An ultrastructural study of cartilaginous zones and surrounding epithelium in mixed tumors of salivary glands and skin.

Cartilaginous areas and adjacent cellular clusters in mixed tumors of the parotid gland, palate, and skin were studied ultrastructurally. A minority of cells in the cartilaginous matrix occupied lacunar spaces and were indistinguishable from normal chondrocytes. The matrix was also structurally similar to that of hyaline cartilage. The majority of matrix cells did not occupy lacunae but were essentially identical to the chrondrocytes. They rarely contained tonofilaments or had specialized attachments to a neighboring cell. Cells at the periphery of cellular clusters closely resembled matrix cells, although tonofilaments and cell attachments were more numerous. Within cellular clusters, there was a transformation to smaller cells that formed small luminal structures or had squamous features. All cells contained tracts, often perinuclear, of 7- to 10-nm. filaments that were most prominent in the nonlacunar matrix cells and cells at the edges of the cellular clusters. These differed from myofilaments; myoepithelial cells were absent. Mixed tumor does not consist of distinct cell populations. Rather, there is a continuum of cytoplasmic features from epithelial cells to mesenchymal cells. The nonlacunar cells present in the matrix and at the edges of the cellular clusters have both mesenchymal and epithelial features and, we believe, an ability to differentiate bidirectionally. Chondrocytic transformation of these cells mimics embryologic chondrogenesis.