Biochemical analysis indicated that the human nail plate contains two distinct types of keratin (skin-type and hair-type keratins), and several population of keratinocytes are thought to be associated with development of the nail. To elucidate the nature of the differentiation occurring in nail development, we examined the patterns of molecular markers relevant to the course of differentiation in the skin and hair in the nail matrix as well as in cultured nail cells. The nail matrix was characterized by the mutually exclusive localization of skin-type and hair-type markers, while in the apical matrix the localization of two groups of keratins partially overlapped. Double-label immunofluorescence showed the existence of unusual cells coexpressing both keratins, thereby indicating that the nail matrix consists of skin-type and hair-type and additionally intermediate-type differentiating cells. The cultured cells taken from the ventral matrix which develop under hair-type differentiation in vivo were found to express skin-type along with hair-type keratins, suggesting alteration of the pathway of differentiation in vitro. The cellular diversification as seen in in vitro cultured cells provides further insight into nail differentiation which is related to the multiple patterns of keratin expression that generate in the nail matrix.
