Apoptosis: Programmed Cell Death

Apoptosis

• Programmed cell death that is active and physiological, ensuring cells live only when and where needed.
• Deprivation of survival factors activates the suicide program.

Apoptosis Characteristics

• Cell shrinkage occurs, but cells do not lyse, and plasma membrane remains intact.
• Blebbing occurs, where portions of the plasma membrane bud off.
• Phosphatidylserine in the inner membrane phospholipid flips to the cell surface, serving as an "eat-me" signal to phagocytic cells.
• Mitochondria releases cytochrome c in an ATP-dependent process, but remains within blebs.
• Chromatin segments and condenses.

Apoptosis Characteristic

• Apoptotic bodies are formed from apoptotic cells and engulfed by phagocytic cells (e.g., macrophages, dendritic cells).

Necrosis

• Passive, pathological process induced by acute injury or disease.
• Characteristics:
  • Affects groups of cells in a localized region simultaneously.
  • Cells increase in volume and lyse (burst).
  • Release of intracellular contents, including mitochondria.
  • Often induces a damaging inflammatory response.
  • Necrotic process completes within several days.

Apoptosis vs. Necrosis

• Apoptosis is an active, physiological process that occurs individually, within a few hours.
• Necrosis is a passive, pathological process that affects groups of cells, completing within several days.

Apoptosis Characteristic

• Apoptotic bodies are recognized for engulfment by the presence of phosphatidylserine.
• Scramblase removes the membrane phospholipids phosphatidylserine from the inner membrane leaflet.
• Phagocytic cells also release cytokines, including interleukin-10 (IL-10) and transforming growth factor (TGF-β), that inhibit inflammation.
• Apoptosis is completed within a few hours.

Biological Significance of Apoptosis

1. Removal of Damaged Cells

• Removes cells damaged beyond repair, infected, or starved, saving nutrition and preventing viral spread.
• p53 protein halts cell cycle and stimulates apoptosis.

2. During Development

• Extensive cell division and differentiation during embryonic development often result in excess cells, which are removed through apoptosis.