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.

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.