Fig. 3. Vascular dilatation in mice lacking Ccm1. (A-D) Immunohistochemical stains for the endothelial antigen Pecam on cross-sections taken from caudal regions of the embryo (as indicated in diagrams to the left). (A,B) The paired dorsal aortae and midline vitelline arteries are apparent at E8.5. Significant enlargement is observed in the dorsal aortae of Ccm1^-/- embryos. (C,D) A comparison of wild-type (C) and Ccm1^-/- (D) embryos at E9.0. There is marked enlargement and midline fusion of the dorsal aortae in the Ccm1^-/- embryo. The enlarged vessel occupies almost the entire volume of the embryo and distorts the closed neural tube. (E-G) Endothelial proliferation at E8.5 in vivo as determined by immunofluorescent double-labeling with antibodies for Pecam and phosphorylated histone 3, a marker of mitosis. Sections were counterstained with DAPI to define cell nuclei. (E,F) Sections taken from wild-type and homozygous mutant embryos, respectively. Two double-positive (mitotic) endothelial nuclei are demonstrated in the Ccm1^-/- embryo (arrows in F). (G) A significantly increased endothelial cell proliferative rate is observed for the dilated aortae of Ccm1^-/- embryos, distal to the heart (light gray portion of embryo diagram to left). Proliferative rates from more rostral sections of aorta and branchial arch arteries were similar between Ccm1^+/+ and Ccm1^-/- embryos. Data bars represent the mean values from three separate embryo pairings, and a total of 137 Ccm1^+/+ and 192 Ccm1^-/- aortic cross-sections. Error bars represent s.e.m. Scale bars: 100 µm.