Light and electron microscopic study of mucoepidermoid tumor of clear cell type.

A rare type of mucoepidermoid tumor of hard palate was found in a 42-year-old man. By light microscopic examination, the nests of the tumor consisted almost entirely of clear cells and partly of epidermoid cells observed at the periphery of the nests. In addition, only a few isolated mucous cells were dispersed in sheets of the clear cells. Transition between the epidermoid cells and clear cells was apparent, characterized by a progressive clearing of the cytoplasm. It was shown by both the PAS reaction and removal by diastase digestion that the clear cytoplasm is due to the presence of significant amount of glycogen granules. By electron microscopic examination, adhesiveness of the epidermoid cells and the clear cells was characterized by numerous desmosomes. There were observed bundles of cytoplasmic tonofilaments and desmosome-tonofilament attachment complexes in these cells. There was a variable amount of amorphous lucent substance in the epidermoid and clear cells, particularly in the clear cells. It was assumed that glycogen is lost from cytoplasm during en bloc staining with uranyl acetate and the matrix of the glycogen areas appears as the amorphous feature.