Deficient EpoR-HM reticulocyte production in response to Epo, and abnormal representation of anucleated red cells in EpoR-HM marrow. (A) For wt-EpoR, EpoR-HM, and EpoR-H mice, time courses of Epo induced in vivo reticulocyte production are graphed (left panel; means ± SE, n = 5 per group, 1200 U/kg). Frequencies of PODXL⁺ immature reticulocytes (IRF; day 3 after Epo) also are illustrated (right panel). (B) Wild-type (wt-EpoR), EpoR-HM, and EpoR-H mice were treated with Epo at 0, 1200, and 1800 U/kg. At day 3.5, levels of PODXL expression among immature reticulocytes were determined. (C) In bone marrow of wt-EpoR, EpoR-HM, and EpoR-H mice (at day 3 after Epo injection, 1500 U/kg), relative frequencies (ratios) of anucleated versus nucleated Ter119⁺ cells were determined based on DRAQ5 staining of Ter119⁺ cells. Error bars indicate the standard error from the mean value for n = 5 mice examined per group. (D) Model for Epo regulation of erythroid progenitor cell adhesion and migration within a proposed stromal niche. Epo's actions on Kit⁺CD71^high proerythroblasts are depicted to involve an Epo dose–dependent repression of Cxcr4 expression and an induction of PODXL. This Epo response is sustained as progenitors advance to a Kit⁻CD71^high erythroblast stage and exit a proposed stromal niche. Epo-dependent PODXL expression further persists among immature reticulocytes and is hypothesized to enhance their release to blood.