Peripheral nerve grafts after cervical spinal cord injury in adult cats

Peripheral nerve grafts (PNG) into the rat spinal cord support axon regeneration after acute or chronic injury, with synaptic reconnection across the lesion site and some level of behavioral recovery. Here, we grafted a peripheral nerve into the injured spinal cord of cats as a preclinical treatment approach to promote regeneration for eventual translational use. Adult female cats received a partial hemisection lesion at the cervical level (C7) and immediate apposition of an autologous tibial nerve segment to the lesion site. Five weeks later, a dorsal quadrant lesion was performed caudally (T1), the lesion site treated with chondroitinase ABC 2 days later to digest growth inhibiting extracellular matrix molecules, and the distal end of the PNG apposed to the injury site. After 4-20 weeks, the grafts survived in 10/12 animals with several thousand myelinated axons present in each graft. The distal end of 9/10 grafts was well apposed to the spinal cord and numerous axons extended beyond the lesion site. Intraspinal stimulation evoked compound action potentials in the graft with an appropriate latency illustrating normal axonal conduction of the regenerated axons. Although stimulation of the PNG failed to elicit responses in the spinal cord distal to the lesion site, the presence of c-Fos immunoreactive neurons close to the distal apposition site indicates that regenerated axons formed functional synapses with host neurons. This study demonstrates the successful application of a nerve grafting approach to promote regeneration after spinal cord injury in a non-rodent, large animal model.     

Essentiality of a specific cellular terrain for growth of axons into a spinal cord lesion

To date, there are no reports of growth of significant numbers of axons into or across a lesion of the mammalian spinal cord. However, recent studies showing that CNS axons will grow into PNS environments indicate that comparable growth into spinal cord lesions could be achieved if ischemic necrosis could be prevented and the lesion site repopulated by astrocytes and ependymal cells rather than by the macrophages, lymphocytes, and fibroblasts that generally accumulate at sites of CNS injury. To examine this possibility, we made a laminectomy at T5 in rats and crushed the spinal cord for 2 s with a smooth forceps (leaving the dura mater intact to prevent ingrowth of connective tissue). At 1 week, the lesion was filled with mononuclear cells, degenerating nerve fibers, and capillaries that were oriented parallel to the long axis of the spinal cord. By 2 weeks, longitudinally oriented cords of ependymal cells and astrocytes had migrated into the lesion from the adjacent spinal cord, and similarly oriented nerve fibers had begun to grow into the lesion along these capillaries and cellular cordons. The mononuclear cells had now assumed phagocytic activity and were engorged with myelin and other cellular debris. After 3 weeks, the astrocytes had elaborated thick cell processes. The nerve fibers in the lesion were still oriented longitudinally but had increased in number and were often arranged in small fascicles. These observations provide the first histological evidence of growth of nerve fibers into a lesion of the rat spinal cord. We conclude that the intrinsic regenerative capacity of the spinal cord can be expressed if ischemic necrosis and collagenous scarring are prevented and the spinal cord parenchyma is first reconstructed by its nonneuronal constituents.     

NT-3 gene delivery elicits growth of chronically injured corticospinal axons and modestly improves functional deficits after chronic scar resection

Nervous system growth factors promote axonal growth following acute spinal cord injury. In the present experiment, we examined whether delivery of neurotrophic factors after chronic spinal cord injury would also promote axonal growth and influence functional outcomes. Adult Fischer 344 rats underwent mid-thoracic spinal cord dorsal hemisection lesions. Three months later, primary fibroblasts genetically modified to express human neurotrophin-3 (NT-3) were placed in, and distal to, the lesion cavity. Upon sacrifice 3 months later (6 months following the initial lesion), NT-3-grafted animals exhibited significant growth of corticospinal axons up to 15 mm distal to the lesion site and showed a modest but significant 1.5-point improvement in locomotor scores (P < 0.05) on the BBB scale, compared to control-grafted animals. Thus, growth factor gene delivery can elicit growth of corticospinal axons in chronic stages of injury and improves functional outcomes compared to non-growth-factor-treated animals.     

