Cell and Tissue Injury

Question 1

[5-minute video]: Liquefactive and Caseous Necrosis

Answer 1

✔

Question 2

Click on Answer for diagrams showing the schema of cellular response to stress. [Very important for understanding the bigger picture of this topic.]

Answer 2

[Diagram 1] [Diagram 2]

Question 3

What are some features seen in reversibly damaged cells?

Answer 3

(1) Acute cellular swelling/hydropic change, which may be as a result of:
✓ Vacuolar degeneration seen as clear vacuoles on light microscopy and ER dilation with degranulation. On electron microscopy, dissociation of polyribosomes into single ribosomes is observed.
✓ Ischaemia leading to hypoxia and hence a reduction in ATP availability and the cell switches to anaerobic glycolysis. Build up of lactic acid and failure of sodium pump results in inflow of water. Break down of cellular constituents results in intracellular water accumulation.
✓ Direct plasma membrane injury can lead to increased permeability.
(2) Changes seen on the cell surface include microvilli loss, surface blebs and myelin figures due to degenerating lipid membrane.
(3) Nuclear changes are mild i.e. mild clamping of chromatin, separation of nucleolus into fibrillar and granular components.
(4) Breakage of microfilaments and intermediate filaments. This will result in perturbation of membrane skeleton, resulting in formation of surface blebs [which are bulges in the plasma membrane often seen in cells undergoing stress or apoptosis.]

[Diagram 1] [Diagram 2]

Question 4

What are the two main phenomena that consistently characterize irreversibility of cell injury?

Answer 4

(1) the inability to reverse mitochondrial dysfunction (lack of oxidative phosphorylation and ATP generation) [even after resolution of the original injury]
(2) profound disturbances in membrane function

Question 5

What are some features of irreversibly damaged cells?

Answer 5

(1) severe vacuolations
(2) wooly or flocculent density in mitochondria
(3) plasma membrane damage is more obvious with loss of intracellular susbtance into extracellular space
(4) karyolysis, karyorrhexis, pyknosis
[Diagram]

Question 6

What are some characteristic morphological features of necrosis?

Answer 6

(1) Cells show increased eosinophilia, with a glassy homogenous appearance.
(2) Swelling of organelles e.g. mitochondria
(3) Plasma membrane rupture
(4) Severe nuclear changes; pyknosis, karyorrhexis, karyolysis

Question 7

What are typical patterns of nuclear degeneration as witnessed in necrosis?

Answer 7

(1) karyolysis: basophilia of chromatin may fade
(2) pyknosis: nuclear shrinkage and increased basophilia. Here the chromatin condenses into a dense, shrunken basophilic mass.
(3) karyorrhexis: the pyknotic nucleus undergoes fragmentation. With the passage of time (1 or 2 days), the nucleus in the necrotic cell totally disappears.
[Diagram 1] [Image 1] [Image 2]

Further notes:
The term karyolysis comes from Greek roots:
✓ karyon: This means “nut” or “kernel,” and in biological terms, it refers to the nucleus of a cell.
✓ lysis: This means “dissolution” or “loosening,” derived from the verb “lyein”, which means “to separate” or to “dissolve”.
So karyolysis literally translates to the “dissolution of the nucleus,” which accurately describes the process where the chromatin in a cell’s nucleus breaks down due to enzymatic activity, often occuring during cell death.

The term pyknosis originates from the Ancient Greek word πύκνωσις (púknōsis), which means “thickening.” This is derived from πυκνός (puknós), meaning “compact” or “dense”.

Pyknosis refers to the irreversible condensation of chromatin in the nucleus of a cell undergoing necrosis or apoptosis

Question 8

What is coagulative necrosis?

Answer 8

Coagulative necrosis is a form of necrosis where the architecture of dead tissue is preserved for at least some days, and the affected tissue has a firm texture.
[Slide 1] [Slide 2] [Slide 3]

Question 9

What causes the preservation of tissue architecture in coagulative necrosis?

Answer 9

The injury denatures structural proteins and enzymes, blocking the proteolysis of dead cells, resulting in intensely eosinophilic cells with indistinct or reddish nuclei persisting for days or weeks.

