Targeted elimination of the follicular label-retaining cells by photo-induced cell killing caused a defect on follicular renewal on mice

Background: One of the most well-known ways to identify stem cells is to search for the slow-cycling cells (label-retaining cells; LRCs), by labelling their DNA. The bulge of hair follicle is assumed to identify the location of the follicular stem cells. While this assumption was supported by previous analyses of cell tracing, it is still not clear whether the LRCs are critical for follicular renewal. Results: To solve this problem, we tried to selectively eliminate the bromodeoxyuridine (BrdU)-labelled follicular LRCs by a photo-induced cell killing method, in combination with Hoechst 33258 treatment. We labelled the slow-cycling cells in the follicular bulge by repeated administration of the BrdU, followed by a chase period. When the labelled mice skin was irradiated at the telogen phase after the Hoechst 33258 treatment, we observed apoptotic cells in the bulge area 18 h after the irradiation and a defect on the follicular renewal at the next anagen phase. The time course of appearance of hair defect suggests that only the late telogen follicle is sensitive to irradiation. Conclusions: This finding suggests that the LRCs contribute to the renewal of the hair follicle, and they might proliferate at a restricted point in the hair cycle to supply hair germinative cells as previously predicted.