Fig. 7. Impaired arterial identity in mice lacking Ccm1. (A,B) Immunohistochemical staining for -smooth muscle actin shows that the arterial endothelium in Ccm1^-/- embryos fails to recruit vascular smooth muscle cells. Sections at the level of the atrium of the heart at E9.5 show actin-positive cells along the medial aspect of the wild-type dorsal aorta (arrows in A) without staining of the adjacent cardinal vein. No actin staining is present in the enlarged dorsal aorta of the Ccm1^-/- embryo (B), although cardiac myocytes stain positive. da, dorsal aorta; v, vein; g, gut; pp, pericardial-peritoneal canal; hrt, heart. (C-F) Staining with X-Gal to examine the expression of a tau-lacZ transgene driven by the mouse Efnb2 promoter (Wang et al., 1998). Embryos shown are all heterozygous for the transgene at the Efnb2 locus. (C,D) Efnb2 is expressed in the somites, the nephrogenic cords (arrowheads) and the hindbrain of both genotypes, with arterial expression observed in the dorsal aorta and vitelline artery (arrows, C) of the wild-type embryo. No arterial expression is observed in the Ccm1^-/- embryo. (E,F) Efnb2 transgene expression is detected in the yolk sac arteries of a Ccm1^+/+ embryo at E9.0, with no expression observed in the Ccm1^-/- yolk sac. (G-J) In situ hybridization for Efnb2 in E8.5 yolk sacs. (G,H) Efnb2 transcript is detected in the arterial endothelial network of the caudal pole of the yolk sac (asterisks) of a Ccm1^+/+ embryo, with no endothelial stain observed from the same region of a Ccm1^-/- yolk sac. (I,J) Higher magnification view of yolk sacs (corresponding to boxes in G,H). Efnb2 stain is observed in individual endothelial cells lining the yolk sac arterial network, with no stain observed in yolk sacs lacking Ccm1. Scale bars: 100 µm.