Keratin 19 as a biochemical marker of skin stem cells in vivo and in vitro: keratin 19 expressing cells are differentially localized in function of anatomic sites, and their number varies with donor age and culture stage.

This study was undertaken to evaluate keratin 19 (K19) as a biochemical marker for skin stem cells in order to address some long standing questions concerning these cells in the field of cutaneous biology. We first used the well-established mouse model enabling us to identify skin stem cells as [3H]thymidine-label-retaining cells. A site directed antibody was raised against a synthetic peptide of K19. It reacted specifically with a 40 kDa protein (K19) on immunoblotting. It labelled the bulge area of the outer root sheath on mouse skin by immunohistochemistry. Double-labelling revealed that K19-positive-cells were also [3H]thymidine-label-retaining cells, suggesting that K19 is a marker for skin stem cells of hair follicles. K19-expression was then used to investigate the variation in mouse and human skin stem cells as a function of body site, donor age and culture time. K19 was expressed in the hair follicle and absent from the interfollicular epidermis at hairy sites (except for some K18 coexpressing Merkel cells). In contrast, at glabrous sites, K19-positive-cells were in deep epidermal rete ridges. K19 expressing cells also contained high levels of alpha 3 beta 1 integrin. The proportion of K19-positive-cells was greater in newborn than older foreskins. This correlated with keratinocyte culture lifespan variation with donor age. Moreover, it could explain clinical observations that children heal faster than adults. In conclusion, K19 expression in skin provides an additional tool to allow further characterization of skin stem cells under normal and pathological conditions in situ and in vitro.