Figure 2

Medial longitudinal fasciculus axons shift away from the midline in Netrin1 mutants. Views of the medial longitudinal fasciculus (MLF) axons growing through hindbrains from mouse embryos, shown as open-book whole mounts of the anterior hindbrain, with the ventral midline in the middle. (A,C,E) MLF axons in Netrin1^+/+, Netrin1^+/−, and Netrin1^−/− embryos on embryonic day 10.5 visualized by whole-mount βIII-tubulin antibody labels. The pair of MLF bundles on either side of the midline are indicated by the arrow heads (A). Netrin1^+/+ and ^+/− embryos had normal MLF projections parallel to and close to the floor plate. (E) Netrin1^−/− embryos had aberrant MLF projections, including an increased distance from the floor plate, and truncated axon bundles (arrow) that split off from the main tract. (B,D,F) MLF axons labeled by a single crystal of the fluorescent axon tracer diI in the midbrain MLF nucleus in control and Netrin1^−/− embryos. Dashed lines mark the ventral midline, centered in the floor plate. (G) Quantification of MLF distance from the midline. The distance of the ventral-most MLF axons from the midline in diI-labeled embryos was normalized for embryo size to compensate for slightly different developmental stages (normalized distance = MLF distance/width of neural tube). (Quantification strategy is further described in Additional file 1). MLF axons were nearly twice as far away from the midline in Netrin1^−/− mutants (n = 7), compared to ^+/+ (n = 5) and ^+/− (n = 7) embryos. Error bars show SEM; significance was measured using the t-test. **P < 0.01. ns, not significant. Scale bars in (E) and (F) indicate 200 μm, and apply to each row of images.