Histological sections of anti–PECAM-1 monoclonal antibody-stained E8.5 dpc wt and SCL^lacZ/lacZ(−/−) embryos and yolk sacs. Anti–PECAM-1 stains both the endothelium and a subset of the hematopoietic cells in a wt yolk sac (A), but is restricted to the endothelium in the −/− yolk sac (C). (B) Section through the trunk of a wt embryo showing anti–PECAM-1 staining of the endocardium and endothelia. (D) In the −/− embryo, the anti–PECAM-1 antibody recognizes the endocardium of a more linear heart tube and dilated intraembryonic vessels. Original magnification: (A and C), ×400; (B and D), ×200.