Spinal cord transplants permit the growth of serotonergic axons across the site of neonatal spinal cord transection

These experiments were designed to determine whether transplants of fetal spinal cord tissue into lesioned spinal cord in newborn rats provide a terrain that supports the growth of serotonergic (5-HT) axons across the site of the lesion. Although descending serotonergic axons can regenerate after chemical lesions in adult animals, they show little regrowth after surgical lesions. In newborn animals, 5-HT axons do not regrow after either chemical or mechanical lesions since the axotomized raphe-spinal neurons die. After partial spinal cord lesions made in developing animals, immature axons can take an aberrant route around the site of the lesion to reach normal target areas. Even these robust, late-growing, uninjured axons, however, are unable to grow through the site of the spinal cord lesion. Immunocytochemical labeling was used to determine if descending serotonergic axons grow into fetal spinal cord transplants, and whether these axons cross the transplant to reach spinal cord levels caudal to the lesion. Spinal cord transection at a mid-thoracic spinal cord level on the day of birth resulted in a dramatic decrease in 5-HT immunoreactivity caudal to the lesion by one day postoperative. 5-HT immunoreactivity caudal to the lesion was abolished by 5 days postoperative and did not return after acute or chronic (6 months) survival periods. When a transplant of fetal spinal cord tissue was placed into the lesion site, 5-HT axons were identified throughout the transplant. At spinal cord levels caudal to the transection and transplant, the serotonergic axons were identified in the host spinal cord in both the white and gray matter. This 5-HT innervation was not confined to spinal cord segments adjacent to the lesion site but extended to spinal cord segments as far as lower lumbar levels. The reinnervation of the host spinal cord caudal to the transection was far less than that seen in unlesioned adult rat spinal cord. Horseradish peroxidase (HRP) injected caudal to the transection and transplant, retrogradely labeled neurons within the medullary raphe nuclei. The HRP and 5-HT results both depended on apposition of the transplant with the rostral and caudal stumps of the host spinal cord; without such apposition, labeling was abolished. These results indicate that the presence of a transplant at the site of the neonatal lesion modifies the environment at the lesion site in such a manner as to support the elongation of identified axons across the site of the lesion and into the host cord caudal to the lesion.     

Adult neural progenitor cells provide a permissive guiding substrate for corticospinal axon growth following spinal cord injury

Adult neural progenitor cells (NPC) are an attractive source for cell transplantation and neural tissue replacement after central nervous system (CNS) injury. Following transplantation of NPC cell suspensions into the acutely injured rat spinal cord, NPC survive; however, they migrate away from the lesion site and are unable to replace the injury-induced lesion cavity. In the present study we examined (i) whether NPC can be retained within the lesion site after co-transplantation with primary fibroblasts, and (ii) whether NPC promote axonal regeneration following spinal cord injury. Co-cultivation of NPC with fibroblasts demonstrated that NPC adhere to fibroblasts and the extracellular matrix produced by fibroblasts. In the presence of fibroblasts, the differentiation pattern of co-cultivated NPC was shifted towards glial differentiation. Three weeks after transplantation of adult spinal-cord-derived NPC with primary fibroblasts as mixed cell suspensions into the acutely injured cervical spinal cord in adult rats, the lesion cavity was completely replaced. NPC survived throughout the graft and differentiated exclusively into glial cells. Quantification of neurofilament-labeled axons and anterogradely labeled corticospinal axons indicated that NPC co-grafted with fibroblasts significantly enhanced axonal regeneration. Both neurofilament-labeled axons and corticospinal axons aligned longitudinally along GFAP-expressing NPC-derived cells, which displayed a bipolar morphology reminiscent of immature astroglia. Thus, grafted astroglial differentiated NPC promote axon regrowth following spinal cord injury by means of cellular guidance.     

