Journal of Neuroscience, Vol 5, 920-929, Copyright © 1985 by Society for Neuroscience
Development of the ipsilateral retinothalamic projection in the frog Xenopus laevis. II. Ingrowth of optic nerve fibers and production of ipsilaterally projecting retinal ganglion cells
SG Hoskins and P Grobstein
We have studied the development of the ipsilateral retinothalamic
projection in the frog Xenopus laevis by analyzing patterns of
histochemical reaction product resulting from anterograde transport of
horseradish peroxidase (HRP) applied to cut optic nerves in animals of
various ages. We have also determined the stages during which ipsilaterally
projecting ganglion cells are born using a combination of [3H] thymidine
autoradiography and retrograde marking of ganglion cells following
injection of HRP into the thalamus. Projections to ipsilateral thalamic
terminal zones were first detectable beginning at about larval stage 54.
There was a clear asynchrony in innervation, with projections to some
terminal zones appearing before projections to others; projections to all
terminal zones were present by late metamorphic stages. Within individual
terminal zones there were progressive increases in the density of the
projections as well as changes in their distribution. By these criteria,
development of the ipsilateral projection was not complete at the end of
metamorphosis but continued for some months thereafter. Our birth dating
studies show that ipsilaterally projecting cells are produced relatively
late in development and that, like the development of the projection, the
production of ipsilaterally projecting ganglion cells continues
postmetamorphically. The vast majority of ipsilaterally projecting cells
are born over a period beginning at stage 54/55, when the projection first
appears. This stage is significant, since it is at approximately this time
that thyroxine-dependent metamorphic events begin. In the following paper
(Hoskins, S.G., and P. Grobstein (1985) J. Neurosci. 5: 930-940) we report
studies on the involvement of thyroxine in the development of the
ipsilateral retinothalamic projection.