Development of spinal neurons and tracts in the zebrafish embryo.

We have analyzed pathfinding by growth cones in the spinal cord of the early zebrafish embryo, because it is an extremely simple system. At 18-20 hours of development the spinal cord contains approximately 18 lateral and presumably post-mitotic cell bodies per hemisegment. Of these 8-11 have projected growth cones by 18 hr of development and fall into five classes of neurons (Bernhardt et al., J. Comp. Neurol, preceding paper), including a set of mechanosensory (RB) neurons, three classes of interneurons (DoLA, ascending commissural, and VeLD), and previously characterized primary motor neurons (Eisen et al., '86: Nature 320:269-271). Of these five classes we analyzed pathfinding by the RB, DoLA, early ascending commissural, and VeLD neurons. These neurons are distinguishable at the earliest stages of axonogenesis based on the location of their somata and the number and initial directionality of their growth cones. In each case they follow stereotyped, cell-specific pathways to reach their termination sites. Up through larval stages exuberant axons have not been observed. The longitudinal axons of each neuronal class form bundles in the early cord. This apparently occurs because growth cones extend in close association with the longitudinal axons of the same neuronal class. At later stages spatially discrete commissural tracts are found in the cord suggesting that commissural growth cones may follow earlier commissural axons as well.