The complete transection of the adult mammalian spinal cord is believed to lead to irreparable, permanent loss of connectivity.
The execution of coordinated movements depends on control exerted by higher centers in the brain.
After transection in the rat spinal cord, the neurons responsible for contraction of muscles of the hindlimb, which are located in the lumbar enlargements of the cord, are left intact but are not functional because of lack of connections from the supraspinal control pathways.
One necessary element of recovery in post-traumatic spinal cord is long tract axonal regeneration.
Pathological changes after spinal cord injury (including scar formation, myelin inhibition, neuronal apoptosis, or necrosis) limit axonal regeneration after the injury.
Several successful repair strategies have demonstrated that regenerated axons can grow beyond the lesion site if provided an appropriate environment.
A favorable environment for axon regeneration can be created by surgical repair of peripheral nerve transplantation and acidic fibroblast growth factor (aFGF) treatment.
These results demonstrated better hindlimb functional recovery in a treatment that combined aFGF and nerve grafts than the individual treatment (either aFGF or nerve graft alone) (Fig. 1).
The partial reestablishment of descending tracts and partial re-establishment of ascending tracts also were demonstrated.
Schematic drawing showing the procedure of aFGF and nerve graft treatment.
Repair Strategies to Promote Recovery Following Spinal Cord Damage
Evidence for axonal regeneration and partial functional recovery after spinal cord injury has been demonstrated by using a variety of different treatments.
These treatments all provide for an environment more conducive to axon growth.
The present study followed the transplantation techniques to set up a tissue bridge at the point of injury in order to allow damaged axons to grow across the gap created by the injury.
The data of tracing studies in the present study corroborate the results of several studies that have demonstrated that the central nervous system axons could grow into the peripheral nerve grafts.
In addition to peripheral nerve graft, several investigators also used fetal tissue, olfactory ensheathing glia, and Schwann cells transplantation to promote axonal regeneration in different spinal cord injury models.
These repair strategies all provided a favorable environment to support axonal growth through the damage site.
The data obtained from previous and present study suggest that axons could establish connections with the distal spinal cord to restore partial locomotion recovery.
A previous study reported labeled neurons in the cortex and brainstem and labeled corticospinal tract fibers below the damage site when using the peripheral nerve graft with aFGF repair strategy.
Furthermore, peripheral nerve graft and aFGF treatment has been shown to promote axonal growth into graft, although not beyond the lesion site, in spinal hemisection of primate model.
The present results also demonstrate the improvement in hindlimb locomotion in behavioral assessment scores correlated with the number of retrograde labeled neurons.
The results support previous study, which demonstrated motor evoked responses in hindlimb muscles to cortical stimulation in nerve graft with aFGF-treated animals, and that the improvement of behavioral assessment scores correlated with the amplitude of motor evoked potentials.
Similarly, the positive relationship between the number of labeled supraspinal nuclei (including sensorimotor cortex, red nucleus, vestibular nuclei, and raphe nuclei) and behavioral assessment scores in spinal cord-repaired neonatal rats were showed.
One of the important factors that could limit the axonal regeneration after spinal cord injury was glia scar formation.
Subsequent to spinal cord injury was the proliferation and hypertrophy of astrocytes around the injury site and increased immunoreactivity to glial fibrillary acidic protein.
Astrocytes not only present a physical barrier to growth, but also a chemical barrier.
After injury, astrocytes express some of the boundary molecules, including chondroitin sulphate proteoglycan and heparin sulphate proteoglycan, which are able to restrict axonal growth.
Using chondroitinase treatment to minimize the scar formation has been demonstrated to facilitate axonal regeneration after spinal cord injury.
In addition to scar formation, the nonpermissive environment in the white matter (including myelin-associated glycoprotein) and Nogo are identified as a barrier for axonal regrowth.
Nogo-A is a transmembrane protein and expressed by oligodendrocytes in white matter of the central nervous system.
