MoD S1+2 - Cell Injury Flashcards
What is the difference in cell response to mild versus severe changes in environment?
What if the cell cannot respond appropriately?
Cells can maintain homeostasis when subject to mild changes
Cells must undergo physiological and morphological changes to attempt to stay viable in the face of severe change
If the cell cannot respond appropriately (have reached the limits of their adaptive response) they show evidence or reversible injury or irreversible injury and death
What are the major causes of cell injury?
Hypoxia Physical agents Chemical agents and drugs Microorganisms Immune mechanisms Dietary insufficiency or excess Genetic abnormalities
What is the result of hypoxia in general terms?
Results in decreased aerobic respiration (although glycolytic respiration can continue)
If this is persistent this can cause Atrophy, cell injury or cell death
What are the 4 major types of hypoxia?
Give a brief description and example of each
Hypoxaemic:
Arterial pO2 is low E.g. Reduced inspired PO2 at altitude
Anaemic:
Decreased ability for haemoglobin to carry O2 E.g. anaemia, CO poisoning
Ischaemic:
Interruption to blood supply E.g. Vessel blockage
Histiocytic: Inability to utilise O2 in cells due to disabled oxidative phosphorylation E.g. Cyanide poisoning
What is meant by Ischaemia and what causes it?
Loss of blood supply caused by:
Reduced arterial supply (obstruction of artery, hypotension)
Reduced venous drainage
Why does Ischaemia cause more severe damage than other forms of hypoxia?
Reduced supply of oxygen AND respiratory substrates E.g. Glucose leads to damage occuring more rapidly and more severe damage than hypoxia
How does the length of time cells can tolerate hypoxia vary? Give examples
Differs greatly between cell types
Eg. Some neurones can only tolerate minutes, while fibroblasts can tolerate a number of hours
What occurs during reversible hypoxic injury?
Hint: One direct result of hypoxia leads to 3 main effects of this, each with their own concequences
As the cell becomes deprived of oxygen, there is a decrease in ATP production by oxidative phosphorylation.
When ATP concentrations fall to 5-10% of normal concentrations, cellular process are interrupted:
- Loss of Na+/K+ pump activity causing intracellular Na+ concentration to rise causing swelling. Ca2+ also enters the cell and damages cellular components
- Cell switches to anaerobic glycolysis for energy resulting in reduced pH intracellularly, low pH affects many enzymes, chromatin clumping is also seen
- Ribosomes detach from RER and protein synthesis is disrupted, this results in accumulations of fat and denatured proteins in the cell
When does cell injury progress from reversible to irreversible?
Not really known/not well understood
What is the result of irreversible hypoxic injury?
Hint: Don’t go into detail on the effects of Calcium
Usually appears as necrosis
Profound disturbances in membrane integrity
Massive cytosolic accumulation of Ca2+
Intracellular substances leak out into circulation (E.g. Enzymes such as transaminases from liver cells) and can indicate cellular damage is occuring
How is Ca2+ involved in irreversible cell damage?
What is the effect of Ca2+ accumulation on a cell experiencing irreversible damage?
Ca2+ enters cells across the damaged plasma membrane and is released from stores in the RER and mitochondria causing Cytosolic build-up
Activates a host of enzymes such as:
ATPases - reduce ATP concentration further
Phospholipases - Cause further membrane damage
Proteases - Breakdown of membrane and cytoskeletal proteins
Endonucleases - Breakdown DNA
Causes leakage of lysosymes (by causing enzymes to damage their membranes) and hence further cell damage
What is Ischaemia-reperfusion injury?
Tissue injury can be worse if blood flow is restored and the tissue is not yet necrotic
This may be due to:
- Increased production of ROS with reoxygenation
- Increased number of neutrophils causing more inflammation and injury
- Delivery of complement proteins and activation of complement pathway
Give examples of physical agents that may cause cell damage
Direct Trauma Extreme temperatures Sudden change in atmospheric pressure Electric currents Radiation
Give examples of chemical agents and drugs that might cause cell damage
Glucose or salt in hypertonic solutions O2 at high concentrations Poisons Insecticides Herbicides Asbestos Alcohol Illicit drugs Therapeutic drugs
How might immune mechanisms lead to cell damage?
Hypersensitivity reactions where the host tissue has an overly vigorous immune reaction and causes injury (E.g. Urticaria)
Autoimmune reactions where the self is mis-identified as the non-self (E.g. Grave’s disease)
What are the 4 components of cells that are the principal targets for cell injury?
Cell membranes
Nucleus
Proteins
Mitochondria
How does chemical damage to a cell occur?
