Cortical axon guidance by the glial wedge during the development of the corpus callosum

Growing axons are often guided to their final destination by intermediate t argets. In the developing spinal cord and optic nerve, specialized cells at the embryonic midline act as intermediate targets for guiding commissural axons. Here we investigate whether similar intermediate targets may play a role in guiding cortical axons in the developing brain. During the developm ent of the corpus callosum, cortical axons from one cerebral hemisphere cro ss the midline to reach their targets in the opposite cortical hemisphere. We have identified two early differentiating populations of midline glial c ells that may act as intermediate guideposts for callosal axons. The first differentiates directly below the corpus callosum forming a wedge shaped st ructure (the glial wedge) and the second differentiates directly above the corpus callosum within the indusium griseum. Axons of the corpus callosum a void both of these populations in vivo. This finding is recapitulated in vi tro in three-dimensional collagen gels. In addition, experimental manipulat ions in organotypic slices show that callosal axons require the presence an d correct orientation of these populations to turn toward the midline. We h ave also identified one possible candidate for this activity because both g lial populations express the chemorepellent molecule slit-2, and cortical a xons express the slit-2 receptors robo-1 and robo-2. Furthermore, slit-2 re pels-suppresses cortical axon growth in three-dimensional collagen gel cocu ltures.