Figure 4. The geography of stem cells within the hair follicle.

Several distinct and overlapping populations of cells have been identified within the telogen hair follicle. The cells that serve as a reservoir for the anagen hair follicle, including McSCs, reside in the sHG and bulge. The sHG sits above the DP during telogen and expresses specific markers (e.g., P-cadherin), while the cells of the bulge upregulate bulge-specific markers (e.g., CD34, Nfatc1, S100A4). Some markers are expressed in both the sHG and bulge in an overlapping manner (e.g., K15, Lgr5). In the isthmus region (between the bulge and opening of the sebaceous gland into the pilary canal), there are other stem cell populations that maintain the sebaceous gland and/or epidermis. Lgr6 and Lrig1 are expressed in isthmus cells and give rise to the sebaceous gland and epidermis. In adult mouse hair follicles, Blimp1 cells are located near or at the opening of the sebaceous gland to the hair canal and appear to act as unipotent sebaceous gland progenitors.