DEMONSTRATION OF THE POTENTIAL FOR CHRONICALLY INJURED NEURONS TO REGENERATE AXONS INTO INTRASPINAL PERIPHERAL-NERVE GRAFTS

Experiments were carried out to determine if neurons damaged by injury to the spinal cord retain the ability to regenerate their axonal process for a prolonged period of time after the initial response to injury and if peripheral nerve (PN) grafts could support the regrowth of these processes. True blue (TB) was injected into one side of the adult rat lumbar spinal cord to label neurons with axons coursing through this region. Seven days later spinal cord tissue surrounding the injection sites was removed by aspiration to create a hemisection cavity 3-4 mm in length. Four weeks later scar tissue lining the lesion cavity was removed prior to grafting 1 cm segments of autologous tibial nerve to the rostral and the caudal surfaces of the cavity wall. The distal end of each graft was ligated and left unapposed to spinal cord tissue. Four weeks later the distal end of each PN graft was exposed to nuclear yellow (NY) to retrogradely label neurons that had grown an axon into the graft. Neurons containing both TB and NY were deemed capable of axonal regeneration while in a chronically injured state. Double-labeled ( TB NY) neurons were found in the ipsilateral spinal cord in laminae IV through X, excluding IX, and in Laminae VI and VII contralateral to the lesion. Most neurons were located within 10 mm of the lesion, with the majority caudal to the lesion. Nearly 50% (range 24-74%) of lumbar dorsal root ganglion neurons containing TB also were labeled with NY. These double-labeled, regenerating ganglion neurons had a mean cell area of 947 μm2, which was significantly lower than that of the total ganglion cell population. This indicated that primarily small neurons were involved in the regenerative response elicited by the present experimental paradigm. No double-labeled neurons were found in the cervical spinal cord or within regions of the brain known to project to the spinal cord. These results indicate that certain neurons associated with a chronic spinal cord injury have the potential to regenerate their axonal process for at least 4 weeks after sustaining a direct injury. © 1991.