The human hair follicle contains two distinct K19 positive compartments in the outer root sheath: a unifying hypothesis for stem cell reservoir?

Up to now, the localization of stem cells in human anagen hair follicle relied on three complementary approaches; namely, detection of slow cycling cells, detection of high colony forming cells, and differential immunohistochemical staining. These techniques, however, gave conflicting results since stem cells were localized either as long label retaining cells in the so-called bulge area or as high colony forming cells in the lower third of the follicle. In the present study we investigated the expression of cytokeratin 19, a marker for putative stem cell-containing epithelial compartments, in order to characterize stem cell distribution in the human hair follicle throughout the hair cycle. We found that anagen human hair follicles contain two distinct reservoirs for stem cells located in the upper and lower thirds of the follicle. These two reservoirs fuse during the catagentelogen transition phase and individualize again in the newly forming anagen hair follicle.