Proliferating trichilemmal cyst with apocrine-acrosyringeal and sebaceous differentiation.

An adnexal tumor on the scalp of a 74-year-old woman is described. Histologically, the tumor was composed of cystic structures showing typical trichilemmal keratinization. The tumor cells in the cyst walls often formed duct-like or squamous eddy-like structures and occasionally showed vacuolation or poroma-like change. Ultrastructurally, some tumor cells showed differentiation either toward the acrosyringium or sebaceous cells. From these findings, the present tumor is considered to differentiate toward various parts of the hair follicle including infundibulo-isthmus epithelium, apocrine acrosyringium, and sebaceous cells.