Fibroblasts genetically modified to produce BDNF support regrowth of chronically injured serotonergic axons

Cells genetically modified to release a variety of growth and/or neurotrophic factors have been used for transplantation into the injured spinal cord as a means to deliver therapeutic products. Axon growth into and through such transplants has been demonstrated after intervention after an acute injury. The present study examined their potential to support regeneration in a chronic injury condition. Five weeks after a cervical hemisection in adult rats, the lesion site was debrided of scar tissue and expanded in both rostral and caudal directions. Animals received a transplant of cultured normal fibroblasts (control) or fibroblasts genetically modified to produce brain-derived neurotrophic factor (BDNF). Six weeks later, animals were killed to determine the extent of growth of serotonergic axons into the transplant. Axons immunoreactive for serotonin (5-HT-ir) were found to cross the rostral interface of host spinal cord readily with either type of fibroblast cell transplant, but the number and density of 5-HT-ir axons extending into the BDNF-producing transplants was markedly greater than those in the control fibroblasts. Axons coursed in all directions among normal fibroblast transplants, whereas growth was more oriented along a longitudinal plane when BDNF was being released by the transplanted cells. The length of growth and the percentage of the transplant length occupied by 5-HT-ir axons were significantly greater in BDNF-producing transplants than in the normal fibroblasts. Many serotonergic axons approached the caudal end of the BDNF-producing cell transplants, although most failed to penetrate the host spinal cord distal to the lesion. These results indicate that whereas fibroblast cell transplants alone can support regrowth of axons from chronically injured supraspinal neurons, modification of these cells to produce BDNF results in a significant increase in the extent of growth into the transplant.     

Self‐assembling peptide amphiphile promotes plasticity of serotonergic fibers following spinal cord injury

Injection into the injured spinal cord of peptide amphiphile (PA) molecules that self‐assemble and display the laminin epitope IKVAV at high density improved functional recovery after spinal cord injury (SCI) in two different species, rat and mouse, and in two different injury models, contusion and compression. The improvement required the IKVAV epitope and was not observed with the injection of an amphiphile displaying a nonbioactive sequence. To explore the mechanisms underlying these improvements, the number of serotonergic fibers in the lesioned spinal cord was compared in animals receiving the IKVAV‐PA, a nonbioactive PA (PA control), or sham injection. Serotonergic fibers were distributed equally in all three groups rostral to the injury but showed a significantly higher density caudal to the injury site in the IKVAV PA‐injected group. Furthermore, this difference was not present in the subacute phase following injury but appeared in the chronically injured cord. The IKVAV PA‐injected groups also trended higher both in the total number neurons adjacent to the lesion and in the number of long propriospinal tract connections from the thoracic to the lumbar cord. IKVAV PA injection did not alter myelin thickness, total axon number caudal to the lesion, axon size distribution, or total axon area. Serotonin can promote stepping even in complete transection models, so the improved function produced by the IKVAV PA treatment may reflect the increased serotonergic innervation caudal to the lesion in addition to the previously demonstrated regeneration of motor and sensory axons through the lesion. © 2010 Wiley‐Liss, Inc..     

Extension of the critical period for developmental plasticity of the corticospinal pathway

The corticospinal tract (CST) of the rat undergoes a prolonged period of postnatal development. Lesions of the presumptive CST pathway at birth are followed by the aberrant rerouting of the developing corticospinal axons around the lesion site through adjacent undamaged CNS tissue. This developmental plasticity becomes severely restricted by 5-6 days of age, so the axons are no longer capable of growth around the site of injury. The aim of the current study was to determine whether altering the environment at the site of injury by filling the lesion with transplanted fetal spinal cord tissue could prolong the critical period for developmental plasticity of the corticospinal pathway. The spinal cord was damaged (overhemisection) at three stages in the development of the corticospinal (CS) pathway: 1) prior to the arrival of CS axons, 2) after the axons elongated through the cord but prior to synaptogenesis, and 3) after both axonal elongation and synaptogenesis were completed. One to 9 months later, anterograde neuronal tracing with horseradish peroxidase was used to assess the growth of the corticospinal pathway with or without a fetal transplant at the site of injury, and the pattern of labeling was compared with that observed in adult nonlesioned control animals. Our results indicate that the presence of a transplant prolongs the critical period for developmental plasticity of the CST. Transplants elicited growth of CST axons throughout the postnatal period examined. CST axons damaged prior to synaptogenesis exhibited more robust growth than those lesioned after synaptogenesis had been completed. These results suggest that both environmental and neuronal factors interact to regulate the response of immature CS neurons to injury.     

