The proposed studies of axonal morphology and its developmental dynamics, in relation to the cellular and molecular environment of the CNS, promise to further our understanding of the mammalian visual system and of how it is assembled. In the hamster, retinal ganglion cells form morphologically distinct and topographically organized pathways and arbors on both sides of the brain. Our current work is revealing a hitherto unknown level of complexity in parallel processing within these retinal efferent projections. We will continue our investigations into the specificity of these connections. In the developing animal, the relationship between the end-arbors of retinal cells and the glial environment of thalamic and midbrain target zones, will be investigated. Timelapse video microscopy will be used, in combination with a flatmount preparation of the brainstem, to watch the dynamics of developing optic tract fibers. Our studies have shown that non-neuronal cells at the tectal midline serve to maintain the laterality of retinal connections; the specificity of this function and possible molecular mechanisms which could mediate this process will be explored both in the living brain and under tissue culture conditions. It has been shown that mature CNS neurons can be induced to regenerate with the provision of an appropriate environment. We will apply this phenomenon to the retinofugal system. Ganglion cells will be axotomized, then allowed to regenerate into the visual thalamus via a peripheral nerve bridge. Spatiotemporal variations and target specificity of this regeneration will be probed. With these experiments, we will be in a position to test new hypotheses about the specificity of axon growth at different developmental stages and in the adult animal, and add to an increasingly rich picture of the central representation of vision.
