FETAL SPINAL-CORD TRANSPLANTS SUPPORT GROWTH OF SUPRASPINAL AND SEGMENTAL PROJECTIONS AFTER CERVICAL SPINAL-CORD HEMISECTION IN THE NEONATAL RAT

Cervical spinal cord injury at birth permanently disrupts forelimb fun ction in goal-directed reaching. Transplants of fetal spinal cord tiss ue permit the development of skilled forelimb use and associated postu ral adjustments (Diener and Bregman, 1998, companion article). The aim of this study was to determine whether transplants of fetal spinal co rd tissue support the remodeling of supraspinal and segmental pathways that may underlie recovery of postural reflexes and forelimb movement s. Although brainstem-spinal and segmental projections to the cervical spinal cord are present at birth, skilled forelimb reaching has not y et developed. Three-day-old rats received a cervical spinal cord overh emisection with or without transplantation of fetal spinal cord tissue (embryonic day 14); unoperated pups served as normal controls. Neuroa natomical tracing techniques were used to examine the organization of CNS pathways that may influence target-directed reaching. In animals w ith hemisections only, corticospinal, brainstem-spinal, and dorsal roo t projections within the spinal cord were decreased in number and exte nt. In contrast, animals receiving hemisections plus transplants exhib ited growth of these projections throughout the transplant and over lo ng distances within the host spinal cord caudal to the transplant. Rap hespinal axons were apposed to numerous propriospinal neurons in contr ol and transplant animals; these associations were greatly reduced in the lesion-only animals. These observations suggest that after neonata l cervical spinal cord injury, embryonic transplants support axonal gr owth of CNS pathways and specifically supraspinal input to propriospin al neurons. We suggest that after neonatal spinal injury in the rat, t he transplant-mediated reestablishment of supraspinal input to spinal circuitry is the mechanism underlying the development of target-direct ed reaching and associated postural adjustments.