Give an example
Some chemicals will act by combining with a cellular component
E.g. Cyanide binds to mitochondrial cytochrome and blocks oxidative phosphorylation
What types of injury produce high levels of free radicals?
Chemical Radiation Ischaemia reperfusion Cellular ageing High O2 concentration
What are some of the effects of free radicals on cells?
Attack membrane lipids and cause lipid peroxidation
Damage proteins and nucleic acids
Mutagenic
What are the three free radicals of biological significance?
OH* - Most dangerous
O2- (superoxide)
H2O2
How are OH* free radicals formed?
Radiation can directly lyse water
Fenton and Haber-Weiss reactions produce OH* from H2O2 and O2- (Note: This is one reason to remove these ROS quickly, to prevent OH* being formed)
Briefly, how is the body protected against free radicals and why it important for this to exist?
The anti-oxidant system protects against free radicals
Imbalance in free radial production and scavenging leads to ROS build up and cell injury (This is oxidative stress)
What 3 elements is the anti-oxidant system comprised of?
Give a brief explanation of each
Enzymes:
- Superoxide dismutase (SOD) catalyses the O2- —–> H2O2 reaction (H2O2 is less toxic)
- Catalases and peroxidases complete the process of ROS removal (H2O2 —–> O2 + H2O
Free radical scavengers such as:
Vitamins A, C and E
Glutathione
In the extracellular matrix storage proteins (E.g. Transferrins and ceruloplasmin) sequester transitional metals (E.g. Copper and iron) which catalyse the formation of free radicals
What is the function of heat shock proteins?
When protein folding or denaturation they ensure they are refolded correctly
If not possible, the protein is destroyed
Heat shock response plays a role in maintaining viability of proteins and thus maximising cell survival
What are the three main changes seen in cells seen with cell injury?
Cytoplasmic changes:
- Reduced pink staining due to accumulation of water (reversible)
- Followed by increased pink staining due accumulation of denatured proteins and ribosomes that have detached from the RER (irreversible)
Nuclear changes:
- Chromatin clumped slightly (reversible)
- Pyknosis (shrinkage), Karryohexis (fragmentation) and karryolysis (dissolution) of the nucleus (irreversible)
Abnormal cellular accumulations
What reversible changes to injured cells can be seen with an electron microscope?
Give a brief reason for each
Swelling of both cell and organelles due to Na+/K+ pump failure
Blebs - (small area of detachment of the plasma membrane from cytoskeleton, appears as a small swelling)
Clumped Chromatin due to low pH
Ribosomes separation from RER due to failure of energy dependant process that keeps ribosomes attached.
What irreversible changes to injured cells can be seen with an electron microscope?
Further cell swelling
Nuclear changes (pyknosis, karryohexis, karryolysis)
Swelling or rupture of lysosymes
Membrane defects/damage
Appearance of myelin figures (which are damaged membranes)
Lysis of ER due to membrane defects
Amorphous densities
Define Oncosis
The spectrum of changes that occurs in injured cells prior to death
Define Necrosis
The morphological changes that follow cell death in living tissue, largely due to the progressive degradative action of enzymes on a lethally injured cell
Where is necrosis found?
What does it cause?
What kind of timescale does it occur on?
Give an example of this
Necrosis is found where there is damage to the cell membranes and lysosomal enzymes are released into the cytoplasm and digest the cell
As a result cell contents often leak out and inflammation is often seen, the ultrastructural changes are those seen in irreversible cell injury
Necrotic changes develop over a number of hours (E.g. after Myocardial infarction it takes 4-12 hours before microscopic changes are seen)
What happens to necrotic tissue?
Normally removed by enzymatic degradation and phagocytosis by white cells
Any remnants may calcify (dystrophic calcification)
What are the two main types of necrosis?
What are the two special types that may only occur in a limited set of circumstances?
Normal:
Liquefactive
Coagulative
Special:
Caseous
Fat necrosis
Describe coagulative necrosis
Hint: What it is, progression
Where is it commonly seen?
Denaturation of proteins dominates over release of active proteases
Dead tissue has a solid consistency due to protein clumping
Cellular architecture is somewhat preserved, this forms a ‘ghost outline’ of cells
This ghost appearance is only seen for a few days, after that the appearance is modified by acute inflammatory reaction to the dead tissue with consequent infiltration of phagocytes
Commonly seen in solid organs that have experienced ischaemia
Describe Liquefactive necrosis
Hint: What, Where, Concequences
Active enzyme degradation is substantially greater than denaturation and this leads to enzymatic digestion of the tissues (liquefaction)
Often seen in massive neutrophil infiltration (E.g. In abscesses) because neutrophils release protease
Also commonly found in bacterial infection and the brain (Fragile tissue without support from collagenous matrix)
Tissue becomes a viscous mass and if there is acute inflammation there is pus present
Describe the appearance of Caseous necrosis
What is it commonly associated with?
