Fig. 2. The dorsal funiculus is disorganized in the developing spinal cord of netrin 1 mutant mice. Whole-mount preparations (A,B) and transverse sections (C-H) stained with anti-neurofilament antibody from wild-type (A,C,G), netrin 1 heterozygous (E) and homozygous (B,D,F,H) mice. (A,B) E11.5. In both animals, DRG axons correctly project from the DRG to the spinal cord to form dorsal rootlets (arrowheads) and extend longitudinally in the dorsal spinal cord. DF, dorsal funiculus; SPC, spinal cord. (C-F) E12.5. DRG axons grow to the dorsolateral margin of the spinal cord forming the dorsal funiculus, and very few axons invade the dorsal mantle layer of wild-type and netrin 1 heterozygous mice (arrowheads in C,E). By contrast, the dorsal funiculus is severely disorganized with the loss of a clear inner border in netrin 1 mutant mice (arrowheads in D). Many axon bundles directly enter the dorsal mantle layer (arrows in D,F). Sections in E and F are processed for lacZ in situ hybridization followed by neurofilament immunohistochemistry. In netrin 1 mutants, direct invasion of axons into the dorsolateral part of the spinal cord in which netrin 1 expression is lost (demarcated by lacZ expression) is observed (arrows in F). (G,H) E13.5. In wild-type mice (G), the dorsal funiculus extends to the most dorsomedial region of the spinal cord (arrowheads in G), and some collaterals enter the dorsal spinal cord from the dorsomedial region of the dorsal funiculus (arrows in G). In netrin 1 mutant mice (H), the dorsal funiculus in the marginal zone is thinner than that in wild-type mice (arrowheads in G,H), and the inner border is less sharp. Moreover, thick axon bundles are observed within the dorsal mantle layer of netrin 1 mutants (open arrowheads in H). As observed in wild-type animals, some neurofilament-positive fibers extend ventrally from this ectopic funiculus (G,H arrows). Scale bar: 250 µm in A,B; 200 µm in C,D; 100 µm in E-H.