Analysis of BMP4^R stress erythroid progenitors during and after the recovery from acute anemia. (A) C57BL/6 mice were injected with PHZ to induce acute anemia. Spleen cells were isolated on the indicated days. BMP4^R cells were measured by determining the fold increase in stress BFU-E when the number of stress BFU-E generated when cells were plated in Epo (3 U/mL) plus BMP4 (15 ng/mL) was compared with the number of stress BFU-E generated when cells were plated in Epo alone. (B) C57BL/6 mice were treated with PHZ to induce anemia and allowed to recover for 7 days. The mice were then challenged with a second dose of PHZ as indicated. At 36 hours after the second dose, stress BFU-E were measured by plating spleen cells in methylcellulose media containing Epo (3 U/mL) alone. The response of an untreated mouse 36 hours after the initial dose of PHZ is shown in black. For all assays, at least 3 mice were used at each time point.