BM erythroid cells develop in vivo as discrete Kit⁺CD71^highTer119⁻ “E1,” Kit⁻CD71^highTer119⁻ “E2,” and Kit⁻ CD71^highTer119⁺ “E3” cohorts, among which stage E2 proerythroblasts compose a uniquely expandable pool. (A-B) Defining of stage E1, E2, and E3 BM cohorts and their EPO-induced expansion potentials. At 1 and 24 hours, mice were dosed with saline (control) or EPO (1200 U/kg). At days 1.5, 3, and 4.5, femoral BM cells were isolated and analyzed for KIT, transferrin receptor (CD71), and Ter119 marker expression (via flow cytometry). Kit⁺CD71^highTer119⁻ cells are designated as stage “E1,” Kit⁻CD71^highTer119⁻ as stage “E2,” and Kit⁻CD71^highTer119⁺ cells as stage “E3.” (A) At days 1.5 to 3, levels of CFUe-like stage E1 cells increased ∼ 1.6-fold. Stage E2 progenitors, in contrast, expanded up to 6.7-fold, whereas frequencies of stage E3 erythroblasts (∼ 30%) were modulated 1.3- to 1.8-fold. Values are mean frequencies ± SE (n = 3). (B) Representative flow cytometric analyses. (C) Dynamics of E1, E2, and E3 pools at steady state and as affected by EPO (at day 3) are summarized.