KIT expression reveals a population of precursor melanocytes in human skin

Human skin is believed to harbor a reservoir population of precursor melanocytes. It has been difficult to identify these putative cells experimentally, because they lack phenotypic features that define mature melanocytes. We have evaluated expression of the KIT tyrosine kinase receptor, which is critical for melanocyte development, as a possible marker of these cells. Sections of human skin were evaluated with single- and double-immunolabeling techniques. KIT-reactive dendritic cells were identified in the basal layer of the epithelia and were most numerous in the follicular infundibula and the rete ridges. These cells were located on the epithelial side of the basement membrane and lacked expression of cytokeratin and mast cell tryptase. The location of the KIT-reactive cells was distinctly different from that of Langerhans cells (identified with anti-CD1a) or Merkel cells (identified with CAM 5.2). Within the epidermis and upper follicular infundibulum the majority of the KIT-reactive dendritic cells also coexpressed TRP-1, a marker present in differentiated melanocytes. In the deeper follicular regions, the coexpression of TRP-1 in the KIT-reactive cells was absent. Throughout the epidermis and follicle, however, the KIT-reactive cells coexpressed BCL-2, a marker known to be increased in melanocytes. Thus, KIT expression reveals a population of intraepithelial cells that have immunophenotypic characteristics of mature melanocytes within the upper epithelial regions, but lack the differentiated melanocytic phenotype within the deeper follicular regions. We propose that these KIT(+), BCL-2(+), and TRP-1(-) cells constitute a precursor melanocyte reservoir of human skin.