Figure 1. Heterogeneity of epidermal and hair follicle stem cells.
Skin epithelia feature distinct stem cell populations both in epidermis and, most prominently, in hair follicles. Epithelial stem cells in different micro-anatomical locations have different lineage potentials. Stem cells in the follicular infundibulum and inter-follicular epidermis physiologically are restricted to epidermal fate. Inter-follicular epidermal stem cells can be identified as slow-cycling in label retention studies, but distinct markers remain elusive. The isthmus and junctional zone of hair follicles harbor several distinct epithelial cell populations. Most prominent among them are Lrig1+ (yellow), Gli1+ and Lgr6+ stem cells (green), all of which physiologically maintain the isthmus and contribute to sebaceous gland, infundibulum and in some instance to inter-follicular epidermis. Blimp1 identifies unipotent sebaceous gland progenitors (orange). The bulge stem cells (blue) normally contribute to all hair follicle lineages and can be identified based on the expression of Krt15, CD200, Lgr5, CD34, Sox9, Lhx2, Tcf3 and Nfatc1. The secondary germ of telogen hair follicles (purple) contains committed hair follicle-fated progenitors that express CD200, Gli1 and Lgr5.