The pathway by which apoptosis is induced vary depending on the stimulus and cell type, and the sequence of events that result in the ultimate cellular changes is far from clear. This pattern contrasts with the diffuse smear pattern of random DNA breakdown that occurs in necrosis. It is mediated by a calcium sensitive endonuclease. This enzyme causes cross-linking of cytoplasmic proteins, forming a shell under the plasma membrane similar to that of keratinised squamous cells. Some apoptosis specific genes have been identified.
Some oncogenes inhibit apoptosis and extend cell life and others stimulated it.
It seems that p53 is required for apoptosis following DNA damage by irradiation, however apoptosis induced by glucocorticoids or aging is p53 independent. It should be noted that in many forms of apoptosis new gene expression is not required and indeed inhibition of gene expression causes apoptosis. Summary Apoptosis is a distinctive form of cell death manifested by characteristic chromatin condensation and DNA fragmentation, whose function is the deletion of cells in normal development, organogenesis, immune function and tissue growth, but which can also be induced by pathologic stimuli. In some instances gene transcription and protein synthesis are required for induction of apoptosis and the process is regulated by a set of genes that are involved in normal cell growth and differentiation.