Induction of let-7 in FL hematopoietic cells. (A) Pregnant females carrying iLet-7 embryos and littermate controls were exposed to doxycycline for 2–4 d before harvesting at E14.5. FL mononuclear cells were cultured in methylcellulose for 14 d in the presence of doxycycline, when colony morphology was scored. Proportions of each colony type are presented. *, P < 0.05 by Student’s t test for each colony type comparison. n = 8 control and 4 iLet-7 embryos across three experiments. CFU-M, CFU monocyte; CFU-G, CFU granulocyte; CFU-GM, CFU granulocyte–monocyte. (B) let-7g induction in the FL at E14.5 after 4-d maternal doxycycline exposure as measured by qPCR. *, P < 0.0001 by Student’s t test. n = 7 WT and 2 iLet-7 embryos from two independent experiments. (C and D) FL cells from the indicated genotypes were stained with the indicated antibodies, and myeloid progenitor populations were analyzed by flow cytometry. Representative histograms are shown. (E and F) FL cells were stained with the indicated antibodies to assess mature granulocyte populations. P = 0.05 by Student’s t test for control and iLet-7 comparison of Gr1⁺/Mac-1⁺ cells. (G and H) Whole FL cells were stained with anti-CD71 and anti-Ter119 antibodies to analyze red blood cell progenitor populations by flow cytometry. (C–H) *, P < 0.05 comparing CD71⁺/Ter119⁻ cells. A total of 10 control and 8 iLet-7 embryos were evaluated, obtained from three independent litters. Error bars represent SEM.