Colocalization of ACE and KDR in the human embryo. (A-C) Three successive transverse sections in the caudal portion of a 23-day human embryo stained by anti-KDR (green) and ACE/BB9 (red) Abs. Subpanels ii through iv show a higher magnification of the boxed area in subpanel i. All of the endothelial cells in the aorta and umbilical vein are stained by KDR (i,iii; arrows). No colocalization of ACE is observed in these KDR⁺ endothelial cells (ii-iv). At this stage, ACE (ii) stains some cells scattered in the splanchnopleura (head arrows and asterisks). Only some of these ACE⁺ cells also express KDR (iii-iv, head arrows). Nuclei were visualized by 4′,6-diamidino-2-phenylindole staining (blue). Scale bar indicates 20 μm in subpanel i and 40 μm in subpanels ii through iv. Ao indicates aorta; UV, umbilical vein; NT, neural tube; Hg, hindgut.