The application of the monoclonal antibody mAb IN-1 was demonstrated to promote axonal regeneration and functional recovery in spinal cord-injured adult rats.
However, Nogo-A-deficient mice studies show different results in axonal regeneration.
One of them shows that Nogo-A-deficient mice improved corticospinal tract axonal regrowth after spinal cord injury, while the other demonstrates that the elimination of Nogo-A does not support extensive axonal regeneration.
In the present study was observed that labeled axons could enter the distal end of host spinal cord.
This positive result may be due to the aFGF treatment and the white matter to gray matter reconnecting of nerve grafts.
aFGF may downregulate the expression of, or allow axons to overcome, the known inhibitors of axon regeneration such as proteoglycan or tenascins.
In brain and spinal cord, aFGF has been shown to induce nerve growth factor production within the injured neocortex and nerve growth factor has been shown to promote the growth of adult axons across reactive scar tissue.
Furthermore, placement of nerve grafts to the gray matter in the distal end of spinal cord may guide the axons regrow into the more permissive gray matter environment instead of nonpermissive environment in the white matter.
The Regeneration of Spinal Tracts after the Injury
The corticospinal tract is one of the important brain tracts projecting to spinal cord that controls motor function in the human, but its role is still not well defined in rats.
Several studies have investigated corticospinal tract recovery following spinal cord damage.
The treatment of stem cell or ensheathing cells transplantation, the monoclonal antibody IN-1, and neurotrophic factors have been demonstrated to facilitate corticospinal tract fiber regeneration in different acute spinal cord injury models.
Furthermore, preligated sural nerve auto-transplantation, olfactory ensheathing cells, neurotrophin-3 with monoclonal antibody IN-1, and the fetal tissue transplantation with neurotrophins all have been used to enhance the corticospinal tract fibers in chronic spinal cord injury.
Previous work also indicated the corticospinal tract fibers could grow into a graft with aFGF or neurotrophin-3 treatment, although another report showed no corticospinal tract fibers growing into the nerve graft with brain-derived neurotrophic factor treatment.
Additionally, using Schwann cell or fetal tissue transplantation and neurotrophic factors could facilitate supraspinal nuclei (e.g. cerebral cortex, reticular formation, red nucleus, and raphe nucleus) to regenerate axons following contusive or transected spinal cord injury.
It is not know, at present, which of these projection systems is essential for the behavioral recovery.
However, even in the normal rat the current level of understanding is incomplete regarding the roles of the different descending motor systems in regulating hind limb performance.
Future studies can examine this issue by selective ablation of each system, both in normal and in repaired rats.
Neurotrophic/Neurotrophin Factors in Axonal Regeneration
Neurotrophic factors have been widely used to treat spinal cord injury.
These factors can promote axon growth in general and may facilitate axons to cross the damage site.
The fibroblast growth factor family members have multiple functions, including being potent modulators of cell proliferation, migration, differentiation, and survival.
aFGF is normally produced in the spinal cord by motor neurons and primary sensory neurons.
The aFGF acts through tyrosine kinase-type receptor, FGF-R1, and aFGF binds with high affinity to heparin sulfate proteoglycans, which are located on the surface of most cells and within the extracellular matrix.
Although aFGF alone can enhance axonal growth, results are optimized when aFGF is used in combination with other supportive factors.
The previous work, both in vitro and in vivo, demonstrated that aFGF combined with peripheral nerve transplants provided better axonal regeneration and functional recovery than the individual treatment (either aFGF or peripheral nerve alone).
Anatomical evidence of axonal regeneration from different neuronal populations in aFGF and peripheral nerve graft treatment exist.
This also confirmed previous work regarding the electrophysiological data and spinal cord retransection result using the same repair strategy.
This surgical technique demonstrated the possibility of axonal regeneration in the completely transected adult mammalian spinal cord.
However, the application of this strategy in the chronic spinal cord injury model or with the other advanced techniques still need to be studied to provide more information for the future clinical work.