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Metaplasia is the conversion of one differentiated cell type to another, the most common sequence being the replacement of a glandular epithelium by a squamous one. It is almost invariably a response to persistent injury and can be thought of as an adaptive mechanism. Columnar or cuboidal lining cells committed to differentiated functions, such as mucus production, assume a simpler form providing more protection against a pernicious chemical action or the effects of chronic inflammation. Prolonged exposure of the bronchi to tobacco smoke leads to squamous metaplasia of the bronchial epithelium. A comparable response, associated with chronic infection, is seen in the endocervix. Metaplasia is not restricted to squamous differentiation. It may consist of replacement of one glandular epithelium by another. In chronic gastritis, a disorder of the stomach characterized by chronic inflammation, atrophic gastric glands are replaced by cells resembling those of the small intestine. The adaptive value of this condition,known as intestinal metaplasia, is not apparent. One also sees metaplasia of transitional epithelium to glandular epithelium in chronic inflammation of the bladder (cystitis glandularis). It should be emphasized that metaplasia is not necessarily a harmless process, even though this response may be thought of as adaptive. Squamous metaplasia can impair bronchial function and predispose an individual to recurrent pneumonia. Furthermore, neoplastic transformation may occur in metaplastic epithelium. Cancers of the lung, cervix, stomach, and bladder have their origins in such areas. It is unlikely that the metaplastic epithelium itself is responsible for cancer formation. More probably, the noxious stimuli leading to metaplasia are also carcinogenic cells. Metaplasia is fully reversible. If the stimulus is removed for example, when one stops smoking - the metaplastic epithelium returns to normal.
Summary: Metaplasia is a reversible process in which one adult cell type (epithelial or mesenchymal) is replaced by another adult cell type. It is limited to the same embryonic layer (epithelium to epithelium, connective tissue to connective tissue). (I) Epithelial metaplasia : Specialized epithelial cells are usually replaced by more resistant squamous epithelium. Example: Ciliated columnar respiratory epithelium is replaced by more resistant squamous epithelium in cigarette smokers. Squamous epithelium in lower oesophagus is changed to gastric / intestinal type of columnar epithelium in chronic gastric reflux. Transitional epithelium of urinary tract is changed to squamous epithelium in urinary calculi. (II) Mesenchymal metaplasia : Fibroblasts to osteoblasts / chondroblasts. Eg: Myositis ossificans
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