The corpus callosum provides a massive transitory input to the visual cortex of cat and rat during early postnatal development.

Studies of corpus callosum development in cat revealed that the callosum must be intact during postnatal month 1 if normal visual development is to occur [11-20,25]. The use of DiI, a lipophilic carbocyanine dye that is an in vitro membrane tracer, permits a detailed search for morphological evidence to account for these functional results because many cells can be simultaneously labeled in their entirety. To search for morphological evidence, the corpus callosum was labeled in vitro with DiI in tissue from cats aged 2-277 days old [21]. To determine whether there was consistent callosal development in mammals, similar studies were carried out in tissue from rats aged 0 days old through adult [22]. Hemispheres were coronally sectioned 1-24 months later. Sections were reconstructed in photomontages to show the overall distribution of corpus callosum projections, as well as provide details about the locations of individual corpus callosum axons and their presumed terminals. The distribution of corpus callosum projections, examined in visual cortex of cat and rat, changed significantly during development. During early postnatal development, callosal axons extended throughout visual cortex to layer I. Numerous varicosities on callosal axons were located en passant and at axon terminals in layer I. In the following weeks, the density of callosal projections was reduced in all cortical areas, although many axons still extended to layer I. By postnatal month 2 the callosal axons were predominantly near the borders between adjacent cortical areas. Thus, for several postnatal weeks, many elaborately formed transitory corpus callosum axons are distributed throughout visual cortex. The transitory callosal axons appear to have terminals in layers I-VI. If some of these terminals were to for synapses, the corpus callosum could provide an extensive input to layers I-VI throughout visual cortex while the majority of cortical microcircuitry is being established.