Response of CD34KO epidermis and hair follicles to short-term TPA exposure. To test the ability of CD34KO skin to develop epidermal hyperplasia, 7-wk-old WT and CD34KO mice were subjected to short-term TPA exposure. Mice were dosed twice weekly for 2 wks with either 5 or 10 μg of TPA, and tissues were collected at 24 and 48 h after the last dose. In addition, beginning at postnatal day 3, CD34KO and WT pups were injected with BrdUrd twice daily for 3 d to generate LRCs. Tissues were fixed in formalin, sectioned, and stained with anti-BrdUrd to identify the quiescent LRCs both under steady-state conditions and after TPA treatment. A, H&E-stained, high-magnification photomicrographs of the interfollicular epidermis in WT (left) and CD34KO (right) to both doses of TPA, 48 h only. Bar, 20 μm. Arrowheads, basal cells. Compare the simple cuboidal cells of the CD34KO mouse with the irregular layers of larger elongated basal cells of WT mice. B and C, photomicrographs of H&E-stained skin taken 24 and 48 h after the last dose of either 4 × 5 μg TPA or 4 × 10 μg TPA. Bar, 100 μm. In WT mice, previously resting hair follicles enter the growing stage in response to TPA treatment. In contrast, hair follicles in CD34KO mice remain in telogen, the resting phase of the hair cycle. Left, WT images; right, CD34KO images. Compare the small inactive hair follicles and thin epidermis of CD34KO mice with the large active growing hair follicles extending deep into the thick dermis of WT mice. D, LRC localization in WT and CD34KO hair follicles after a 7-wk chase. Bar, 25 μm. E, LRC response after TPA treatment. Left, WT; right, CD34KO. Bar, 25 μm. Arrows, bulge. Note in (E) the highly cellular, active hair follicles in WT mice.