coagulative necrosis?
Denaturation of proteins dominates over release of active proteases, so the dead tissue is solid
- Appears white to the naked eye
- The cells proteins uncoil and become less soluble
- The cellular architecture is somewhat preserved, creating a ‘ghost outline’ of cells (only seen in first few days, after that the appearances are modified by an acute inflammatory reaction incited by the dead tissue, with consequent infiltration by phagocytes)
- Most common type of necrosis in most solid organs when the cause of death is ischaemia
What happens in liquefactive necrosis?
Active enzyme degradation is substantially greater than denaturation and this leads to enzymatic digestion of tissues
- Seen in massive neutrophil infiltration because neutrophils release proteases (so often seen in bacterial infections)
- The tissue becomes a viscous mass
- If there is acute inflammation, pus will be present
What is caseous necrosis?
- ‘Cheesy’ appearance by the naked eye
- Characterised by amorphous (structureless) debris
- Particularly associated with infections, especially TB
- Often associated with granulomatous inflammation
What is fat necrosis?
- Occurs when there is destruction of adipose tissue
- Most typically seen as a consequence of acute pancreatitis
- Causes release of free fatty acids which can react with calcium to form chalky deposits (calcium soaps) in fatty tissue
- Can also occur after direct trauma to fatty tissue
When can apoptosis occur?
Can be a normal physiological process occurring when cells which are no longer needed are removed to maintain a steady state
- During hormone-controlled involution and in cytotoxic T-cell killing of virus-infected or neoplastic cells
- Also seen in embryogenesis
Occurs when a cell is damaged
- Particularly when the damage affects the cells DNA
- Can be seen with some forms of toxic injury and in tumours
What does apoptosis look like under a light microscope?
Shrunken and intensely eosinophilic
- Chromatin condensation, pyknosis and karyorrhexis are sen
What does apoptosis look like under an electron microscope?
Shows cytoplasmic budding
- Progresses to fragmentation into membrane-bound apoptotic bodies which contain cytoplasm, organelles and often nuclear fragments
- These are eventually removed by macrophage phagocytosis
What are the key phases of apoptosis?
- Initiation
- Execution
- Degradation
Where can apoptosis occur?
Can be intrinsic:
- All apoptotic machinery is within the cell
- Mitochondria as a key player
- Triggers cause cytochrome C release from mitochondria
- This interacts with capsase 9 and APAF1 to form apoptosome
Or extrinsic
- Caused by external death ligands
What are the important apoptotic molecules and what do they do?
- p53 = ‘guardian of the genome’, mediates apoptosis in response to DNA damage
- Cytochrome C, APAF1, capsase q = the apoptosome
- Bcl-2 = prevents cytochrome c release from the mitochondria so inhibits apoptosis
- Death ligands (e.g. TRAIL)
- Death receptors (e.g. TRAIL-R)
- Capsases = effector molecules of apoptosis
What are the differences between encases/necrosis and apoptosis?
- Pattern: o/n = contiguous group of cells, a = single cells
- Cell size: o/n = enlarged (swelling), a = reduced (shrinkage)
- Nucleus: o/n = pyknosis - karyorrhexis - karyolysis, a = fragmentation into nucleosome size fragments
- Plasma membrane: o/n = disrupted, early lysis, a = intact, altered structure, especially orientation of lipids
- Adjacent inflammation: o/n = frequent, a = no
- Cellular contents: o/n = enzymatic digestion, may leak out of cell, a = intact, may be released in apoptotic bodies
- Physiologic or pathologic role: o/n = invariably pathologic, a = often physiologic (means of eliminating unwanted cells) but may be pathologic (after some forms of cell injury)
What is gangrene?
A clinical term used to describe necrosis that is visible to the naked eye
- Can be further classified into dry or wet gangrene depending on how the necrosis is modified
What is dry gangrene?
Necrosis that is modified by exposure to air resulting in drying
- Responsible for the dry crisp appearance of the gangrenous umbilical cord stump after birth, autumn leaves and gangrenous toes
- Underlying process = coagulative necrosis
What is wet gangrene?
Necrosis that is modified by infection with a mixed bacterial culture
- Underlying process = liquefactive necrosis
- Very serious as bacteria can easily get into the bloodstream and it can result in septicaemia
What is gas gangrene?
Wet gangrene where the tissue has become infected with anaerobic bacteria
- Produce visible and palpable bubble of gas within the tissues
- Most commonly seen in clinical practice in ischaemic limbs
- Gangrenous tissue is dead and so cannot be salvaged
What is infarction?
