KIT EXPRESSION REVEALS A POPULATION OF PRECURSOR MELANOCYTES IN HUMANSKIN

Human skin is believed to harbor a reservoir population of precursor m elanocytes. It has been difficult to identify these putative cells exp erimentally, because they lack phenotypic features that define mature melanocytes. We have evaluated expression of the KIT tyrosine kinase r eceptor, which is critical for melanocyte development, as a possible m arker of these cells. Sections of human skin were evaluated with singl e- and double-immunolabeling techniques. KIT-reactive dendritic cells were identified in the basal layer of the epithelia and were most nume rous in the follicular infundibula and the rete ridges. These cells we re 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 ce lls (identified with anti-CD1a) or Merkel cells (identified with CAM 5 .2). Within the epidermis and upper follicular infundibulum the majori ty of the KIT-reactive dendritic cells also coexpressed TRP-1, a marke r present in differentiated melanocytes. In the deeper follicular regi ons, the coexpression of TRP-1 in the KIT-reactive cells was absent, T hroughout 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 ha ve immunophenotypic characteristics of mature melanocytes within the u pper epithelial regions, but lack the differentiated melanocytic pheno type within the deeper follicular regions. We propose that these KIT(), BCL-2(+), and TRP-1(-) cells constitute a precursor melanocyte rese rvoir of human skin.