The term apoptosis, an ancient Greek word used to describe the "falling off" of petals from flowers or leaves from trees, was proposed by Kerr, Wyllie and Currie in 1972 to refer to the peculiar morphology of physiologically occurring cell death which plays a complementary but opposite role to mitosis in the regulation of animal cell populations. Apoptosis is opposed to necrosis-the appearance of accidental and pathological cell death. Apoptosis involves loss of microvilli, smooth-surfaced protuberances, chromatin condensation, nuclear and cytoplasmic condensation, loss of cell volume, and nuclear fragmentation. At an early stage, condensed chromatin tends to marginate in crescents around the nuclear envelope in most cell types, but in certain cells such as thymocytes, it often occupies much of the nuclear volume. Contrasted to necrosis, in apoptotic cells there are neither swelling and rupture of cytoplasmic organelles and plasma membranes nor inflammatory reaction. Apoptotic cells break up into membrane bound apoptotic bodies and they are phagocytosed by nearly resident tissue cells. These morphological changes are often accompanied by the internucleosomal DNA fragmentation. Apoptosis is a representative morphology of programmed cell death (PCD) which occurs within a developmental context in response to a definable physiological stimulus and requires de novo gene expression. However, apoptosis should not be considered synonymous with PCD. Because, there are examples of non-apoptotic PCD and pathological stimuli such as mild cell injury can induce apoptosis.