Apoptosis is the programmed self-destruction of specific cells within a multicellular organism. This process is controlled by proteolytic enzymes known as caspases which, when specific cell signals are present, dismantle the cell from within through cleavage of proteins in the cytoplasm and nucleus. The cell is broken into fragments, known as apoptotic bodies, which are then engulfed by phagocytes to prevent them damaging surrounding cells. This differs from necrosis, as it is programmed cell death, rather than unintended death through cellular damage.
Apoptosis is an inexorable process, meaning it cannot stop once it has begun; because of this apoptosis is highly regulated. It can be initiated through two separate pathways; the intrinsic pathway occurs when internal cell stress causes the self-destruction, whereas the extrinsic pathway occurs when external signals from other cells cause the onset of apoptosis.
Many different aspects of multicellular life rely on apoptosis, with the average human losing an estimated 50-70 billion cells each day through the process. Examples of apoptosis include eliminating cells damaged beyond recovery, removal of cells between digits of a developing hand allowing the formation of fingers, or activating onset of apoptosis in tumour cells as a form of non-surgical cancer treatment.