MoD S1+2 - Cell Injury

Question 1

What is the difference in cell response to mild versus severe changes in environment?

What if the cell cannot respond appropriately?

Answer 1

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

Question 2

What are the major causes of cell injury?

Answer 2

Hypoxia
Physical agents
Chemical agents and drugs
Microorganisms
Immune mechanisms
Dietary insufficiency or excess
Genetic abnormalities

Question 3

What is the result of hypoxia in general terms?

Answer 3

Results in decreased aerobic respiration (although glycolytic respiration can continue)

If this is persistent this can cause Atrophy, cell injury or cell death

Question 4

What are the 4 major types of hypoxia?

Give a brief description and example of each

Answer 4

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

Question 5

What is meant by Ischaemia and what causes it?

Answer 5

Loss of blood supply caused by:

Reduced arterial supply (obstruction of artery, hypotension)

Reduced venous drainage

Question 6

Why does Ischaemia cause more severe damage than other forms of hypoxia?

Answer 6

Reduced supply of oxygen AND respiratory substrates E.g. Glucose leads to damage occuring more rapidly and more severe damage than hypoxia

Question 7

How does the length of time cells can tolerate hypoxia vary? Give examples

Answer 7

Differs greatly between cell types

Eg. Some neurones can only tolerate minutes, while fibroblasts can tolerate a number of hours

Question 8

What occurs during reversible hypoxic injury?

Hint: One direct result of hypoxia leads to 3 main effects of this, each with their own concequences

Answer 8

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

Question 9

When does cell injury progress from reversible to irreversible?

Answer 9

Not really known/not well understood

Question 10

What is the result of irreversible hypoxic injury?

Hint: Don’t go into detail on the effects of Calcium

Answer 10

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

Question 11

How is Ca2+ involved in irreversible cell damage?

What is the effect of Ca2+ accumulation on a cell experiencing irreversible damage?

Answer 11

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

Question 12

What is Ischaemia-reperfusion injury?

Answer 12

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

Question 13

Give examples of physical agents that may cause cell damage

Answer 13

Direct Trauma
Extreme temperatures
Sudden change in atmospheric pressure
Electric currents
Radiation

Question 14

Give examples of chemical agents and drugs that might cause cell damage

Answer 14

Glucose or salt in hypertonic solutions
O2 at high concentrations
Poisons
Insecticides
Herbicides
Asbestos
Alcohol
Illicit drugs
Therapeutic drugs

Question 15

How might immune mechanisms lead to cell damage?

Answer 15

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)

Question 16

What are the 4 components of cells that are the principal targets for cell injury?

Answer 16

Cell membranes
Nucleus
Proteins
Mitochondria

Question 17

How does chemical damage to a cell occur?

Give an example

Answer 17

Some chemicals will act by combining with a cellular component

E.g. Cyanide binds to mitochondrial cytochrome and blocks oxidative phosphorylation

Question 18

What types of injury produce high levels of free radicals?

Answer 18

Chemical
Radiation
Ischaemia reperfusion
Cellular ageing
High O2 concentration

Question 19

What are some of the effects of free radicals on cells?

Answer 19

Attack membrane lipids and cause lipid peroxidation

Damage proteins and nucleic acids

Mutagenic

Question 20

What are the three free radicals of biological significance?

Answer 20

OH* - Most dangerous

O2- (superoxide)

H2O2

Question 21

How are OH* free radicals formed?

Answer 21

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)

Question 22

Briefly, how is the body protected against free radicals and why it important for this to exist?

Answer 22

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)

Question 23

What 3 elements is the anti-oxidant system comprised of?

Give a brief explanation of each

Answer 23

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

Question 24

What is the function of heat shock proteins?

Answer 24

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

Question 25

What are the three main changes seen in cells seen with cell injury?

Answer 25

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

Question 26

What reversible changes to injured cells can be seen with an electron microscope? Give a brief reason for each

Answer 26

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.

Question 27

What irreversible changes to injured cells can be seen with an electron microscope?

Answer 27

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

Question 28

Define Oncosis

Answer 28

The spectrum of changes that occurs in injured cells prior to death

Question 29

Define Necrosis

Answer 29

The morphological changes that follow cell death in living tissue, largely due to the progressive degradative action of enzymes on a lethally injured cell

Question 30

Where is necrosis found? What does it cause? What kind of timescale does it occur on? Give an example of this

Answer 30

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)

Question 31

What happens to necrotic tissue?

Answer 31

Normally removed by enzymatic degradation and phagocytosis by white cells Any remnants may calcify (dystrophic calcification)

Question 32

What are the two main types of necrosis? What are the two special types that may only occur in a limited set of circumstances?

Answer 32

Normal: Liquefactive Coagulative Special: Caseous Fat necrosis

Question 33

Describe coagulative necrosis Hint: What it is, progression Where is it commonly seen?

Answer 33

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

Question 34

Describe Liquefactive necrosis Hint: What, Where, Concequences

Answer 34

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

Question 35

Describe the appearance of Caseous necrosis What is it commonly associated with?

