Cellular Adaptation and Reversible Cellular Injury
Cellular adaptation refers to reversible, functional, and structural responses to changes in the environment, allowing cells to survive and maintain function. When stress is eliminated, cells can return to normal (homeostasis).
Types of Cellular Adaptation
- Hypertrophy
- Definition: Increase in cell size → increased organ size.
- Mechanism: Increased synthesis of structural components and proteins.
- Examples:
- Skeletal muscle hypertrophy from exercise.
- Left ventricular hypertrophy from hypertension.
- Hyperplasia
- Definition: Increase in number of cells in an organ or tissue.
- Mechanisms: Growth factor-driven proliferation of mature cells and possibly new cells from stem cells.
- Examples:
- Hormonal hyperplasia: proliferation of glandular epithelium in female breast during pregnancy.
- Atrophy
- Definition: Decrease in cell size and number → reduced organ size.
- Mechanisms: Decreased protein synthesis and increased protein degradation (via ubiquitin-proteasome pathway).
- Causes:
- Decreased workload (immobilization of limb).
- Loss of innervation.
- Reduced blood supply (ischemia).
- Inadequate nutrition.
- Aging (senile atrophy).
- Metaplasia
Definition: Reversible change where one differentiated cell type is replaced by another.
Mechanisms: Reprogramming of stem cells to produce a different cell type better adapted to stress.
Examples:
- Barrett esophagus (squamous → columnar with goblet cells due to acid reflux).