| Summary: | The development of the avian retinotectal projection is a well established model for the study of axon guidance and topographic map formation. Here we employed irnrnunoflourescence to demonstrate that the guidance molecule, Semaphorin 5B, is expressed differentially across the tectum during the time that retinal ganglion cell axons extend into and establish their connections. Beginning on embryonic day 5, a robust anterior to posterior and dorsal to ventral gradient was observed. Highest expression was localized to the superficial SGFS cell layer and continued until embryonic day 12. In vitro analysis using a cell island assay indicated that retinal ganglion cell neurites make significantly fewer contacts and avoid cells expressing Semaphorin 5B in comparison to control cells. No temporal or spatial disparity in response was observed, as RGC’s from all ages examined (E5-E8) and all locations of the retina, responded similarly to Semaphorin 5B. Interestingly, irnrnunoflourescence examination revealed that Semaphorin 5B is also highly expressed within the developing RGC layer of the avian retina. Further analysis indicated that Semaphorin 5B protein is spatially restricted to RGC perikarya and was not observed in RGC neurites at this stage of development. Together this data implicates Semaphorin 5B as an inhibitory guidance molecule expressed in both the retina and optic tectum in regions adjacent to developing RGC neurites. This study identifies Semaphorin 5B as a novel RGC guidance cue and provides the first evidence of transmembrane semaphorin involvement in RGC axon arbourization and target selection. === Medicine, Faculty of === Graduate
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