Apoptosis

Apoptosis is a tightly regulated form of cell death that a cell carries out on itself, triggered either by signals from other cells or by internal damage it cannot repair. The cell shrinks, its chromatin condenses, its DNA is cut into fragments, and its membrane bulges into blebs before the cell breaks into small, membrane-wrapped pieces called apoptotic bodies. Macrophages and neighboring cells quickly engulf these pieces, so the process causes no inflammation. This sets it apart from necrosis, in which injured cells swell and burst.

The work is done by caspases, protein-cutting enzymes that switch one another on in a cascade. They can be activated through the intrinsic pathway, when stress makes mitochondria release cytochrome c, or through the extrinsic pathway, when death receptors such as Fas on the cell surface are triggered.

Apoptosis shapes the developing embryo, for example by removing the tissue between the fingers, and in adults it clears infected, damaged or self-attacking immune cells. Too little apoptosis helps cancers grow, while too much contributes to neurodegenerative diseases. The term was introduced in 1972 from a Greek word for falling off, as leaves fall from a tree.