Apoptosis is a cell suicide mechanism that enables metazoans to control cell number in tissues and to eliminate individual cells that threaten the animal's survival. Certain cells have unique sensors, termed Death Receptors, on their surface, which detect the presence of extracellular death signals and, in response, rapidly ignite the cell's intrinsic apoptosis machinery. Cytotoxic T cell with Fas ligand binds to the Fas receptor on target cell and induces apoptosis through a cytoplasmic Death Domain that interacts with signaling adaptors like FADD (Fas-Associated Death Domain). FADD recruits the inactive Procaspase8, which is proteolytically activated to Caspase8, which further cleaves and activate downstream effector Caspase3.
Activated BAX within the mitochondrial membrane leads to creation or alteration of membrane pores, resulting in Mitochondrial-Outer-Membrane Permeabilization that causes release of Cytochrome-C into the cytosol. Cytochrome-C associates with APAF1 protein and Procaspase9 to form the apoptosome, which activates Caspase9, leading to activation of downstream effector caspases, including Caspase3. Caspase3 cleaves ICAD-CAD heterodimer to free CAD to cause DNA fragmentation, thus causing apoptosis. This animation illustrates the detailed process of apoptosis by extrinsic and intrinsic pathways.