The Journal of Neuroscience, May 1, 1998, 18(9):3297-3313
Selective Fasciculation and Divergent Pathfinding Decisions of
Embryonic Chick Motor Axons Projecting to Fast and Slow Muscle
Regions
Louise D.
Milner,
Victor F.
Rafuse, and
Lynn T.
Landmesser
Department of Neurosciences, Case Western Reserve University,
Cleveland, Ohio 44106-4975
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 is established by selective innervation during development or
later via motoneuron-muscle fiber interactions. To examine whether
chick motoneurons 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
axons projecting to fast and slow muscle regions sorted into separate
but adjacent fascicles proximally in the limb, long before they reached
the muscle. More distally, these fascicles made divergent pathfinding
decisions to course directly to the appropriate muscle fiber region. In
contrast, axons projecting to different areas of an all-fast muscle did not fasciculate separately and became more intermingled as they coursed
through the limb. Selective fasciculation of fast- and slow-projecting
motoneurons was similar both before and after motoneuron cell death,
suggesting that motoneurons specifically recognized and fasciculated
with axons growing 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 fasciculate, pathfind to, and branch within the correct
muscle fiber region from the outset of neuromuscular development.
Key words:
selective fasciculation; selective innervation; motoneuron; muscle; specificity; axon pathfinding; guidance; neuromuscular development; fast-slow innervation
Copyright © 1998 Society for Neuroscience 0270-6474/98/1893297-17$05.00/0
Previous Article | Next Article
