Apoptosis is a regulated form of cell death that removes damaged, superfluous or potentially dangerous cells without provoking inflammation. It is essential for embryonic development, tissue homeostasis and immune system function, and its disruption contributes to cancer, neurodegenerative diseases and autoimmunity.

Morphologically, apoptotic cells shrink, condense their chromatin, fragment their DNA and form membrane bound apoptotic bodies that are cleared by phagocytes. Biochemically, apoptosis is orchestrated by caspases, a family of cysteine proteases produced as inactive zymogens and activated in cascades.

Two main pathways initiate apoptosis. The intrinsic or mitochondrial pathway responds to intracellular stress such as DNA damage, hypoxia or growth factor deprivation. It is controlled by Bcl 2 family proteins that balance pro survival members against pro apoptotic members like Bax and Bak. When the balance tips toward death, mitochondria release cytochrome c, which combines with Apaf 1 and procaspase 9 to form the apoptosome, activating executioner caspases such as caspase 3.

The extrinsic or death receptor pathway is triggered by extracellular ligands such as FasL or TNF binding to receptors on the cell surface. This leads to assembly of a death inducing signaling complex that activates initiator caspase 8, which can directly activate executioner caspases or engage the mitochondrial pathway via Bid cleavage.

Cells also integrate survival signals through kinases like PI3K Akt and transcription factors such as NF kappa B, which upregulate anti apoptotic proteins and inhibit caspases. Many cancers acquire mutations that block apoptosis, for example in p53 or Bcl 2, while some therapies seek to restore or enhance apoptotic responses to eliminate malignant cells.