What are the mechanisms of apoptosis?

Apoptosis is programmed cell death. It is a normal and highly regulated process that takes place to intentionally eliminate damaged or unused cells from the body in order to maintain homeostasis

There are two main mechanisms of apoptosis –intrinsic and extrinsic mechanisms. Both mechanisms are highly sophisticated and complex, involving a cascade of molecular events. Recent studies suggest that the two mechanisms are linked with molecules in one mechanism being able to influence the other. 

Intrinsic mechanism, also known as the mitochondrial mechanism, is activated when the cell is stressed internally. This internal stress may be caused by several different factors including but not limited to DNA damage from exposure to X-Rays or UV light, hypoxia, chemotherapeutic agents, or buildup of misfolded proteins inside the cell. 

When the cell is stressed due to any of the above factors, it transmits signals that induce mitochondria to release apoptosis-inducing proteins. This leads to the degradation and condensation of the organelles and other cellular components, which causes the cell to shrink and then break down into smaller membrane-enclosed fragments known as apoptotic bodies. Simultaneously, the cell also transmits distress signals to macrophages, which completely clean away the shrunken apoptotic cells, leaving no trace behind. This prevents the apoptotic cells from being able to cause any cellular damage. 

Extrinsic mechanism, or death receptor mechanism, is activated when the cell receives death signals from other cells such as when chemical substances bind to specific receptors on the surface of the cell. White blood cells use the extrinsic pathway to combat infection and activate apoptosis in cells that are infected.