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The expression of cytokeratins varies with the type of epithelium, the state of differentiation, and pathological conditions. In this study, the differential expression of cytokeratins in external meatal skin and middle ear epithelium was used for a pathogenetic study of cholesteatoma lesions and infection-induced epidermoid formations in the middle ear of the rat. Immunocytochemistry generally revealed an epidermal-type cytokeratin profile in the cholesteatoma matrix, except for the focal expression of nonepidermal cytokeratins at the invasion front. Comparable observations were made in the middle ear of the rat after an infection-induced invasion of epidermal cells from the meatal skin. An infection-induced-cornifying metaplastic lesion of the middle ear epithelium revealed nonepidermal cytokeratin expression. The results of this combined study suggested that the cholesteatoma specimens studied had an epidermal origin. The expression of nonepidermal cytokeratins was considered to result from a state of hyperproliferation rather than from metaplasia.

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