CONGENITAL MESOBLASTIC NEPHROMA IN A YOUNG BEAGLE DOG

A 10-month-old male beagle dog had a congenital tumor in the right kidney. Histopathologically, the tumor was characterized by fingerlike extensionsprojecting into the adjacent normal tissue, and often entrapping the normal glomerulo-tubular structure. The tumor was composed of spindle cells arranged in an interlacing fascicular pattern with some collagen fibers. Ultrastructurally, thetumor cells had long projections on the surface, rarely in contact with the junctional structures among the neighboring cells, and formed thin discontinuous basement membranes at the cell ends. Immunohistochemically, the tumor cells showed positive reactions with vimentin and S-100 protein antisera, and some weak reactions with actin, desmin, or myoglobin, but no reactions with cytokeratin. This present tumor should be diagnosed as a congenital mesoblastic nephroma (CMN) on the basis of characteristic features in HE staining, other immunohistochemical reactions, and electromicroscopic findings, as well as the young age of the dog.