Mucoepidermoid mammary carcinoma. Immunohistochemical and biochemical analyses of intermediate filaments.

The histological features of mucoepidermoid mammary carcinomas (MMCs) are presented, and criteria for distinguishing these tumours from squamous epithelial metaplasia in other mammary carcinomas are considered. Immunohistochemical and gel-electrophoretic analyses of the intermediate-filament proteins in one MMC case revealed a complex pattern of cytokeratin polypeptide expression. The simple-epithelium-type cytokeratins 7, 8, 18, and 19 were detected mainly in nonsquamous (including mucinous) cells, while the stratified-epithelium-type cytokeratins 5, 6, 14, 16, and 17 were present in squamous cells. However, in both the nonsquamous and squamous regions of the tumour, cytokeratins of the "reverse" type were detected in individual cells. This pattern of single-cell heterogeneity with respect to cytokeratin polypeptide expression suggests that the mixed phenotype of this tumour is not caused by the clonal divergence of tumour cell types. Rather, histogenetically, a pluripotent stem cell with the ability to differentiate into squamous (epidermoid) or mucinous cells might be the starting-point of such a tumour and such differentiation processes may continue to occur during tumour growth. The present case also revealed that mucoepidermoid tumours are not necessarily of low malignancy; there are highly malignant forms with rapid metastasis.