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The discovery of long-lived epithelial stem cells in the bulge region of the hair follicle led to the hypothesis that epidermal renewal and epidermal repair after wounding both depend on these cells. To determine whether bulge cells are necessary for epidermal renewal, here we have ablated these cells by targeting them with a suicide gene encoding herpes simplex virus thymidine kinase (HSV-TK) using a Keratin 1–15 (Krt1-15) promoter. We show that ablation leads to complete loss of hair follicles but survival of the epidermis. Through fate-mapping experiments, we find that stem cells in the hair follicle bulge do not normally contribute cells to the epidermis which is organized into epidermal proliferative units, as previously predicted. After epidermal injury, however, cells from the bulge are recruited into the epidermis and migrate in a linear manner toward the center of the wound, ultimately forming a marked radial pattern. Notably, although the bulge-derived cells acquire an epidermal phenotype, most are eliminated from the epidermis over several weeks, indicating that bulge stem cells respond rapidly to epidermal wounding by generating short-lived 'transient amplifying' cells responsible for acute wound repair. Our findings have implications for both gene therapy and developing treatments for wounds because it will be necessary to consider epidermal and hair follicle stem cells as distinct populations. [ABSTRACT FROM AUTHOR]

The hair follicle bulge possesses putative epithelial stem cells. Characterization of these cells has been hampered by the inability to target bulge cells genetically. Here, we use a Keratin1-15 (Krt1-15, also known as K15) promoter to target mouse bulge cells with an inducible Cre recombinase construct or with the gene encoding enhanced green fluorescent protein (EGFP), which allow for lineage analysis and for isolation of the cells. We show that bulge cells in adult mice generate all epithelial cell types within the intact follicle and hair during normal hair follicle cycling. After isolation, adult Krt1-15-EGFP-positive cells reconstituted all components of the cutaneous epithelium and had a higher proliferative potential than Krt1-15-EGFP-negative cells. Genetic profiling of hair follicle stem cells revealed several known and unknown receptors and signaling pathways important for maintaining the stem cell phenotype. Ultimately, these findings provide potential targets for the treatment of hair loss and other disorders of skin and hair. [ABSTRACT FROM AUTHOR]