Adult stem cells (SCs) are tissue-specific resident cells that serve to replenish and maintain the tissue. Two significant SC populations co-exist in the hair follicle: hair follicle SCs (HFSCs) and melanocytes SCs (McSCs). These populations are localized to the lower part of the follicle during the resting phase. Upon hair cycle activation these SCs divide generating differentiated progenitors for hair construction and pigmentation. As long-lived cells, SCs are exposed to environmental stress for long periods of time. Therefore, they are equipped with pro-survival mechanisms which prevent their loss by cell death processes, primarily via apoptosis. While the anti-apoptotic nature of HFSCs has been previously investigated, very little is known regarding apoptotic regulation in McSCs.
We found that chemical inhibition of apoptotic related proteins in vivo result in the rapid elimination of McSCs, leading to irreversible coat-depigmentation (whitening). Interestingly, local damage to HFSCs niche by hair depilation acquire McSCs with resistance to cell death. Furthermore, we found that hair depilation induces release of pro-survival factors that enhance McSC resistance to cell death. These findings pave the way for therapeutic approaches for pigmentation disorders that originate from McSC-pool depletion.