Journal of Neuroscience, Vol 11, 3481-3492, Copyright © 1991 by Society for Neuroscience
Guidance of callosal axons by radial glia in the developing cerebral cortex
CR Norris and K Kalil
Neuroscience Training Program, University of Wisconsin, Madison 53706.
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.
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