Has a cheesy appearance macroscopically
Characterised by amorphous debris (no ghost outlines)
Particularly associated with infection, especially Tuberculosis.
Often associated with granulomatous inflammation
Describe Fat necrosis
Hint: Appearance, 2 causes
Occurs where there is destruction of adipose tissue
Causes release of fatty acids which can react with calcium to form chalky deposits (calcium soaps) in fatty tissue, these can been seen in X-rays and with the naked eye in surgery or autopsy
Typically seen as a consequence of acute pancreatitis as there is release of lipases from the pancreatic acinar cells, causing fat necrosis in the abdominal cavity.
Also occurs after direct trauma to fatty tissue, especially breast tissue, can leave an irregular scar after healing which may mimic a cancerous breast nodule
What is gangrene?
NOT a ‘type’ of necrosis
It’s the clinical term used to describe necrosis that is visible to the naked eye
Gangrene can be ‘dry’ (coagulative necrosis) or ‘wet’ (liquefactive)
Wet gangrene is normally due to infection and can result in septicaemia
Gangrene is seen most commonly clinically as a result of ischaemic limbs
Gangrenous tissue is dead and cannot be salvaged
What is an Infarct?
What can cause an infarct?
An area of Ischaemic necrosis
Can result in gangrene
Mostly due to thrombosis or embolism
Can also be due to:
External compression of a vessel (by tumour or within a hernia)
Twisting of a vessel (E.g. Testicular torsion)
How does necrosis due to infarction present?
Give examples of the presentations
How else can infarcts be classified?
Necrosis due to infarct can present as liquefactive (E.g. Cerebral infarction) or coagulative (E.g. Myocardial infarction)
Can be described by colour, white or red
Describe how white infarct appear and how/where they commonly present
What type of necrosis is present?
White occurs in solid organs with good stromal support after the occlusion of an ‘end’ artery (i.e. an artery which is the sole arterial supply for a segment of the organ
The solid nature of the tissue limits the amount of haemorrhage that can occur into the infarct from adjacent capillaries
Appears white due to the lack of blood into the tissue
Commonly occur in the heart, spleen and kidneys
Most are wedge shaped with the occluded edge at the apex
Coagulative necrosis
What is a red infarct?
An infarction that results in significant haemorrhage
In what situations can red infarcts appear?
Hint: 5 appearances
Provide examples of where each may occur if possible/applicable
In organs with dual blood supply (E.g. Lungs) Occlusion of the main artery causes infarct, the secondary supply is insufficient but does allow blood to enter, hence red infarct
If numerous anastomoses (capillary beds of two separate arterial supplies merge) are present (E.g. Intestines) red infarct appears for same reason as above
In loose tissue (E.g. Lung) where there is poor stromal support for capillaries there is therefore more than normal haemorrhage into the dead tissue
Where there has been previous congestion (E.g. Congestive heart failure) there is more than normal amount of blood in the necrotic tissue
Where there is raised venous pressure, pressure is transmitted to capillary beds, as tissue pressure rises there is reduced arterial filling pressure in the tissue that causes ischaemia and necrosis, because tissue was previously engorged with blood a red infarct appears
What does the severity of an infarct depend on?
Whether the tissue has an alternative blood supply (E.g. In the lung and forearm)
How quickly the ischaemia occurs (if slowly there is time for additional perfusion pathways to develop)
How vulnerable a tissue is to hypoxia
The oxygen content of the blood (an infarct occurring in an anaemic patient will be more serious)
Define Apoptosis
Cell death induced by a regulated intracellular program where a cell activates enzymes that degrade its own nuclear DNA and proteins
Where is apoptosis seen?
When cells are no longer needed to maintain a steady state
During hormone controlled involution
Cytotoxic T cell killing of virus infected cells or neoplastic cells
Embryogensis
When does apoptosis occur?
When a cell is particularly damaged, particularly when damage is to the cell’s DNA or proteins
How do apoptic cells appear under a light microscope?
Shrunken
Intensely eosinophilic
Chromatin shrinkage
Pyknosis, karryohexis
Affects single cells or small groups
How do apoptic cells appear under an electron microscope?
How does this appearance change as apoptosis progresses?
Cytoplasmic blebbing is seen in addition to those features found in light microscopy
Blebbing progresses to fragmentation into membrane bound apoptic fragments which contain organelles and nuclear fragments
These fragments are eventually removed by macrophages
Why does apoptosis not cause inflammation?
