Spontaneous mandibular follicular ameloblastoma in a female Sprague-Dawley rat

Abstract

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.

Keywords: Sprague-Dawley rat; mandible; odontogenic tumor.

Fig. 1.

Gross appearance of ameloblastoma in the mandibular mass of an SD rat. An arrow points to the right incisor. The delineating line indicates the cutting direction along with the sagittal plane of the mass. Scale bar=5 mm.

Fig. 2.

Histological features of ameloblastoma in the mandibular mass of an SD rat. (A)Epithelial cells formed islands, nests or anastomosing strands, embedded in interstitial tissue. The alveolar bone (asterisk) is visible at the bottom of the tumor. Hematoxylin and eosin (HE) stain. Scale bar=500 μm. (B) Epithelial cells are surrounded by fibrous connective tissue (arrows). Some bone trabeculae (▲) are scattered within the mass. The alveolar bone (asterisk) is observed at the bottom of the tumor. Masson’s trichrome stain. Scale bar=500 μm. (C) Fibrous connective tissue (arrows) and bone trabeculae (▲) from Fig. 2B are observed at higher magnification. Masson’s trichrome stain. Scale bar = 50 μm.

Fig. 3.

Histological features of ameloblastoma in the mandibular mass of an SD rat with hemotoxylin and eosin stain. (A) Cuboidal to tall columnar epithelial cells forms a peripheral palisade (arrows). Central epithelial cells are arranged loosely, resembling stellate reticulum, and/or clustered densely, resembling basal cells. The square indicates the specific region being magnified in Fig. 3B. Scale bar=50 μm. (B) Stellate reticulum cells with long intercellular bridges (arrows) from Fig. 3A are observed at higher magnification. Scale bar=10 μm. (C) Areas of cystic degeneration, necrotic tissue, bleeding, and inflammatory cell infiltration. Scale bar=100 μm. (D) Epithelial cells show plexiform pattern (arrows). Scale bar=50 μm. (E) Epithelial islands show squamous metaplasia with keratin pearl formation (arrows). Scale bar=50 μm. (F) Tall columnar epithelial cells form peripheral palisade with numerous mitotic figures (arrows). Scale bar=50 μm.

Fig. 4.

Immunohistochemical staining of ameloblastoma in the mandibular mass of an SD rat. (A) Epithelium islands show weak-to-moderate cytoplasmic positivity for pan-cytokeratin. Scale bar=50 μm. (B) Interstitial tissues show strong cytoplasmic positivity for vimentin. Scale bar=50 μm.