Journal of Neuroscience, Vol 3, 1216-1225, Copyright © 1983 by Society for Neuroscience
Position-dependent motor innervation of the chick hindlimb following serial and parallel duplications of limb segments
V Whitelaw and M Hollyday
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.