Mats made from fibronectin support oriented growth of axons in the damaged spinal cord of the adult rat

A variety of biological as well as synthetic implants have been used to attempt to promote regeneration into the damaged spinal cord. We have implanted mats made from fibronectin (FN) into the damaged spinal cord to determine their effectiveness as a substrate for regeneration of axons. These mats contain oriented pores and can take up and release growth factors. Lesion cavities 1 mm in width and depth and 2 mm in length were created on one side of the spinal cord of adult rats. FN mats containing neurotrophins or saline were placed into the lesion. Mats were well integrated into surrounding tissue and showed robust well-oriented growth of calcitonin gene-related peptide, substance P, GABAergic, cholinergic, glutamatergic, and noradrenergic axons into FN mats. Transganglionic tracing using cholera toxin B indicated large-diameter primary afferents had grown into FN implants. Schwann cells had also infiltrated FN mats. Electron microscopy confirmed the presence of axons within implants sites, with most axons either ensheathed or myelinated by Schwann cells. Mats incubated in brain-derived neurotrophic factor and neurotrophin-3 showed significantly more neurofilament-positive and glutamatergic fibers compared to saline- and nerve growth factor-incubated mats, while mats incubated with nerve growth factor showed more calcitonin gene-related peptide-positive axons. In contrast, neurotrophin treatment had no effect on PGP 9.5-positive axons. In addition, in some animals with neurotrophin-3-incubated mats, cholera toxin B-labelled fibers had grown from the mat into adjoining intact areas of spinal cord. The results indicate that FN mats provide a substrate that is permissive for robust oriented axonal growth in the damaged spinal cord, and that this growth is supported by Schwann cells.     

Laminin distribution during corticospinal tract development and after spinal cord injury

The glycoprotein laminin is a prominent constituent of basal laminae and has been suggested to play an important role in axonal growth. We have tested this hypothesis, by examining the temporal and spatial distribution of laminin in the rat spinal cord, relative to elongating corticospinal tract (CST) axons, during normal development and after newborn and adult spinal lesions. The distribution of laminin was demonstrated in spinal cord sections from animals ranging in age from 14 days embryonic to adult using immunocytochemistry. Anti-laminin immunolabeling was seen around blood vessels and meninges in all the animals examined. However, within the grey and white matter its distribution was age-dependent. In the normal cord, immunostaining appeared in small amounts in early embryos, but was absent from all postnatal animals even at ages when the CST was growing down the cord. Following injury, intense immunostaining was associated with lesions in both newborn and adult operates at all postoperative periods examined. Within the matrix of the lesion laminin immunostaining was especially prominent. In the intact cord it was prominent only around blood vessels near the lesion site. Our results indicate that the distribution of laminin does not closely correlate with axonal growth of the CST either during normal development or after spinal injury.     

Regeneration of corticospinal axons in the rat.

In the rat, a few long descending motor tracts capable of carrying an impulse and causing a propagated impulse in the ipsilateral sciatic nerve will regenerate after complete spinal cord transection. In this experiment such regeneration was found in both treated and control animals. Orthograde axonal transport of tritiated proline injected into the motor cortex labels only the corticospinal tracts in the rat spinal cord. Scintillation counts of measured lengths of spinal cord can be used as a measure of the number of labeled corticospinal axons. Comparison of radioactivity per unit length of measured cord segments taken from above and below the site of a previous spinal cord transection can give a reliable estimate of the number of labeled axons that regenerated and crossed the site of injury. Using this test we have demonstrated that some corticospinal axons had regenerated six months after spinlal cord transection in control animals, animals made tolerant to degenerating spinal cord antigens, and animals treated with cyclophosphamide. A group treated with a single 75 mg per kilogram dose of cyclophosphamide 24 hours after spinal cord transection showed the best evidence of corticospinal tract regeneration.     