No leakage of cell contents
What are the three key stages of apoptosis?
Initiation
Execution
Degradation/Phagocytosis
Describe the initiation phase of apoptosis
Triggered by intrinsic and/or extrinsic processes which both result in the activation of capases
Extrinsic apoptosis is ligand driven (as a result of ligand binding) such as TRAIL or Fas that bind to death receptors leading to capase activation
Intrinsic apoptosis has mitochondria as a central player, all apoptic machinery is internal to the cell
Various triggers including DNA damage, withdrawal of growth factors, hormones or p53 protein
Triggers lead to increased mitochondrial permeability resulting in the release of cytochrome c from mitochondria, this interacts with APAF1 and capase 9 to form an apotosome the activates various downstream capases
Describe the execution phase of apoptosis
Capases are released
They are proteases that mediate the cellular effects of apoptosis
They act by cleaving proteins breaking up the cytoskeleton and initiating the degradation of DNA
Describe the Degradation/Phagocytosis phase of apoptosis
Cell breaks into membrane bound fragments called apoptic bodies
They express molecules on their surface that induce their phagocytosis by neighbouring cells or phagocytes
What are the important apoptic molecules?
p53 - ‘Guardian of the genome’ that mediates apoptosis in response to DNA damage
Cytochrome c, APAF1, capase 9 - together they are the apoptosome
Bcl-2 - Prevents cytochrome c release from mitochondria therefore inhibits apoptosis
Death ligands - E.g. TRAIL
Death receptors - E.g. TRAIL-R
Capases - Effector molecules of apoptosis
What are abnormal cellular accumulations?
What might they consist of?
Occurs with sublethal or chronic injury when metabolic processes become deranged.
May be reversible or toxic
Can consist of:
Normal cellular constituents - E.g. Water, Lipids, Proteins, Carbohydrates
Abnormal substances - exogenous such as minerals or endogenous such as the products of abnormal metabolism
Pigments - Endogenous or exogenous
What are two common types of abnormal lipid accumulation?
Steatosis
Cholesterol accumulation
What is steatosis?
What are the causes of steatosis and where it is commonly found?
Does it affect cell function?
Accumulation of triglycerides
It is often seen in the liver (major site of lipid metabolism)
Common causes of liver steatosis include:
Alcohol abuse
Diabetes
Obesity
toxins (E.g. Carbon tetrachloride)
Mild steatosis doesn’t appear to affect function, more severe will have an effect
Where is cholesterol likely to be found abnormally accumulating?
What effect does it have on cells?
Accumulates in smooth muscle cells and macrophages within athersclerotic plaques
Microscopically these cells appear to have foamy cytoplasm, so are known as ‘foam cells’
Also seen in macrophages in the skin of people with familial and acquired hyperlipidaemias.
The macrophages form small masses known as xanthomas
How do abnormal protein accumulations appear in cells?
As eosinophilic droplets or aggregates in the cytoplasm
Describe two examples of abnormal protein aggregation
Mallory’s hyaline is a damged protein seen in hepatocytes of those with alcoholic liver disease due to accumulation of altered keratin fliaments
alpha1-antitrypsin deficiency is a genetic disorder where the liver produces incorrectly folded alpha1-antitrypsin that accumulates in the ER and is not secreted
This leads to proteases in the lung acting unchecked and breaking down lung tissue, leading to emphysema
Give examples of exogenous pigment accumulation
In tattoos, pigment is phagocytosed by macrophages and remains there indefinitely
Carbon coal dust inhalation, Dust particles phagocytosed by macrophages in the lung, this gives rise to blackened lung tissue (anthracosis)
If high exposure occurs lungs can become fibrotic or emphysema can develop, this is known as ‘coal workers pneumoconiosis’
Give examples of endogenous pigment accumulation
Lipofusin:
Brown pigment in aging cells a sign of ROS injury and lipid peroxidation
Haemosiderin:
Derived from Hb, Yellow brown and contains iron, forms in local or systemic excess of iron. A common example of this is in a bruise
Systemic iron overload leads to deposition of this pigment in all organs (haemosiderosis), if severe can lead to heart, pancreas and liver damage
Haemosiderosis can be seen in haemochromatosis (increased intestinal iron absorption), blood transfusions and haemolytic anaemias
Bilirubin:
Bile pigment, can cause jaundice if in excess (as in abnormal liver function or haemolytic anaemias)
What is pathological calcification?
Abnormal deposition of calcium salts in tissues
Can be dystrophic or metastatic
What is dystrophic calcification?
