Fig. 2. Left side views of whole-mount transgenic Tie2-lacZ (A-C,G-I) (see Materials and Methods) and ink injections (D-F,J-L) demonstrate abnormal arch artery patterning in Fgf8 mutant embryos. At E9.5, (A-F) endothelial cells of the first through third arch arteries are normally well developed in wild type embryos (A), while in Fgf8 mutants the arch arteries were often hypoplastic (arrowheads in B). In addition, the second arch was often proximally interrupted (arrowhead, C). These findings were essentially confirmed by ink injection (compare D with E,F). At E10.5 (G-L), the third, fourth and sixth arch arteries are well formed in wild type (G), while in Fgf8 mutants they were often hypoplastic (H) or completely missing, as the second and fourth are in I (note embryo in I was also ink injected). Similarly, ink injections demonstrate hypoplastic (arrowhead in K) or absent (L) fourth arch artery in Fgf8 mutants when compared with control (J).