Epo rapidly induces the formation of PODXL⁺(pro)erythroblasts and reticulocytes within bone marrow. (A) Epo induction of the sequential formation of PODXL⁺ bone marrow Kit⁺CD71^high and CD71^highTer119⁺ erythroblasts. Wild-type mice were injected with Epo (1500 U/kg) at 1 and 24 hours. At days 0.5, 1.5, 2.5, 3.5, and 4.5, levels of marrow resident PODXL⁺Kit⁺CD71^high (pro)erythroblasts, NH₄CI-resistant PODXL⁺CD71^highTer119⁺ erythroblasts, and PODXL⁺CD71^highTer119⁺ erythroblasts were determined (left panels). For these populations, note the sequential waves of Epo-induced PODXL⁺ erythroblast formation (arrows). Total levels of these erythroblast cohorts also were determined (right panels). (B) Epo-induced formation of marrow-resident R5- and R4-PODXL⁺ reticulocytes (top panels). For R4 reticulocytes, also note the rapid Epo-induced decrease of this cohort within marrow (arrow, bottom right panel).