Cell Death

Question 1

Describe and explain the cause of the morphological features of necrosis

Answer 1

Necrosis is uncontrolled cell death that happens in living tissues (like during injury, infection, or lack of oxygen). The dead cells show a series of visible changes under the microscope.

Morphological changes following cell death in living tissue:
1. Increased eosinophilic staining-denatured protein and loss of RNA
2. Vacuolation-digested cytoplasmic organelles
3. Swelling of ER and mitochondria
4. Myelin figures-whorls of phospholipid from damaged membranes
5. Discontinuous plasma and organelle membranes
6. Nuclear change due to the breakdown of DNA and chromatin
- Pyknosis-nuclear shrinkage and increased basophilia (condensed)
- Karyorrhexis-nuclear fragmentation
- Karyolysis-decreased basophilia from DNA breakdown

Question 2

Describe the different types of necrosis, with examples

Answer 2

1. Coagulative
   - Structure maintained, cells dead
   - Protein denaturation
   - Occurs in heart & solid organs
   - associated with severe ischemia
2. Liquefactive
   - Dead cells digested
   - ischemia in brain
   - inflammation
   - Due to bacterial infection
3. Fibrinoid
   - Necrosis associated with leakage of fibrin and inflammatory cells
   - Associated with damage to blood vessels in response to the deposition of immune complexes
4. Fat
   - Refers to focal areas of fat destruction
   - Enzymes liquefy membranes of fat cells
   - Release fatty acids which combine with calcium to cause patchy white lesions (fat saponification)
   - Associated with acute pancreatitis
5. Gangrenous
   - Coagulative necrosis involving multiple tissue planes
   - Liquefactive/more digestion with bacterial infection
   - Wet gangrene
   - Associated with necrosis in a lower limb which has lost its blood supply
6. Caseous
   - Fragmented lysed cells with amorphous granular appearance
   - Tissue architecture obliterated
   - Associated with infection with mycobacterium tuberculosis
   - Large numbers of organisms and degenerating tissue

Question 3

Describe the two main pathways that lead to apoptosis

Answer 3

1. Intrinsic
   - BAX/BAK are activated - pro-apoptotic enzymes
   - BCL-2/BCL-XL are inhibited - anti-apoptotic enzymes
   - BAX/BAK will create pores in mitochondria which releases cytochrome C and activates caspases for apoptosis
2. Extrinsic
   > Death-receptor binding
   - Infected cells have MHC-I (pathogen fragment molecules) indicate cell is infected to T cells
   - FAS-L on cytotoxic T cells (immune cells) binds to FAS receptor on infected cells
   - activates caspase 8 and active caspase 8 leading to apoptosis
   > Perforin and Granzyme release
   - Cytotoxic T-cells create holes in cell through perforin (like a drill)
   - T cells release granzymes through perforins
   - granzymes activate caspase 8 and cause apoptosis

Question 4

Explain the key differences between apoptosis and necrosis

Answer 4

Necrosis
- Cell size: Swell
- Nucleus: Pyknosis/Karyohexis/Karyolysis
- Plasma membrane: Disrupted
- Cellular contents: Enzymatic digestion, may leak
- Inflammation?: Yes
- Pathologic

Apoptosis:
- Cell size: Shrink
- Nucleus: Pyknosis/Karyohexsis
- Plasma membrane: intact but altered
- Cellular contents: intact, released by apoptotic bodies
- Inflammation?: No
- Physiologic mainly

Question 5

Be aware of other mechanisms of cell death (necroptosis, ferroptosis, pyroptosis) and survival (autophagy)

Answer 5

1. Necroptosis
   - programmed necrosis
   - When virus blocks apoptosis
   - Occurs by RIPK1, RIPK3, MLKL proteins
2. Ferroptosis
   – a form of cell death associated with high levels of iron and excess ROS causing lipid peroxidation
   → Destroys cell membranes from the inside out.
3. Pyroptosis
   a form of cell death associated with the release of fever-induced IL-I
   - A very inflammatory form of cell death
   - The cell swells, bursts, and releases fever-causing chemicals like IL-1β (interleukin-1 beta).
   - Usually happens in response to infections — the cell sacrifices itself to alert and activate the immune system.

Autophagy:
- Autophagy – a survival mechanism induced under stress conditions by recycling metabolites

Steps:
Detection: The cell senses damaged organelles, proteins, or is under stress (like starvation).

Packaging: A special double-membrane bubble called an autophagosome forms around the junk.

Fusion: The autophagosome fuses with a lysosome (which contains digestive enzymes).

Digestion: The lysosome breaks down the contents into basic building blocks (amino acids, fatty acids, etc.).

Recycling: These pieces are reused to build new cell parts or make energy.

Question 6

Explain the origins of intracellular accumulations

Answer 6

Intracellular Accumulations:
- Accumulations within cells occur when a cell is unable to metabolise a substance, causing it to accumulate within the cytoplasm, organelles, or nucleus of the cell

Why accumulate?
- A normal endogenous substance is produced at a normal or increased rate, but the rate of metabolism is inadequate to remove it
- Accumulation due to defects in folding, packaging, or degradation, typically due to mutation
- Failure to degrade due to enzyme deficiency (mutation)
- Deposition of exogenous substance (foreign substances enter cell)

Types of accumulations:
1. Water
- transport proteins/pumps fail (Na/K pump)
- Vacuolated (hydropic) of cytoplasm
2. Fat
- Liver cells (process fat)
- called steatosis or fatty change
- fat droplets push nucleus to the side
- caused by alcohol (toxin) or obesity (nutritional)
3. Proteins -
4. Cholesterol
- macrophages and smooth muscle cells take up cholesterol creating foamy look
- foam cells clump together forming plaques
- can cause atherosclerosis
5. Exogenous -
- Caused by carbon/soot from air pollution
- taken up by alveolar cells of lungs
- causes arachosis (black pigmentation of lungs)
6. Endogenous
> Lipofuscin
- In aging cells
- Yellow-brown pigment cells due to ROS buildup
> Hemosiderin
- Iron storage molecule that accumulates because of blood breakdown
- Brownish granules appearance

Question 7

Describe physiologic and pathological induction of apoptosis

Answer 7

1. Physiological induction of apoptosis:
   - Embryogenesis
   - Involution
   - Cell loss in proliferating cell population
   - Elimination of cells that have reached their ‘used by date’
   - Self-reactive T-lymphocytes
2. Pathological induction of apoptosis:
   - DNA damage
   - Accumulation of misfolded protein
   - Cell death in infection
   - Pathologic atrophy

Question 8

Describe phagocytosis

Answer 8

1. Phagocytosis
   - A form of endocytosis in which specialised endocytic vesicles (phagosome) are used to ingest large organisms (Microbes or dead cells)
   - Phagocytes
   > Macrophages-professional phagocytes that reside in tissues throughout the body
   > Neutrophils-short-lived cells, abundant in blood but not present in normal tissues