Salivary Gland, Duct - Metaplasia, Squamous

Metaplasia is a change in which one terminally differentiated cell type is replaced by another cell type of the same germ line. Usually, a specialized type of epithelium is replaced by less specialized type of epithelium. Metaplasia is often, but not always, an adaptive change that occurs in response to repeated epithelial damage and is therefore often accompanied by other lesions such as inflammation or necrosis. Squamous metaplasia may be reversible if the cause is removed. Squamous metaplasia is a common form of metaplasia, presumably because squamous epithelium is more resistant to damage than other forms of epithelia. Squamous metaplasia is usually the result of chronic irritation, but it can have other causes (e.g., hypovitamnosis A). In the salivary ducts, metaplasia of the normally cuboidal ductal epithelium to stratified squamous epithelium has been seen in response to chemicals, ionizing radiation, viral infections, vitamin A deficiency, and blockage of ducts by salivary calculi. Squamous metaplasia of the ductular epithelium may be a preneoplastic lesion progressing to squamous cell carcinoma.