Histogenesis of benign pleomorphic adenoma (mixed tumor) of the major salivary glands. An ultrastructural and immunohistochemical study.

Twenty-two benign pleomorphic adenomas of the major salivary glands were studied by transmission electron microscopy and immunohistochemical techniques (three cases) in order to characterize the cell types comprising the epithelial and so-called mesenchymal regions of the tumors. Light- and electron-microscopic studies showed the tumors to consist of variable mixtures of neoplastic ductular epithelial cells, rare acinar cells, and metaplastic myoepithelial cells. Many of the loosely organized "stromal cells" contained structures indicative of their myoepithelial origin, e.g., perinuclear tonofilaments, ectoplasmic actin microfilaments, and remnants of basement membrane. Polyclonal antikeratin antisera strongly stained ductular epithelial and myoepithelial cells, squamoid cell nests, and periductular myoepithelial cells, whereas myxoid and chondroid cells were less intensely stained. Monoclonal cytokeratin antibody AE1 stained only the ductular epithelial cells in both the normal glands and tumors. In contrast, S-100 protein, which is present only in scattered acinar cells and myoepithelial cells in the normal parotid gland, was found in the ductular and periductular myoepithelial cells, isolated myxoid cells, and chondroid and cartilagenous cells in the tumors. Actin was found in all the cell types of the tumor but staining was strongest in the ducts. Double immunofluorescence staining for cytokeratin and vimentin revealed coexpression of both types of intermediate filaments in occasional normal acinar and intercalated duct myoepithelial cells, and in some cells in the myxoid and chondroid regions of the tumors. In the tumors, vimentin was present in occasional periductular myoepithelial cells, stellate myxoid cells, and especially in chondroid cells and chondrocytes. Our findings indicate that benign pleomorphic adenomas of the major salivary glands are pure epithelial cell tumors. The histologic complexity of these neoplasms is due to the ability of the neoplastic ductular myoepithelial cell to modulate its morphologic appearance and intermediate filament composition, and to produce large amounts of matrix substances. We further postulate that these tumors arise from neoplastically transformed intercalated ducts.