An autoradiographic study of the retinal projection in Xenopus laevis with comparisons to Rana

The retinal projection to the brain in Xenopus has been examined using autoradiographic techniques. The labelled moieties used were 3H-proline and 3H-fucose. There are three anterior thalamic optic terminal sites in Xenopus. These are the nucleus of Bellonci, the corpus geniculatum thalamicum and the rostral visual nucleus. The first two of these are similar to the homonymous nuclei in Rana while the last, the rostral visual nucleus, appears to correspond to the rostral end of the posterior entopeduncular nucleus of Rana. Both the nucleus of Bellonci and the rostral visual nucleus receive strong ipisilateral retinal inputs while the corpus geniculatum receives only a weak ipsilateral projection. In the posterior thalamic: pretectal region there is only one clearly defined optic terminal nucleus--the uncinate field. This structure receives a strong ipsilateral input. In addition there are two other labelled fields in this region whose nature (fibers v. terminals) cannot be clearly discerned in autoradiographs. These are the thalamopretectal field--a horizontal band which extends in the dorsomedial neuropil from midthalamus to the pretectum, and the pretectal field where diffuse labelling is evident, especially ipsilateral to the injected eye. The thalamopretectal field also receives an ipsilateral input. The optic tectum in Xenopus receives a dense contralateral retinal innervation but only a weak ipsilateral projection. Finally, the basal optic nucleus receives both contralateral and ipsilateral retinal innervation with the latter being, by far, the weaker of the two. In addition to other differences between the two species, it is obvious that there is a much denser and more extensive ipsilateral retinal projection to the thalamic and pretectal brain structures in Xenopus than in Rana. This is presumably due to the greater size of the binocular field in Xenopus.