U01.04.001 Cellular adaptations

Cellular adaptations refer to reversible changes in cells that can be either physiological or pathological. These adaptations allow cells to survive and function under varying conditions. If stress becomes excessive or persistent, these adaptations may progress to cell injury.

Types of Cellular Adaptations

Hypertrophy
- Definition: Increase in the size of cells due to an increase in structural proteins and organelles.
- Examples:
  - Physiological: Uterine enlargement during pregnancy.
  - Pathological: Cardiac hypertrophy in response to systemic hypertension.
- Outcome: If excessive, may lead to cell injury (e.g., significant left ventricular hypertrophy leading to injury to myofibrils and eventually heart failure).
Hyperplasia
- Definition: Controlled proliferation of stem cells and differentiated cells, resulting in an increase in the number of cells.
- Examples:
  - Physiological: Liver regeneration.
  - Pathological: Endometrial hyperplasia due to excessive hormonal stimulation, which may progress to dysplasia and cancer.
  - Example: Benign prostatic hyperplasia (BPH).
Atrophy
- Definition: Decrease in tissue mass due to a reduction in the size and/or number of cells.
- Mechanisms:
  - Cytoskeleton degradation via the ubiquitin-proteasome pathway and autophagy.
  - Decreased protein synthesis.
- Causes:
  - Disuse (e.g., muscle atrophy due to immobilization).
  - Denervation.
  - Loss of blood supply.
  - Loss of hormonal stimulation.
  - Poor nutrition.
Metaplasia
- Definition: Reprogramming of stem cells leads to the replacement of one cell type by another that can better withstand the new stress.
- Examples:
  - Barrett esophagus: Replacement of normal esophageal squamous epithelium with intestinal-type columnar cells due to gastric acid exposure.
  - Respiratory epithelium: Replacement of ciliated columnar epithelium with stratified squamous epithelium in response to cigarette smoke.
  - Connective tissue metaplasia: Formation of bone within muscle after trauma (e.g., myositis ossificans).
- Outcome: May progress to dysplasia and malignant transformation if the irritant persists.
Dysplasia
- Definition: Disordered, precancerous epithelial cell growth characterized by a loss of uniformity in cell size and shape (pleomorphism), loss of tissue orientation, and nuclear changes (e.g., increased nuclear-to-cytoplasmic ratio, clumped chromatin).
- Stages:
  - Mild to moderate dysplasia may regress if the inciting cause is removed.
  - Severe dysplasia often becomes irreversible, progressing to carcinoma in situ.
- Preceding Conditions: Usually preceded by persistent metaplasia or pathologic hyperplasia.

Table: Summary of Cellular Adaptations

Adaptation	Definition	Physiological Example	Pathological Example
Hypertrophy	Increase in cell size	Uterine enlargement during pregnancy	Cardiac hypertrophy due to hypertension
Hyperplasia	Increase in cell number	Liver regeneration	Endometrial hyperplasia, BPH
Atrophy	Decrease in tissue mass due to reduced size/number of cells	Muscle atrophy due to disuse	Denervation atrophy
Metaplasia	Replacement of one cell type with another	–	Barrett esophagus, respiratory epithelium changes
Dysplasia	Disordered, precancerous cell growth	–	Dysplasia of cervix, esophageal dysplasia

Points to Remember:

Cellular adaptations are initially reversible.
Pathological adaptations can progress to irreversible changes and lead to diseases like cancer.
Hypertrophy and hyperplasia often occur together but are distinct processes.
Atrophy involves both a decrease in cell size and number, often due to decreased demand or adverse conditions.
Metaplasia is a response to chronic irritation and can be a precursor to dysplasia and malignancy.

USMLE Step 1 Review Series

Curriculum

U01.04.001 Cellular adaptations

Types of Cellular Adaptations

Table: Summary of Cellular Adaptations

Points to Remember:

Modal title