Axonal outgrowth by identified neurons in the spinal cord of zebrafish embryos.

The spinal cord of early zebrafish embryos contains a small number of neurons per hemisegment. The earliest neurons are identifiable as individual neurons or small groups of homogeneous neurons and project growth cones that follow stereotyped, cell-specific pathways to their targets. These growth cones appear to bypass some axons but follow others during pathfinding, suggesting that they can distinguish among the different axons they normally encounter. Furthermore, identified growth cones exhibit cell-specific behaviors in apparent contact with the floor plate cells, which are found at the ventral midline of the early cord. These observations suggest the testable hypothesis that the floor plate may mediate multiple, cell-specific actions on identified growth cones in the zebrafish cord. One hypothesized action is inhibition of specific growth cones to prevent them from crossing the ventral midline.