Specification And Function Of Early Hair Follicle Stem Cells

In adult skin, multipotent epithelial stem cells (SCs) reside in a quiescent niche associated with each hair follicle termed the “bulge†and are essential for cyclic bouts of hair growth. Bulge stem cells can also contribute to sebaceous glands and the intefollicular epidermis when it is injured. While hair follicles begin to develop during embryogenesis, niche architecture becomes pronounced postnatally at the start of the first hair cycle. Whether SCs exist or function earlier is unknown. To determine if a stem cell population exists during follicle development, I conducted embryonic pulse-chase experiments to identify putative stem cells based on slow-cycling character. Slow-cycling cells appeared early in follicle development, where they expressed numerous SC markers, and later gave rise to the adult bulge SC population. Furthermore, early slow-cycling cells displayed a similar pattern of divisions during follicle development as activated adult bulge stem cells displayed during the postnatal hair cycle. To test whether early slow-cycling cells function as SCs, I focused on Sox9, an essential adult SC gene also expressed by early slow-cycling cells. Using Sox9-Cre for genetic marking and K14-Cre to embryonically ablate Sox9 revealed that the progeny of Sox9-expressing cells contribute to all skin epithelial lineages and that Sox9 is required for early SC specification. In the absence of early SCs, hair follicle and sebaceous gland morphogenesis is blocked and epidermal wound repair is compromised. To begin dissecting the molecular pathways that are required for early SC specification, a microarray screen was conducted to identify genes regulated by Sox9 during in vivo SC specification. Finally, purification of Sox9 expressing cells from the skin using a novel, double transgenic mouse reporter system directly demonstrated that early Sox9-positive cells display extensive colony forming ability in vitro, a behavior consistent with SC character. These findings establish the existence of early hair follicle SCs and reveal their physiological importance in tissue morphogenesis. Future studies of these early SCs should deepen our understanding of how SC populations are established in different tissues, and the molecular mechanisms that regulate their activity in development, adult homeostasis, and disease