Figure 1. The location of LRCs during the hair cycle.

(A and B) During spontaneous anagen phase (A) or depilation-induced anagen phase (B), actively dividing sHG and bulge cells incorporate a pulse of BrdU nucleotide label. During anagen phase, cells within the sHG/bulge cycle infrequently and give rise to rapidly proliferating TA cells that migrate down to the bulb, which contains matrix cells. In contrast, slow-cycling cells from the sHG/bulge area retain the BrdU label and remain in the telogen sac and upper portion of the anagen ORS. During the chase period, rapidly dividing cells lose the BrdU label and, by the late anagen phase, can no longer be detected, while those slow-cycling cells in the upper hair follicle retain the BrdU label (LRCs). During the catagen phase, cells within the matrix and lower ORS undergo apoptosis, and the hair follicle retracts upward with the associated DP. LRCs remain as the permanent population of the ORS and can also be seen in the epithelial strand during the catagen phase. As the catagen phase proceeds, LRCs in the forming club hair follicle begin to upregulate bulge and sHG markers while entering the telogen phase. In the telogen phase, the sHG rests above the DP, while the bulge surrounds the club hair as an epithelial sac. (B) In depilation-induced anagen phase, nearly all bulge cells undergo apoptosis after the club hair is plucked and are quickly replenished by remaining proliferating sHG cells. As sHG cells repopulate the bulge, cells within the bulge region upregulate bulge-specific markers.