Apoptosis: a product of programmed and unprogrammed cell death.

Apoptosis is a form of cell death defined by morphological and biochemical characteristics. Although originally described in 1972, it is only very recently that significant interest in the subject has occurred, possibly as a result of the identification of genes that may either positively or negatively regulate the process. With the rapid expansion of knowledge, it has become apparent that there are multiple pathways that induce apoptosis; some of these may represent programmed events but others are clearly unprogrammed. To clarify the terminology used, it is recommended that apoptosis be used as originally defined to refer only to the end product of these pathways. Furthermore, the realization that a cell can die by multiple pathways suggests caution when translating experimental results. Many potential intermediates may be identified but they may represent components of different pathways. Concern for the use of various inhibitors of apoptosis is also presented. Future directions will be aimed at the definitive identification of the signal mechanisms regulating apoptotic cell death.