Merkel cells in ontogenesis of human nails

Digital skin of human fetuses is known to contain a particularly high concentration of Merkel cells. Using antibodies against the simple epithelial cytokeratins (CK) 18 and 20, which are sensitive and specific Merkel cell markers, we studied imiminohistochemically the main adnexal structure of digital skin, the nail anlage, in human fetuses (9–22 weeks of gestation) for the presence of Merkel cells. As early as week 9 some clustered Merkel cells were detected in the early matrix primordium. In specimens of week 12–15, abundant Merkel cells were found in the nail anlagen, particularly in the epithelium of the proximal nail-fold and the dorsal and ventral side of the apex region. In contrast, Merkel cells were essentially absent from the epithelium of the ventral matrix (surface-near portion), lunula and nail bed. Correspondingly, in these regions, the adjacent dermis contained hardly any nerve fibres, whereas such fibres, as detected by neurofilament antibodies, were quite numerous adjacent to the proximal nail-fold epithelium. At week 22, the Merkel cell number in the nail anlage had decreased, and in adult nail matrix such cells were very rare. No Merkel cells were found in the dermal tissue surrounding the nail anlage while finger-tip skin of week 15, and particularly of week 22, exhibited single Merkel cells in the upper dermis next to clusters of such cells in the glandular ridges. We also found that Merkel cells were negative for CK 17. These results suggest a possible role for fetal Merkel cells in the proper development of sensory nerve fibres, while they do not seem to be important for the growth of trichocytic tissues, i.e. the hair bulb and the trichocytic portion of the nail anlage.