This study of spinal cord injury in bullfrog (Rana catesbeiana) tadpoles using the neuroanatomical tracer horseradish peroxidase (HRP) was undertaken to determine (1) whether the same anatomical regions that normally give rise to ascending or descending spinal tracts do so following complete spinal cord transection and (2) whether the course of behavioral recovery could be related to the anatomical results. The results of this study show that (1) spinal cord continuity is readily restored in tadpoles subjected to spinal cord transection, but nerve fibers crossing the site of injury end within 1 to 2 mm of the lesion site; (2) tadpoles with spinal cord transections held through metamorphosis show, as juvenile frogs, restoration of lumbar projections from all brainstem regions that normally project to the lumbar spinal cord; (3) neither long ascending projections from dorsal root ganglion cells nor those from spinal neurons caudal to the transection traverse the transection site, even after metamorphosis; and (4) consistent with the anatomical results, tadpoles show only minimal behavioral recovery, but these same animals as juvenile frogs show recovery of behaviors that are dependent upon connections to supraspinal regions. In other experiments, [3H]thymidine or [3H]apo-HRP was combined with HRP histochemistry to determine if new brainstem neurons projecting to the spinal cord are born in the metamorphic period and if, in normal animals, brainstem projections to the lumbar spinal cord persist through metamorphosis. We found no evidence that neurons with lumbar spinal cord projections are born during metamorphosis; however, evidence was found that most brainstem neurons that project to the lumbar spinal cord before metamorphosis retain this projection in the juvenile frog.

The segmental distribution of regenerating bullfrog motor axons was mapped in advanced tadpoles and juvenile frogs by stimulating selected muscle nerves and recording from the distal ends of the 3 lumbar ventral roots (VRs) that innervate the hindlimb. When motoneurons were axotomized by VR transection, they reestablished their original innervation fields, rarely, if ever, growing beyond the territory normally supplied by their spinal segment. However, when motoneurons were axotomized in the spinal nerves at the level of the hindlimb plexus, some of them regenerated into limb nerves that lay outside the axons' normal segmental boundaries, and many regenerated into the medial femoral cutaneous nerve, a pathway normally limited to sensory axons. These observations suggest that the ultimate destinations of regenerating axons are largely determined by structures the axons encounter as they penetrate the distal nerve stumps. Thus, axons regenerating from a severed VR grow into that root's own distal stump and reinnervate the hindlimb in a manner that is segmentally appropriate; axons transected near the plexus have access to the pathways of sensory, as well as motor, axons in all 3 lumbar segments, and establish innervation fields that are inappropriate for their segment of origin and their motor function.

The purpose of this study was to examine the effects of axon transection on the development and differentiation of spinal motoneurons in the bullfrog (Rana catesbeiana) tadpole. The 3 ventral roots (VRs) that innervate the hindlimb were transected, and the animals were killed 6-7 weeks later (reinnervation took place within 3 weeks). At early stages of development, axotomy resulted in an increase in the number of spinal motoneurons on the operated side. By histological criteria, these motoneurons appeared more differentiated than those in normal tadpoles. Axotomy was effective in increasing motoneuron number only during the period of naturally occurring cell death. Similar effects were seen when the transected VRs were ligated to prevent regeneration. Hindlimb amputation without VR transection had no effect on motoneuron number or differentiation. Thus, target removal is neither a necessary nor a sufficient condition for hyperplasia of the lateral motor column. An extreme loss of spinal motoneurons was seen if the operated tadpole entered into metamorphic climax during the 6-7-week postoperative survival period. Motoneuron loss occurred although the injured motoneurons had reconnected to the hindlimb. In contrast, tadpoles allowed to survive up to 6 months showed no loss of motoneurons if they did not enter metamorphic climax. From these data, it appears axon transection in developing spinal motoneurons exerts its effects on motoneuron number and differentiation by altering the metabolic state of the motoneuron (axon reaction) rather than by depriving it of contact with its target.