Development of the nucleus isthmi in Xenopus, II: Branching patterns of contralaterally projecting isthmotectal axons during maturation of binocular maps.

The tectum of Xenopus frogs receives input from both eyes. The contralateral eye's projection reaches the tectum directly, via the optic nerve, and the ipsilateral eye's projection reaches the tectum indirectly, via the nucleus isthmi. Under normal conditions, the topography of the ipsilateral map relayed from the nucleus isthmi is in register with the topography of the retinotectal map from the contralateral eye. During development, the process of aligning the two maps is complicated by the dramatic changes in binocular overlap of the two eyes' visual fields which take place during late tadpole and juvenile stages. The goal of this study is to determine the branching patterns of contralaterally projecting isthmotectal axons before, during, and after the period of rapid eye migration. Isthmotectal axons were filled by anterograde transport of horseradish peroxidase (HRP) from the nucleus isthmi. The results show that crossed isthmotectal axons enter the entire extent of the tectum before binocular overlap begins to increase. Therefore, binocular overlap is not necessary for the initial isthmotectal projection to span the tectum. The density of isthmotectal branches rises dramatically at the same time that the eyes begin to shift. During the period when eye migration is most rapid, many isthmotectal axons form arbors which resemble adult arbors but which extend over greater proportions of the tectal surface. The axons appear to be directed toward appropriate mediolateral positions as they enter the tectum. Their trajectories are roughly rostocaudal, with relatively little change along the mediolateral dimension. These data, when combined with available physiological data, suggest that mediolateral order is initially established by vision-independent mechanisms but can be altered by vision-dependent mechanisms. Rostrocaudal order becomes discernable only at the time when binocular visual cues become available and appears to be established primarily on the basis of the activity of the retinotectal and isthmotectal axons.