The ganglion cells of the mature retina project to the visual nuclei of the brain in a highly organized fashion. We have shown that during early development these projections are much less organized. This study will further characterize the optic projections in the immature visual system and investigate possible developmental mechanisms for the establishment and refinement of these early projections. There are five major lines of investigation. First, the degree of order between growing optic axons in the optic nerve, tract and tectum will be examined. This is to determine if there is sufficient order between ingrowing axons to account for the initial order in the terminal field. Second, the retinal ganglion cells giving rise to the earliest tectal projection will be identified. This is to evaluate the role fo temporal gradients in orienting the retinal map on the tectum. Third, timing will again be evaluated as a mechanism for determining the laminar distribution of ganglion cell axons in the tectum. This will involve determining the time and pattern of ingrowth of axons from the different ganglion cell classes to the tectum. Fourth, we will determine whether cell death is responsible for removing aberrantly projecting ganglion cells from the developing retina or if these cells can retract their aberrant collaterals. This will require labeling aberrantly projecting cells early in development and determining whether these labeled cells survive through the development of the visual system. Fifth, we will further characterize the role of the central visual nuclei in developmental cell death in the retina. This will involve attempts to alter the cell death in the ganglion cell layer by increasing the size of the tectum and attempts to demonstrate a specific trophic action of the tectum on retina.
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