The histological and ultrastructural features of five major salivary gland tumours, which have little or no evidence of duct- or gland-type differentiation in routine sections, are described. Four of the cases have the tumour cells organized as narrow, anastomosing cords of cells separated by a myxoid and vascularized stroma; we have designated such lesions as reticular-type
myoepitheliomas. The fifth case has a solid growth pattern and is largely composed of hyaline cells, that is, a plasmacytoid
myoepithelioma. Ultrastructurally, one reticular
myoepithelioma reveals myoepithelial cell differentiation with microfilament aggregates, while the other three examples are composed of modified myoepithelial cells displaying widened intercellular spaces, prominent synthesis of extracellular
glycosaminoglycans, distinct basal lamina development, and obvious accumulations of cytoplasmic intermediate filaments. In electron micrographs, the modified myoepithelial cells of the plasmacytoid variant closely resemble the tumour cells in the reticular form. Three cases had expression of both
glial fibrillary acid protein (GFAP) and
vimentin, but only one of the
myoepitheliomas contained muscle-specific actin. At least focally, each of the cases exhibited a considerable spectrum of
cytokeratin filaments. Using double-labeled immunofluorescent microscopy of one reticular variant and the plasmacytoid
myoepithelioma, there was individual tumour cell co-expression of GFAP and
vimentin focally in the plasmacytoid
myoepithelioma, but co-expression of cytokeratins 13, 16 and GFAP were not noted in either case. As expected, co-expression of high- and low-molecular weight
cytokeratin filaments was widespread in both
myoepitheliomas. Most described
myoepitheliomas have a solid growth pattern and are composed of spindle and plasmacytoid cells, but based on cytological features and growth patterns in this series, it is apparent that polygonal-shaped cells with novel architecture can occur in
myoepitheliomas. The results also indicate the close relationship between
pleomorphic adenoma and such variants of
myoepithelioma.