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 callos um must be intact during postnatal month 1 if normal visual developmen t is to occur [11-20,25]. The use of DiI lipophilic carbocyanine dye t hat is an in vitro membrane tracer, permits a detailed search for morp hological evidence to account for these functional results because man y 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 whethe r there was consistent callosal development in mammals, similar studie s 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 o f corpus callosum projections, as well as provide details about the lo cations of individual corpus callosum axons and their presumed termina ls. The distribution of corpus callosum projections, examined in visua l cortex of cat and rat, changed significantly during development. Dur ing early postnatal development, callosal axons extended throughout vi sual cortex to layer I, Numerous varicosities on callosal axons were l ocated en passant and al axon terminals in layer I. In the following w eeks, the density of callosal projections was reduced in all cortical areas, although many axons still extended to layer I. By postnatal mon th 2 the callosal axons were predominantly near the borders between ad jacent cortical areas. Thus, for several postnatal weeks, many elabora tely formed transitory corpus callosum axons are distributed throughou t visual cortex. The transitory callosal axons appear to have terminal s in layers I-VI. If some of these terminals were to form synapses, th e corpus callosum could provide an extensive input to layers I-VI thro ughout visual cortex while the majority of cortical microcircuitry is being established.