Trichilemmal tumor undergoing specific keratinization: "keratinizing trichilemmoma".

Numerous tumors with follicle-like direction of differentiation have been described. At least three of them connected to the overlying epidermis are related to the outer root sheath or trichilemma: the tumor of follicular infundibulum (Mehregan & Butler 1961) and the inverted follicular keratosis (Helwig 1955) or follicular poroma (Duperrat & Mascaro 1963) have been related to the infundibulum, the tricholemmoma (Headington & French 1961) to the lower outer root sheath. On the occasion of a case first diagnosed as proliferating trichilemmal cyst of the scalp, we think we have individualized a tumor exhibiting ability to undergo trichilemmal keratinization. We describe its histogenesis and stress its peculiarties compared with other lesions showing trichilemmal keratinization.