Assembly,maintenance and repair of nervous systems rely on the precise coordination in the presentation of guidance signals and the correct reception and processing of these signals.During embryonic development,considerable progress has already been made in identifying the extracellular cues and the receptors mediating axonal guidance(Araújo and Tear,2003).Axons are particularly vulnerable to injury and disease and axonal Assembly, maintenance and repair of nervous systems rely on the precise coordination in the presentation of guidance signals and the correct reception and processing of these signals. During embryonic development, considerable progress has been made in identifying the extracellular cues and the receptors mediating axonal guidance (Araújo and Tear, 2003) .Axons are particularly vulnerable to injury and disease and axonal