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Expression of chick semaphorin 5B during neuronal development Legg, Arthur Terrence

Abstract

During development, the formation of a functional nervous system requires precise pathfinding of axons to their targets. Growth cones at the leading edge of these axons use the information supplied by a variety of cues in the environment to navigate their course. The semaphorins, comprising a large family of neuronal guidance cues, were first identified in the grasshopper limb bud and were shown to be important for proper Ti1 axon pathfinding. Most of the studies from invertebrates and vertebrates have demonstrated an inhibitory role but more recent studies have shown them to have a bi-functional nature, also acting as attractive cues. The transmembrane semaphorin, chick semaphorin 5B (cSEMA5B), is unique from other semaphorin in that it contains both an inhibitory SEMA domain and a region of thrombospondin type-1-like repeats that have been associated with neuronal outgrowth. To determine whether this semaphorin plays a role in axon guidance in the developing chick nervous system, its expression was analyzed using a variety of techniques including in situ hybridization, RT-PCR, and immmunocytochemistry. Expression of cSEMA5B is first clearly identified at E5 in the spinal cord, DRGs, retina, and in a variety of neuroepithelia associated with the tectum, ventricular regions, and the olfactory system. Expression within the spinal cord is dynamic being first broadly expressed in both dorsal and ventral regions at E5 with the ventral expression persisting through E11. At this later stage the expression is associated with large-diameter cells in the lateral motor column. Expression within the retina occurs along the retinal ganglion cell layer and is relatively uniform in distribution and is maintained from E5 through E10, while the expression in the tectum appears to occur in a gradient with highest levels in the anterior region. Results from these studies, along with some important in vitro studies performed by others in our lab, have suggested that cSEMA5B is important for the neuronal pathfinding within the spinal cord, retina, and developing tectum.

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