Pleomorphic adenoma, II: Ultrastructural organization of "stromal" regions.

Findings from an ultrastructural study of 24 major and minor salivary gland pleomorphic adenomas suggest that the principal cell type in the myxoid and chondromyxoid regions of these tumors is a structurally modified myoepithelial cell. This interpretation is based on findings in the transitional zone between myxoid regions and compact cellular areas that have a ductal-acinar organization, that is, are composed of luminal epithelial and modified myoepithelial cells. Survey-type low-power electron micrographs allowed appreciation of the original orientation of the major proliferating component of these tumors to the perimeter of ductal-acinar units. The low-power electron micrographs also revealed residual features of this organization, the early development and subsequent sequential alteration of matrix compartments as tumor cells became increasingly separated by extracellular products, and a variety of myoepithelial cell modifications, such as squamous and chondroid metaplasia, resulting from neoplastic induction. According to the authors' interpretation, modified myoepithelial cells in myxoid and chondromyxoid regions form a continuum with similar tumor cells in transitional and solid areas, forming what can be visualized as markedly expanded and merging ductal-acinar units that tend to converge with similarly altered adjacent neoplastic ductal-acinar units. Thus, a multiplicity of processes are involved in the formation of the complex and varied histologic patterns that characterize pleomorphic adenomas.