Differentiation kinetics of BM and spleen erythroid cells under steady-state and stress conditions. (A) Comparison of hematopoietic lineage Cre specificity between G1B- and EpoR-Cre mice. The percentage of TdT⁺ cells among erythroid precursors and progenitors, as well as among lymphoid (B220) and myeloid (CD11b and Gr1) cells, was calculated in G1B:R26T mice 5 days after the last Tx injection (T+) (orange bars) and in EpoR:R26T mice (green bars). Untreated (T-) G1B:R26T mice (purple bars) were included as controls. Data are shown as mean ± standard deviation from 3 mice. (B) Erythroid differentiation kinetics for PreMegE, PreCFU-E, CFU-E, and ProBasoE cells under steady state (5-FU-; left column) or 5-FU-induced stress erythropoiesis (5-FU+; middle and right columns). The percentages of TdT⁺ cells in G1B:R26T BM (left and middle columns) or spleen (SP; right column) were quantified by flow cytometry at various time points (1, 4, 7, 14, and 21 days after the final Tx injection). Red circles represent data points and green triangles denote T_C50. For each time point, 3 to 4 mice were analyzed. (C) Erythroid differentiation time (days) from PreMegE to PreCUF-E, PreCFU-E to CFU-E, CFU-E to ProBasoE, and PreMegE to ProBasoE. Red bars represent the time of erythroid differentiation (days) of the BM cells under steady-state conditions. Blue bars represent the erythroid differentiation time of BM cells under 5-FU-induced stress conditions. Green bars represent the erythroid differentiation time of splenic cells in the setting of 5-FU-induced stress. Data are shown as mean ± standard deviation from 3 or 4 experiments. ∗P < .05; ∗∗P < .01; ∗∗∗P < .001; unpaired Student t test. LSK, lineage- scal1+ kit+ cells; n.s., not significant; PreGM, pre-colony-forming unit-granulocyte-macrophage.