Figure 1.

Skin stem cells and their niches. The skin is a complex organ composed of cell types from diverse lineages. With the exception of higher primates, whose eccrine sweat glands and associated stem cells are highly abundant, most mammals have three major skin stem cell populations—epidermal stem cells, hair follicle stem cells, and melanocyte stem cells. These stem cells regenerate in rich microenvironments (niches) replete with diverse cell types, including distinct fibroblast populations, sensory and sympathetic innervations, a vast array of immune cells, blood vessels, lymphatic capillaries, and subcutaneous adipocytes. In addition to their specialized functions, complex interactions among these cell types underlie tissue formation, homeostasis, and repair in the skin. (TAC) Transit-amplifying cell, (DP) dermal papilla.