A cause of necrosis, namely ischaemia
- An area of tissue death caused by obstruction of a tissue’s blood supply = an infarct
- Can result in gangrene
- Most are due to a thrombosis or embolism
- Can occasionally be due to external compression of a vessel or by twisting of vessels
- The necrosis in infarcted tissue can be coagulative or liquefactive
How can infarcts be described?
By their colour (white or red)
- Indicates how much haemorrhage there is into the infarct
Describe a white infarct
- Occurs in ‘solid’ organs
- After occlusion of an end artery
- The solid nature of the tissue limits the amount of haemorrhage that can occur into the infarct
- The tissue supplied by the end artery dies
- It appears pale/white because of a lack of blood in the tissue
- They occur in the heart, spleen and kidney
- Most are wedge-shaped with the occluded artery at the apex of the wedge
- Appears as coagulative necrosis
When and where does a red infarct occur?
Occurs where there is extensive haemorrhage into dead tissue
Can occur in a number of situations:
- In organs with a dual blood supply (the secondary arterial supply is insufficient to rescue the tissue but does allow blood to enter the dead tissue)
- If numerous anastomoses (where the capillary beds of 2 separate arterial supplies merge) are present within the tissue
- In loose tissue, where there is poor stromal support for capillaries
- When there has been previous congestion
- Where there is raised venous pressure
What does the consequence of an infarct depend on?
- Whether the tissue affected has an alternative blood supply
- How quickly the ischaemia occurred
- How vulnerable a tissue is to hypoxia
- How quickly the ischaemia has occurred
- How vulnerable a tissue is to hypoxia
- The oxygen content of the blood
What is the consequence of molecules leaking out of injured cells?
The molecules can
- Cause local irritation and local inflammation
- Have general toxic effects on the body
- Appear in high concentrations
What are the principal molecules released by injured cells?
- Potassium
- Enzymes (enzymes with the smallest molecular weight are released first)
- Myoglobin (released from dead myocardium and striated muscle)
What is the effect of potassium leaking out of injured cells?
- Usually in high concentrations in cells compared to the extracellular fluid
- The heart stops with high potassium concentrations (potassium can reach the heart from a MI or massive necrosis elsewhere in the body or tumour lysis syndrome)
What is the significance of enzymes leaking out of injured cells?
Can indicate the organ involved and the extent, timing and evolution of the tissue damage
What is the effect of myoglobin leaking out of injured cells?
If large amounts ate released by damaged striated muscle a condition called rhabdomyolysis occurs
- Can be seen in severe burns or trauma, strenuous exercise, with potassium depletion and with alcohol + drug abuse
The myoglobin can plug the renal tubules causing renal failure
What happens if a cell cannot metabolise something and when is it seen?
It will remain within the cell
- Seen as metabolic processes become deranged
- Often occur with sublethal or chronic injury
What are the main groups of intracellular accumulations?
- Water and electrocytes
- Lipids
- Proteins
- Pigments
- Carbohydrates
How do intracellular accumulations of fluid appear?
- As discrete droplets or diffuse waterlogging of the entire cell resulting in cell swelling (hydropic swelling, due to osmotic disturbance)
- Cells are enlarged but not hypertrophic
When do intracellular accumulations of fluid occur?
When energy supplies to cells are cut off
- Indicates severe cellular damage, but it may also cause further problems
What is steatosis?
Accumulation of triglycerides
- Often seen in the liver
- It is reversible
What are the common causes of steatosis?
Alcohol abuse, diabetes mellitus, obesity and toxins
What is the effect of steatosis?
Liver is golden yellow If mild: - Has no effect on cell function - Is clinically asymptomatic If advanced: - Increases the size of the organ
How does cholesterol accumulate within a cell?
- Excess cholesterol in the cell is stored in membrane-bound droplets
- It accumulates within smooth muscle cells and macrophages within atherosclerotic plaques
- Also seen in macrophages within the skin and tendons of people with acquired and hereditary hyperlipidaemias (the macrophages form small masses called xanthomas)
How do phospholipids accumulate within a cell?
- From disrupted cell membranes
- Form myelin figures in cells or tissue spaces
How do proteins accumulate within a cell?
