Ld. Milner et al., SELECTIVE FASCICULATION AND DIVERGENT PATHFINDING DECISIONS OF EMBRYONIC CHICK MOTOR AXONS PROJECTING TO FAST AND SLOW MUSCLE REGIONS, The Journal of neuroscience, 18(9), 1998, pp. 3297-3313
Proper motor function requires the precise matching of motoneuron and
muscle fiber properties. The-lack of distinguishing markers for early
motoneurons has made it difficult to determine whether this matching i
s established by selective innervation during development or later via
motorneuron-muscle fiber interactions. To examine whether chick moton
eurons selectively innervate regions of their target containing either
fast or slow muscle fibers, we backlabeled neurons from each of these
regions with lipophilic dyes. We found that motor a)tons projecting t
o fast and slow muscle regions sorted into separate but adjacent fasci
cles proximally in the limb; long before they reached the muscle. More
distally, these fascicles made divergent pathfinding decisions to cou
rse directly to the appropriate muscle fiber region. In contrast, axon
s projecting to different areas of an all-fast muscle did not fascicul
ate separately and became more intermingled as they coursed through th
e limb. Selective fasciculation of fast-and slow-projecting motoneuron
s was similar both before and after motoneuron cell death, suggesting
that motoneurons specifically recognized and fasciculated with axons g
rowing to muscle regions containing the appropriate muscle fiber type.
Taken together, these results strongly support the hypothesis that ''
fast'' and ''slow'' motoneurons are molecularly distinct before target
innervation and that they use these differences to selectively fascic
ulate, path-find to, and branch within the correct muscle fiber region
from the outset of neuromuscular development.