Apoptosis

Apoptosis is initiated through two distinct pathways. The intrinsic or mitochondrial-mediated pathway is triggered by intracellular death signals from DNA damage, hypoxia, and oxidative stress. The extrinsic apoptotic pathway is initiated when extracellular death-inducing ligands, such as cytokines, bind to death receptors on the cell surface. Both pathways ultimately activate caspases to execute cell death.

Caspases are proteases that serve as key mediators of apoptosis. Initiator caspases are activated by both intrinsic and extrinsic signals through different mechanisms. Each activated initiator caspase then activates multiple executioner caspases, which cleave cellular proteins. This cleavage causes morphological changes including cell shrinkage, nuclear fragmentation, and plasma membrane blebbing.

During apoptosis, cells undergo distinct morphological changes as executioner caspases cleave cellular proteins. These changes include cell shrinkage, nuclear fragmentation, and plasma membrane blebbing. Subsequently, cellular components are enclosed into membrane vesicles called apoptotic bodies. This controlled process prevents the release of potentially damaging molecules to neighboring cells.

After cellular components are enclosed into membrane vesicles called apoptotic bodies, these structures are engulfed by phagocytic cells through phagocytosis of apoptotic cells. This removal process is essential for maintaining tissue integrity and preventing inflammation. The contained nature of apoptosis ensures that neighboring cells remain unharmed during this controlled cell death.

Apoptosis allows cells to die in a controlled manner that prevents damage to surrounding tissues. Many internal checkpoints monitor cell health, and if abnormalities are detected, cells can spontaneously initiate apoptosis. This regulated process is essential for maintaining normal physiology and tissue homeostasis. Without proper apoptosis, cells may proliferate uncontrollably, as occurs in cancer.

Normal animal cells have receptors that interact with the extracellular matrix, a network of glycoproteins providing structural support. When cells bind to this matrix, signaling cascades are initiated within the cell. However, if a cell moves away from the extracellular matrix, signaling ceases and the cell undergoes apoptosis. This system prevents uncontrolled cell proliferation and metastasis.

Dysregulated apoptosis leads to various diseases depending on whether apoptosis is decreased or increased. Decreased apoptosis contributes to cancer and cardiovascular diseases, allowing abnormal cells to survive. Increased apoptosis occurs in autoimmune diseases such as Hashimoto's thyroiditis, systemic lupus erythematosus, and rheumatoid arthritis, where healthy cells are destroyed.