Histological slides: [Slide 1] [Slide 2] [Slide 3]

Further notes:
The “intensely eosinophilic” appearance is as a result of:
(1) Denaturation of proteins: When cells undergo coagulative necrosis, the injury denatures structural proteins. This denaturation makes the proteins more accessible to eosin.
(2) Loss of basophilic components: Normally, the nucleus and other cellular components stain blue due to the presence of nucleic acids and ribosomes. However, in necrotic cells, these components are degraded or lost, reducing the basophilic staining.

Question 10

How are necrotic cells ultimately broken down in coagulative necrosis?

Answer 10

Necrotic cells are broken down by lysosomal enzymes from infiltrating leukocytes, which also remove the debris of dead cells by phagocytosis.

Question 11

What can cause coagulative necrosis?

Answer 11

Ischemia caused by obstruction in a vessel can lead to coagulative necrosis of the supplied tissue in all organs except the brain.

Question 12

Distinguish between hypoxia and ischemia.

Answer 12

Ischemia refers to a lack of blood flow to a tissue or organ, whereas hypoxia refers to a lack of oxygen in the tissues, regardless of blood flow. [Hypoxia can occur even if blood flow is normal but the oxygen content in the blood is low, such as in cases of respiratory diseases or high altitudes.]

Question 13

What is an infarct?

Answer 13

A localized area of coagulative necrosis is called an infarct.

Question 14

What is liquefactive necrosis?

Answer 14

Liquefactive necrosis is characterized by the digestion of dead cells, transforming the tissue into a viscous liquid.

Question 15

In what conditions is liquefactive necrosis commonly seen?

Answer 15

It is commonly seen in focal bacterial or occasionally fungal infections.

Question 16

What stimulates liquefactive necrosis in infections?

Answer 16

Microbes stimulate the accumulation of leukocytes and the liberation of enzymes from these cells.

Question 17

What is the appearance of necrotic material in liquefactive necrosis?

Answer 17

The necrotic material is frequently creamy yellow due to the presence of leukocytes and is called pus.

Histological images:
[Image 1] [Image 2] [Image 3]

Question 18

What is gangrenous necrosis?

Answer 18

Gangrenous necrosis is a type of tissue death that occurs when a large area of tissue dies due to severe hypoxia (lack of oxygen), often complicated by bacterial infection.
There are two main types of gangrene:
(a) Dry gangrene: This type results from reduced blood flow (ischemia) and is characterized by dry, shriveled, and darkened tissue. It often affects the extremeties, such as toes and fingers.
(b) Wet gangrene: This type involves bacterial infection and is characterized by swollen, blistered, and moist tissue. It can spread rapidly and is often accompanied by a foul odor.
[Image]

Question 19

What is caseous necrosis?

Answer 19

This is a type of necrosis characterized by a cheese-like appearance of the affected tissue.
[Image 1] [Image 2] [Image 3]

Further notes:
It is most commonly associated with tuberculosis, where granulomas form in the lungs; the necrotic tissue is often enclosed within a granuloma, a structure formed by the immune system to wall off the infection.

Question 20

Fat necrosis refers to focal areas of fat destruction. What condition is commonly associated with fat necrosis? Also explain how it causes fat necrosis.

Answer 20

Fat necrosis is commonly associated with acute pancreatitis, an abdominal emergency where pancreatic enzymes leak out of damaged acinar cells, liquefying the membranes of fat cells in the peritoneum, releasing triglyceride esters that are split by pancreatic lipases.
[Slide 1] [Slide 2]

Question 21

What is one definitive sign of fat necrosis in a tissue or organ?

Answer 21

Fatty acids combine with calcium to produce grossly visible chalky-white areas.
[Image 1] [Image 2]

Question 22

Distinguish between programmed cell death and apoptosis.

Answer 22

Programmed cell death is a broader term that encompasses various forms of regulated cell death, including apoptosis, necroptosis, pyroptosis, and autophagy, whereas apoptosis is a form of programmed cell death characterized by specific morphological and biochemical features.

Question 23

Apoptosis results from the activation of enzymes called ________.

Answer 23

caspases

Question 24

Briefly discuss the three types of caspases.

Answer 24

(a) Initiator caspases
These include caspase-8 and caspase-9, which are responsible for the initial steps of the apoptotic signaling cascade.

