Development of EPCs from EpoR-HM mice is attenuated at a Ter119-positive, CD71^high stage. (A) EPCs in marrow preparations from EpoR-HM and wt-EpoR mice were expanded in the presence of Epo at 0.1, 0.4, or 1.6 U/mL. At 42 hours of culture, Ter119 and CD71 expression levels were assayed. Indicated in representative flow profiles (as percents of total events) are frequencies of Ter119-positive CD71⁺ populations (H gate, Ter119-positive CD71^high; L gate, Ter119-positive CD71^low EPCs). (B) Ter119-positive CD71^high and CD71^low EPCs were isolated by FACS and stained as cytospin preparations. For each, representative sets of cells are shown (predominantly proerythroblasts and basophilic erythroblasts as CD71^high EPCs and chromatophilic erythroblasts as CD71^low EPCs) (original magnification, × 400). (C) Graphed are mean ratios (plus or minus standard deviation) of CD71^low/CD71^high Ter119-positive cells formed in the presence of Epo at 0.1, 0.4, or 1.6 U/mL for EpoR-HM versus wt-EpoR mice (n = 3 + n = 3 mice).