Ultrastructure of the chondroid regions of benign mixed tumor of salivary gland

Methods of light and electron microscopy were employed to determine the exact nature of the chondroid portions of the benign mixed tumor of salivary gland. Although these regions could not be distinguished from normal hyaline cartilage when examined with the light microscope, no normal or metaplastic chondrocytes could be identified when the ultrastructure of these regions was examined with the electron microscope. Instead, neoplastic myoepithelial cells embedded in a matrix quite similar to that of immature hyaline cartilage were found to be present. It is concluded that in the benign mixed tumor neoplastic myoepithelial cells undergo partial metaplasia enabling them to produce a typical chondroid matrix. Further, because of the similarity in structure and function between myoepithelial cells and visceral smooth muscle cells, it is suggested that the myoepithelial cell is of mesenchymal origin and, therefore, after metaplasia entirely capable of producing a mesenchymal mucin.