Spontaneous mandibular follicular ameloblastoma in a female Sprague-Dawley rat.

Ameloblastoma is a locally aggressive tumor derived from the odontogenic epithelium of the developing tooth germ. It is rarely reported in experimental Sprague-Dawley (SD) rats. In this 90-day percutaneous repeated-dose toxicity study, mandibular nodules were observed from day 56 to 90. Upon necropsy, a well-demarcated nodule, approximately 1.2×1.0×1.0 cm, was found attached to the mandibular bone, alongside the unerupted left incisor. Histopathologically, the epithelial cells formed islands, nests, or anastomosing strands. The epithelial islands were surrounded by a peripheral layer of tall columnar cells with antibasilar nuclei arranged in a palisading pattern. Several mitotic cells were observed. Some epithelial islands displayed cystic degenerative changes and squamous metaplasia. Necrotic tissue with inflammatory cell infiltration was observed at the tumor margins. Immunohistochemically, the neoplastic epithelial islands and mesenchymal components exhibited positivity for pan-cytokeratin and vimentin, respectively. Based on these features, the case was diagnosed as follicular ameloblastoma in an SD rat.