WEEK 2: Cellular basis of disease including cell injury and cell death

Question 1

Cellular injury occurs when a stress exceeds the cell’s ability to adapt.

What does the likelihood of injury depend on?

Outline the common causes of cellular injury.

Answer 1

Cellular injury occurs when a stress exceeds the cell’s ability to adapt.

The likelihood of injury depends on the type of stress, its severity, and the type of cell affected.

Common causes of cellular injury include inflammation, nutritional deficiency or excess, hypoxia, trauma, and genetic mutations.

Question 2

What is hypoxia?
How does it result in cell injury?

Answer 2

*Low oxygen delivery to tissue; important cause of cellular injury.

Decreased oxygen impairs oxidative phosphorylation; Oxygen is the final electron acceptor in the electron transport chain of oxidative phosphorylation.

This result in decreased ATP production.

Lack of ATP leads to cellular injury.

Question 3

Outline causes of hypoxia.

Answer 3

1.Ischemia
2.Hypoxemia
3.Decreased oxygen carrying capacity of blood

Question 4

What is ischemia?
Outline some factors resulting in ischemia.

Answer 4

Ischemia is decreased blood flow through an organ.

*Decreased arterial perfusion (e.g., atherosclerosis)
*Decreased venous drainage (e.g., Budd-Chiari syndrome)
*Shock - generalized hypotension resulting in poor tissue perfusion

Question 5

What is hypoxemia?
Outline some causes of hypoxemia.

Answer 5

1.Hypoxemia is a low partial pressure of oxygen in the blood (PaO2 < 60 mm Hg, SaO2< 90%).

2.Hypoventilation- Increased PAC02 results in decreased PAO2.

*Interstitial lung disease (ILD) isan umbrella term used for a large group of diseases that cause scarring (fibrosis) of the lungs.

The scarring causes stiffness in the lungs which makes it difficult to breathe and get oxygen to the bloodstream. Lung damage from ILDs is often irreversible and gets worse over time.

*Diffusion defect

PAO2 not able to push as much O2 into the blood due to a thicker diffusion barrier.

*V/Q mismatch

Blood bypasses oxygenated lung. (circulation problem, e.g., right-to-left shunt), or oxygenated air cannot reach blood (ventilation problem, e.g., atelectasis).

Question 6

Describe causes of decreased oxygen carrying capacity of blood.

Answer 6

1.Anemia (decrease in RBC mass)-PaO2 = Normal. SaO2=Normal.

2.Carbon monoxide poisoning

i. CO binds hemoglobin more avidly than oxygen-Pao2 normal; Sao2 decreased.

ii. Exposures include smoke from fires and exhaust from cars or gas heaters.

iii. Classic finding is cherry-red appearance of skin.

iv. Early sign of exposure is headache; significant exposure leads to coma and
death.

3. Methemoglobinemia
4. Iron in heme is oxidized to Fe3+, which cannot bind oxygen-Pao2 normal; Sao2 decreased.
5. Seen with oxidant stress (e.g., sulfa and nitrate drugs) or in newborns.
6. Classic finding is cyanosis with chocolate-colored blood.

iv. Treatment is intravenous methylene blue, which helps reduce Fe3+ back to Fe2+ state.

Question 7

What is the hallmark of irreversible injury?

Answer 7

The hallmark of irreversible injury is membrane damage.

Question 8

What are the effects of plasma membrane damage and mitochondrial membrane damage?

Answer 8

*Plasma membrane damage results in:

i. Cytosolic enzymes leaking into the serum (e.g., cardiac troponin).
ii. Additional calcium entering the cell.

*Mitochondrial membrane damage results in

i. Loss of the electron transport chain.
ii. Cytochrome c leaking into cytosol (activates apoptosis).

*Pancreatic lipase and amylase may raise in acute pancreatitis.

Irreversible injury and cell death in these tissues elevate the serum levels of these proteins, which makes them clinically useful markers of tissue damage.

*Lysosome membrane damage results in hydrolytic enzymes leaking into the cytosol, which, in turn, are activated by the high intracellular calcium.

