Murine Epidermal Label-Retaining Cells Isolated by Flow Cytometry do not Express the Stem Cell Markers CD34, Sca-1, or Flk-1

Keratinocyte stem cells are present in the murine epidermis, based on both in vitro and in vivo evidence, and better characterization of these cells remains an active goal. Because keratinocyte stem cells are believed to cycle slowly, a good method for identification is based on their ability to retain nucleoside analog, such as bromodeoxyuridine. Adult stem cells have been identified in other tissues, including hematopoietic, neural, and skeletal muscle, and stem cell surface markers have been characterized. We wanted to determine if cell-surface markers present on both hematopoietic and skeletal muscle stem cells (CD34, Sca-1, and Flk-1) were also present on keratinocyte stem cells, and could be used to identify them. The cell-surface expression of cells that retained bromodeoxyuridine label for at least 21 d was compared with that of nonlabel-retaining cells. Double-labeling for flow cytometric analysis was employed, and label-retaining cells were found to lack expression of the tested markers. β1 integrin levels were also evaluated, and although high expression was found on label-retaining cells, it was not specific for these cells.