Cell Biology Chapter on Stress Responses

Cellular Responses to Stress and Noxious Stimuli

• Cells actively adjust their structure and function to adapt to changing environmental and extracellular stresses.
• Cells adapt to physiological or pathological stimuli, achieving a new steady state and maintaining viability and function.
• Adaptive responses include hypertrophy, hyperplasia, atrophy, and metaplasia.
• If the adaptive capacity is exceeded or the stress is harmful, cell injury occurs.

Cellular Injury

• Cells exhibit reversible or irreversible injury.
• Reversible injury is characterized as mild and transient, severe and progressive.
• Irreversible injury leads to cell death through necrosis or apoptosis.

Cellular Adaptations to Stress

• Adaptations are reversible changes in cell number, size, phenotype, metabolic activity, or function in response to environmental changes.
• Physiological adaptations are responses to normal stimulation by hormones or endogenous chemical mediators, such as hormone-induced breast and uterine enlargement during pregnancy.
• Pathological adaptations are responses to stress that help cells avoid injury.

Hypertrophy

• Hypertrophy increases cell size, leading to an increase in organ size.
• Existing cells increase in size, not the number of cells.
• Increased functional demand or growth factors/hormones stimulate hypertrophy.
• Physiological examples: uterine enlargement during pregnancy.
• Pathological examples: cardiac enlargement due to hypertension or aortic valve disease.

Hyperplasia

• Hyperplasia involves an increase in cell number due to cell proliferation or replacement by tissue stem cells.
• Occurs in tissues with cells capable of replication.
• Can occur concurrently with hypertrophy.
• Physiological examples: proliferation of female breast glandular epithelium due to hormones during puberty and pregnancy and compensatory hyperplasia after partial liver resection.
• Pathological examples: hyperplasia from excessive hormonal or growth factor stimulation (e.g., uterine proliferation after a menstrual period).

Atrophy

• Atrophy is a decrease in cell size due to loss of cell substance.
• Causes include decreased workload, loss of innervation, diminished blood supply, inadequate nutrition, loss of endocrine stimulation, and aging.
• Examples: limb immobilization to permit healing, cerebral atrophy in Alzheimer's disease.

Metaplasia

• Metaplasia is a change from one adult cell type (epithelial or mesenchymal) to another adult cell type.
• A cell type sensitive to a particular stress is replaced by another cell type better able to tolerate the adverse environment.
• Examples: squamous metaplasia of respiratory epithelium in habitual smokers. Often a precursor to neoplasia.