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In mammalian skin, stem cell factor (SCF) regulates the proliferation and maturation of mast cells and melanocytes, which are thought to be the only cutaneous cells that express the Kit-tyrosine kinase receptor (Kit) and respond to epithelial and mesenchymal-derived SCF. We previously had noted, however, the presence of Kit þ cells in murine hair follicles, in an introepithelial tissue compartiment devoid of melanocytes and mast
cells. Here we have identi¢ed the nature of this Kit þ population of cells in hair follicles of C57BL/6 mice. Anagen hair follicles showed strong Kit immunoreactivity not only in the pigmentary unit above the follicular dermal papilla but also in a much more proximally located, homogenous group of nondendritic, nonmelanized cells. By immunohistochemistry (desmoplakin þ/ Trp-1^) and electron microscopy (presence of tono¢laments, desmosomes, lack of melanosomes), these Kit þ cells were shown to be hair matrix keratinocytes and were also found in melanocyte-de¢cient hair follicles (KitSl/KitSl^d mice, Kit-neutralizing antibody-treated C57BL/6 mice). Expression of Kit and SCF was strongly hair-cycle-dependent, suggesting a functional role of epithelial Kit expression in hair growth control. This was supported by the observation that mice unable to respond to SCF stimulation (KitW/KitW^v) showed a signi¢cant retardation of anagen development compared to their wild-type littermates. The expression of Kit in the most rapidly proliferating compartment of the hair follicle epithelium suggests intriguing, as yet unexplored new functions of Kit signaling in epithelial cell biology.