Anatomical tracing was used to determine the extent and distribution of CC axons in mammalian visual cortex. Postnatal development of rat CC was studied by in vitro callosal labeling with the lipophilic carbocyanine dye Dil in 59 rats. Solid Dil crystals were placed in the midsagittal region of the CC in aldehyde-fixed brain slabs. Coronal sections through visual cortex were photographed and reconstructed to show the overall distribution of Dil-labeled callosal projections as well as the locations of individual callosal axons and their presumed synaptic boutons.

During postnatal weeks 1 and 2, CC axons were found to project to layer I throughout the entire mediolateral extent of areas 17, 18a, and 18b. Numerous varicosities on callosal axons are located en passant and at axon terminals in layer I. During postnatal week 3 the tangential density of callosal projections was significantly reduced, so that fewer callosal axons extended to layer I throughout areas 17, 18a, and 18b than in younger postnatal rats. However, at this age some CC axons could still be found extending to layer I throughout the mediolateral extent of areas 17, 18a, and 18b. By postnatal week 4 the tangential distribution of callosal projections was greatly restricted; most callosal axons projecting to layer I were located at the borders of the visual cortical areas. Nevertheless, there were still callosal axons projecting into cortex and terminating in supragranular and infragranular layers in areas 17, 18a, and 18b; this was most pronounced in area 18a.

Thus, in the rat there are many elaborately formed transitory CC axons projecting throughout visual cortex for several weeks postnatal. These projections extend to layer I and have varicosities in all cortical layers. With increasing age, fewer axons extended to layer I; subsequently most axons not at cytoarchitectonic borders fail to extend to layer I. If some of the varicosities on the transitory rat callosal axons were to form synapses, there would be extensive opportunities for the CC to provide input to all layers of visual cortical areas while cortical microcircuitry is being established. The same type of study in the cat has shown similar results during early postnatal development. Cat CC axons project to all parts of primary and association visual cortical areas; even in regions found to be acallosal in the adult, the neonatal callosal axons extend through all layers of cortex to reach layer I (Elberger, 1993). The parallel results for rat and cat suggest a common mammalian pattern for CC development.

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