The fate of severed corticospinal axons.

The potential for regeneration of severed corticospinal axons was examined by labeling these axons with horseradish peroxidase following thoracic spinal cord transections in mice. Shortly after severance, the proximal ends of corticospinal axons formed terminal bulbs that persisted for weeks and were associated with axonal retraction. There were few signs of corticospinal axonal sprouting or elongation. By 2 months after injury, corticospinal axons near the transection site showed an increased number of probable labeled terminals in the adjacent gray matter. These new terminals may contribute to the persistence of many corticospinal axons near the injury site long after a spinal cord transection.