Development of retinofugal neuropil areas in the brain of the alpine newt, Triturus alpestris. II. Topographic organization and formation of projections.

The development of retinal projections and the formation of their retinotopic organization were studied by means of anterograde transport of horseradish peroxidase in the newt, Triturus alpestris. All tracts found in the adult on the contralateral brain side are established during embryonic stages. At this stage a few uncrossed fibers are also detectable. Retinal fibers project first to the contralateral optic tectum. These are followed by contralateral projections to the thalamic recipient areas. Beginning at embryonic stages, the projections from the retinal quadrants into the optic tectum are topographically organized. The other terminal areas innervated by the marginal optic tract (MaOT) show a topographic order from midlarval stages. The terminal areas innervated by the medial optic tract (MeOT) show no clear topographic organization at any stage. The contralateral projection of the MeOT originates from the central area of the retina, whereas the uncrossed projection originates from the temporal peripheral retina. Ipsilateral (uncrossed) retinal projections develop during metamorphic climax. The MeOT is more distinct than the MaOT. The latter shows a clear retinotopic organization. The topography of the ipsilateral MaOT and its corresponding terminal areas are mirror-symmetric to the contralateral tract and terminal areas.