Cellular Adaptations

Cellular Adaptations

• Cells must adapt to changes in their environment, which can be physiological or pathological.

Types of Cellular Adaptations

• Hyperplasia: increase in the size of an organ or tissue due to an increase in the number of constituent cells.
  • Physiological Hyperplasia: e.g. hormonal (female breast during puberty and pregnancy), compensatory (hyperplasia of liver hepatocytes after partial hepatectomy).
  • Pathological Hyperplasia: increased level of circulating hormones → hyperplasia in target organs, e.g. endometrium due to estrogen, senile prostatic hyperplasia due to androgen and estrogen.

Hypertrophy

• Definition: increase in the size of an organ or tissue due to enlargement of individual cells without an increase in cell number.
• Types of Hypertrophy:
  • Physiological Hypertrophy: e.g. pregnant uterus, muscles of athletes.
  • Pathological Hypertrophy (Adaptive Hypertrophy): e.g. stomach in pyloric stenosis, cardiac muscle hypertrophy in hypertension.

Atrophy

• Definition: reduction of the size of an organ after reaching its normal adult size, usually due to a decrease in both the number and size of cells.
• Types of Atrophy:
  • Physiological Atrophy: e.g. atrophy of the ovary and breast after menopause, senile atrophy of the heart.
  • Pathological Atrophy: e.g. ischemic atrophy (decrease in blood supply), pressure atrophy (long continued pressure on a tissue leading to decreased blood supply), neuropathic atrophy (e.g. poliomyelitis), decreased work load (e.g. immobilized limb), starvation (leading to generalized atrophy).

Metaplasia

• Definition: reversible change in which an adult cell type is replaced by another adult cell type of the same tissue.
• Examples: squamous metaplasia due to chronic irritation in transitional epithelium of urinary bladder (e.g. bilharziasis or stones), columnar epithelium of gall bladder (e.g. stones), pseudostratified ciliated columnar respiratory epithelium (e.g. smoking).
• Importance: adaptive metaplastic epithelium is better able to survive, but may lead to loss of important protective mechanisms, and potentially cancer transformation if the inducing influences persist.