Pleomorphic adenoma of the labial gland, characterized by reticular pattern of myoepithelial cells in a dog.

An 11-year-old female golden retriever dog had a mass at the right corner of the upper lip, which gradually increased in size and protruded into the oral cavity. The mass was removed surgically. The cut surface of the mass was smooth, whitish and solid, and covered by the oral mucosal membrane. Histopathologically, the mass consisted mainly of reticular pattern of short spindle cells that stained positively for cytokeratin AE1/AE3, α-smooth muscle actin and p63, suggestive of a myoepithelial cell phenotype. Between the neoplastic cords, there was myxoid or edematous connective tissue. Additionally, neoplastic cells with luminal epithelial and basal cell phenotypes were arranged in ducts and small islands, respectively. Based on the diverse histological and immunohistochemical features, the tumor was diagnosed as pleomorphic adenoma of the labial gland. To our knowledge, the reticular proliferation pattern of myoepithelial cells has not been described in salivary gland tumors of domestic animals.