Axon guidance: mice and men need rig and robo

For many growing axons, navigating across the midline of the nervous system is a crucial stage of their development. New studies on mice and humans show that the axon guidance receptor Robo3/Rig1 is indispensable for axons to accomplish this task. The central nervous system develops as an axon scaffold arranged fairly symmetrically around a ventral midline. As axons grow, they maintain strict relationships to the midline in order to form correct pathways and functional connections. Some axons grow either away from or parallel to the midline, and never cross. Other axons cross the midline once, forming a commissure, and then continue to grow on the other side without recrossing. In humans and other mammals, many major axon tracts have crossed projections. These include the corticospinal tract, which descends from the cerebral cortex, crosses the midline in the brainstem and innervates motor neurons in the brainstem and spinal cord. Two large sensory tracts conveying information from the periphery also cross the midline as they ascend via the spinal cord to the brain. The dorsal column-medial lemniscus pathway crosses within the brainstem, while the spinothalamic tract contains commissural neurons which send axons across the midline at spinal levels.