Figure 5.

Thinner endocardium and increased number of fenestrations in β -catenin–null embryos. In electron micrographs, the endocardial lining of the heart appear continuously thinner in the null embryos compared with control littermates (compare A with B), whereas the myocardium is equally organized (not depicted). Ultrathin sections of yolk sac vessels of control and mutant embryos show similar features as the heart endocardium, with thinner endothelial walls and less organized junctions in β-catenin–deficient embryos. In the latter, the inter-endothelial junctions show a reduced junctional overlapping (see arrows), as compared with controls (C and D; asterisks point to fenestrations). Furthermore, a higher incidence of fenestrations (asterisks) in the yolk sac vasculature of null embryos can be observed (compare D, E, and F), which are quantified as number of fenestrae per length unit of endothelial wall (G; 10 pictures counted for each genotype).