DLL1 expression in fetal arteries. DLL1 (red) is highly expressed in the endothelium (arrows in C′ and E′) and in the smooth muscle layer (arrowheads in C′ and E′) starting at embryonic day (E) 13.5 in arterial vessels, but not in veins (D). At E12.5, there is no detectable expression in arteries or veins (A,B). This pattern persists until E18.5 (E,F). A′ to F′ shows magnifications of panels A through F. Green indicates antibody staining for α-smooth muscle actin.