| Apoptosis, a form of programmed cell death, occurs during normal cellular development
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| It is a fundamentally important biological process that is required to maintain the integrity and homeostasis of multicellular organisms. Whereas inappropriate apoptosis can lead to degenerative conditions, such as neurodegenerative disorders, subversion and disruption of the apoptosis machinery can result in cancer (Table 41.1) or autoimmune disease. Cells that die from damage typically swell and burst (necrosis), but apoptosis is characterized by dramatic morphologic changes in the cell, including shrinkage, chromatin condensation, cleavage, and disassembly into membrane-enclosed vesicles called apoptotic bodies that undergo rapid phagocytosis by neighboring cells. It is the accumulation of sterols in the plasma membrane and translocation of phosphatidyl-serine to the outer leaflet of the plasma membrane that serve to flag the apoptotic cell for elimination via phagocytosis by neighboring cells or macrophages.
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| Recent research has elucidated many of the biochemical signals that regulate apoptosis, and has identified two key families of proteins that appear to be central to the regulation of mammalian cell death by apoptosis. These are the family of cysteine proteases called caspases, and the B cell lymphoma protein 2 (Bcl-2)-related family members, which serve to modulate cell survival by regulating caspase activity.
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