hyperplasia

Hyperplasia is an increase in the number of cells in a tissue or organ, leading to its enlargement. It is distinct from hypertrophy, which results from an increase in cell size rather than cell number. Hyperplasia arises from increased cell proliferation and is often driven by hormonal or growth factor signals. It is usually reversible if the stimulating factor is removed.

Hyperplasia can be categorized as physiologic or pathologic. Physiologic hyperplasia occurs as a normal adaptive or hormonal response, such as endometrial gland proliferation under estrogen during the menstrual cycle, breast tissue development during pregnancy, or liver growth after partial hepatectomy (compensatory hyperplasia) to restore mass.

Pathologic hyperplasia results from abnormal or excessive stimulation and can be associated with disease. Examples include endometrial hyperplasia due to unopposed estrogen exposure, which may be classified as simple or complex and with or without cellular atypia and can carry a risk of progression to endometrial carcinoma; benign prostatic hyperplasia is age-related proliferation of prostatic glands and stroma that can cause urinary symptoms; thyroid goiter from elevated TSH is another form. In some tissues, hyperplasia can predispose to neoplastic change when persistent proliferative signaling or genetic abnormalities are present, though hyperplasia itself is not cancer.

Mechanistically, hyperplasia involves increased entry into the cell cycle and is regulated by growth factors, hormones, and receptor signaling that influence cyclins and cyclin-dependent kinases. The condition is typically reversible with removal of the stimulus and, depending on the tissue and context, may or may not progress to dysplasia or malignancy.