Fig. 3. Defective vasculogenesis in Foxf1^-/- yolk sac. (A-C) Whole-mount Pecam staining of E8.5 wt (A left, B) and Foxf1^-/- (A right, C) mouse embryos. B and C are higher magnifications of parts of the yolk sacs of the embryos in A. A primitive vascular plexus of endothelial tubes is well developed in the wt, but absent from the mutant. (D-G) Pecam staining of paraffin sections of E9.5 wt (D,F) and Foxf1^-/- (E,G) embryos. In the Foxf1 mutant, the yolk sac mesoderm is divided into a thin Pecam⁺ layer that lines the inner surface of the yolk sac endoderm, and a detached thick layer that only makes sporadic contact with the yolk sac endoderm (arrowhead in E). (H-J) Flk1 staining of cryosections of E8.5 wt (H) and Foxf1^-/- (I,J) embryos. The separation of the yolk sac mesoderm has started in the mesometrial pole of the yolk sac (J), but not in the antimesometrial (I). al, allantois; am, amnion; post, posterior part of the embryo proper; yse, yolk sac endoderm; ysm, yolk sac mesoderm.