Faltered Epo-induced reticulocyte formation in EpoR-HM mice and attenuated maturation of EpoR-HM erythroblasts in vitro. (A) Deficient reticulocyte production in Epo-treated EpoR-HM mice. At 1 and 24 hours, Epo was administered to wt-EpoR, EpoR-HM, and EpoR-H mice (2.5 U/g). On day 5, induced levels of reticulocytes were assayed. Illustrated are representative flow cytometric profiles of thiazole orange staining, together with mean reticulocyte values (± SE) (n = 4 wt-EpoR, EpoR-HM, and EpoR-H mice per group). (B) Attenuated formation of low FALS CD71^pos Ter119^pos EpoR-HM erythroblasts in vitro. Bone marrow–derived erythroid progenitor cells were expanded for 3 days in SP34-EX medium and were then transferred to differentiation medium (containing Epo, insulin, and transferrin). At 40 hours of culture, frequencies of maturing CD71^highTer119^pos erythroblasts were analyzed by flow cytometry (Bi, left panels). Maturation was also assessed based on transitions to low side-angle and forward-angle light scatter populations (Bi, right panels). In panel Bii, defects in this transition for EpoR-HM erythroblasts are graphically summarized. The numbers in quadrants (clockwise from bottom left) indicate the percentage of CD71^negTer119^neg, Ter119^negCD71^pos, and CD71^posTer119^high cells among total live-gated cells. Plotted values are mean values plus or minus SE; n = 3 wt-EpoR, EpoR-HM, and EpoR-H mice per group, representative of 3 experiments.