Plasticity in the developing chick visual system: topography and maintenance of experimentally induced ipsilateral projections.

The present work examines the topography of the contra- and ipsilateral centrifugal projections from the isthmooptic nuclei (IONs) to the remaining retina in monocular chick embryos. After removal of the left eyecup at embryonic day (E)1.5, the IONs were investigated at various embryonic stages by the retrograde transport of fluorescent dyes and horseradish perioxidase (HRP) injected into the remaining eye. The projection of the ipsilateral ION was consistently found at E13 and frequently disappeared by E18 to E19. Selective regional labeling of the remaining retina in monocular embryos with DiI (1,1'-dioctadecyl-3,3,3',3'-tetramethyl-indocarbocyanine perchlorate) revealed that the retinotopic order of the enhanced projection from the ipsilateral ION corresponded precisely to the normal one from the contralateral ION. The formation of the projection from the retina to the ipsilateral tectum was also investigated at E18 to E19 by means of intravitreally injected HRP or rhodamine-B-isothiocyanate (RITC) in monocular embryos after early eyecup removal. In cases with persistent ION, the eye enucleations resulted in ipsilateral retinotectal projections consisting of varying numbers of retinofugal fibers. The data are consistent with the view that there is a certain degree of plasticity in the embryonic development of the chick visual system. If an ION projection to the ipsilateral retina is strongly developed, it is retinotopically organized and probably influences the maintenance of the ipsilateral retinotectal projection. The stabilization of the otherwise transiently formed ipsilateral retinotectal projection may be influenced by the tectal neurons which receive retinal input and are efferently connected with persisting ION neurons.