Infundibular and trichilemmal keratinization of a pilar tumor.

Microscopic examination of a cystic epithelial tumor removed from the elbow of a 52-year-old man showed islands of epithelium undergoing keratinization without formation of a visible granular cell layer, as occurs within the follicular isthmus, in trichilemmal cysts, and in pilar tumors (proliferating trichilemmal cysts). Other areas showed keratinization with the formation of a distinct, even prominent, granular cell layer, as seen within the follicular infundibulum and in epidermal cysts. Possible modes of origin and relationships among these tumors and epidermal cysts are presented.