Question 9

What is the result of irreversible injury?

Answer 9

The result of irreversible injury is cell death.

Question 10

What is the morphologic hallmark of cell death?

Answer 10

The morphologic hallmark of cell death is loss of the nucleus.

Question 11

Describe the 3 main nuclear changes that occur during cell death.

Answer 11

*Nuclear condensation (pyknosis: Characterized by nuclear shrinkage and basophilia.

*Fragmentation (karyorrhexis): The pyknotic nucleus undergoes fragmentation. The nucleus starts fading away.

*Dissolution (karyolysis): The basophilia of the chromatin’s fades to reflect loss of DNA due to enzymatic degradation by endonucleases.

Question 12

What are the two mechanisms of cell death?

Answer 12

The two mechanisms of cell death are necrosis and apoptosis.

Question 13

What is necrosis?

Answer 13

*Death of large groups of cells followed by acute inflammation.

Due to some underlying pathologic process; never physiologic.

Divided into several types based on gross features.

Question 14

What is reversible cell injury?

Answer 14

Cell injury is classified as reversible if the injured cell can regain homeostasis and return to a morphologically (and functionally) normal state.

Question 15

What is the classic morphologic change in reversible injury?

Answer 15

Acute cell swelling is the classic morphologic change in reversible injury.

Question 16

What is infarction?

Answer 16

If there is reduction or cessation of the blood supply to a tissue (ischemia), then a zone of that tissue dies by necrosis, and the process is termed infarction.

A particularly important site where this occurs is the heart and a myocardial infarction result.

Question 17

Which of the following changes indicate necrosis?

A. Disappearance of nuclei.
B. Apoptotic body formation.
C. Karyolysis..
D. Nuclear enlargement.
E. Cell swelling

Answer 17

A
C

Question 18

Necrosis occurs in which of the following?

A. In cerebral infarction (‘stroke’).

B. Following attack by complement.

C. When cytotoxic T cells kill virus-infected cells.

D. During embryogenesis.

E. After complete occlusion of an end artery.

Answer 18

A, B, E

Question 19

A tissue deprived of oxygen is hypoxic. Hypoxia is a particularly important cause of cell injury. Many changes induced early in hypoxic injury are reversible on re-oxygenation. Individually sublethal cellular changes caused by hypoxia include which of the following?

A. Ribosomes remain attached to normal-sized endoplasmic reticulum.

B. Cellular swelling.

C. Reduced ATP within the cell.

D. Glycogen depletion and anaerobic glycolysis.

E. Decreased intracellular ph.

Answer 19

B, C, D, E

Question 20

Describe coagulative necrosis.

Answer 20

*Necrotic tissue that remains firm cell: cell shape and organ structure are preserved by coagulation of proteins, but the nucleus disappears.

*Characteristic of ischemic infarction of any organ except the brain.

Area of infarcted tissue is often wedge-shaped and pale.

Red infarction arises if blood re-enters a loosely organized tissue.

Question 21

Describe liquefied necrosis.
What is pus?

Answer 21

Necrotic tissue that becomes liquefied; enzymatic lysis of cells and protein results in liquefaction.

Pattern of cell death characterized by dissolution of necrotic cells.Infiltration of dead tissue by large numbers of neutrophils leads to digestion (rather than coagulation) of cell proteins.

Pus is the liquefied remnants of dead cells, including dead neutrophils.

Characteristic of:

i. Brain infarction - Proteolytic enzymes from microglial cells liquefy the brain.

ii. Abscess - Proteolytic enzymes from neutrophils liquefy tissue.

Question 22

Describe gangrenous necrosis.

Answer 22

Coagulative necrosis that resembles mummified tissue (dry gangrene)

Dry gangrene is coagulative necrosis of an extremity due to slowly developing vascular occlusion. This can be seen in patients with diabetic vascular disease.

Characteristic of ischemia of lower limb and GI tract

If superimposed infection of dead tissues occurs, then liquefactive necrosis ensues (wet gangrene).

Question 23

Describe caseous necrosis.

