Cell Death and Apoptosis

Types of Necrosis

• Fat necrosis: occurs due to trauma to adipose tissue of breast and subcutaneous tissue, causing rupture of fat cells with self-digestion, resulting in a firm white areas
• Fibrinoid necrosis: a special form of necrosis typically seen in immune reactions involving blood vessels (vasculitis), characterized by deposits of “immune complexes” and fibrin that has leaked out of vessels in the walls of arteries

Fibrinoid Necrosis

• Microscopic appearance: homogeneous, eosinophilic (bright pink) and structure-less, resembling fibrin
• Example: fibrinoid necrosis in an artery

Apoptosis

• Definition: programmed cell death, also known as cell suicide, where individual cells or a small group of cells die within the living body to remove unwanted cells
• Characteristics: energy-dependent, active, and programmed
• Physiological causes:
  • During embryogenesis
  • Hormone-dependent (e.g., shedding of endometrium in menstrual cycle, regression of lactating breast)
  • Cell loss in proliferating cell populations (homeostasis)
  • Elimination of potentially harmful self-reactive lymphocytes
  • Death of cells that have served their useful purpose (e.g., neutrophils at the end of acute inflammation)
• Pathological causes:
  • DNA damage (e.g., radiation, anticancer drugs)
  • Pathological atrophy (e.g., organ obstruction)
  • Cell death in infections (e.g., viral infections)
  • Cell death by cytotoxic T lymphocytes
• Morphology:
  • Cell shrinkage
  • Chromatin condensation and fragmentation
  • Formation of cytoplasmic blebs and apoptotic bodies
  • Rapid phagocytosis of apoptotic cells or cell bodies by macrophages
  • No inflammation

Comparison of Necrosis and Apoptosis

• Necrosis: passive, non-programmed, affects a large group of cells, enlarged cells, pyknosis, karyorrhexis, and karyolysis, with adjacent inflammation
• Apoptosis: active, programmed, affects a single cell or small group of cells, reduced cell size, chromatin condensation and fragmentation, with no inflammation

Cell Response to Injury

• Causes of cell injury: depends on the type of injured cells, irritant nature, severity, and duration
• End result of injury: reversible (cloudy swelling, hydropic swelling, fatty change) or irreversible (necrosis, apoptosis)