Hair Development, Growth, and Loss 56 mammalian skin and hair color by initiating a signaling cascade to increase cytoplasmic cyclic AMP levels for eumelanogenesis. By antagonizing with MC1R, agouti protein switches melanocytes to the production of pheomelanin.65 In mice, more than 50 genetic loci have been identified that are involved in the regulation of melanogenesis in hair follicles. In murine pelage hair follicles, these include c-Kit, stem cell factor (SCF), and hepatocyte growth factor. Recently, expression of the fork-head family transcription factors, Foxn1, and activation of its downstream target Fgf2 in pigment-recipient keratinocytes were identified as the major regulators in determining the patterning of hair pigmentation.66 Although the life cycle of a melanocyte is tightly integrated with the cyclical nature of the hair follicle, hair follicle melanocytes and their progenitors appear to age faster than their epidermal counterparts. Hair graying is thus the direct reflection of a Figure 6 The steps of the biosynthetic pathway for melanin including both eumelanins and pheomelanins image courtesy of Dr. Raymond Boissy