mymedschool.orgM08.02.005 Adaptation to Cellular Injury

Cellular adaptation refers to potentially reversible structural and functional changes that occur in response to environmental stress or injury. These changes enable the cell to survive and maintain function in altered conditions. If the stress persists or becomes severe, adaptation may progress to cell injury.

Types of Cellular Adaptations

Type	Definition	Key Outcome
Atrophy	Decrease in cell size and function	Reduced tissue mass
Hypertrophy	Increase in cell size and function	Enlarged organ or tissue
Hyperplasia	Increase in the number of cells	Tissue enlargement
Metaplasia	Reversible replacement of one cell type by another	Adaptation to stress or irritation

Atrophy
- Definition
  - Reduction in the size and metabolic activity of cells, leading to decreased organ size.
- Causes
  - ↓ Workload (disuse): An immobilized limb in a cast
  - Ischemia, Atherosclerosis, and Chronic Vascular Disease
  - ↓ Hormonal or Neural input,t Denervation of muscle, post-menopausal endometrium
  - Malnutrition Protein-calorie deficiency
  - Aging Senile atrophy in the brain, heart, etc.
- Morphologic Features
  - Light Microscopy: Small, shrunken cells with lipofuscin granules
  - Electron Microscopy: Decreased organelles, presence of autophagosomes
Hypertrophy
- Definition
  - Increase in cell size and functional capacity due to increased synthesis of intracellular components.
  - Occurs primarily in non-dividing cells (e.g., skeletal muscle, myocardium).
- Causes
  - Increased mechanical demand Physiologic: Skeletal muscles in weightlifters
  - Pathologic: Cardiac hypertrophy in hypertension
  - Hormonal stimulation: Gravid uterus (estrogen), lactating breast (estrogen, prolactin), puberty (GH, androgens)
- Mechanism
  - Mediated by growth factors, cytokines, and trophic stimuli.
  - Leads to ↑ gene expression, ↑ protein synthesis, and organ enlargement.
Hyperplasia
- Definition
  - Increase in the number of cells in a tissue or organ, resulting in increased mass. Occurs only in cells capable of division (labile and stable cells).
- Causes
  - Physiologic Hyperplasia
    - Hormonal: breast tissue growth during puberty and lactation.
    - Compensatory: liver regeneration after partial hepatectomy.
    - Antigenic: lymphoid hyperplasia after infection.
  - Pathologic Hyperplasia
    - Endometrial hyperplasia (excess estrogen),
    - Benign prostatic hyperplasia (aging, DHT effect).
- Mechanism
  - Involves growth factors, cytokines, and activation of proto-oncogenes
  - ↑ DNA synthesis → ↑ cell division
Metaplasia
1. Definition
  1. Reversible replacement of one differentiated cell type with another, better suited to withstand stress or irritation.
2. Mechanism
  1. Chronic irritation → reprogramming of stem (reserve) cells → differentiation into a new cell type
  2. Mediated by growth factors, cytokines, and extracellular matrix components

Examples

Normal Tissue	Replaced By	Cause
Ciliated columnar epithelium (bronchus)	Stratified squamous epithelium	Chronic tobacco smoke irritation
Squamous epithelium (esophagus)	Columnar epithelium	Chronic GERD → Barrett esophagus

Clinical Correlations

Condition	Adaptation Type	Description
Secondary polycythemia	Hyperplasia	↑ RBC precursors in bone marrow due to chronic hypoxia at high altitude
Barrett esophagus	Metaplasia	Squamous → columnar epithelium due to chronic acid reflux
Cardiac hypertrophy	Hypertrophy	Myocyte enlargement due to pressure overload (e.g., hypertension)
Muscle atrophy in immobilization	Atrophy	Reduced in size from disuse

Key Points to Remember

Adaptations are reversible responses to stress.
Persistent stress may lead to irreversible injury or neoplasia.
Hypertrophy and hyperplasia often occur together.
Metaplasia increases risk of malignant transformation (e.g., Barrett esophagus → adenocarcinoma).