The development of the nucleus isthmi in Xenopus laevis. I. Cell genesis and the formation of connections with the tectum.

The nucleus isthmi (NI) of the amphibian relays visual input from one tectum to the other tectum and thus brings a visual map from the eye to the ipsilateral tectum. This isthmotectal visual map develops slowly; it is first detected electrophysiologically at stages 60-62, the age at which the eyes begin their dorsalward migration and the region of binocular overlap beings to increase in extent. During this critical period of life, normal binocular visual input is required for establishment of normal topographic isthmotectal projections. In this study, we have used anatomical methods to trace cell birth, cell death, and formation of connections by the nucleus isthmi during the critical period. Tritiated thymidine labelling demonstrates that cells in the nucleus isthmi are generated throughout most of tadpole life (stages 29-62). Most cells conform to an orderly ventrodorsal gradient starting from stage 29 and extending to stages 56; later cells are inserted at apparently random locations in the nucleus. We have re-examined the hypothesis of Tay and Straznicky ('80) that the order of cell genesis in the NI and tectum could help establish proper isthmotectal connections, and we find that a timing mechanisms does not explain the two-dimensional topography of the isthmotectal map but that timing may aid in proper mediolateral positioning of isthmotectal axons at the points where they first enter the tectum. Horseradish peroxidase labelling was used to investigate whether anatomical projections from tectum to NI and from NI to tectum are present prior to the onset of eye migration. The results show that there are tectoisthmotectal projections by stage 52. Moreover, isthmotectal axons grow into as yet monocular tectal regions prior to the onset of eye migration. At stage 60, when binocular overlap begins, isthmotectal axons are visible throughout the tectum but are densely branched only at the rostral tectal margin, the location where they are predicted to occur on the basis of electrophysiological maps.