Yolk sac vessels, blood islands, and placenta. Sections of yolk sac vessels from survivin^lox/lox (+/+) (A-B) and tie1-cre/survivin^lox/lox (−/−) (D-E) were stained with H&E (A,D) and antithrombomodulin antibodies (B,E). While the vasculature of the −/− yolk sacs appeared remarkably grossly intact (C), histologic studies (D-E) revealed several areas with discontinuities of the flattened endodermal endothelial layers (e), and blood islands (bi) that were smaller and with fewer hematopoietic precursor cells. (F-I) At E12.5, branching yolk sac vessels were detectable in +/+ and −/− embryos, but they were frequently empty of blood in the −/− embryos (vessels marked with white line and arrows in panel G), and this was most often associated with a pale placenta. By E13.5, all −/− embryos that were detected were nonviable and resorbing (I). Scale bar equals 1 mm (G-I).