Apoptosis is the process of programmed cell death that occurs in multicellular organisms. Different from necrosis, in which cells die due to injury, apoptosis is a highly regulated and controlled process that confers advantages during an organism's life cycle. Unlike necrosis, apoptosis produces cell fragments called apoptotic bodies that phagocytic cells are able to engulf and remove before the contents of the cell can spill out onto surrounding cells and cause damage to them.
Apoptosis can be initiated through one of two pathways. In the intrinsic pathway the cell kills itself because it senses cell stress, while in the extrinsic pathway the cell kills itself because of signals from other cells. Weak external signals may also activate the intrinsic pathway of apoptosis. Both pathways induce cell death by activating caspases, which are proteases, or enzymes that degrade proteins. The two pathways both
activate initiator caspases, which then activate executioner caspases,
which then kill the cell by degrading proteins indiscriminately.