Apoptosis isan important cellular process that allows cells to die in a programmed manner.Cells which undergo apoptosis present a “blebbed” characteristic on its cellmembrane (textbook). Cells which undergo apoptosis go through certain changes,including, cell shrinkage, chromosome condensation (pyknosis), membraneblebbing, nuclear fragmentation (karyohexis) and then the engulfment of thecell by phagosomes (1). Apoptosisplays an important role in the growth and development, for instance, during thedevelopment of the embryo where the fetal hand starts out webbed and thefingers are formed through process of apoptosis between the web spaces(textbook). Apoptosis also plays a role in removing faulty cells, for instancea cell infected with a virus which cannot be repaired (textbook).
For apoptosisto occur, certain proteins known as caspases break the cell down from within(apoptosis). Caspases are activated via two distinct pathways know as intrinsicand extrinsic pathway. As the namesuggests, the intrinsic pathway is initiated by signals from within the cell ifan irreparable damage to the cellular components or DNA occurs (textbook).
Inthe mitochondria, two types of proteins are present- anti-apoptotic proteinssuch as Bcl-2 and Bcl-x and proapoptotic proteins such as Bax and Bak (2). Theproapoptotic proteins called as Bax is activated and is inserted into themembrane of the mitochondria allowing substances such as cytochrome c to leakout (textbook). The leaked cytochrome c then binds with Apaf-1 which is presentin the cytoplasm to form a complex which in turn activates caspase 9, bringingabout the formation of the apoptosome which destroys the cell (3).
In theextrinsic pathway the signal is initiated from the death receptors which belongto the tumor necrosis factor receptor (TNFR) (1). This pathway is initiatedwhen the Fas death receptor binds to the Fas ligand present on the T-cytotoxiccells (textbook). This sets off a chain of intracellular events that willultimately results in apoptosis. The sequence is mediated by a Fas AssociatedDeath Domain (FADD) (2). This interaction activates caspase 8 or 10 which inturn results in the activation of the effector caspases causing a caspasecascade to take place. Apoptosis is then initiated (textbook).