Journal of Neuroscience, Vol 3, 1226-1233, Copyright © 1983 by Society for Neuroscience
Neural pathway constraints in the motor innervation of the chick hindlimb following dorsoventral rotations of distal limb segments
V Whitelaw and M Hollyday
Several studies have demonstrated that motor axons can discriminate between
dorsally and ventrally derived muscles. In this paper we present evidence
that (1) the pathway axons take in the limb constrain their access to
either dorsally or ventrally derived muscles, and therefore (2) the axon's
ability to discriminate between dorsal and ventral is expressed already at
the level of pathway selection into the limb. Surgically manipulated
hindlimbs were produced consisting of a normal host thigh connected to a
dorsoventrally rotated calf or to rotated and duplicated donor limb
segments. The limb rotations were done distal to the level at which axons
select a dorsally or ventrally destined pathway through the limb, such that
at the level of the rotation, axons in each nerve were confronted with the
opposite-from- normal set of muscles. In this situation, the relative
influence of pathway availability versus dorsal/ventral muscle recognition
could be assessed. The innervation of rotated limb segments was, in all
cases, opposite from normal. Motoneurons which normally innervate dorsal
muscles innervated ventrally derived muscles that had been rotated into a
dorsal position. Likewise, normally ventrally destined axons served dorsal
muscles in the rotated segments. Thus, motor axons did not alter their
distal path to reach their normal set of muscles. While these results do
not rule out intrinsic dorsal/ventral differences between muscles, they do
demonstrate that muscle surface recognition is not necessary to account for
dorsal/ventral discrimination in the innervation of normal, supernumerary,
or duplicated limbs, nor is it sufficient to account for the innervation of
rotated limb segments. These results also indicate that pathway guidance
cues are an important influence on innervation patterns.