Deletion of Flk-1 in the erythroid lineage cannot rescue the +Vegf164^Eryth embryonic lethal phenotype. (A) Images of E10.5 embryos in yolk sac. Yolk sacs of +Vegf164^Eryth/Flk-1^ΔEpoRiCre embryos were paler and vessels containing red blood cells were absent. Yolk sac section shows decreased circulating cells in blood island, compared with Flk-1^ΔEpoRiCre embryos (magnification, 200×). (B) Definitive erythroid colonies (BFU-E and CFU-E) from E10.5 yolk sac; control (▭), +Vegf164^Eryth/Flk-1^ΔEpoRiCre (), and Flk-1^ΔEpoRiCre () yolk sac measured by methylcellulose assay. Cells (1 × 10⁵) were plated and results are given as percentage to control absolute number of colonies (100%). (C) Relative mRNA expression levels of erythropoietic transcription factors and Gata1 target genes. Bars represent mean ± SEM; *P < .05, **P < .01, ***P < .001.