The pathology of head and neck tumors: the myoepithelial cell and its participation in salivary gland neoplasia, Part 17.

Sharing a common ectodermal origin with salivary duct epithelium, the myoepithelial cell is two-sided in several respects. It lies between the epithelial cells and the basal lamina, with one side facing the duct or acinar cells and the other facing the stroma. It has a fine structure not unlike that of smooth muscle, and one of its functions is to contract. Despite its electron-optic similarity to smooth muscle, the myoepithelial cell's principal filamentous protein is cytokeratin, which is found only in epithelial cells. Myoepithelial cells do not have a demonstrable secretory capability, but they can store glycogen in abundance. In some tumors of the salivary glands, myoepithelial cells do not participate in histogenesis. These tumors arise from portions of the salivary duct system in which myoepithelial cells are not normally found. In tumors originating from intercalated ducts, myoepithelial cells may assume one of two roles; a passive presence or an active and integral formation of the tumors. Mixed tumors, clear cell tumors, plasmacytoid (hyaline cell) and fibroblastic myoepitheliomas, and terminal duct adenocarcinomas are examples of tumors in which the myoepithelial cell is prominent or dominant.