3. Irreversible Cell Injury

Define irreversible injury → ‘point of no return’

When a stressor is too severe or prolonged, the cell passes a "point of no return" and undergoes cell death. Two critical events define this transition:

1. Irreversible Mitochondrial Dysfunction:

   • Profound, permanent loss of oxidative phosphorylation and ATP generation.

   • Release of pro-apoptotic proteins (e.g., Cytochrome c) into the cytosol.

2. Profound Membrane Damage:

   • Plasma Membrane: Loss of cellular contents and severe disruption of osmotic balance.

   • Lysosomal Membrane: Leakage of highly destructive enzymes (RNases, DNases, proteases) into the cytoplasm, leading to enzymatic digestion of the cell components.

Accidental vs regulated cell death?

• Accidental Cell Death (ACD): Uncontrolled, unprogrammed death in response to severe injury (e.g., severe ischemia, burns, toxins). Necrosis is the primary morphological manifestation.

• Regulated Cell Death (RCD): A genetically and biochemically programmed process executed by specific signaling pathways. Examples include Apoptosis, Necroptosis, and Pyroptosis.

Explain necrosis

• Definition: The morphological manifestation of unprogrammed, accidental cell death (ACD) in living tissue. Always pathologic.

• Key Features:

  • Cell swelling (Oncosis).

  • Loss of membrane integrity → cellular contents leak out.

  • Elicits a prominent inflammatory response (due to leaked Damage-Associated Molecular Patterns - DAMPs).

  • Enzymatic digestion of the cell (via autolysis from its own lysosomes or heterolysis from invading neutrophils).

• Nuclear Changes (Hallmarks):

  1. Pyknosis: Nuclear shrinkage and increased basophilia (dark staining).

  2. Karyorrhexis: Fragmentation of the pyknotic nucleus.

  3. Karyolysis: Fading/dissolution of the nucleus (due to DNAse activity).

• Note on Oncosis: Oncosis (ischemic cell swelling) is technically the pre-lethal pathway leading to necrosis; "necrosis" refers to the dead tissue left behind.

Explain apoptosis

• Definition: Regulated cell death (RCD) driven by the activation of caspases. Can be physiologic or pathologic.

• Key Features:

  • Cell shrinkage.

  • Chromatin condensation and fragmentation.

  • Plasma membrane remains intact (but forms blebs).

  • Formation of apoptotic bodies (membrane-bound fragments containing organelles).

  • NO inflammation. (Apoptotic bodies are rapidly phagocytosed by macrophages before contents leak).

• Pathways:

  • Intrinsic (Mitochondrial) Pathway: Triggered by cellular stress/DNA damage. Cytochrome c leaks from mitochondria → activates Caspase-9. (Regulated by Bcl-2 family proteins).

  • Extrinsic (Death Receptor) Pathway: Triggered by external signals (e.g., FasL binding to Fas receptor, or TNF binding to TNFR). Activates Caspase-8.

Explain necroptosis

• Definition: A hybrid form of cell death. It is programmed/regulated (like apoptosis) but morphologically resembles necrosis (cell swelling, membrane rupture, inflammation).

• Mechanism:

  • Triggered by death receptors (e.g., TNFR1) when Caspase-8 is inhibited or deficient.

  • Depends on the activation of kinases RIPK1 and RIPK3.

  • RIPK1/3 form a complex (the necrosome), which phosphorylates MLKL.

  • MLKL oligomerizes, inserts into the plasma membrane, and causes fatal pores/rupture.

• Clinical Significance: Prominent in ischemia-reperfusion injury, viral infections (where viruses block caspases), and inflammatory bowel disease.

Explain pyroptosis

• Definition: A highly inflammatory form of regulated cell death, typically triggered by intracellular microbial infections.

• Mechanism:

  • Microbial products activate the inflammasome (a cytosolic multi-protein complex).

  • Inflammasome activates Caspase-1.

  • Caspase-1 does two things:

    1. Cleaves and activates pro-inflammatory cytokines (IL-1β and IL-18).

    2. Cleaves Gasdermin D.

  • Gasdermin D inserts into the plasma membrane, forming pores that lead to cell swelling, membrane rupture, and massive release of cytokines.

Explain autophagy ‘self-eating’

• Definition: A survival mechanism during nutrient deprivation where the cell digests its own organelles for energy. If stress is too severe, it culminates in cell death.

• Mechanism:

  • Intracellular organelles are enclosed in a double-membrane vacuole (autophagosome).

  • The autophagosome fuses with a lysosome to form an autophagolysosome.

  • Lysosomal enzymes degrade the contents, recycling metabolites for cell survival.

Which forms of cell death are inflammatory?

• necrosis

• necroptosis

• pyroptosis

Which forms of cell death involve swelling vs shrinkage?

Swelling = necrosis, necroptosis, pyroptosis, oncosis

Shrinkage = apoptosis

What caspase is unique to pyroptosis?

Caspase-1 (activated by inflammasome)