[Histogenetic and morphogenetic problems of pleomorphic adenoma of the salivary glands]

Electron microscopic examinations of pleomorphic adenoma tissue showed epithelial and myoepithelial cell elements to take part in morphogenesis of both epithelial and "mesenchymal" zones. The epithelial component of pleomorphic adenoma is represented by cells showing signs of structural-functional differentiation of the squamous epithelium,, epithelium of terminal secretory parts of the salivary gland, and myoepithelial cells. The "mesenchymal" component is also represented by myoepithelial and epithelial cells. Differences in the structure of the epithelial and "mesenchymal" components are due to the pattern of arrangement and the functional status of the cells rather than to the set of the cell forms. A slow growth of the tumor and the marked functional activity of myoepithelial cells result in the accumulation of the substance of basal membranes released into the intercellular space, separation of the cells and degenerative changes in them. The results of ultrastructural examinations confirm the theory of the epithelial genesis of salivary gland pleomorphic adenoma. In this connection it sould be acknowledged that the term "mixed tumor" does not relate the essence of this neoplasia because the point at issue is a tumor of epithelial nature, whereas the term "pleomorphic adenoma" recommended by the WHO is more acceptable and emphasized the epithelial origin and heterogeneity of the cell composition and structures of the tumor.