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 traveling in the mediolateral plane also utilize the deep gray matter predominately. Layer VII neurons are persistent remnants of the subplate in rats. Based on our retrograde labeling results, they are involved in long distance as well as local corticocortical connections. Layer VII neurons are often labeled in a more continuous pattern after cortical injections of retrograde tracers than neurons of layers II, III and V, which are labeled in a patchy manner.