Cellular Physiology: Cellular Pathology

Cells possess a variety of mechanisms to maintain homeostasis or respond to environmental stressors, ranging from physical injury to genetic mutations. These responses are broadly categorized into cell death pathways, adaptive growth changes, and inflammatory reactions. Understanding these processes is fundamental to identifying how tissues transition from health to disease states.

Cell death occurs primarily through two pathways: necrosis and apoptosis. Necrosis is typically an uncontrolled response to external or internal injury, such as ischemia or toxins, leading to cell swelling and rupture, which triggers inflammation. Common forms include coagulative, liquefactive, caseous, fat, and fibrinoid necrosis. In contrast, apoptosis is a highly regulated, programmed process driven by a caspase cascade. It can be initiated via the intrinsic (mitochondrial) pathway, triggered by cellular stress, or the extrinsic (death receptor) pathway, initiated by external signaling proteins. Unlike necrosis, apoptosis results in the orderly degradation of cellular components without causing significant inflammation.

Cells adapt to persistent stress through structural and numerical changes. Hyperplasia involves an increase in cell number, often driven by hormonal or compensatory signals, while hypertrophy involves an increase in cell size due to heightened functional demand. Metaplasia represents the transformation of one mature cell type into another in response to chronic stress, whereas dysplasia describes the development of immature, precancerous cells. Conversely, atrophy, aplasia, and hypoplasia represent failures in growth or maintenance, often resulting in reduced tissue size or function.

Inflammation is the immune system's response to injury or infection, characterized by heat, pain, redness, and swelling. It is triggered by molecular patterns (PAMPs and DAMPs) that activate immune cells, leading to vasodilation and the recruitment of leukocytes like neutrophils to the site of injury. Additionally, cellular injury is frequently mediated by free radicals—molecules with unpaired electrons that cause lipid peroxidation and protein oxidation. Cells defend against this oxidative stress using antioxidants, metal carrier proteins, and specialized enzymes.