PATHFINDING BY IDENTIFIED ZEBRAFISH MOTONEURONS IN THE ABSENCE OF MUSCLE PIONEERS

To identify the cellular cues that guide zebrafish neuronal growth con es to their targets, we examined interactions between identified motor growth cones and identified muscle fibers and tested whether these fi bers were required for growth cone navigation. Caudal primary motoneur ons (CaPs) and middle primary motoneurons (MiPs) are identified motone urons that innervate cell-specific regions of the myotome. Growth cone s of both cells initially extend along a common pathway and then pause at a set of identified muscle fibers, called muscle pioneers, before diverging along cell-specific pathways. Muscle pioneers are intermedia te targets of both CaP and MIP (Westerfield et al., 1986; Liu and West erfield, 1990); both motoneurons extend their growth cones directly to the muscle pioneers on which the first functional neuromuscular conta cts form, suggesting that muscle pioneers may provide guidance informa tion to these growth cones. We tested this idea by ablating muscle pio neers and observing the resulting motor axonal trajectories. Both CaP and MIP ultimately formed normal axonal arbors after muscle pioneer ab lation, showing that muscle pioneers are unnecessary for formation of correct axonal trajectories; however, although final cellular morpholo gy was correct in the absence of muscle pioneers, MiP growth cones bra nched abnormally or extended ventrally beyond the common pathway. Abla tion of CaP and the muscle pioneers together increased the aberrant be havior of the MIP growth cone. Our results provide evidence that an in termediate target, the muscle pioneers, affects motor axonal extension without altering target choice, suggesting that other cues also contr ibute to proper pathway navigation.