Histogenesis of canine mixed mammary tumors

The origin of different mesenchymal components which characterizes the canine mixed mammary tumours has been controversial since they were first described. Most of the authors have found, through different methods, important evidence that point out the myoepithelial cells as the responsibles ones. In the present study, 15 canine mixed mammary tumours were studied using the avidin-biotin-peroxidase complex method with the 7 primary antibodies: Citokeradne, epithelial membrane antigen, carcinoembrionic antigen, muscle-specific actin, vimentin, S-100 protein and desmin. Commercially produced antibodies were used, and their specificity was previously determined regarding its reaction on control tissues to correlate expression of these antigens with the ultrastructural changes. Specificity was reliable for canine tissues except for S-100 protein. Cytokeratin, muscle-specific actin and vimentin expressed themselves remarkably on epithelial, myoepithelial and mesenchymal elements, respectively. Ultrastructurally, 6 main cellular types were identified and characterized; from those, the transitional cells were outstanding (filamentous, stellate and chondroid cells) and remained characteristic and specific of myoepithelial cell origin with actin filaments (5 to 8 nm thick), desmosomes and dense bodies. These electron microscopic findings, together with the expression of the membrane and structural proteins manifested by immunoperoxidase, pointed out the myoepithelial cell origin of their precursors as responsible for the formation of the heterologous mesenchymal component, typical of these tumors. The possibility that the epithelial component is originated from cells capable to have divergent epi-myoepithelial differentiation, as it apparently occurs in the pleomorphic adenomas of the human mammary and salivary glands, is also considered.