Guidance of callosal axons by radial glia in the developing cerebral cortex.

During development, columns of the mammalian cerebral cortex are formed by migration of neurons along fascicles of radial glia. Subsequently, axons of the corpus callosum connect reciprocal regions of each cerebral hemisphere. To determine whether the radial growth of callosal afferents through the developing cortex may be guided by particular cellular elements, we examined the ultrastructural relationship between callosal afferents and radial fibers in the early postnatal hamster sensorimotor cortex. Developing callosal axons and their growth cones were labeled with HRP injected into the cortex at 3 d postnatal when the growth cones have extended across the callosum and are just entering the contralateral cortex. An EM analysis of 30 HRP-labeled axons and their growth cones revealed that they extended upon fascicles of radial processes associated with migrating neurons. Reconstruction of seven of these growth cones, serially sectioned in their entirety, showed that growth cones were associated with the same radial fascicle as their axon. Growth cones also touched other cellular elements such as axons. However, the finding that callosal afferents, from the point at which they enter the cortex to their growth cones, were apposed to a continuous fascicle of radial fibers suggests that callosal axons are tracking along radial processes. We conclude that the majority of the radial processes within fascicles are likely to be glial, based on their relatively large diameters, electron-lucent cytoplasm with a regular array of microtubules, the presence of glycogen granules, occasional cytoplasmic protrusions lacking microtubules, and their consistent association with migrating neurons. We propose therefore that radial glia may serve a guidance function for growing callosal axons in their radial trajectory through the developing cerebral cortex.