Metaplasia

Metaplasia is a change in which one kind of mature, differentiated cell in a tissue is replaced by another kind that does not normally belong there. The existing cells do not simply turn into the new type; instead, stem or precursor cells in the tissue are reprogrammed to mature along a different path, a switch steered by transcription factors. The new cells usually cope better with the stress that caused the change, but some normal function is lost.

Squamous metaplasia is the most frequent form. In smokers' airways, the tall ciliated cells that sweep mucus upward are replaced by tough, flat, layered squamous cells that resist smoke but cannot clear debris; the cervix undergoes a similar change after puberty. In Barrett's esophagus, years of acid and bile reflux convert the squamous lining of the lower esophagus into column-shaped cells of an intestinal type, which tolerate acid. Metaplasia can also occur in connective tissue, as when bone forms in an injured muscle.

Metaplasia may regress if the stimulus is removed, though how often this happens is uncertain. If the irritation continues, the altered tissue can go on to develop dysplasia and then cancer: Barrett's esophagus raises the risk of esophageal adenocarcinoma, and bronchial squamous metaplasia precedes many squamous cell lung cancers. Barrett's esophagus is therefore often monitored by endoscopy.