Answer 35

Has a cheesy appearance macroscopically Characterised by amorphous debris (no ghost outlines) Particularly associated with infection, especially Tuberculosis. Often associated with granulomatous inflammation

Question 36

Describe Fat necrosis Hint: Appearance, 2 causes

Answer 36

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

Question 37

What is gangrene?

Answer 37

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

Question 38

What is an Infarct? What can cause an infarct?

Answer 38

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)

Question 39

How does necrosis due to infarction present? Give examples of the presentations How else can infarcts be classified?

Answer 39

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

Question 40

Describe how white infarct appear and how/where they commonly present What type of necrosis is present?

Answer 40

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

Question 41

What is a red infarct?

Answer 41

An infarction that results in significant haemorrhage

Question 42

In what situations can red infarcts appear? Hint: 5 appearances Provide examples of where each may occur if possible/applicable

Answer 42

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

Question 43

What does the severity of an infarct depend on?

Answer 43

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)

Question 44

Define Apoptosis

Answer 44

Cell death induced by a regulated intracellular program where a cell activates enzymes that degrade its own nuclear DNA and proteins

Question 45

Where is apoptosis seen?

Answer 45

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

Question 46

When does apoptosis occur?

Answer 46

When a cell is particularly damaged, particularly when damage is to the cell's DNA or proteins

Question 47

How do apoptic cells appear under a light microscope?

Answer 47

Shrunken Intensely eosinophilic Chromatin shrinkage Pyknosis, karryohexis Affects single cells or small groups

Question 48

How do apoptic cells appear under an electron microscope? How does this appearance change as apoptosis progresses?

Answer 48

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

Question 49

Why does apoptosis not cause inflammation?

Answer 49

No leakage of cell contents

Question 50

What are the three key stages of apoptosis?

Answer 50

Initiation Execution Degradation/Phagocytosis

Question 51

Describe the initiation phase of apoptosis

Answer 51

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

Question 52

Describe the execution phase of apoptosis

Answer 52

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

Question 53

Describe the Degradation/Phagocytosis phase of apoptosis

Answer 53

Cell breaks into membrane bound fragments called apoptic bodies They express molecules on their surface that induce their phagocytosis by neighbouring cells or phagocytes

Question 54

What are the important apoptic molecules?

Answer 54

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

Question 55

What are abnormal cellular accumulations? What might they consist of?

Answer 55

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

Question 56

What are two common types of abnormal lipid accumulation?

Answer 56

Steatosis | Cholesterol accumulation

Question 57

What is steatosis? What are the causes of steatosis and where it is commonly found? Does it affect cell function?

Answer 57

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

Question 58

Where is cholesterol likely to be found abnormally accumulating? What effect does it have on cells?

Answer 58

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

Question 59

How do abnormal protein accumulations appear in cells?

Answer 59

As eosinophilic droplets or aggregates in the cytoplasm

Question 60

Describe two examples of abnormal protein aggregation

Answer 60

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

Question 61

Give examples of exogenous pigment accumulation

Answer 61

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'

Question 62

Give examples of endogenous pigment accumulation

Answer 62

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)

Question 63

What is pathological calcification?

Answer 63

Abnormal deposition of calcium salts in tissues Can be dystrophic or metastatic

Question 64

What is dystrophic calcification?

Answer 64

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)

Question 65

What is metastatic calcification? Hint: Four main causes

Answer 65

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

Question 66

What happens to cells as they age?

Answer 66

Accumulate damage to cellular components and DNA May accumulate lipofuscin and abnormally folded proteins Decline in replicative ability (Replicative senescence)

Question 67

How does replicative senescence come about and how is it avoided?

Answer 67

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)

Question 68

How does metabolic tolerance to alcohol come about?

Answer 68

Induction of CYP2E1, increasing rate of ethanol metabolism and other drugs that are metabolised by this enzyme

Question 69

What are the three major effects of Chronic excessive alcohol intake on the liver?

Answer 69

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

Question 70

How is paracetamol normally metabolised by the liver? (Metabolism Semester 1)

Answer 70

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)

Question 71

If a large dose of paracetamol is taken, what happens?

Answer 71

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

Question 72

What makes someone particularly at risk of damage following a paracetamol overdose?

Answer 72

``` Those who took alcohol with the overdose Alcohol dependents Malnourished people People on enzyme inducing drugs (E.g. Carbamazepine) People with HIV or AIDS ```

Question 73

What can be done for people following a paracetamol overdose? How can severity of damage following overdose be gauged?

Answer 73

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

Question 74

What are the effects of aspirin normally?

Answer 74

Acetylates platelet cyclooxygenase and blocks thromboxane A2 production (a substance which activates platelet aggregation)

Question 75

What are the major consequences of aspirin overdose? What indicates serious poisoning?

Answer 75

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

Question 76

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?

Answer 76

Raised bilirubin and aspartate aminotransferase (ASP) and alanine aminotransferase (ALT) in the blood indicated liver damged A liver biopsy

Question 77

What lab tests might be done if acute pancreatitis is suspected? Raised values for these tests indicates what?

Answer 77

A serum amylase and lipase test Raised result indicates cell injury/necrosis of the pancreatic cells

Question 78

What abnormal blood test results might indicate someone is having a myocardial infarction? Why are these good tests?

Answer 78

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