Parallel Morphological and Functional Development in the Xenopus Retinotectal System

The retinotectal projection in Xenopus laevis is topographically organized. During the early development of the Xenopus visual system, the optic tectum increases considerably in volume, and retinotectal axons and dendrites undergo extensive activity-dependent remodeling. We have previously observed marked changes in the three-dimensional layout of the tectal retinotopic functional map over the course of a few days. This raised the question of whether such functional reorganization might be attributable to the migration and structural remodeling of tectal neurons as the brain grows. To examine changes in map topography in the context of individual tectal neuron morphology and location, we performed calcium imaging in the optic tecta of GCaMP6s-expressing tadpoles in parallel with structural imaging of tectal cells that were sparsely labeled with Alexa 594-dextran dye. We performed functional and structural imaging of the optic tectum at two developmental time points, recording the morphology of the dextran-labeled cells and quantifying the changes in their positions and the spanning volume of their dendritic fields. Comparing anatomical growth to changes in the functional retinotopic map at these early stages, we found that dendritic arbor growth kept pace with the overall growth of the optic tectum and that individual neurons continued to receive widespread visual field input, even as the tectal retinotopic map evolved markedly over time. This suggests a period of initial growth during which inputs to individual tectal neurons maintain diffuse connectivity and broad topographic integration.