Journal of Neuroscience, Vol 5, 2336-2344, Copyright © 1985 by Society for Neuroscience
Specificity of early motoneuron growth cone outgrowth in the chick embryo
KW Tosney and LT Landmesser
During development, chick lumbosacral motoneurons have been reported to
form precise topographic projections within the limb from the time of
initial outgrowth. This observation implies, first, that motoneurons select
the appropriate muscle nerve pathway and, second, that they restrict their
ramification within the primary uncleaved muscle masses to appropriate
regions. Several reports based on electrophysiology and orthograde
horseradish peroxidase (HRP) labeling have shown muscle nerve pathway
selection to be fairly precise. However, studies based on retrograde
labeling with HRP have produced conflicting reports on the extent to which
vertebrate motoneurons make projection errors. Since it is difficult to
distinguish between true projection errors and HRP leakage when using
retrograde labeling, we decided to assess the distribution of labeled
growth cones in 25-micron serial plastic sections, following orthograde
labeling of identifiable subpopulations of motoneurons during the period of
initial axon outgrowth. Examination of a large number of muscle nerves
revealed no segmentally inappropriate axons, confirming earlier reports
that muscle nerve pathway selection is very accurate. In addition, we
observed that growth cones take widely divergent trajectories into the same
muscle nerve, suggesting that growth cones are responding independently to
some specific environmental cue rather than being passively channeled at
this point. The distribution of labeled growth cones within the muscle
masses provided direct evidence that motoneurons did not at any time
project to obviously inappropriate muscle regions.(ABSTRACT TRUNCATED AT
250 WORDS)