Apoptosis is programmed cell suicide: a caspase enzyme cascade dismantles the cell from within in a controlled way, so it shrinks, buds off membrane-bound "apoptotic bodies" and gets quietly eaten by neighbours — no membrane rupture, no inflammation (unlike necrosis).
Caspase-3 activation (simplified rate model):
dC/dt = k · S · (1 − I) · (1 − C) − k_off · C
S = stimulus (0–1), I = inhibitor level (0–1, e.g. Bcl-2/IAP)
k ≈ 0.9 /s (extrinsic, direct caspase-8 → 3)
k ≈ 0.5 /s (intrinsic, needs MOMP + apoptosome first)
Fragmentation trigger: C(t) ≥ 0.85 → membrane blebbing → cell buds
into apoptotic bodies, each engulfing a piece of condensed chromatin.
- Pathway — extrinsic: a death ligand (Fas/TRAIL) binds its receptor, forming the DISC, which activates caspase-8 directly. Intrinsic: internal stress (DNA damage, ER stress) tips the Bax/Bak–Bcl-2 balance, permeabilizing the mitochondrial outer membrane (MOMP), releasing cytochrome c, which builds the apoptosome and activates caspase-9 — slower, with a visible mitochondrial-glow step.
- Death stimulus — strength of the initiating signal; higher values push caspase activation faster and higher.
- Bcl-2 / IAP inhibitor — anti-apoptotic proteins that resist the cascade; high enough levels can stall it below the fragmentation threshold (this is exactly how many cancers evade apoptosis).
- Trigger / Reset — start the cascade from a healthy cell, or snap back to the resting state at any time.
Watch the nucleus: chromatin condenses and brightens (pyknosis) as caspases activate nucleases, then the whole cell blebs and fragments into apoptotic bodies that drift apart and fade — the phagocytic-clearance step that keeps apoptosis "clean" compared to necrotic cell death.