The arborization of single callosal axons in the mouse cerebral cortex

After several large cortical injections of horseradish peroxidase, individual callosal axons could be observed in most cortical areas contralateral to the injected hemisphere. They left the white matter and travelled for various distances (up to 2 mm) deep in layer VI, then turned to penetrate the cortex radially or obliquely, giving collaterals to several layers and forming narrow terminal arborisations in supragranular layers. In addition, callosal fibers were seen predominantly in deep cortical layers, which fibers could be interpreted either as collaterals of the thick fibers or as a distinct class of callosal afferents.