Answer 23

Granulomas are aggregates of epithelioid macrophages and giant cell macrophages, often surrounded by lymphocytes.

Granulomas are found as a response to foreign bodies, in some autoimmune diseases, and in mycobacterial infection (e.g. M.tuberculosis).

Necrosis can occur in the center of granulomas, typically in mycobacterial infection. This is described as caseous necrosis because the macroscopic appearance was considered to be cheese-like.

*Combination of coagulative and liquefactive necrosis.

*Characteristic of granulomatous inflammation due to tuberculous or fungal infection

Question 24

Describe fat necrosis.

Answer 24

Necrotic adipose tissue with chalky-white appearance due to deposition of calcium.

Characteristic of trauma to fat (e.g., breast) and pancreatitis-mediated damage of peripancreatic fat.

This can be seen in acute pancreatitis (acute inflammation of the pancreas causing necrosis of pancreatic acinar cells and lipase release), or from trauma to fatty tissues.

When lipases are released into adipose tissue, triglycerides are cleaved into fatty acids, which bind and precipitate calcium ions, forming insoluble salts.

Question 25

Describe fibrinoid necrosis.

Answer 25

Necrotic damage to blood vessel wall.

Leaking of proteins (including fibrin) into vessel wall results in bright pink staining of the wall microscopically.

Characteristic of malignant hypertension and vasculitis

Question 26

Your patient has experienced an acute myocardial infarct and expired due ventricular rupture seven days later. You ask for an autopsy and examine a section of the heart under the microscope.

What type of necrosis do you see?
A. Caseous necrosis
B. Liquefactive necrosis
C. Fibrinoid necrosis
D. Fat Necrosis
E. Coagulative necrosis

Answer 26

E

Question 27

State the 6 major mechanisms of cell injury.

Answer 27

• Depletion of ATP
• Mitochondrial Damage
• Entry of Calcium into the cell
• Increase reactive oxygen species (ROS)
• Membrane Damage
• DNA damage, Protein misfolding.

Question 28

Describe how depletion of ATP result in cell injury.

Answer 28

ATP depletion and decreased ATP synthesis are common with both hypoxic and toxic (or chemical) injury.

• Na+, K+- ATPase pump activity is reduced
• Cellular energy metabolism is changed
• Failure of Ca++ pump
• Reduced protein synthesis

Question 29

Describe how mitochondrial damage result in cell injury.

Answer 29

• Loss of membrane potential via membrane permeability transition
• Results in failed oxidative phosphorylation and loss of ATP
• Membrane damage leads to leakage of Cytochrome c and other proteins which activate apoptotic pathways

Question 30

Describe how entry of calcium in the cell results in cell injury.

Answer 30

Intracellular Ca++ is low and is sequestered in
mitochondria and endoplasmic reticulum
* Extracellular Ca++ is high
* Gradients are maintained by Ca++ Mg++
ATPases

Increased cytosolic Ca++ activates enzymes:
ATPases, phospholipases, proteases and endonucleases

Starts breaking apart ATP, lipid membrane, cellular proteins, and DNA, respectively.

Question 31

What cellular processes consume the most energy on an ongoing basis?
A. protein synthesis
B. DNA synthesis
C. DNA repair
D. phospholipid synthesis
E. ion transport

Answer 31

E

Question 32

Describe the pathologic effect of reactive oxidative species resulting in cell injury.

Answer 32

Fatty acids- lipid peroxidation of
plasma membranes and organelles
* Proteins- oxidation with loss of
enzyme activity, protein misfolding
* DNA – oxidation, mutations, breaks

Question 33

Describe the consequences of membrane damage, how it leads to cell injury.

Answer 33

*Plasma membrane damage causes loss of
osmotic balance, loss of proteins, enzymes and
nucleic acids.
* Injury to lysosome membranes causes leakage of
enzymes with destruction of cellular components

Question 34

State the pathologic effect of DNA damage and protein misfolding resulting in cell injury.

Answer 34

*If DNA damage to cell is too severe, apoptosis is initiated.
* Improperly folded proteins can initiate apoptosis.

Question 35

A 53-year-old man has had marked chest pain for the past 3 hours.

