A morphologically characterised process of programmed cell-death, initiated by various physiological or pathological causes (e.g. cell turnover, hormone-induced atrophy, cell-mediated immune cytolysis, tumour regression), that is characterised by shrinkage of the nucleus and cytoplasm, cell fragmentation and phagocytosis. Apoptosis is contrasted with necrosis, which is a random pathological process initiated by irreversible cell damage. Apoptosis is controlled by extracellular signals or the removal of extracellular suppressors of cell death and is mediated intracellularly by a death-inducing signalling complex (DISC).