: Kuroda, J.; Yoshimura, H.; Ernst, H.

Abstract
A congenital basal cell tumor was found in a 21-old Sprague-Dawley rat. Macroscopically, a pinkish soft nodule about 10 mm in diameter was present on the mandibular skin region of the animal. The surface of the nodule was alopecic and ulcerated. Histologically, nuclei resembled those of the cells in the basal layer of epidermis or the external root-sheat, but tumor cell were more closely packed and had a larger ratio of nucleus to cytoplasma. Mitotic figures were frequent. A large number of tumor cells formed various-sized epithelial nests, which appeared to coalesce into larger solid masses, suggesting a histogenetic derivation of the tumor from hair follicles. Immunohistochemistry revealed that most tumor cells were positive for cytokeratins. Ultrastructurally, the tumor had many desmosomes, hemidesmosoms, and tonofilaments.