Kinetics of erythroid recovery in the 3 compartments of the erythron after sublethal 4 Gy TBI. (A) Erythroid progenitor (d7 BFU-E, d3 BFU-E, CFU-E) kinetics in the bone marrow post–4 Gy TBI. Recovery initiates in the d3 BFU-E progenitor at 4 days after radiation followed by rapid expansion of CFU-E at 5 to 6 days after TBI. (B) Erythroblast precursor (ProE, BasoE, PolyE, OrthoE) kinetics in the bone marrow post–4 Gy TBI. Erythroid precursor recovery follows expansion of late-stage erythroid progenitors. Progenitors and precursors are normalized per femur and expressed as a percent of unirradiated control marrow. (C) Hematocrit (HCT) and reticulocyte (Retic) levels in the peripheral blood post–4 Gy TBI. Circulating red cell recovery occurs by 8 to 10 days in reticulocytes and by 14 days in RBC. Reticulocytes are calculated as absolute reticulocyte index (percent Retic × total RBC and Retic number × %HCT) and expressed as a percent of unirradiated control levels. RBC levels are expressed as percent HCT. Dotted lines represent unirradiated control levels. Dashed data points at 1 to 2 days after radiation represent erythroid injury data previously published⁸  and shown here for clarity. Error bars represent SEM of at least 3 experiments, and 3 or more independently assayed mice were used to determine each data point.