Fig. 2. Endothelial cell differentiation and maturation. Whole-mount immunostaining with antibodies against PE-CAM1 demonstrated that in wild-type yolk sacs (A), endothelial tubes were similar in diameter, evenly distributed and exhibited some `pruning' into smaller vessels (arrows). Endothelial tubes within Raldh2^-/- yolk sacs (B) were dilated (arrows), with no appropriate remodeling. Within wild-type embryos, vascular plexus formation was evident within the head (C) with distinct margins of vascularization (arrows); vascular plexi within the heads of Raldh2^-/- embryos (D) were diffusely distributed. Vessels within the somitic regions of Raldh2^-/- mutants were also dilated and inappropriately patterned (F, arrows) relative to wild type (E). Semi-quantitative RT-PCR analyses revealed that the expression of Flk1, VE-cad, Tie2 and Ang1 mRNA in Raldh2^-/- mutants was similar to wild type, but the expression of Cx40 mRNA in Raldh2^-/- mutants was significantly reduced (G).