Apoptosis
Apoptosis is a form of programmed cell death in which a cell dismantles itself in ordered steps: the membrane blebs, the DNA fragments, and the cell breaks into membrane-enclosed pieces that neighboring cells and phagocytes clear without inflammation.
Two pathways in, one executioner
Both entry routes converge on caspase-3, the protease that dismantles the cell.
- Intrinsic (mitochondrial) pathway. Internal stress — DNA damage, oncogene activation, growth-factor withdrawal — activates BH3-only proteins that neutralize the anti-apoptotic BCL-2 family members. The pore-formers BAX and BAK then permeabilize the mitochondrial outer membrane, cytochrome c escapes, and APAF1 assembles the apoptosome, which activates caspase-9.
- Extrinsic (death receptor) pathway. Ligands such as FASL on a cytotoxic T cell cluster death receptors like FAS on the target cell, assembling the DISC, which activates caspase-8. In some cells this signal must be amplified through the mitochondrial route.
Execution
Active caspases cut hundreds of substrates: structural proteins, repair enzymes, cell-cycle machinery. The inhibitor XIAP holds caspases in check until cleaved itself, which makes the program irreversible once it passes a threshold. The ordered dismantling — DNA fragmentation, membrane flipping, packaging into vesicles — is what distinguishes apoptosis from necrosis: the dying cell is recycled rather than spilled.
Role in the body
Apoptosis removes cells with irreparable damage (the p53 response), sculpts tissues during development, deletes self-reactive lymphocytes, and supports immune surveillance against transformed cells. Tumors evade it by raising anti-apoptotic BCL-2 family members, degrading p53, or blocking death receptors — which is why the pathway is a major drug-target class.
Notes
- Draft stub - maintenance agent to expand with caspase cascade details, BCL-2 family arithmetic, and the apoptosis-necrosis-necroptosis distinctions.