Volume 16, Number 18,
Issue of September 15, 1996
pp. 5749-5761
Copyright ©1996 Society for Neuroscience
Development of an Identified Spinal Commissural Interneuron
Population in an Amniote: Neurons of the Avian Hofmann Nuclei
Received March 21, 1996; revised June 27, 1996; accepted July 2, 1996.
Anne Lill Eide1 and
Joel C. Glover2
1 Department of Physiology, and
2 Department of Anatomy, Institute of Basic Medical
Sciences, University of Oslo, 0317 Oslo, Norway
The commissural interneurons of the Hofmann nuclei (HN) of the
avian spinal cord (The axonal projections of the Hofmann nuclei in the
spinal cord of the late stage chicken embryo, Anat Embryol (Berl), A. L. , Vol 193, pp 543-557) provide a unique opportunity to
describe the development of an identified spinal commissural axon
projection and its terminal collaterals in an amniote vertebrate. Here,
we use the lipophilic tracer DiI to label these and other commissural
projections anterogradely and retrogradely from the time the HN neurons
are born. [3H]thymidine birthdating shows that the final
mitoses of HN neurons occur at stages 21-24 [developmental day (d)
4]. By direct comparison, this follows the generation of motoneurons
and of large, dorsally located commissural interneurons. The first HN
neurons reach the ventrolateral margin of the spinal cord by d6 by a
radial migration through the ventral horn. Radial migration occurs
after the extension of HN axons across the midline. Thus, HN neurons
are determined to be commissural interneurons before attaining their
definitive locations. The HN neurons subsequently aggregate into
segmentally iterated clusters at the ventrolateral margin of the spinal
cord by d8. Also by d8 their longitudinal axons attain mature extent in
the ventral funiculus of the contralateral side and begin to sprout
collaterals. The collaterals are directed predominantly toward the
medial aspect of the ventral horn at all stages, forming by d12 a dense
thicket of terminals that thins out over several segments to each side
of the HN of origin. The initial direction of collateral outgrowth is
largely appropriate for the mature termination pattern of the HN.
Terminal arbors, however, are less focused at early developmental
stages than at later stages.
Key words:
birthdating;
spinal interneuron;
neuronal migration;
axon
outgrowth;
collateral sprouting;
spinal cord;
chicken embryo