Undesired effects of a combinatorial treatment for spinal cord injury--transplantation of olfactory ensheathing cells and BDNF infusion to the red nucleus

Transplantations of olfactory ensheathing cells (OECs) have been reported to promote axonal regeneration and functional recovery after spinal cord injury, but have demonstrated limited growth promotion of rat rubrospinal axons after a cervical dorsolateral funiculus crush. Rubrospinal neurons undergo massive atrophy after cervical axotomy and show only transient expression of regeneration-associated genes. Cell body treatment with brain-derived neurotrophic factor (BDNF) prevents this atrophy, stimulates regeneration-associated gene expression and promotes regeneration of rubrospinal axons into peripheral nerve transplants. Here, we hypothesized that the failure of rubrospinal axons to regenerate through a bridge of OEC transplants was due to this weak intrinsic cell body response. Hence, we combined BDNF treatment of rubrospinal neurons with transplantation of highly enriched OECs derived from the nasal mucosa and assessed axonal regeneration as well as behavioral changes after a cervical dorsolateral funiculus crush. Each treatment alone as well as their combination prevented the dieback of the rubrospinal axons, but none of them promoted rubrospinal regeneration beyond the lesion/transplantation site. Motor performance in a food-pellet reaching test and forelimb usage during vertical exploration (cylinder test) were more impaired after combining transplantation of OECs with BDNF treatment. This impaired motor performance correlated with lowered sensory thresholds in animals receiving the combinatorial therapy - which were not seen with each treatment alone. Only this combinatorial treatment group showed enhanced sprouting of calcitonin gene-related peptide-positive axons rostral to the lesion site. Hence, some combinatorial treatments, such as OECs with BDNF, may have undesired effects in the injured spinal cord.     

Regenerating descending axons preferentially reroute to the gray matter in the presence of a general macrophage/microglial reaction caudal to a spinal transection in adult zebrafish

We analyzed pathway choices of regenerating, mostly supraspinal, descending axons in the spinal cord of adult zebrafish and the cellular changes in the spinal cord caudal to a lesion site after complete spinal transection. Anterograde tracing (by application of the tracer rostral to the spinal lesion site) showed that significantly more descending axons (74%) regenerated in the spinal gray matter of the caudal spinal cord than would be expected from random growth. Retrograde tracing (by application of the tracer caudal to the spinal lesion site) showed that, rostral to the lesion, most of these axons (80%) extended into the major white matter tracts. Thus, ventral descending tracts often were devoid of labeled axons caudal to a spinal lesion but contained many axons rostral to the lesion in the same animals, indicating a pathway switch of descending axons from the white matter to the gray matter. Ascending axons of spinal neurons were not observed regrowing to the rostral tracer application site; therefore, they most likely did not contribute to the axonal populations analyzed. A macrophage/microglia response within 2 days of spinal cord transection, along with phagocytosis of myelin, was observed caudal to the transection by immunohistochemistry and electron microscopy. Nevertheless, caudal to the lesion, descending tracts in the white matter were filled with myelin debris during the time of axonal regrowth, at least up to 6 weeks postlesion. We suggest that the spontaneous regeneration of axons of supraspinal origin after spinal cord transection in adult zebrafish may be due in part to the axons' ability to negotiate novel pathways in the spinal cord gray matter.     

Robust Growth of Chronically Injured Spinal Cord Axons Induced by Grafts of Genetically Modified NGF-Secreting Cells

Little spontaneous regeneration of axons occurs after acute and chronic injury to the CNS. Previously we have shown that the continuous local delivery of neurotrophic factors to the acutely injured spinal cord induces robust growth of spinal and supraspinal axons. In the present study we examined whetherchronicallyinjured axons also demonstrate significant neurotrophin responsiveness. Adult rats underwent bilateral dorsal hemisection lesions that axotomize descending supraspinal pathways, including the corticospinal, rubrospinal, and cerulospinal tracts, and ascending dorsal spinal sensory projections. One to three months later, injured rats received grafts of syngenic fibroblasts genetically modified to produce nerve growth factor (NGF). Control subjects received unmodified cell grafts or cells transduced to express the reporter gene β-galactosidase. Three to five months after grafting, animals that received NGF-secreting grafts showed dense growth of putative cerulospinal axons and primary sensory axons of the dorsolateral fasciculus into the grafted lesion site. Growth from corticospinal, raphaespinal, and local motor axons was not detected. Thus, robust growth of defined populations of supraspinal and spinal axons can be elicited in chronic stages after spinal cord injury by localized, continuous transgenic delivery of neurotrophic factors.     

