ABSTRACT
Introduction Homeostasis of tissues is regulated by a balance between cell death and cell proliferation. Disturbances in this rheostat of cellular turnover may contribute significantly to carcinogenesis. Pathological longevity of cells is a consequence of blocks in programmed cell death (apoptosis). Apoptosis is an evolutionarily conserved form of controlled cellular self-destruction, in contrast to necrosis, which is a form of cell death consequent to acute cellular injury. Apoptosis has many important physiological functions: ■ It is essential for successful embryonic development and for maintenance
of normal cellular homeostasis. During development, many cells are produced in excess and are eliminated through apoptosis: – In a developing embryo, the cells between the fingers undergo apoptosis
so that the fingers can separate. – In the fetal brain, half of the original neurons die in later stages when
the adult brain is formed. ■ Tissue homeostasis relies on apoptosis to eliminate cells that have com-
pleted their life cycle, as in the skin, intestinal mucosa, and blood cells. ■ Apoptosis also plays a pivotal role in the regulation of the immune
system: ineffective cells (the majority of freshly made T lymphocytes fail to connect with foreign antigens) and those showing high affinity for self-antigens are committed to death through apoptosis, allowing tight control over the pool of highly efficient but not self-reactive immune cells.
