Impaired hematopoiesis in Brd1^−/− fetal liver. Appearance of wild-type (A) and Brd1^−/− (B) embryos at 12.5 dpc. H&E-stained transverse sections of 12.5 dpc wild-type (C) and Brd1^−/− (D) embryos. Morphology of 12.5 dpc fetal liver hematopoietic cells from wild-type (E) and Brd1^−/− (F) embryos stained with May-Grüenwald-Giemsa solutions. Arrows and arrowheads indicate mature erythroblasts and nucleated erythrocytes, respectively. Frequency (G) and absolute cell numbers (H) of c-Kit⁺ hematopoietic progenitors, CD45⁺ hematopoietic cells, Ter119⁺ erythroid cells, and Dlk1⁺ hepatoblasts in 12.5 dpc fetal livers from wild-type and Brd1^−/− embryos. The results are shown as the mean ± SE (n ≥ 4). *P < .05, ***P < .0005. (I) Flow cytometric profiles of erythroid differentiation defined by CD71 and Ter119 expression in representative fetal livers at 12.5 dpc. The percentage of each fraction is indicated. (J) Frequency (top) and absolute cell numbers (bottom) of BFU-E, CFU-erythroid, CD71⁻Ter119⁻ cells, CD71⁺Ter119⁻ erythroblasts, CD71⁺Ter119⁺ erythroblasts, and CD71⁻Ter119⁺ erythroblasts in 12.5 dpc fetal livers from wild-type and Brd1^−/− embryos. The results are shown as the mean ± SE (n ≥ 8). **P < .005, ***P < .0005. (K) Massive apoptosis of Brd1^−/− erythroblasts. The percentage of annexin V⁺/7-aminoactinomycin D⁻ (7-AAD⁻) apoptotic cells in each fraction defined by CD71 and Ter119 is shown as the mean ± SE (n ≥ 4). *P < .05, **P < .005.