IGF-I gene delivery promotes corticospinal neuronal survival but not regeneration after adult CNS injury

An unmet challenge of spinal cord injury research is the identification of mechanisms that promote regeneration of corticospinal motor axons. Recently it was reported that IGF-I promotes corticospinal axon growth during nervous system development. We therefore investigated whether IGF-I also promotes regeneration or survival of adult lesioned corticospinal neurons. Adult Fischer 344 rats underwent C3 dorsal column transections followed by grafts of IGF-I-secreting marrow stromal cell grafts into the lesion cavity. IGF-I secreting cell grafts promoted growth of raphespinal and cerulospinal axons, but not corticospinal axons, into the lesion/graft site. We then examined whether IGF-I-secreting cell grafts promote corticospinal motor neuron survival or axon growth in a subcortical axotomy model. IGF-I expression coupled with infusion of the IGF binding protein inhibitor NBI-31772 significantly prevented corticospinal motor neuron death (93% cell survival compared to 49% in controls, P < 0.05), but did not promote corticospinal axon regeneration. Coincident with observed effects of IGF-I on corticospinal survival but not growth, expression of IGF-I receptors was restricted to the somal compartment and not the axon of adult corticospinal motor neurons. Thus, whereas IGF-I influences corticospinal axonal growth during development, its application to sites of adult spinal cord injury or subcortical axotomy fails to promote corticospinal axonal regeneration under conditions that are sufficient to prevent corticospinal cell death and promote the growth of other supraspinal axons. We conclude that developmental patterns of growth factor responsiveness are not simply recapitulated after adult injury, potentially due to post-natal shifts in patterns of IGF-I receptor expression.     

Sensory afferents regenerated into dorsal columns after spinal cord injury remain in a chronic pathophysiological state

Axon regeneration after experimental spinal cord injury (SCI) can be promoted by combinatorial treatments that increase the intrinsic growth capacity of the damaged neurons and reduce environmental factors that inhibit axon growth. A prior peripheral nerve conditioning lesion is a well-established means of increasing the intrinsic growth state of sensory neurons whose axons project within the dorsal columns of the spinal cord. Combining such a prior peripheral nerve conditioning lesion with the infusion of antibodies that neutralize the growth inhibitory effects of the NG2 chondroitin sulfate proteoglycan promotes sensory axon growth through the glial scar and into the white matter of the dorsal columns. The physiological properties of these regenerated axons, particularly in the chronic SCI phase, have not been established. Here we examined the functional status of regenerated sensory afferents in the dorsal columns after SCI. Six months post-injury, we located and electrically mapped functional sensory axons that had regenerated beyond the injury site. The regenerated axons had reduced conduction velocity, decreased frequency-following ability, and increasing latency to repetitive stimuli. Many of the axons that had regenerated into the dorsal columns rostral to the injury site were chronically demyelinated. These results demonstrate that regenerated sensory axons remain in a chronic pathophysiological state and emphasize the need to restore normal conduction properties to regenerated axons after spinal cord injury.     

Regeneration and Sprouting of Chronically Injured Corticospinal Tract Fibers in Adult Rats Promoted by NT-3 and the mAb IN-1, Which Neutralizes Myelin-Associated Neurite Growth Inhibitors

Myelin-associated inhibitors of neurite growth play an important role in the regenerative failure after injury in the adult mammalian CNS. The application of the mAb IN-1, which efficiently neutralizes the NI-250/35 inhibitory proteins, alone or in combination with neurotrophin-3 (NT-3), has been shown to promote axonal regeneration when applied in acute injury models. To test whether IN-1 application can induce axonal growth also in a chronic injury model, we treated rats with IN-1 and NT-3 starting 2 or 8 weeks after injury. Rats underwent bilateral dorsal hemisection of the spinal cord at the age of 5–6 weeks. Regeneration of corticospinal (CST) fibers into the caudal spinal cord was observed in three of eight of those animals with a 2-week delay between lesion and treatment. CST fibers regenerated for 2–11.4 mm. In the control group sprouting occurred rostral to the lesion but no long-distance regeneration occurred. In animals where treatment started at 8 weeks after injury the longest fibers observed grew up to 2 mm into the caudal spinal cord. The results show that transected corticospinal axons retain the ability to regenerate at least for a few weeks after injury. Functional analysis of these animals showed a slight improvement of functional recovery.     

