Identified primary motoneurons in embryonic zebrafish select appropriate pathways in the absence of other primary motoneurons.

Accurate pathfinding is a crucial step in formation of a functional nervous system. Individually identified zebrafish primary motoneurons undergo a stereotyped temporal sequence of axonal outgrowth and pathway selection during which their growth cones follow a common pathway to a "choice point" and then select divergent cell-specific pathways that lead to separate muscle territories. The characteristic sequence of cell-specific pathway selection raises the possibility that the sequence of growth cone arrival at the choice point might determine pathway selection. To test this idea, we ablated identified primary motoneurons by laser irradiation, labeled the remaining primary motoneuron in the same hemisegment with a fluorescent dye, and followed its development through the end of embryogenesis. We found that the growth cone of each primary motoneuron has an independent ability to pioneer the common pathway and select its appropriate cell-specific pathway, even in the absence of all other primary motoneurons in the same hemisegment.