Spontaneous malignant trichoepithelioma in a female CD1/ICR mouse

A spontaneous malignant trichoepithelioma is diagnosed in a CD1/ICR outbred, six-month-old, intact, nulliparous female mouse belonging to a breeding colony. Postmortem examination revealed a solitary, locally extensive, tumour mass, 3.5 × 3,2 centimetres in size, affecting the skin and subcutaneous tissues on the left side of the neck, shoulder, and chest. The overlying skin was focally ulcerated. At sagittal section, the mass was pseudolobulated, grayish pink in colour with soft consistency and adhered to deeper tissues. Regional lymph nodes were not enlarged, and no other lesions were found at necropsy. Microscopically, a pseudolobulated, well circumscribed but non-capsulated highly cellular epithelial tumour with trichogenic differentiation extends from superficial dermis, in contact and replacing hair follicles and raise the skeletal muscle was found. More than 60% of the tumour was composed by small buds of basaloid epithelial cells with incipient keratinisation, (resembling hair bulbar papilla's), admixed with cords and trabecula of squamous epithelium with central tricolemmal keratinitation (hair intermediate segment differentiation) as well as, multiple cysts, variable in size, lined by squamous epithelium, with or without a granular cell layer, and containing lamellar keratin and/or amorphous cell debris, resembling the infundibular segment of hair follicle. Nuclear atypia and the mitotic account were moderate. Emboli of tumours cells were found in the skin, pleura and lung. Nests and cords of basaloid cells with trichogenic differentiation were observed throughout ipsilateral skeletal muscles, soft tissues of the neck, thoracic wall, adventitia layers of the thyroid, pleura and lung parenchyma. The histological features and immunohistochemical profile were consistent with malignant trichoepithelioma. Although trichoepitheliomas are majority benign tumours in domestic animal and humans, this study represents the first description of malignant trichoepithelioma in a CD1/ICR mouse.