Reorganization of the corticospinal tract following neonatal unilateral cortical ablation in rats.

The corticospinal tract in the rat after neonatal ablation of the unilateral cerebral cortex was studied morphologically and histochemically using the retrograde and antegrade horseradish peroxidase (HRP) tracing methods. The normal corticospinal tract in the lumbar cord was composed of a number of small and some large axons. In the atrophic corticospinal tract related to the ablated cerebral cortex, the small axons were decreased in number two weeks after the operation. However, new myelinated small axons appeared around day 28 and their diameters increased gradually from after day 56 to day 84. The original large axon in the atrophic corticospinal tract was much more increased in size than that in the corticospinal tract of the non-operated-on control. When HRP was injected into the left cervical cord of the adult rat whose right cerebral cortex had been ablated during the neonatal period, a considerable number of HRP-labeled neurons was seen in the healthy left cerebral cortex. When the corticospinal tract was traced antegradely by injecting HRP into the healthy left cerebral cortex, an aberrant corticospinal tract reaching into the ipsilateral dorsal funiculus was observed. These results give a morphological basis for the well known fact that children who have had brain damage during the neonatal period and early infancy have the capacity for recovery of motor function.