• adult skin;
  • stem cells;
  • K15;
  • rete ridges;
  • flow cytometry

Abstract The location and identity of interfollicular epidermal stem cells of adult human skin remain undefined. Based on our previous work in both adult murine and neonatal human foreskin, we demonstrate that cell surface levels of the α6 integrin and the transferrin receptor (CD71) are valid markers for resolving a putative stem cell, transit amplifying and differentiating compartment in adult human skin by flow cytometry. Specifically, epidermal cells expressing high levels of α6 integrin and low levels of the transferrin receptor CD71 (phenotype α6briCD71dim) exhibit several stem cell characteristics, comprising a minor population (2%–5%) of the K14bri fraction, enriched for quiescent and small blast-like cells with high clonogenic capacity, lacking the differentiation marker K10. Conversely, the majority of K14bri K10neg epidermal cells express high levels of CD71 (phenotype α6briCD71bri), and represent the actively cycling fraction of keratinocytes displaying greater cell size due to an increase in cytoplasmic area, consistent with their being transient amplifying cells. The α6briCD71bri population exhibited intermediate clonogenic capacity. A third population of K14dim but K10 positive epidermal cells could be identified by their low levels of α6 integrin expression (i.e. α6dim cells), representing the differentiation compartment; predictably, this subpopulation exhibited poor clonogenic efficiency. Flow cytometric analysis for the hair follicle bulge region (stem cell) marker K15 revealed preferential expression of this keratin in α6bri cells (i.e., both stem and transient amplifying fractions), but not the α6dim population. Given that K15 positive cells could only be detected in the deep rete ridges of adult skin in situ, we conclude that stem and transient amplifying cells reside in this location, while differentiating (K15 negative) cells are found in the shallow rete ridges.