Metaplasia

Metaplasia is the reversible replacement of one differentiated cell type by another, often as an adaptive response to chronic irritation or inflammation. The new cell type is typically better suited to withstand the prevailing stress, but the change can reduce the original tissue’s specialized functions. Metaplasia is distinct from hyperplasia, which is an increase in cell number, and from dysplasia, which involves abnormal cellular morphology and architecture and can be a step toward cancer. While metaplasia itself is not cancer, persistent or severe irritation can increase the risk of premalignant changes in some tissues.

Common examples include squamous metaplasia of the respiratory tract, in which the normal ciliated columnar epithelium is replaced by nonciliated squamous cells in smokers or individuals with chronic bronchitis. In the stomach, chronic gastritis or Helicobacter pylori infection can lead to intestinal metaplasia, where intestinal-type cells replace normal gastric mucosa. Barrett esophagus is a specific form of intestinal metaplasia in the distal esophagus, associated with gastroesophageal reflux and an elevated risk of esophageal adenocarcinoma. Osseous metaplasia refers to bone formation within soft tissues after injury or chronic inflammation.

Mechanistically, metaplasia is thought to involve reprogramming of resident stem or progenitor cells and shifts in signaling environments driven by cytokines and growth factors. If the irritant is removed, some metaplastic changes may reverse; otherwise, continued stress may lead to dysplasia or neoplasia in susceptible tissues.