How cells die: Apoptosis pathways

Generally speaking, there are 2 types of cell death: apoptosis and necrosis . Necrotic cell death is considered an accidental type or death, caused by gross cell injury, and results in the death of groups of cells within a tis sue. In contrast, apoptotic cell death may be induced or Is preprogrammed i nto the cell (eg, during development) and results in the death of the indiv idual cells. Apoptotic cells may be characterized by specific morphologic a nd biochemical changes orchestrated by a family of cysteine proteases known as caspases. At the molecular level, apoptosis is tightly regulated. There are 2 main pathways to apoptotic cell death. One involves the interaction of a death receptor, such as the TNF receptor-1 or the Fas receptor with it s ligand, and the second pathway depends on the participation of mitochondr ia. Proapoptotic and antipoptotic members or the Bcl-2 family regulate the mitochondrial pathway. The end result of either pathway is caspase activati on and the cleavage of specific cellular substrates, resulting in the morph ologic and biochemical changes associated with the apoptotic phenotype.