Pathfinding in the central nervous system and periphery by identified embryonic Drosophila motor axons.

We have studied the pattern of axon outgrowth from the identified embryonic Drosophila motorneurons, RP1, RP3, RP4 and RP5, from the onset of axonogenesis to the time of arborization over target muscles. Lucifer Yellow was intracellularly injected into each of these neurons to obtain a detailed description of the morphology of their growth cones and of the pathways that they follow. We have divided the sequence of axon growth from these neurons into five major phases. In the first phase, the growth cone of each RP axon grows medially along its contralateral homologue along the anterior commissure. Each RP axon follows a separate path across the midline in the anterior commissure. After crossing the ventral midline, the axons wrap around specific contralateral RP somata. In the second phase, each axon grows posteriorly and dorsally down the contralateral longitudinal connective, fasciculating with the other RP axons. In the third phase, the axons turn into the intersegmental nerve via the anterior nerve root, then cross over to the segmental nerve, before contacting the external surfaces of intermediate muscles 15/16. They do not fasciculate with the pioneering aCC and RP2 axons at this time. In the fourth phase, the axons advance laterally across the ventral muscle group. During this phase, each axon extends processes over a number of inappropriate muscles as well as contacting its correct, target muscle. In the final phase, the processes to inappropriate muscles are withdrawn, generating the mature pattern of motor axon projections. There is no consistent, clear difference between the RP motorneurons in the relative timing of axon outgrowth.