Abstract: Nestin is an intermediate filament protein transiently expressed in neural stem/progenitor cells. We previously demonstrated that outer root sheath (ORS) keratinocytes of adult hair follicles (HFs) in mice descend from nestin-expressing cells, despite being an epithelial cell lineage. This study aimed to define the exact stage when nestin-expressing ORS stem/precursor cells or their descendants appear during HF morphogenesis, and whether they are also recognized in adult HFs. Using Nes-Cre/CAG-CAT-EGFP mice, in which enhanced green fluorescent protein (EGFP) is expressed following Cre-based recombination driven by the nestin promoter, we found that EGFP+ cells appeared in the epithelial layer of embryonic HFs as early as the peg stage. EGFP+ cells in hair pegs were positive for keratin 5, but not vimentin, Sox2, Sox10, or S100 alpha 6. Tracing of tamoxifen-induced EGFP+ cells in postnatal Nes-CreERT2/CAG-CAT-EGFP mice revealed labeling of some isthmus HF epithelial cells in the first anagen stage. EGFP+ cells in adult HFs were not immunolabeled for keratin 15, an HF multipotent stem cell marker. However, when hairs were depilated to induce the anagen stage after tamoxifen injection, the majority of ORS keratinocytes in depilation-induced anagen HFs were labeled for EGFP. Our findings indicate that nestin-expressing unipotent progenitor cells capable of differentiating into ORS keratinocytes are present in both the HF primordia and adult HFs.

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