- As eosinophilic droplets/aggregates in the cytoplasm
- Mallory’s hyaline: a damaged protein which is seen in hepatocytes in alcoholic liver disease, due to accumulation of altered keratin filaments
- Due to a1-antitrypsin deficiency (a genetically inherited disorder): the liver produces a version of a1-antitrypsin that is incorrectly folded so it cannot be packaged by the ER and accumulates within this organelle
What are some examples of endogenous pigments?
- Lipofuscin (age pigment/wear and tear pigment)
- Haemosiderin
- Bilirubin (bright yellow bile pigment)
What is haemosiderin?
- An iron-storage molecule
- Yellow/brown, derived from haemoglobin
- Forms when there is local or systemic excess of iron
- It is deposited in organs if there is systemic overload (haemosiderosis)
- Seen in conditions such as haemolytic anaemias, blood transfusions and hereditary haemochromatosis
What is lipofuscin?
- A brown pigment seen in ageing cells which doesn’t cause any injury to the cell
- A sign of previous radical injury
- Seen in long-lived cells
- Appears as yellow-brown grains within the cytoplasm under a microscope
What happens in haemochromatosis?
Iron is deposited in: - Skin - Liver - Pancreas - Heart - Endocrine organs Can be treated by repeated bleeding
What are some symptoms of haemochromatosis?
- Liver damage
- Heart dysfunction
- Multiple endocrine failures
What is haemochromatosis associated with?
Scarring in the liver (cirrhosis) and pancreas
What happens when bilirubin levels rise?
- Bilirubin is deposited in tissues either extracellularly or intracellularly (in macrophages)
- Jaundice results
- It is very toxic
How do bilirubin levels rise?
When the bile flow is obstructed or overwhelmed
- E.g. in impacted gallstones, in liver disease or in haemolytic anaemia
What are some exogenous pigments?
Carbon, coal dust and soot
- Once inhaled, it is phagocytes by macrophages within lung tissue
- Seen as blackened lung tissue or as blacked peribronchial lymph nodes
- It discolours these tissues for life
Tattoo pigments
- Pigments are pricked into the skin
- They are phagocytosed by macrophages within the dermis, which remain there indefinitely
- Some pigment will reach the draining lymph nodes and remain there
What are pigments?
Substances produced by living organisms that have a colour
Normal cellular constituents
- Some only collect in unusual constituents
What is pathological calcification?
The abnormal deposition of calcium salts within tissues
- 2 roads to it: dystrophic (local, most common) and metastatic (general)
Describe dystrophic calcification
- Occurs in an area of dying tissue, in atherosclerotic plaques, in ageing/damaged heart valves and in tuberculous lymph nodes
- No abnormality in calcium metabolism or serum calcium/potassium concentrations
- A local change/disturbance in the tissue favours the nucleation of hydroxyapatite crystals
- Can cause organ dysfunction
Describe metastatic calcification
- Disturbance is body-wide
- Hydroxyapatite crystals are deposited in normal tissues throughout the body when there is hypercalcaemia secondary to disturbances in calcium metabolism
- Usually asymptomatic
- Can be lethal
- Can potentially regress if the cause of hypercalcaemia if corrected
What are the principle causes of hypercalcaemia?
- Increased secretion of parathyroid hormone resulting in bone resorption
- Destruction of bone tissue
What happens as cells age?
They accumulate damage to cellular constituents and DNA
- May also accumulate lipofuscin pigment and abnormally folded proteins
- There is a decline in their ability to replicate (replicative senescence)
What is replicative senescence related to?
The length of chromosomes
- With every replication the telomere is shortened
- When the telomeres reach a critical length, the cell can no longer divide
- Germ cells and stem cells contain an enzyme (telomerase) which maintains the original length of the telomeres, allowing them to continue replicating
- Many cells produce telomerase
What are the main effects of excessive alcohol intake on the liver?
Fatty change:
- Excessive alcohol intake can affect fat metabolism within the liver
- Results in steatosis (can be so marked as to cause hepatomegaly)
- Happens acutely
- It is irreversible and generally asymptomatic
Acute alcoholic hepatitis:
- As alcohol and its metabolites are directly toxic, a binge of alcohol can result in acute hepatitis with focal hepatocyte necrosis, the formation of Mallory bodies and a neutrophilic infiltrate
- Symptoms = fever, liver tenderness, jaundice
- Usually reversible
Cirrhosis:
- Results in a hard, shrunken liver
- Appears as micro nodules of regenerating hepatocytes surrounded by bands of collagen (histologically)
- Irreversible, serious and sometimes fatal
