Fig. 6. Cdx expression and early vascular development in the allantois. (A-C) In situ hybridization on E7.5 embryo sections reveals strong expression of Cdx1 in the primitive streak and weak expression at the base of the allantois (arrowhead in A). Expression is absent in the chorion (asterisk in A). (B) Cdx2 is strongly expressed in the primitive streak and in the outgrowing allantois (arrowhead) and in the chorion (triangle). (C) Cdx4 expression is restricted to the primitive streak and allantois (arrowhead), and is absent in the chorion (asterisk). (D,E) Flk1 staining marks endothelial precursors that are present throughout the E7.5 wild-type (D) and Cdx2^+/-/Cdx4^-/0 mutant allantois (E). The allantois of the wild type was slightly smaller than that of the compound mutant, explaining the lower Flk1 staining in D. (F-I) Pecam staining of allantoises isolated at E8.25 just after chorio-allantoic fusion. Primary endothelial networks in wild-type (F and H) and Cdx2^+/-/Cdx4^-/0 mutant (G and I) allantoises are visible. The wild-type E8.25 allantois in F shows organization of endothelial cells into vessels with numerous branching and connecting vessels (arrowheads). Endothelial cells are less well organized in Cdx2^+/-/Cdx4^-/0 compound mutant littermate shown in G, with clumps of Pecam-positive cells (arrows). The Cdx2^+/-/Cdx4^-/0 allantois in I is, in this case, smaller than the wild-type control (H). Scale bars: 200 µm in A-C; 40 µm in D,E; 20 µm in F,G; 80 µm in H,I. D, distal; P, proximal.