The notochord and floor plate guide growth cones in the zebrafish spinal cord.

Elimination of the floor plate, a row of distinctive cells at the ventral midline of the spinal cord, dramatically increased the frequency of errors made by specific growth cones in the zebrafish embryo. This demonstrated that the floor plate participated in guiding specific growth cones at the ventral midline of the spinal cord. However, since a significant proportion of these growth cones followed their normal pathway despite the absence of the floor plate, we hypothesized that a second source of cues must exist at or near the ventral midline. We tested whether the notochord, which is located just ventral to the spinal cord, was an additional source of pathfinding cues by eliminating it prior to axonogenesis. Laser ablation of the notochord in wildtype embryos increased errors by spinal growth cones. Likewise, spinal growth cones made errors in no tail mutants that are missing the notochord but not the floor plate. These results demonstrate that the notochord also participates in guiding growth cones at the ventral midline. Furthermore, removal of both the floor plate and notochord resulted in a higher error rate than removal of either one alone. These results suggest that the notochord and floor plate normally act in concert to guide spinal growth cones at the ventral midline.