Effect of Epo treatment on bone marrow stem and progenitor cells. Mice were treated with human recombinant Epo for 3 days. Epo dose-dependently increased the number of erythroid CFUs in mice (A). A similar increase in response to treatment with Epo was observed for GM-CFUs (B). FACS analysis showed that Epo significantly enhanced the total number of lin^–/sca-1⁺ stem cells (C). Further analysis using BrdU uptake revealed that Epo treatment caused a significant increase in the percentage of proliferating lin^–/sca-1⁺ stem cells (D). (E) The percentage of total bone marrow cells that are in S-phase was significantly enhanced by Epo (iii). Cells in S-phase are defined as a typical horseshoelike population after staining with BrdU and 7AAD (i-ii). (F) The stimulatory effect of Epo on stem cell proliferation caused a significant increase in the percentage of EPCs defined as CD34⁺/flk-1⁺ cells. (G) Murine EPCs can also be identified by positive staining for sca-1 and flk-1. Results for sca-1⁺/flk-1⁺ cells showed a similar increase after 3 days of treatment with Epo. Data are shown as means ± SD.