AUTORADIOGRAPHIC DEMONSTRATION OF SHIFTING RETINOTECTAL PROJECTION DURING LARVAL DEVELOPMENT IN THE TOAD DISCOGLOSSUS-PICTUS

It has been known that in Anurans, the retina projects onto the tectum opticum, the main retinorecipient center, in some rough topographical order at all stages of the development, although the two interconnect ed optic structures continue to enlarge in topologically dissimilar pa tterns. Previous studies in Rana and Xenopus have suggested that a cha nge in the primary established connections was required for the re adj ustment of the developing retinotectal map. The purpose of this work i n Discoglossus pictus, is to clarify the evolution of the tectal map o f a restricted (the oldest central) part of retina by means of radioau tographic tracing of the oldest retinal axons. The tectal projection s ites of the primary formed central part of the retina were determined at different stages of metamorphosis. The early retinotectal projectio n moved from the anterior to the posterior part of the tectum in the t adpoles. These results support the hypothesis of << shifting terminals >> (GAZE et al., 1974) of retinal fibres on the tectum opticum during growth. It was proposed that the course of growing retinal fibres res ults from a double shift : the first corresponds to the rostrocaudal a xonal growth whereas the second is related to tectal neurogenesis whic h pushes away the initial part of the tectum to the lateral side. A sp atiotemporal ordering of optic fibres is seen in the diencephalon and only transiently observed in the tectal neuropil. These observations h ave implications for the role of adhesive mechanisms in the developmen t of retinotectal connections.