(b) Executioner caspases
These include caspase-3, caspase-6, and caspase-7, which carry out the degradation of cellular components during apoptosis.

(c) Inflammatory caspases
These include caspase-1, caspase-4, caspase-5 and caspase-12, which are involved in the processing of pro-inflammatory cytokines and the inflammatory response.

Question 25

What is the role of apoptosis in development?

Answer 25

Removal of supernumerary cells and remodeling of maturing tissues.

Question 26

What happens to hormone-dependent tissues on hormone withdrawal?

Answer 26

They undergo involution, such as endometrial cell breakdown during the menstrual cycle and regression of the lactating breast after weaning.

Question 27

Why is apoptosis important for B lymphocytes in germinal centers?

Answer 27

To eliminate those that fail to express useful antigen receptors.

Question 28

What is the role of apoptosis in preventing immune reactions against one’s own tissues?

Answer 28

Elimination of potentially harmful self-reactive lymphocytes.

Question 29

What happens to neutrophils after an acute inflammatory response?

Answer 29

They undergo apoptosis after serving their useful purpose.

Question 30

What is the role of phagocytes in apoptosis?

Answer 30

They remove apoptotic cells.

Question 31

What is the role of apoptosis in pathologic conditions?

Answer 31

It eliminates cells that are injured beyond repair without eliciting a host reaction, thus limiting collateral tissue damage.

Question 32

How does apoptosis protect against malignant transformation?

Answer 32

By preventing the survival of cells with DNA mutations that can lead to cancer.

Question 33

What can trigger DNA damage leading to apoptosis?

Answer 33

radiation and cytotoxic anticancer drugs

Question 34

What happens if DNA repair mechanisms cannot correct damage?

Answer 34

The cell triggers intrinsic mechanisms that induce apoptosis.

Question 35

Which cells are responsible for inducing apoptosis in infected cells during viral infections?

Answer 35

Cytotoxic T lymphocytes (CTLs) specific for viral proteins.

Question 36

List some morphological changes seen in cells undergoing apoptosis when viewed under the electron microscope.

Answer 36

cell shrinkage, chromatin condensation, formation of cytoplasmic blebs and apoptotic bodies, phagocytosis of apoptotic cells or bodies [usually by macrophages]
[Diagram]

Question 37

Apoptotic cells provoke an inflammatory response.
True or False?

Answer 37

False

Question 38

What are the two pathways that lead to apoptosis?

Answer 38

(a) Mitochondrial pathway (Intrinsic pathway)
(b) Death Receptor Pathway (Extrinsic pathway)

Question 39

Discuss the mitochondrial pathway of apoptosis.

Answer 39

(1) Initiation: The pathway is triggered by various internal stress signals such as DNA damage, oxidative stress, or the absence of growth factors.

(2) Mitochondrial Outer Membrane Permeabilization (MOMP): This is a critical event where the mitochondrial outer membrane becomes permeable. This process is tightly regulated by the BCL-2 family of proteins, which includes both pro-apoptotic (e.g. BAX, BAK) and anti-apoptotic (e.g. BCL-2, BCL-Xl and MCL1) members.

(3) Release of cytochrome c: Once MOMP occurs, cytochrome c is released from the mitochondria into the cytosol. Cytochrome c then binds to APAF-1 (apoptotic protease activating factor-1) and ATP, forming the apoptosome.

(4) Activation of caspases: The apoptosome activates caspase-9, which in turn activates executioner caspases such as caspase-3 and caspase-7. These caspases are responsible for cleavage of various cellular components, leading to the morphological and biochemical changes associated with apoptosis.

(5) Execution phase: The activated caspases degrade key structural and regulatory proteins, resulting in cell shrinkage, chromatin condensation, DNA fragmentation, and the formation of apoptotic bodies. These apoptotic bodies are then phagocytosed by neighbouring cells or macrophages.

[Diagram 1] [Diagram 2] [Diagram 3]

Further notes:
🩺 The anti-apoptotic proteins mentioned above keep the outer mitochondrial membrane impermeable, thus preventing leakage of cytochrome c and other death-inducing proteins into the cytosol. [They possess 4 BH domains.]
🩺 The pro-apoptotic proteins increase the permeability of the outer mitochondrial membrane. They contain the first three BH domains (BH1-3).
🩺 There is another group of BCL family proteins called regulated apoptosis initiators [e.g. BAD, BIM, BID, Puma, Noxa]. These proteins contain only one BH domain, and hence are sometimes called BH3-only proteins. The activity of BH3-only proteins is modulated by sensors of cellular stress and damage; when upregulated and activated, they can initiate apoptosis.

