Anti–PECAM-1 monoclonal antibody staining of wholemount E8 to 9.5 dpc wild-type (wt) and SCL^lacZ/lacZ(−/−) embryos and yolk sacs. (A) through (C) E8 embryos showing a similar pattern of anti–PECAM-1 staining in wt and −/− embryos (A), but a poorly formed primary vascular plexus in the −/− yolk sac (C) compared with the wt yolk sac (B). (D) and (E) At E8.5, somitic arteries are beginning to develop from the dorsal aorta. A dilated pericardial sac (arrowed), delayed cardiac looping, and diminished cerebral vasculature are evident in the −/− embryo. The pericardial sac was dissected away in (E) to display anti–PECAM-1 staining in the endocardium. (F) E9 embryos showing incomplete somitic vessels in the −/− embryo, which is undergoing apoptosis. (G) Preterminal, severely growth-retarded E9.5 mutant embryo (−/−) displaying loss of vascular architecture. (H) Large branching vitelline vessels in wt yolk sac at E9.5 contrast with the −/− yolk sac (I), which retains a poorly formed capillary plexus.