Motor innervation of dorsoventrally reversed wings in chick/quail chimeric embryos

In the limb plexus, motor axons destined for limb muscles diverge along separate pathways to innervate muscles derived from either the dorsal or ventral premuscle masses. We have examined the axonal guidance cues involved in this initial, specific pathway choice at the plexus by making dorsoventral (D/V) limb bud reversals prior to innervation. Chick/quail chimeras were used to determine the proximodistal level of the reversal in tissue sections. The specificity of the projections to dorsal or ventral nerve trunks was assessed by retrograde HRP labeling at ages prior to motoneuron death. Axons corrected for the reversal when the level of the graft was proximal to the plexus, and when the reversed limb and its gross nerve pattern were normal. If all of these conditions were not satisfied, aberrant innervation patterns were observed. Axonal trajectories were analyzed within the host tissue, at the host-graft border, and within rotated tissue to determine where along the pathway guidance cues might be located. Special attention was given to cases in which axons compensated for the reversal to project in accord with the positions of their soma in the lateral motor column. In these correcting cases, after normal D/V sorting in the spinal nerves of the host, motor axons altered their trajectories upon entering rotated graft tissue as they approached and traversed the plexus. Because corrections were within rotated tissue and not proximal to it, the D/V pathway cues are unlikely to be long-range target-derived signals, but rather appear to be closely associated with positional information in the plexus region and also more proximally in the tissue surrounding the distal spinal nerves.