Hair follicle-like change over histiocytomas.

Epidermal hyperplasia is a common finding overlying histiocytomas. Occasionally, a distinctive type of basaloid hyperplasia occurs, with differentiation into primitive hair follicle tissue. This study attempts to characterize this change and determine whether it is associated with any particular clinical or histological feature. Data from patients with histiocytomas displaying induction of pilar epithelium in the epidermis above the lesions was compared with data from controls who did not show this change. The size, site, and duration of histiocytomas were similar in both groups, as were the predominant cell type and distance of histiocytoma from the epidermis. Serial sectioning of histiocytomas showed that the induction of pilar epithelium was a focal phenomenon, often missed in routine sections. It is suggested that histiocytoma cells may produce a "hair organizing mediator."