Development of the ipsilateral retinothalamic projection in the frog Xenopus laevis. I. Retinal distribution of ipsilaterally projecting cells in normal and experimentally manipulated frogs.

The distribution of ipsilaterally and contralaterally projecting cells within the retina in Xenopus laevis was studied by injection of horseradish peroxidase into the thalamus on one side of the brain and subsequent determination of the locations of retrogradely labeled cells in both retinas. In normal animals, contralaterally projecting cells were found throughout the retina. Ipsilaterally projecting cells, in contrast, were most frequent in temporoventral retina and largely absent from dorsonasal retina as well as from a region surrounding the nerve head. A similarly restricted distribution of ipsilaterally projecting cells was observed in retinas of animals after regeneration of one optic nerve as well as in animals from which one eye was removed prior to the time when the ipsilateral projection first develops. The restricted distribution of ipsilaterally projecting cells in normal animals raises the possibility that these cells may be produced relatively late in development. This hypothesis is explored in the following paper (Hoskins, S.G., and P. Grobstein (1985) J. Neurosci. 5: 920-929). The fact that similar distributions were seen in normal and experimental animals implies that organization of the ipsilateral retinothalamic projection in X. laevis is not critically dependent either on particular patterns of axonal organization which may be present during normal development or on interactions among fibers from the two eyes.