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Cellular Adaptation I: Introduction and Atrophy

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Cells adapt to physiological demands or pathological stress by adjusting their size, number, or function.

Major forms of cellular adaptation include atrophy, hypertrophy, hyperplasia, and metaplasia.

Atrophy involves a reduction in cell size and, in some tissues, a reduction in cell number, often leading to tissue or organ shrinkage. Commonly affected organs include skeletal muscle, the brain, the heart, and reproductive tissues.

Physiological atrophy develops naturally with aging, such as the shrinking of the thymus after childhood.

Pathological atrophy often develops in response to chronic disease or prolonged stress.

For example, brain atrophy may result from conditions such as chronic cerebral ischemia and is often associated with atherosclerosis.

Atrophic cells contain fewer organelles, such as mitochondria and rough endoplasmic reticulum, which reduces overall metabolic activity.

Additionally, autophagy, a process in which cells degrade internal components, contributes to atrophy, especially during nutrient deprivation or stress.

Overview

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Cells can adapt to environmental changes to maintain function and avoid injury, a process called cellular adaptation. Adapted cells exist in a reversible intermediate state with changes in size, number, phenotype, metabolism, or function. These responses help cells meet altered physiological or pathological demands; for example, enlargement of breast and uterine tissues during pregnancy. Early adaptations may enhance function, but persistent stress eventually causes tissue damage.

Types of Cellular Adaptations

Major adaptations include atrophy (decreased cell size causing tissue shrinkage), hypertrophy (increased cell size without new cells), hyperplasia (increase in cell number), and metaplasia (reversible replacement of one cell type by another due to chronic irritation). Dysplasia, which is not a true adaptation, involves disordered growth with loss of uniformity and may progress to neoplasia.

Atrophy

Atrophy is characterized by a reduction in cell size, leading to shrinkage of affected tissues or organs such as skeletal muscle, heart, brain, and reproductive organs. It may be physiological, occurring during normal development or aging (e.g., thymic involution, postpartum uterine reduction, and decreased lymphoid tissue in adolescence), or pathological, resulting from abnormal conditions such as disuse, sustained pressure, reduced blood supply (ischemia), or loss of nerve supply (denervation).

Mechanisms of Atrophy

Atrophic cells show reduced organelles, including mitochondria, rough endoplasmic reticulum, and myofilaments, lowering metabolic demand. Key mechanisms include decreased protein synthesis, increased protein degradation, and autophagy, a process that degrades and recycles cellular components. In prolonged atrophy, indigestible materials accumulate as residual bodies, notably lipofuscin, a yellow-brown pigment that builds up in long-lived cells (heart, liver, brain) and serves as a marker of aging and oxidative stress, also contributing to age-related skin pigmentation.

Transcript

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