Epithelial neoplasia resembling an odontogenic tumor from a wild bullhead (Cottus gobio, Linnaeus 1758).

A wild bullhead (Cottus gobio) was captured from the Valturcana, a stream in Veneto region of Northern Italy. An irregular nodular mass was detected on the dorsum of left opercular region. The bilobulated mass was approximately 2 cm in diameter and covered by intact skin. It filled the gill chamber extending from underneath the operculum to the caudal margin of the cavity and medially it was minimally infiltrative of the dorsal aspect of the oral cavity. A portion of the lesion was attached to the medial margin of the operculum while caudally two gill arches were incorporated in the mass. The mass was homogeneously white, compact, with a multinodular smooth surface. At the histological examination, the lesion had irregular margins covered by a stratified keratinizing epithelium that was continuous with the epithelium on the inner surface of the gill chamber. The neoplastic tissue infiltrated the musculature below the operculum and the proximal margin of the pseudobranch. The multilobular mass was composed by irregular sheets and nests of elongated to polygonal cells admixed with stromal tissue. The cells had poorly defined margins, a moderate to abundant slightly eosinophilic cytoplasm, and round to oval vesicular nuclei. Multifocally the cells cytoplasm showed squamoid differentiation and keratinised foci with occasional formation of keratin pearls-like structures were evident. Occasionally the cells had a palisading pattern and appeared to lie on a basal lamina. In these areas, there was marked spongiosis and the cells had apically located nuclei. Anisokaryosis and anisocytosis were marked and mitoses vary from 1 to 2 per hpf. Multifocal necrosis was present and there were disseminated pyknotic nuclei. The superficial epithelium showed diffuse exocytosis of mononuclear inflammatory cells and multifocal ulceration. Severe hyperaemia and chronic moderate inflammation was distributed throughout the stroma where pleomorphic elongated cells were present (hypertrophic/reactive fibroblasts). Immunohistochemistry showed diffuse positive cytoplasmic staining of the neoplastic population with an anti-human pancytokeratins antibody. Vimentin was negative and exclusively stained the stroma. On the basis of the morphological and immunohistochemical results an odontogenic tumor (i.e. ameloblastoma) was considered the primary differential diagnosis although a carcinoma of other origin (glandular or surface epithelium) could not be ruled out.