Myoepithelial cell
tumors of the mammary gland have been observed in several mammalian species and are composed of a single cell type (myoepithelium) or, more often, present as a biphasic process including neoplastic ductal epithelial cells. In dogs, these are common
tumors, but in humans they are rare
neoplasms of the breast, and little is yet known of their pathogenesis, particularly with respect to myoepithelial origin. The present report describes bicellular mammary gland
tumors arising from the duct epithelium that were induced in (C57BL/6NCr X DBA/2NCr)F1 (B6D2F1) mice by four weekly oral applications of 1 mg 7,12-dimethylbenz[a]
anthracene (DMBA) starting at 8 weeks of age. Mammary
tumors developed 7 to 8 months later in 14 of 57 mice, and most showed great morphologic resemblance to human
adenomyoepitheliomas and myoepithelial
carcinomas. Ultrastructurally, the induced
tumors were composed of cuboidal epithelium with a microvillous border originating from the lining duct epithelium and plump oval or highly elongated cells that were identified as myoepithelial in origin. These spindle cells contained abundant microfilaments in parallel orientation, some with focal densities and intermediate filaments that frequently formed loose bundles or compact tonofibrils. The myoepithelial cells possessed well-developed desmosomes and plasma membrane caveolae and were regularly bordered by single or reduplicated basement membranes. By immunohistochemistry, strong immunoreactivity was observed for actin in the
myoepithelial tumor component only, whereas
cytokeratin was variably present in both duct epithelium and myoepithelium. Neoplastic myoepithelial cells stained purple with
phosphotungstic acid hematoxylin (PTAH) and brilliant red with Masson's trichrome. It is suggested that DMBA-induced mouse mammary gland
adenomyoepitheliomas and myoepithelial
carcinomas may serve as very useful animal models to study myoepithelial
tumorigenesis.