MOTOR INNERVATION OF DORSOVENTRALLY REVERSED WINGS IN CHICK QUAIL CHIMERIC EMBRYOS

In the limb plexus, motor axons destined for limb muscles diverge alon g separate pathways to innervate muscles derived from either the dorsa l or ventral premuscle masses. We have examined the axonal guidance cu es involved in this initial, specific pathway choice at the plexus by making dorsoventral (D/V) limb bud reversals prior to innervation. Chi ck/quail chimeras were used to determine the proximodistal level of th e reversal in tissue sections. The specificity of the projections to d orsal or ventral nerve trunks was assessed by retrograde HRP labeling at ages prior to motoneuron death. Axons corrected for the reversal wh en the level of the graft was proximal to the plexus, and when the rev ersed limb and its gross nerve pattern were normal. If all of these co nditions were not satisfied, aberrant innervation patterns were observ ed. Axonal trajectories were analyzed within the host tissue, at the h ost-graft border, and within rotated tissue to determine where along t he pathway guidance cues might be located. Special attention was given to cases in which axons compensated for the reversal to project in ac cord 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 rota ted 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, bu t rather appear to be closely associated with positional information i n the plexus region and also more proximally in the tissue surrounding the distal spinal nerves.