Stem cells and the skin.

Stem cells live long lives, renew themselves, and differentiate into more mature, less potent, specialized cells, such as epidermal keratinocytes and dermal fibroblasts. Stem cells can be embryonic, if derived from an embryo, or adult/somatic if derived from postembryonic tissue. By producing new skin cells, stem cell division and differentiation can potentially rejuvenate skin and restore hair. To reproduce, stem cells can undergo symmetric nondifferentiative or differentiative divisions, or asymmetric differentiative divisions. Asymmetric divisions reproduce the stem cell and provide a more differentiated, but less potent transient amplifying cell. Divisions and differentiation of transient amplifying cells regenerate tissues by producing cells of a specific lineage, for example, keratinocytes. Epidermal stem cells lie in niches in the interfollicular epidermis, sebaceous gland, and in the bulge regions of hair follicles. These epidermal stem cells renew the epidermis, the sebaceous glands, and hair follicles after mature cells die. Dermal stem cells lie in the hair papillae, around pericytes, and elsewhere among other dermal cells. These form pericytes, myoblasts, fibroblasts, chondrocytes, and other specialized dermal cells. Along with other signaling pathways, the Wnt signaling pathway controls stem cell fate. Wnt signals enlist two functionally and chemically different gene coactivators to direct the time and type of replicative divisions. Stem cells may help to heal wounds, repair damaged tissues, regenerate aged skin, and reinvigorate growth of skin, hair, nails, and mucous membranes.