Cellular Adaptations

Study Notes

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

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.

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.

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).

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.