The biology and pathology of programmed cell death (apoptosis).

Apoptosis is a process by which cells die in a controlled and programmed manner in response to specific stimuli, often following extrinsic and intrinsic signals which ultimately cause the "switching on" of cell death regulatory genes. Condensation of chromatin and cytoplasm, fragmentation of the cell and formation of membrane-bound bodies containing intact organelles (apoptotic bodies), and phagocytosis of these bodies by resident cells are the major structural changes associated with apoptosis. Biochemically, activation of a nonlysosomal endonuclease is a cardinal feature of this mode of cell death. Several genes have been implicated in the execution of apoptosis. A signal transduction mechanism is suspected to regulate the phenomenon. Although apoptosis is widely considered as an adaptive response to physiological or near physiological stimuli, several noxious agents can initiate the reaction and thus it is often a toxicological response.