Merkel cells in the vellus hair follicles of human facial skin: a study using confocal laser microscopy.

Many cases of Merkel cell carcinoma have recently been reported, and most of them have been localized on the facial skin. In this study, we investigated Merkel cells in the vellus hair follicles of facial region to characterize these cells in human subjects. Skin specimens doubly stained with cytokeratin (CK) 20 and either protein gene product (PGP) 9.5 or vasoreactive intestinal polypeptide (VIP) were examined by confocal laser microscopy. Many of the Merkel cells in the vellus hair follicles of the facial skin were localized in the bulge area. Some of these cells were attached to nerve terminals, although most of them were not associated with them. Our results suggest that there are two types of Merkel cells in the bulge area of the vellus hair follicles of facial skin: cells wholly unassociated with the nerve terminals and cells associated with thin nerve fibers. We postulate that the former cells may be undifferentiated (immature) and the latter differentiated (mature). If this is so, there is a chance that Merkel cell carcinoma originates from the undifferentiated Merkel cells in the bulge of the vellus hair with the formation of tumor masses in the dermis and no involvement of the epidermis. The Merkel cells connected with nerve fibers may secrete endocrine substances via a regulation of autonomic nerves.