Fig. 3. Magnetic resonance diffusion tensor imaging of Robo1 knockout brains. To survey the entire brain for axonal tract defects, we scanned Robo1 and control littermate brains in a 9.4 Tesla magnet. In these images, orientations of axonal tracts are pseudo-coloured according to their orientation within the brain. Axonal tracts projecting mediolaterally, such as commissures, are green; tracts projecting in the dorsoventral axis are blue; and tracts in the rostrocaudal axis are red. Brains from wild-type (A-C), heterozygote (D-F) and Robo1 knockout (G-I) littermates were analyzed at E17. (A'-I') Higher power views of the boxed regions in A-I, respectively. Three planes of section were extracted from the 3D data and are shown at the level of the rostral corpus callosum: horizontal (A,D,G); coronal (B,E,H) and sagittal (C,F,I). The corpus callosum is indicated by the arrows in the wild-type brain in A'-C' and in the knockout in G'-I'. The anterior commissure is observed as a green spot in cross-section and indicated by the arrowheads in C',F',I'. From this analysis, it is evident that the corpus callosum (arrow in A') and hippocampal commissure (arrowhead in A') are greatly reduced in the Robo1 knockout (compare A' with G', and C' with I') and that the orientation of the fibres has changed from mediolaterally (green) to dorsoventrally (blue) projecting. Scale bar: 650 µm for A,D,G; 400 µm for B,E,F; 500 µm for C,F,I; 250 µm for B',C',E',F',H',I'; 350 µm for A',D',G'.