Hair Development, Growth, and Loss 54 melanin can help buffer the cellular stress induced by reactive oxygen in the hair and skin. In addition, the transfer of melanin from melanocytes to keratinocytes may promote differentiation of the melanin-receiving keratinocytes in the hair matrix.61 In mice, the progenitor cells of melanocytes are neural crest derived melanoblasts.62 Melanoblasts migrate through the dermis and invade the overlying epidermis, where they proliferate and then enter developing hair placodes, in which they finally become localized during morphogenesis. Once in the hair follicles, the development melanoblasts are segregated into two populations—one consisting of mature melanocytes in the hair bulb, which are responsible for pigmentation of the initial hairs, the other consisting of melanocyte stem cells, which are localized in the bulge region and are responsible for the maintenance of the hair follicle pigmentary system in the subsequent hair cycles. The melanocytes and melanoblasts that come to reside in the ORS normally do not engage in pigment production. Melanocyte stem cells are needed for replacing the vast majority of differentiated, melanotic hair follicle melanocytes that undergo apoptosis during catagen.63 Preservation of the hair follicle melanocyte stem cell pool and maintenance of the delicate balance between proliferation and differentiation Figure 5 Mature melanocytes around dermal papilla cells at the base of anagen hair follicle image courtesy of Dr. Raymond Boissy