Formation of an ipsilateral corticospinal tract after ablation of cerebral cortex in neonatal rat.

The formation of an aberrant ipsilateral corticospinal tract after unilateral cerebral cortical ablation during the neonatal period has been confirmed in the rat. The tract was chronologically studied using the antegrade horseradish peroxidase (HRP) tracing method. An aberrant ipsilateral tract is not observed 3 days after the operation. However, ipsilateral HRP positive fibers become apparent on day 7 and progressively more prominent until day 14. These results suggest that the ipsilateral corticospinal tract is composed of collateral axons originating from pyramidal neurons in the healthy ipsilateral cerebral cortex. These results also indicate that, when cerebral cortex has the damage during early postnatal life, the remaining cortical neurons which have been freed from the damage show considerable plasticity in terms of the collateral axons.