Postnatal development of biotinylated dextran amine-labeled corpus callosum axons projecting from the visual and...

The distribution and morphology of developing corpus callosum (CC) axons in rat visual cortex was studied by unilateral application of the in vivo anterograde tracer biotinylated dextran amine (BDA) to the visual or auditory cortex of newborns through adults. Changes in the distribution and morphology of CC axons during development were observed. Following BDA placement only in visual cortex, nearly all CC projections were to visual cortex (homotopic CC projections). At postnatal day (PND) 5-8, labeled CC axons were found throughout the contralateral visual cortex, including area 17; these CC axons could be followed from the white matter to layer I. By PND 13, few CC axons were found in medial area 17, indicating the existence of transitory CC axons in area 17 at younger ages. Morphological changes were investigated at the area 17/18a border and showed that CC axon collaterals were not formed until PND 8, and terminal arbors were not visible until PND 13; by PND 17, the adult CC-axon terminal pattern was present. At all ages, only a few heterotopic CC projections from visual to auditory cortex were found in the gray matter, although many labeled CC axons extended laterally into the white matter underlying the auditory cortex. Following BDA placement only in auditory cortex, CC projections to both auditory (homotopic CC projections) and visual (heterotopic CC projections) cortex were observed. At all ages, the homotopic CC projections were present throughout the auditory cortex, but were not distributed homogeneously; densely labeled CC axons showed a distinct columnar organization. The heterotopic CC projections were present in all visual cortical areas, including medial area 17, in significant numbers until PND 24, but were mostly eliminated by PND 28, at which time a labeling pattern similar to the adult was found. Thus, most of the heterotopic CC projections were transitory. The present study confirms the existence of transitory CC axons projecting through all layers of the visual cortex, as revealed by DiI, and extends the DiI results by showing that these transitory CC axons arise from both homotopic and heterotopic origins. Furthermore, different sources of transitory CC axons have different timetables for elimination.