Mammary adenoma with sebaceous differentiation in a dog.

The current report describes a complex canine mammary adenoma with a rare histological feature characterized by sebaceous differentiation of tumor cells. A 13-year-old, mixed-breed, intact female dog had mammary tumors on the right mammary chain. Histologically, one of the masses was composed of bilayered ductal structures with luminal epithelial cells together with basaloid or myoepithelial cell components. Within the tumor, there were a number of lobules and nests of large foamy cells associated with basaloid reserve-like cells similar to sebaceous gland. Squamous metaplasia was also seen within the tumor. Immunohistochemical staining indicated that the tumor cells with sebaceous differentiation were positive for cytokeratin (CK)14 and that the associated basaloid reserve-like cells were positive for p63. In contrast, other luminal epithelial tumor cells were positive for CK18 and CK19, but not for CK14 and p63. The myoepithelial cells were positive for α-smooth muscle actin and p63. The expression of p63 in both sebaceous basaloid reserve-like cells and myoepithelial cells, and their structural continuity within the tumor tissue, suggested a common origin of these 2 components.