Spontaneous trichoepithelioma in a laboratory mouse: gross, microscopic and immunohistochemical findings.

A spontaneous trichoepithelioma occurred in a Swiss OF1 outbred, four-month-old, intact, nulliparous female mouse from a breeding colony. At necropsy, the tumour was a single, well-delineated mass measuring 4.2 cm in major diameter, located in the thoracic region and had an intact haired surface. The regional lymph nodes were not enlarged and no other abnormalities were found. Microscopically, it was composed of a random admixture of budding epithelial islands and cystic structures variable in size. The epithelial islands were composed of basaloid cells. The cystic structures were lined by squamous epithelium with or without a granular cell layer and contained lamellar or amorphous keratin, as well as wide areas of matrical keratinization (ghost cells) with or without a peripheral layer of basaloid cells and calcified contents. Mitotic activity of basaloid cells was moderate to high, but nuclear or mitotic atypia were not observed. High and low molecular weight cytokeratins, profilaggrin and involucrin expression were observed in the tumour. The immunohistochemical profile of this rare type of tumour of the skin of mice, which includes a first-time description of involucrin expression, confirms the histological evidence of differentiation towards more than one segment of follicular epithelium.