Laboratory findings include elevated serum creatine kinase-MB. He is given a thrombolytic drug, and the CK-MB rises further.

Which of the following is the most likely biochemical basis for this observed rise in CK-MB?

A. Reduced protein synthesis
B. Generation of reactive oxygen species
C. Increased activity of Catalase
D. Reduced oxidative phosphorylation.
E. Release of calcium from endoplasmic reticulum

Answer 35

B. Generation of ROS
Delivery of drug causes reperfusion -> further injury -> generation of ROS

Question 36

What is apoptosis?

Answer 36

A. Energy (ATP) -dependent, genetically programmed cell death involving single cells or small groups of cells.

Question 37

Outline at least 3 examples of apoptosis.

Answer 37

Endometrial shedding during menstrual cycle.

Removal of cells during embryogenesis.

CD8+ T cell-mediated killing of virally infected cells.

Question 38

Describe the morphology of cells in apoptosis.

Answer 38

Dying cell shrinks, leading cytoplasm to become more eosinophilic (pink).

Nucleus condenses and fragments in an organized manner.

Apoptotic bodies fall from the cell and are removed by macrophages; apoptosis is not followed by inflammation.

Question 39

Apoptosis is mediated by caspases that activate proteases and endonucleases.

What are caspases?

What are the functions of proteases and endonucleases?

Answer 39

1. They are a family of protease enzymes playing essential roles in programmed cell death.
2. Proteases break down the cytoskeleton.
   Endonucleases break down DNA.

Question 40

Caspases are activated by multiple pathways.

Describe the intrinsic mitochondrial pathway.

Answer 40

1. Intrinsic mitochondrial pathway:

i. Cellular injury, DNA damage, or decreased hormonal stimulation leads to inactivation of Bcl-2.

ii. Lack of Bcl-2 allows cytochrome c to leak from the inner mitochondrial matrix into the cytoplasm and activate caspases.

Question 41

2. Describe the Extrinsic receptor-ligand pathway of activating caspases.

Answer 41

2. Extrinsic receptor-ligand pathway:

i. FAS ligand binds FAS death receptor on the target cell, activating caspases.
(e.g., negative selection of thymocytes in thymus).

ii. TNF binds TNF receptor on the target cell, activating caspases.

Question 42

Describe how Cytotoxic CD8 + T cell-mediated pathway activate caspases.

Answer 42

*Perforins secreted by CD8+ T cell create pores in membrane of target cell.

*Granzyme from CD8+ T cell enters pores and activates caspases.

WHAT ARE GRANZYMES?
Granzymes are serine proteases released by cytoplasmic granules within cytotoxic T cells and natural killer (NK) cells.

They induce programmed cell death (apoptosis) in the target cell,

Question 43

What are free radicals?
How are they generated physiologically?

Answer 43

Free radicals are chemical species with an unpaired electron in their outer orbit.

Physiologic generation of free radicals occurs during oxidative phosphorylation:

Superoxide (02 ∙),
Hydrogen peroxide (H2O2),
Hydroxyl radicals (OH).

Question 44

Describe the pathological generation of free radicals.

Answer 44

Arises with:

*Inflammation - NADPH oxidase generates superoxide ions during oxygen-dependent killing by neutrophils.

*Metals - Fe2+ generates hydroxyl free radicals.

*Drugs and chemicals-P450 system of liver metabolizes drugs generating free radicals.

Question 45

What are the negative effects of free radicals?

Answer 45

Free radicals cause cellular injury via peroxidation of lipids and oxidation of DNA and proteins.

DNA damage is implicated in aging and oncogenesis.

Question 46

Describe ways of removing free radicals.

Answer 46

1.Antioxidants- Vitamins A and E, glutathione and
ascorbic acid.

2. Iron and Copper ions catalyze formation of ROS
   and are bound to transport proteins - transferrin, ferritin, ceruloplasmin.

3.Enzymes scavenge free radicals.

• Catalase in peroxisomes
  *Superoxide dismutase in mitochondria and cytosol
  *Glutathione peroxidase in cytosol.