The need for cellular elements during axonal regeneration in the sea lamprey spinal cord.

Spinal axons in the larval sea lamprey regenerate following a complete spinal transection. It is not known whether regenerating growth cones require contact with cellular elements or whether the basement membrane and collagenous meninx primitiva which surround the spinal cord are sufficient for neurite out-growth. To determine this, a freeze lesion was made which severed axons, destroyed neuronal perikarya, and greatly reduced the number of glial cells. After at least 10 weeks of recovery, 50 neurites from 31 Müller and Mauthner axons were labeled by intracellular injection of HRP. Eighty-six percent of these neurites did not regenerate into the lesion site. No neurites grew through the lesion. No animals recovered coordinated swimming. These results suggest that glial and/or neuronal surfaces are required for axonal regeneration. Moreover, a monolayer of glial cells appears to be suboptimal and a three-dimensional matrix of cells may be necessary to promote regeneration in the lamprey spinal cord.     

Biopolymers and biodegradable smart implants for tissue regeneration after spinal cord injury

PURPOSE OF REVIEW: This article reviews recent experimental advances in the development of biosynthetic implants for repair of spinal cord injury. RECENT FINDINGS: Various important advances in the development of biosynthetic conduits for spinal cord repair have recently been reported. It was found that implantation of freeze dried alginate sponge into completely transected spinal cord supports axonal regeneration across the lesion site. A poly(lactic-co-glycolic acid) scaffold seeded with neural stem cells has been developed that promotes axonal regeneration across the gap. It was found that polyethylene glycol can reseal damaged spinal cord axons and restore impulse conduction. Findings have been reported that poly-beta-hydroxybutyrate conduits in combination with alginate and fibronectin provide neuroprotection for axotomized descending neurones. It has been reported that conduits made of fibronectin mats or fibrin in combination with neurotrophic growth factors promote axonal growth into the grafts. Finally, magnetic resonance imaging after experimental spinal cord injury has been used to monitor regeneration in biosynthetic conduits in vivo over time. SUMMARY: Biosynthetic conduits carrying extracellular matrix molecules and different cell lines, and supplemented with neurotrophic growth factors have yielded encouraging results in the treatment of experimental spinal cord injury. These findings provide a basis for further development of techniques aimed at spinal cord repair in humans.     

Regeneration of descending spinal axons after transection of the thoracic spinal cord during early development in the North American opossum, Didelphis virginiana

Opossums are born in an immature, fetal-like state, making it possible to lesion their spinal cord early in development without intrauterine surgery. When the thoracic spinal cord of the North American opossum, Didelphis virginiana, is transected on postnatal day 5, and injections of Fast Blue (FB) are made caudal to the lesion site 30-40 days or 6 months later, neurons are labeled in all of the spinal and supraspinal areas that are labeled after comparable injections in age-matched, unlesioned controls. Double-labeling studies document that regeneration of cut axons contributes to growth of axons through the lesion site and behavioral studies show that animals lesioned on postnatal day 5 use their hindlimbs in normal appearing locomotion as adults. The critical period for developmental plasticity of descending spinal axons extends to postnatal day 26, although axons which grow through the lesion site become fewer in number and more restricted as to origin with increasing age. Animals lesioned between postnatal day 12 and 26 use the hindlimbs better than animals lesioned as adults, but hindlimb function is markedly abnormal and uncoordinated with that of the forelimbs. We conclude that restoration of anatomical continuity occurs after transection of the spinal cord in developing opossums, that descending axons grow through the lesion site, that regeneration of cut axons contributes to such growth, and that animals lesioned early enough in development have relatively normal motor function as adults.