Ipsilateral corticocortical projections of fibers which course within Probst's longitudinal bundle seen in the brains of mice with congenital absence of the corpus callosum: a study with the horseradish peroxidase technique.

In the brains of mice with congenital absence of the corpus callosum, Probst's longitudinal bundle is always present in both cerebral hemispheres. In this research, the ipsilateral corticocortical projections by the fibers in this bundle were studied by iontophoretical injections of horseradish peroxidase into different loci in the neocortex of acallosal mouse brains. Our results show that the 'intrinsic' association fibers seen in normal mice are preserved unchanged in acallosal mice and that in these mice, a considerable portion of the fibers arising from a particular cortical region and coursing in Probst's bundle terminate ipsilaterally in the same area as do the 'intrinsic' association fibers from that region. Furthermore, careful comparison of the labeling patterns in acallosal and normal mice revealed that ipsilaterally labeled cells in acallosal mice are distributed as in similarly injected normal mice. This fact strongly suggests that in spite of the presence of ipsilateral corticocortical projections by the fibers in Probst's bundle, the overall pattern of intrahemispheric connections in the neocortex of acallosal mice is not different from that of normal mice. Thus, from the present findings it can be concluded that in congenitally acallosal mouse brains, the ipsilateral corticocortical projections of fibers which arise from a given cortical region and course within Probst's longitudinal bundle are formed only in areas that also have the 'intrinsic' association projections from that region.