A craniopharyngioma in a Wistar rat most likely originated in a Rathke's cleft cyst.

We examined a 110-week-old RccHanTM: WIST Wistar male rat from a carcinogenicity study. No clinical signs were observed, and the rat was sacrificed at the end of the study. Macroscopically, within the midline of the sphenoid bone, was a 10 mm, non-infiltrative, soft, heterogeneous mass. Microscopic evaluation showed an expansile, cystic proliferation, consisting of two patterns of epithelial lining: well-differentiated areas lined by a single layer to a pseudostratified, ciliated-cuboidal epithelia with Goblet cells compatible with Rathke's cleft cyst; and poorly differentiated ones that formed irregular papillary projections, covered by atypical epithelia with squamous differentiation and hyperkeratosis compatible with areas of craniopharyngioma. Pleomorphisms were high in atypical areas with up to 2-3 mitotic figures per high power field. Within the cystic cavities, there was abrupt keratinization, mucus, cholesterol clefts, and foci of foamy macrophages. Immunohistochemistry revealed strong pancytokeratin immunolabelling of neoplastic cells confirming the epithelial origin. Well-differentiated epithelial lining showed cytokeratin-20 and cytokeratin-8 immunoreactivity, whereas the atypical squamous epithelium presented with a loss of cytokeratin-20 positive signal and weak to moderate positivity with cytokeratin-8. Areas compatible with a Rathke's cleft cyst and craniopharyngioma were considered to co-exist in the same mass.