Apoptosis is an actively regulated process of cell death necessary for proper control of tissue growth. The phenomenon is characterized by plasma membrane blebbing, cell shrinkage, chromatin condensation, and degradation of DNA. Apoptosis is distinctly different from necrosis. Unlike necrosis, which is a passive form of cell death, apoptosis appears to be an active physiological process using a controlled genetic “program” of gene expression. Furthermore, apoptosis does not involve severe tissue damage or inflammation. Membrane integrity is preserved, and there is no loss of cellular contents before phagocytosis. Apoptosis is also distinct from necrosis in that it can be triggered or suppressed by tissue-specific hormones and growth factors.

Apoptosis plays a critical role in tissue homeostasis by counterbalancing mitosis. It is an essential component of many physiological processes, including embryonic development, clonal selection in thymocytes, and in protection against disease.