Occurs in areas of dying tissue, athersclerotic plaques or damaged heart valves
There is no abnormality in serum Ca2+ concentration or calcium metabolism
Can cause organ dysfunction (E.g. Athersclerosis or calcified heart valves)
What is metastatic calcification?
Hint: Four main causes
Calcium deposited in tissue secondary to hypercalcaemia and disturbances in calcium metabolism
Usually asymptomatic
Four main causes:
Increased PTH, for example due to parathyroid tumour or ectopic parathyroid tissue in tumours
Destruction of bone secondary to primary tumours of bone, for example in leukaemia, metastases in bone or immobilisation
Vit D disorders
Renal failure
What happens to cells as they age?
Accumulate damage to cellular components and DNA
May accumulate lipofuscin and abnormally folded proteins
Decline in replicative ability (Replicative senescence)
How does replicative senescence come about and how is it avoided?
Ends of chromosomes (telomeres) shorten with every division, when they reach critical length that cell can no longer divide
Germ cells, stem cells and some malignant cells produce telomerase to maintain telomeres and avoid senescence (indefinitely for germ cells)
How does metabolic tolerance to alcohol come about?
Induction of CYP2E1, increasing rate of ethanol metabolism and other drugs that are metabolised by this enzyme
What are the three major effects of Chronic excessive alcohol intake on the liver?
Fatty change:
Toxicity of alcohol results in steatosis, which can cause hepatomegaly
This is acute change and is reversible and generally asymptomatic
Acute alcoholic hepatitis:
Alcohol and metabolites can cause hepatitis as they are toxic
A binge can result in AAH and focal hepatocyte necrosis the formation of mallory bodies and and neutrophilic infiltrate
Gives symptoms of fever, liver tenderness and jaundice, is usually reversible
Cirrhosis:
Occurs in 10-15% of alcoholics, can result in a hard, shrunken liver and histologically appears as micronodules of regenerating hepatocytes surrounded by bands of collagen
It is irreversible, serious and sometimes fatal
How is paracetamol normally metabolised by the liver? (Metabolism Semester 1)
Detoxified in the liver by phase 2 sulphonation or glucuronidation
Small amounts are metabolised by CYP2E1 to NAPQI (phase 1) which is detoxified by glutathione (phase 2)
If a large dose of paracetamol is taken, what happens?
NAPQI accumulates
Glutathione is depleted by NAPQI
NAPQI binds to sulphydryl groups on liver cell membranes causing hepatocyte necrosis and liver failure
Massive liver necrosis can occur 3-5 days after the overdose with a large dose, this can be fatal
What makes someone particularly at risk of damage following a paracetamol overdose?
Those who took alcohol with the overdose Alcohol dependents Malnourished people People on enzyme inducing drugs (E.g. Carbamazepine) People with HIV or AIDS
What can be done for people following a paracetamol overdose?
How can severity of damage following overdose be gauged?
Can be given an antidote (N-acetyl cysteine / NAC) to increase availability of hepatic glutathione
To decide wether this is required 4 hours after overdose the serum concentration of paracetamol is measured
Prothrombin time after 24 hours indicates severity of liver damage
What are the effects of aspirin normally?
Acetylates platelet cyclooxygenase and blocks thromboxane A2 production (a substance which activates platelet aggregation)
What are the major consequences of aspirin overdose?
What indicates serious poisoning?
Aspirin stimulates the respiratory centre leading to respiratory alkalosis
Compensatory mechanisms result in metabolic acidosis
INterferes with carbohydrate, fat and protein metabolism and oxidative phosphorylation, resulting in an increase in lactate, pyruvate and ketones bodies, contributing to acidosis
Fall in serum pH indicates serious poisoning
Platelet cyclooxygenase inhibition causes decreased platelet aggregation and petechaie (small red/purple spot on skin indicating minor haemorrhage) may be present
Can also cause acute erosive gastritis leading to GI bleeding
What is a common indication of liver damage due to hepatitis or alcoholic liver disease?
If you see this indication, what might be the next step towards diagnosing hepatitis?
Raised bilirubin and aspartate aminotransferase (ASP) and alanine aminotransferase (ALT) in the blood indicated liver damged
A liver biopsy
What lab tests might be done if acute pancreatitis is suspected?
Raised values for these tests indicates what?
A serum amylase and lipase test
Raised result indicates cell injury/necrosis of the pancreatic cells
What abnormal blood test results might indicate someone is having a myocardial infarction?
Why are these good tests?
Raised troponin - Only released from cardiac cells
Raised Creatine kinase MB (CKMB) - Found mainly in the heart
Good tests as they are fairly specific to MI
