Chapter 3: Cellular Adaptation, Injury, and Death

Cells employ various mechanisms for cellular adaptation to maintain internal stability, including changes in size (atrophy or hypertrophy), number (hyperplasia), and type (metaplasia). When adaptation is deranged, dysplasia may occur, which is considered a potential precursor to cancer. Furthermore, cells may suffer from intracellular accumulations of substances—such as lipids, pigments, or products of genetic disorders—and abnormal pathologic calcifications, categorized as dystrophic (in damaged tissue) or metastatic (due to elevated serum calcium). When stressors are overwhelming, cells undergo injury and death. Causes of cell injury are diverse, encompassing physical agents like trauma and electrical forces, various forms of radiation (ionizing and nonionizing), chemical toxicity (including lead and mercury), biological agents, and nutritional imbalances. Most injurious agents exert their effects through three major mechanisms: the creation of highly damaging free radicals leading to oxidative stress; hypoxic cell injury, which depletes ATP and causes cellular swelling; and disturbances in calcium homeostasis, which activate destructive cellular enzymes. Cell death follows two distinct patterns: regulated, non-inflammatory apoptosis (programmed cell destruction essential for normal development and turnover, utilizing caspase pathways) or uncontrolled necrosis (cell swelling, membrane rupture, and release of contents that trigger the inflammatory response). Specific types of necrosis include coagulative, liquefactive, and caseous, with massive tissue death known as gangrene. Finally, the chapter explores theories of cellular aging, linking senescence to factors like the shortening of telomeres and cumulative free radical damage.