Pleomorphic adenoma, I: Ultrastructural organization of "epithelial" regions.

Twenty-four major and minor salivary gland pleomorphic adenomas were studied ultrastructurally to determine the growth patterns, organization, and cytologic modifications of the proliferating neoplastic cells. In compact and highly cellular regions, two cell types--luminal epithelial and myoepithelial--could often be identified; their organization mimicked that of the normal salivary gland duct or acinar unit. Results of the study indicate that the principal proliferating tumor cell is a structurally modified myoepithelial cell that frequently shows squamous differentiation. At the immediate margins of cellular regions of many tumor cells, gradual dedifferentiation of modified myoepithelial cells with a loss of squamous features occurs, although in some cells the squamous features are retained to varying degrees. Within cellular regions, the earliest development of matrix occurs in relation to small, basal lamina-lined extracellular spaces between myoepithelial-like cells. Modifications of such intercellular spaces are helpful in tracing the development of myxoid zones and the evolution of cell types in this unique region. The authors postulate that salivary gland pleomorphic adenomas result from the neoplastic transformation of the complete ductal-acinar unit rather than from one particular ductal "reserve" cell.