Apoptosis

What is apoptosis?

Apoptosis is a form of regulated cell death in which cellular signaling activates a controlled dismantling process. Characteristic changes include condensation of nuclear material and fragmentation into membrane-bound fragments called apoptotic bodies that can be cleared by other cells.

How it works

The 2018 Nomenclature Committee on Cell Death recommendations distinguish intrinsic apoptosis, involving mitochondrial membrane changes, from extrinsic apoptosis initiated through cell-surface signaling. Executioner caspases contribute to the dismantling process.

Apoptosis can occur during normal development or in response to damage. It is not a synonym for every type of regulated cell death. The committee uses programmed cell death more narrowly for physiological settings.

Examples of apoptosis

Separating developing digits

During limb development, cells in the tissue between forming digits are removed. In the developing mouse paw, this helps separate the individual digits from an initially paddle-like structure. This is a normal developmental use of cell death, rather than a response to an injury. Alberts and colleagues illustrate the process.

Removing self-reactive immune cells

Some immature lymphocytes have receptors that react strongly to the body’s own molecules. Apoptosis can remove such cells before they join the mature immune-cell population. This selective removal is one route to immune tolerance. Other outcomes, including changing a receptor or becoming unresponsive, are also possible; self-reactivity does not mean every cell follows the same route. Janeway and colleagues describe these different fates.

Why it matters

Controlled cell removal is part of normal development and can also respond to damage. Other cells often clear the fragments promptly, although the process does not always occur without an immune response. In disease research, identifying how cells died matters: cell loss alone does not tell us whether apoptosis or another process caused it.