Position-dependent motor innervation of the chick hindlimb following serial and parallel duplications of limb segments.

In this paper we report investigations of the innervation of duplicated limb segments to test whether the addition of limb segments along the proximodistal axons could stimulate the growth of appropriate motoneurons into double occurrences of these muscles. Our evidence indicates that it does not. Using retrograde horseradish peroxidase nerve-tracing techniques and reconstructions of experimental limbs, we investigated the motor projection to parallel and serially duplicated legs. In all cases, host limb segments were normally innervated. In a control experiment involving a host thigh connected to a graft calf, the innervation of both segments was normal. In serially duplicated limb segments, however, we found abnormal innervation. In limbs of the type thigh-thigh-calf-foot, the innervation of the second thigh was accomplished by calf motoneurons. In limbs consisting of thigh-calf-calf-foot, the duplicated calf was served by foot motoneurons. The general pattern was that muscles were innervated as a function of their position along the proximodistal axis, irrespective of their identity. In no limb were axons found distal to the third limb segment even before the period of normal cell death. Despite the mismatched innervation with regard to the thigh/calf/foot distinction, axons retained their characteristic selectivity for either dorsally or ventrally derived muscles. The findings suggest that the projection of axons along the proximodistal axis of the limb is influenced by proximal growth cues associated with the formation of the limb plexus as well as by competitive interactions in the distal limb tissue.