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

Stephane Commo, Olivier Gaillard and Bruno A. Bernard Hair Biology Group, L’Oréal, 90 rue du général Roguet, 92110 Clichy, France.

Up to now, the localisation 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 localised 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 characterise stem cell distribution in human hair follicle throughout the hair cycle. We found that anagen human hair follicle contained two distinct reservoirs for stem cells, located in the upper and lower thirds of the follicle. These two reservoirs fuse during catagen-telogen transition phase and individualise again in the newly forming anagen hair follicle.