Flk1 KO mice are deficient in vascular ECs. (A) Flk1⁺/GFP and Flk1 GFP/GFP embryos at 8.5 dpc; (top) bright field images and (bottom) fluorescent images. EPC indicates ectoplacental cone; and BI, blood island. Arrows indicate dorsal aorta. Scale bar indicates 100 μm. (B) Reduced expression of the PECAM1 and Tie2 proteins in Flk1 KO embryos. Whole-mount IHC shows PECAM1 and Tie2 are expressed in ECs of the dorsal aorta and yolk sac of WT but not Flk1 KO embryos. Arrows indicate dorsal aorta. (C) Reduced expression of endothelial marker mRNAs, such as VE-cadherin and Pecam1, in Flk1 KO embryos at 8.5 dpc. Semiquantitative RT-PCR was performed on serially diluted cDNAs from WT and Flk1 KO embryos proper. Images were captured with a Leica DC500 CCD camera attached to a Leica MZ FLIII microscope (Plan APO 1.0/0.125 NA lens).