⚔ When cells are deprived of survival signals, suffer DNA damage, or develop ER stress due to the accumulation of misfolded proteins, BH3-only proteins are upregulated through increased transcription and/or post-translational modifications (e.g., phosphorylation).
⚔ These BH3-only proteins in turn directly activate the two critical pro-apoptotic family members, BAX and BAK, which form oligomers that insert into the mitochondrial membrane and allow proteins from the inner mitochondrial membrane to leak out into the cytoplasm.
⚔ BH3-only proteins may also bind to and block the function of BCL2 and BCL-XL. At the same time, synthesis of BCL2 and BCL-XL may decline because their transcription relies on survival signals. The net result of BAX-BAK activation coupled with loss of the protective functions of the anti-apoptotic BCL2 family members is the release into the cytoplasm of several mitochondrial proteins such as cytochrome c that can activate the caspase cascade.

About the domains…
“Domain” refers to specific regions or segments within a protein that have distinct structural or functional properties.

For the anti-apoptotic proteins BCL2, BCL-XL, and MCL1, the “BH domains” (BH1-4) are particular regions within these proteins that are crucial for their function. Here’s a breakdown:

BH Domains (BCL-2 Homology Domains): These are conserved regions found in BCL-2 family proteins. The BH domains are important for the protein’s ability to regulate apoptosis. There are four BH domains, named BH1, BH2, BH3, and BH4.
🧪 BH1 and BH2: These domains are involved in the formation of the hydrophobic groove that binds to the BH3 domain of pro-apoptotic proteins.
🧪 BH3: This domain is critical for the interaction between pro-apoptotic and anti-apoptotic Bcl-2 family members.
🧪 BH4: This domain is often involved in the anti-apoptotic function of the protein.

These domains enable the anti-apoptotic proteins to interact with other proteins and maintain the integrity of the mitochondrial outer membrane, thereby preventing the release of cytochrome c and other pro-apoptotic factors into the cytosol, which would otherwise trigger cell death.

Question 40

Discuss the extrinsic pathway of apoptosis.

Answer 40

(1) Initiation: The process begins when specific extracellular death ligands, such as Fas ligand (FasL) or Tumor Necrosis Factor (TNF), bind to their corresponding death receptors on the cell surface.

(2) Formation of DISC: This binding leads to formation of the Death-Inducing Signaling Complex (DISC). The DISC is a multi-protein complex that induces the death receptors, adaptor proteins like FADD (Fas-Associated Death Domain), and initiatior caspases such as caspase-8.

(3) Activation of caspases: The formation of the DISC results in the activation of initiator caspases (e.g. caspase-8). These caspases then activate executioner caspases (e.g. caspase-3), which dismantle the cell by cleaving various cellular components.

(4) Execution phase: The activated executioner caspases degrade key structural and regulatory proteins within the cell, leading to the characteristic morphological changes of apoptosis, such as cell shrinkage, chromatin condesation, and DNA fragmentation.

[Diagram 1] [Diagram 2] [Diagram 3] [Diagram 4]

Question 41

Pyogenic bacterial infections and ischemic brain infarcts most characteristically produce which type of necrosis?
(a) Liquefactive
(b) Caseous
(c) Coagulative
(d) Fat
(e) Fibroid

Answer 41

(a) Liquefactive

Further notes:
The term pyogenic refers to the production of pus. It is often used to describe bacteria or infections that lead to the formation of pus.

Question 42

Which of the following is the earliest change seen in irreversible cell injury?
(a) Mitochondrial change
(b) Myelin figures
(c) Nuclear pyknosis
(d) Karyorrhexis
(e) Cell membrane blebs

Answer 42

(a) Mitochondrial change

Further notes:
The earliest changes in irreversible cell injury typically involve the mitochondria. These changes include mitochondrial swelling, the appearance of amorphous densities, and the loss of mitochondrial membrane potential. These mitochondrial alterations are critical because they lead to the failure of ATP production, which is essential for cell survival.