TRANSITORY CORPUS-CALLOSUM AXONS PROJECTING THROUGHOUT DEVELOPING RATVISUAL-CORTEX REVEALED BY DII

Anatomical tracing was used to determine the extent and distribution o f CC axons in mammalian visual cortex. Postnatal development of rat CC was studied by in vitro callosal labeling with the lipophilic carbocy anine dye Dil in 59 rats. Solid Dil crystals were placed in the midsag ittal 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 t he locations of individual callosal axons and their presumed synaptic boutons. During postnatal weeks 1 and 2, CC axons were found to projec t to layer I throughout the entire mediolateral extent of areas 17, 18 a, and 18b. Numerous varicosities on callosal axons are located on pas sant and at axon terminals in layer I. During postnatal week 3 the tan gential 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 he found extending to layer I throughout the mediol ateral extent of areas 17, 18a, and 18b. By postnatal week 4 the tange ntial distribution of callosal projections was greatly restricted; mos t callosal axons projecting to layer I were located at the borders of the visual cortical areas. Nevertheless, there were still callosal axo ns projecting into cortex and terminating in supragranular and infragr anular layers in areas 17, 18a, and 18b; this was most pronounced in a rea 18a. Thus, in the rat there are many elaborately formed transitory CC axons projecting throughout visual cortex for several weeks postna tal. 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 exte nd to layer I, If some of the varicosities on the transitory rat callo sal axons were to form synapses, there would be extensive opportunitie s for the CC to provide input to all layers of visual cortical areas w hile cortical microcircuitry is being established. The same type of st udy in the cat has shown similar results during early postnatal develo pment. Cat CC axons project to all parts of primary and association vi sual cortical areas; even in regions found to be acallosal in the adul t, the neonatal callosal axons extend through all layers of cortex to reach layer I (Elberger, 1993). The parallel results for rat and cat s uggest a common mammalian pattern for CC development.