apoptosis (programmed cell death) The process of cell death that occurs naturally as part of the normal development, maintenance, and renewal of tissues within an organism. During embryonic development it plays a vital role in determining the final size and form of tissues and organs. For example, the fingers are ‘sculpted’ on the spadelike embryonic hand by apoptosis of the cells between them; the tubules of the embryonic kidney are hollowed out by a similar process. Apoptosis follows an orderly sequence of events, and involves the action of enzymes called caspases. These are cellular proteins that when activated cleave target proteins within the cell, including other ‘executioner’ enzymes, which digest the cytoskeleton, DNA, and other cell components. The cell shrinks as apoptosis proceeds and components are dismembered, until finally the cell is condensed into fragments called apoptotic bodies. These are degraded by scavenging phagocytic cells. Apoptosis is normally suppressed as long as cells continue to receive extracellular survival signals, in the form of trophic factors – an example being nerve growth factor. In the absence of such signals, the cell embarks on a ‘suicide’ programme. Sometimes, other cells, for example immune cells, release specific ‘murder’ signals, which activate apoptosis in target cells. Cancer is associated with the suppression of apoptosis (see also growth factor), which also occurs when viruses infect cells – in order to inhibit the activity of killer cells. Apoptosis differs from cell necrosis, in which cell death may be stimulated by a toxic substance.