Environmental Regulation of Skin Pigmentation and Hair Regeneration.

Skin bears essential appendages and diverse cell types that function importantly in protection, thermoregulation, mechanosensation, piloerection, and social communication. The hair follicle is a tiny skin appendage with intricate structure and has versatile functions in mammals. Hair follicles evolve stem cells that regenerate cyclically to produce hairs and to accommodate the rapidly changing environment. Sharing the same bulge niche with hair follicle stem cells (HFSCs), melanocyte stem cells (McSCs) undergo cyclic activation in synchrony with HFSCs, to pigment the hairs, which can protect the skin from ultraviolet radiation. Disorders of HFSCs or McSCs result in skin diseases such as hair loss, canities, vitiligo, and even melanoma, compromising senses of well-being and posing psychosocial distress to the affected individuals. The activation or quiescence of these stem cells is not only regulated by intrinsic factors within the follicle, but is also largely influenced by the extrinsic environmental factors, including the neighboring cells, systemic factors, and the external environment. Although great progress has been made to elucidate the intrinsic regulation of HFSCs or McSCs, understanding the environmental modulation of these stem cells can provide novel insight for the development of new therapeutic strategies for treating alopecia and skin pigmentation-related disorders.