Epidermal stem cells do not communicate through gap junctions.

Although enrichment of putative epidermal stem cells has been achieved, a need for additional markers that can enable isolation of live keratinocytes is crucial for characterization of these cells. Earlier work has shown that connexin proteins are absent from basal cells in the limbal epithelium, a region of the corneal epithelium enriched in corneal stem cells. Accordingly, we investigated whether connexin 43, a gap junction protein present in the basal layer of normal human epidermis, can serve as a negative marker for keratinocyte stem cells. In humans, cells with immunohistochemically undetectable levels of connexin 43 are found in the epidermal basal layer of neonatal foreskin and in the follicular bulge region. About 10% of the basal keratinocytes are connexin 43 negative, as determined by flow cytometry. These cells are uniformly small and low in granularity. Restricted gap junction communication was confirmed by the failure of low molecular weight dyes to transfer between cells. Experiments carried out in mouse epidermis demonstrated that most of the slowly cycling cells, detected as label-retaining cells, do not express connexin 43. Thus, presumptive keratinocyte stem cells can be identified and separated based on connexin 43 expression.