Apoptosis is a regulated form of programmed cell death that shapes tissues during development, maintains adult tissue balance, and removes damaged or dangerous cells. It is distinct from necrosis, which is an uncontrolled, often inflammatory process. In apoptosis, cells shrink, their DNA is fragmented in an orderly way, and the cell is packaged into membrane-bound fragments that are efficiently cleared by phagocytes without triggering inflammation.

Two main molecular pathways control apoptosis. The extrinsic pathway begins at the cell surface when death ligands bind to death receptors, forming signaling complexes that activate initiator caspases, particularly caspase 8. The intrinsic or mitochondrial pathway responds to internal stress such as DNA damage, growth factor withdrawal, or oxidative stress. This pathway is governed by the Bcl 2 family, which includes both pro apoptotic and anti apoptotic members. When pro apoptotic signals prevail, mitochondrial outer membrane permeabilization occurs, releasing cytochrome c that helps form the apoptosome and activates caspase 9.

Both pathways converge on effector caspases, including caspases 3, 6, and 7, which cleave key structural and regulatory proteins, driving the morphological changes of apoptosis. The tumor suppressor p53 links cellular stress to apoptosis by inducing pro apoptotic genes and repressing survival factors. Disruption of apoptotic regulation is central to disease. Insufficient apoptosis promotes cancer by allowing survival of genetically unstable cells, while excessive apoptosis contributes to neurodegenerative disorders and some immune diseases. Because of this dual role, components of apoptotic pathways, such as Bcl 2 family proteins and caspases, are active targets for anticancer drugs and for therapies aimed at protecting cells in degenerative conditions.