Apoptosis

Apoptosis is a mechanism of programmed cell death involved in the regulation of tissue maintenance and development of organisms. It is a defense mechanism to remove excess or dangerous cells.

In this process, the chromatin condenses, cells individualize and many organelles remain intact and metabolically active for a long period. The nuclear and plasma membranes are destroyed and the nucleus is then broken up into fragments that are surrounded by the cytoplasmic membrane, producing apoptotic bodies that are degraded by macrophages. This mechanism is very distinct of necrosis, because it has no inflammatory reaction.

There are extrinsic and intrinsic apoptosis. The extrinsic pathway is triggered by the ligation of extracellular apoptotic signals to receptors in plasma membrane that lead to activation of proteins caspases in a sequence called cascade that activate other caspases or the release of cytochrome c  by the mitochondria. Intracellular apoptotic signals activate the mitochondria to release cytochrome c that indirectly activate many caspases. These proteins activate endonucleases and proteases that degrade DNA and proteins, leading to morphological changes of cells, formation of apoptotic bodies and, ultimately, cell death.

Apoptosis is a reversible process, because the blocking of apoptotic genes enhance cell survival when they are subjected to weak apoptotic signals, mutations in killer genes allow the survival of programmed cells and elimination or inhibition of macrophages result in the survival of cells.