K5-tetVP16/TRE-H2BGFP double transgenic mice were chased at E18.5 to shut off H2B-GFP expression and identify label retaining cells (LRCs) within the early postnatal skin epithelium (P2-P8). (A) Appearance of LRCs during HF morphogenesis. Shown are representative skin sections with green H2B-GFP epifluorescence and counterlabeling with indicated antibodies (Abs) (red). After 3d of chase, the brightest H2B-GFP LRCs in the HF reside in a narrow zone of the upper ORS (brackets in 1A-1B) and are distinct from rapidly proliferating (Ki67-positive) cells, abundant in the infundibulum (In) and matrix (Mx). Arrowheads indicate a trail of slightly dimmer H2B-GFP LRCs extending down the ORS. Dotted lines denote the basement membrane that separates the interfollicular epidermis (Epi) and HF from the underlying dermis (Der) and dermal papilla (DP). (B) Immunofluorescence reveals partial co-localization (brackets, arrowheads) of LRCs with Abs specific for transcription factors expressed preferentially in adult bulge cells. (C) Immunofluorescence shows that after 22d of chase, LRCs labeled during embryogenesis are found exclusively in the adult bulge (Bu) niche (CD34-positive) and secondary hair germ (HG), and not in the K5/K14-positive basal layer of the sebaceous gland or interfollicular epidermis (Epi). Scale bars, 50 μm.