Feline inferior tricholemmoma: Histological and immunohistochemical characterisation

Abstract Feline inferior tricholemmoma is a benign hair follicular tumour with minimal outer root sheath differentiation. The diagnosis is rarely suggested clinically, and this may contribute to frequent underrecognition by the histopathologist. A spayed female Munchkin cat exhibited a subcutaneous growth of a mass, 1.7 cm in diameter, located on the neck region. Histological examination revealed that the sole tumour consisted of clearly defined circular lobules, demonstrating a well‐developed basement membrane, a cuboidal or columnar morphology, and a vacuolated and/or pale eosinophilic cytoplasm. Additionally, these cells displayed round to ovoid nuclei with a peripheral palisade arrangement, which could be diagnosed as feline inferior tricholemmoma. Moreover, the tumour cells showed extremely positive staining for periodic acid–Schiff, immunoreactive SRY sex determining region Y‐box 9 (Sox9), and CD34. In this case, the diagnosis was established based on histological characteristics and supported a definitive diagnosis originating from cells with distinctive Sox9 and CD34 expression.