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 vent ral midline of the spinal cord, dramatically increased the frequency o f errors made by specific growth cones in the zebrafish embryo, This d emonstrated that the floor plate participated in guiding specific grow th cones at the ventral midline of the spinal cord, However, since a s ignificant proportion of these growth cones followed their normal path way despite the absence of the floor plate, we hypothesized that a sec ond source of cues must exist at or near the ventral midline, We teste d whether the notochord, which is located just ventral to the spinal c ord, was an additional source of pathfinding cues by eliminating it pr ior to axonogenesis, Laser ablation of the notochord in wildtype embry os increased errors by spinal growth cones, Likewise, spinal growth co nes made errors in no fail 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. Furtherm ore, removal of both the floor plate and notochord resulted in a highe r error rate than removal of either one alone, These results suggest t hat the notochord and floor plate normally act in concert to guide spi nal growth cones at the ventral midline.