Pathway selection by ectopic motoneurons in embryonic zebrafish.

Primary motoneurons in embryonic zebrafish innervate cell-specific muscles. During pathfinding, motoneuronal growth cones encounter three distinct regions: a common pathway, a choice point, and separate cell-specific pathways. To learn whether the order in which these regions are encountered influences pathway choice, we transplanted individual motoneurons to the choice point region. These cells selected their appropriate cell-specific pathways. Thus, the sequence in which pathway regions are encountered may not be important for accurate path-finding, and the cell-specific pathways may be delineated by distinct cues that individual growth cones recognize. Moreover, these cues are unlikely to be general ones, since primary sensory neurons transplanted to the same location do not extend growth cones along the motoneuronal pathways.