Ipsi- and contralateral commissural growth cones react differently to the cellular environment of the ventral zebrafish spinal cord.

Early commissural axons in the zebrafish spinal cord extend along a pathway consisting of a ventrally directed ipsilateral, a contralateral diagonal, and a contralateral longitudinal segment. The midline floor plate cell is one important cue at the transition from the ipsilateral to the contralateral pathway segments. In order to identify additional guidance cues, the interactions between commissural growth cones and their substrates were examined at the electron microscopic level in the different pathway segments. The growth cones extended near the superficial margin of the spinal cord, within filopodial reach of three bilateral longitudinal axon pathways that were ignored irrespective of whether other axons were already present. Ultimately the commissural growth cones pioneered an additional independent longitudinal pathway in the dorsolateral spinal cord. Neuroepithelial cells were extensively contacted in the lateral marginal zone of the dorsal spinal cord and are thus in a position to contribute to the establishment of the longitudinal commissural pathway segment. The extent of contact with neuroepithelial cells in the ventral spinal cord was dependent on whether commissural growth cones had already crossed the ventral midline: ipsilateral, but not contralateral, growth cones showed extensive contacts with neuroepithelial processes and minor contacts with the basal lamina. In marked contrast, commissural growth cones that had already crossed the ventral midline and entered the diagonal pathway segment showed major appositions to the basal lamina. Extensive contact with the basal lamina was first established in the ventral midline region, where crossing growth cones always inserted between the basal lamina and the base of the midline floor plate cells. This indicates that a change occurs in the response characteristics of commissural growth cones as they cross the ventral midline of the spinal cord. Such a change could help to explain why the growth cones extend first toward but then away from the ventral midline.