Hair follicle lineage and niche signals regulate hair follicle stem cells. (a) HFSCs can exist in two states. Quiescent bulge stem cells (Bu-SCs) are located in the outer layer of this niche and contribute to the generation of the outer root sheath. Primed stem cells reside in the hair germ, sandwiched between the bulge and a specialized dermal cluster known as the dermal papilla. They are responsible for generating the transit amplifying cell (TAC) matrix, which then gives rise to the hair shaft and its inner root sheath (IRS) channel. Although matrix and IRS are destroyed during catagen, many of the outer root sheath (ORS) cells are spared and generate a new bulge right next to the original one at the end of catagen. The upper ORS contributes to the outer layer of the new bulge, and the middle ORS contributes to the hair germ. Some of the lower ORS cells become the differentiated inner keratin 6+ (K6+) bulge cells, which provide inhibitory signals to Bu-SCs, raising their activation threshold for the next hair cycle. (b) During telogen, K6+ bulge cells produce BMP6 and FGF-18, dermal fibroblasts (DFs) produce BMP4 and subcutaneous adipocytes express BMP2. Together, these factors maintain Bu-SCs and hair germ in quiescence. At the transition to anagen, BMP2 and BMP4 are downregulated, whereas the expression of activation factors including noggin (NOG), FGF-7, FGF-10 and TGF-2 from dermal papillae and PDGF- from adipocyte precursor cells (APCs) is elevated. This, in turn, stimulates hair germ proliferation, and a new hair cycle is launched. Bu-SCs maintain their quiescent state until TAC matrix is generated and starts producing SHH.

Go here to read the rest:
Emerging interactions between skin stem cells and their ...