LAYER-VII AND THE GRAY-MATTER TRAJECTORIES OF CORTICOCORTICAL AXONS IN RATS

The trajectory of long distance intrahemispheric corticocortical axons has been investigated using the anterograde fluorescent axonal tracer fluororuby. Most axons of this kind were found to travel through the gray matter of layers VI and VII rather than in the white matter. The cell-sparse zone immediately superficial to layer VII contains a dense aggregate of longitudinally directed axons. Corticocortical axons tra veling in the mediolateral plane also utilize the deep gray matter pre dominately. Layer VII neurons are persistent remnants of the subplate in rats. Based on our retrograde labeling results, they are involved i n long distance as well as local corticocortical connections. Layer VI I neurons are often labeled in a more continuous pattern after cortica l injections of retrograde tracers than neurons of layers II, III and V, which are labeled in a patchy manner.