DISTRIBUTION OF TRANSITORY CORPUS-CALLOSUM AXONS PROJECTING TO DEVELOPING CAT VISUAL-CORTEX REVEALED BY DII

Functional studies of the development of the corpus callosum in the ca t have shown that an intact callosum during postnatal month 1 is neces sary for normal visual development. In vivo tracing techniques have no t provided enough information on corpus callosum connectivity to fully evaluate the evidence for a morphological mechanism for the functiona l effects of neonatal callosum section. However, lipophilic in vitro m embrane tracers permit a more detailed search for such evidence becaus e the entire limit of many cells can be labeled simultaneously. To inv estigate the morphological basis for the observed functional results i n cats, the corpus callosum war, labeled in vitro with the carbocyanin e dye, DiI. Crystals of DiI were placed in the midsagittal callosum in tissue from 2 to 277-day-old cats. Tissue was coronally sectioned 3-2 2 months later. Sections were photographed and reconstructed to show t he overall distribution of corpus callosum projections, as well as the locations of individual corpus callosum axons and their presumed term inals. The distribution of corpus callosum projections, examined in co rtical areas 17-19, 7, and posterior medial lateral suprasylvian corte x, changes significantly during development. During postnatal week 1, callosal axons extend throughout these cortical areas to layer I. Nume rous varicosities on callosal axons are located en passant and at axon terminals in layer I. During postnatal week 2, the density of callosa l projections is reduced in all cortical areas, although many axons st ill extend to layer 1. By postnatal month 2, the callosal axons extend ing to layer I are predominantly near the border with adjacent cortica l areas; in the nonborder regions of these areas, many axons extend to layer VI while a much smaller number of axons extend to layers II-V. By postnatal month 3, the callosal projections to supragranular layers are almost exclusively restricted to cytoarchitectonic border regions ; in the remaining regions, including medial area 17, there are occasi onal axons extending to the supragranular layers and only a moderate n umber of axons extending to infragranular layers.Thus, a substantial n umber of elaborately formed transitory corpus callosum axons, distribu ted throughout visual cortex, exist for several weeks during postnatal development; in area 17, these axons are found in central through per ipheral visual field representations. The transitory callosal axons ap pear to have axon terminals in layer I as well as en passant terminals while extending through layers II-VI. If some of these terminals were to form synapses, there would be extensive opportunities for the corp us callosum to provide input to layers I-VI throughout visual cortex d uring the period of development in which cortical microcircuitry is be ing established. (C) 1993 Wiley-Liss, Inc.