Journal of Neuroscience, Vol 11, 2598-2613, Copyright © 1991 by Society for Neuroscience
An experimental analysis of in vivo guidance cues used by axons of spinal interneurons in the chick embryo: evidence for chemotropism and related guidance mechanisms
H Yaginuma and RW Oppenheim
Department of Neurobiology and Anatomy, Bowman Gray School of Medicine, Wake Forest University, Winston-Salem, North Carolina 27103.
To clarify axonal guidance mechanisms involved in pathway formation by
spinal interneurons, the pattern of axonal outgrowth was examined following
three kinds of perturbations: (1) rotation of three segments of the neural
tube around the dorsal-ventral axis by 90 degrees or 180 degrees (D-V
rotation), (2) rotation of three segments of the neural tube around the
rostral-caudal axis (R-C rotation), and (3) transplantation of brainstem
into thoracic spinal segments (BS transplantation). Following D-V rotation,
it was observed that circumferential axons near the junction between
rotated graft (RG) and host cord changed their course so as to project
toward the ectopic floor plate. This supports the notion that the floor
plate exerts a chemotropic-like effect on the directional projection of
circumferential axons. The longitudinal fibers in the ventral funiculus
(VF) were able to grow through the transplant junction when the VF was well
apposed to the VF of the RG. However, in most cases, the longitudinal
fibers in the VF that were not apposed to the VF of the RG made a medial
turn at the junction. After turning, some of them made a U-turn into the
contralateral VF, whereas others grew circumferentially around the junction
zone, then exited the spinal cord; still others directly entered the RG.
The majority of fibers that entered the RG were located in the VF, though a
small number of such fibers were also located in both the lateral and
dorsal funiculus (LF and DF, respectively). The fibers that entered the LF
shifted ventrally, whereas the fibers in the DF remained within the DF.
These results suggest that there may be some matching mechanism between
specific fibers and the funiculus through which the fibers normally
project. Moreover, there may be a barrier between the LF and the DF that
prevents the fibers in the DF from shifting ventrally. In R-C rotation
experiments, the projection of axons of dorsolateral border cells (DLB-
cells), which are known to give rise almost exclusively to commissural
ascending axons, was examined. Following the injection of HRP into the
rostral junctional region, retrogradely labeled DLB-cells were observed in
the rotated segments only on the side contralateral to the injection.
Labeled DLB-cells were also observed in the segments caudal to the rotated
segments. These results suggest that putative rostrocaudal directional cues
that guide the axons of DLB-cells may arise from outside of the segments in
which the decision to turn is made.(ABSTRACT TRUNCATED AT 400 WORDS)
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