KLF2-/-KLF4-/- primary head vein lacks continuous endothelial layer at E9.5. Light micrographs of 6 μm sections were taken at 200X magnification. A) WT and B) KLF2-/- (n = 4) shows normal looking primary head vein; C) Two of the four KLF2-/-KLF4+/- embryos have gaps in the endothelial layer of the primary head vein (n = 4); D) KLF2-/-KLF4-/- primary head vein lacks a continuous endothelial layer (n = 3). Red arrows indicate apparent gaps in the endothelial layer. Black arrows point at the lumen of the Primary Head Vein (PHV).