Peripheral nerve grafts lacking viable Schwann cells fail to support central nervous system axonal regeneration

Nerve Bypass Surgery for Spinal Cord Reconstruction

OBJECTIVE

Spinal cord injury (SCI) is a devastating condition that significantly decreases the patient's quality of life. Therefore, treatments that can facilitate nerve regeneration, reduce complications, and increase quality of life are valuable for these patients. In this study, we aimed to assess nerve bypass surgery's feasibility and clinical outcomes by transferring the intercostal nerves into the spinal cord.

METHODS

Eight patients with complete thoracic SCI and delayed presentation more than a year after the injury were analyzed retrospectively. All patients underwent nerve bypass surgery with the transfer of 2 pairs of intercostal nerves from proximal to the injury site to the anterolateral spinal cord, followed by duraplasty with fascia grafting to close the dura.

RESULTS

Six of the 8 (75%) patients demonstrated motor and sensory improvements, based on the American SCI Association score. Three patients demonstrated a limited recovery of motor function that could be independently triggered without ICN initiation. Five patients demonstrated evidence of cerebrospinal fluid leakage after surgery; however, only 1 patient complained of a headache.

CONCLUSIONS

Spinal cord bypass surgery is a potential reconstruction method to treat chronic complete thoracic SCI with functional improvements, and is worth further investigation.

Natural Biomaterials as Instructive Engineered Microenvironments That Direct Cellular Function in Peripheral Nerve Tissue Engineering

Nerve tissue function and regeneration depend on precise and well-synchronised spatial and temporal control of biological, physical, and chemotactic cues, which are provided by cellular components and the surrounding extracellular matrix. Therefore, natural biomaterials currently used in peripheral nerve tissue engineering are selected on the basis that they can act as instructive extracellular microenvironments. Despite emerging knowledge regarding cell-matrix interactions, the exact mechanisms through which these biomaterials alter the behaviour of the host and implanted cells, including neurons, Schwann cells and immune cells, remain largely unclear. Here, we review some of the physical processes by which natural biomaterials mimic the function of the extracellular matrix and regulate cellular behaviour. We also highlight some representative cases of controllable cell microenvironments developed by combining cell biology and tissue engineering principles.

Dissecting spinal cord regeneration

The inability to recover functions lost after severe spinal cord injury has been recognized for millennia and was first attributed to a failure of spinal cord neural regeneration over 100 years ago. The last forty years have seen intense research into achieving such regeneration, but in spite of conceptual advances and many reports announcing successful interventions, progress has been slow and often controversial. Here, I examine consequential advances and setbacks, and critically consider assumptions underlying certain approaches. I argue that expanding mechanistic knowledge about multiple forms of neural regeneration, why they fail and how they can restore function will resolve conceptual contentions and push the field forward.

Cell biology of spinal cord injury and repair

Spinal cord injury (SCI) lesions present diverse challenges for repair strategies. Anatomically complete injuries require restoration of neural connectivity across lesions. Anatomically incomplete injuries may benefit from augmentation of spontaneous circuit reorganization. Here, we review SCI cell biology, which varies considerably across three different lesion-related tissue compartments: (a) non-neural lesion core, (b) astrocyte scar border, and (c) surrounding spared but reactive neural tissue. After SCI, axon growth and circuit reorganization are determined by neuron-cell-autonomous mechanisms and by interactions among neurons, glia, and immune and other cells. These interactions are shaped by both the presence and the absence of growth-modulating molecules, which vary markedly in different lesion compartments. The emerging understanding of how SCI cell biology differs across lesion compartments is fundamental to developing rationally targeted repair strategies.

Sciatic nerve regeneration by transplantation of in vitro differentiated nucleus pulposus progenitor cells

Aim: To assess the applicability of mouse intervertebral disc-derived nucleus pulposus (NP) progenitor cells as a cell source for sciatic nerve regeneration. Materials & methods: P0-Cre/Floxed-EGFP-transgenic mouse-derived NP progenitor cells were differentiated to Schwann-like cells in conventional induction medium. Schwann-like cells were subsequently transplanted into a mouse model of sciatic nerve transection, and nerve regeneration assessed by immunohistochemistry, electron microscopy and functional walking track analysis and heat stimulus reflex. Results & conclusion: NP progenitor cells differentiated into Schwann-like cells. Transplantation of these cells promoted myelinated axon formation, morphology restoration and nerve function improvement. NP progenitor cells have the capacity to differentiate into neuronal cells and are candidates for peripheral nerve regeneration therapy.

Phosphodiesterase Inhibitors as a Therapeutic Approach to Neuroprotection and Repair

A wide diversity of perturbations of the central nervous system (CNS) result in structural damage to the neuroarchitecture and cellular defects, which in turn are accompanied by neurological dysfunction and abortive endogenous neurorepair. Altering intracellular signaling pathways involved in inflammation and immune regulation, neural cell death, axon plasticity and remyelination has shown therapeutic benefit in experimental models of neurological disease and trauma. The second messengers, cyclic adenosine monophosphate (cyclic AMP) and cyclic guanosine monophosphate (cyclic GMP), are two such intracellular signaling targets, the elevation of which has produced beneficial cellular effects within a range of CNS pathologies. The only known negative regulators of cyclic nucleotides are a family of enzymes called phosphodiesterases (PDEs) that hydrolyze cyclic nucleotides into adenosine monophosphate (AMP) or guanylate monophosphate (GMP). Herein, we discuss the structure and physiological function as well as the roles PDEs play in pathological processes of the diseased or injured CNS. Further we review the approaches that have been employed therapeutically in experimental paradigms to block PDE expression or activity and in turn elevate cyclic nucleotide levels to mediate neuroprotection or neurorepair as well as discuss both the translational pathway and current limitations in moving new PDE-targeted therapies to the clinic.

Immunohistochemical assessment of rat nerve isografts and immunosuppressed allografts

OBJECTIVE

Autologous peripheral nerve grafts are commonly used clinically as a treatment for peripheral nerve injuries. However, in research using an autologous graft is not always feasible due to loss of function, which in many cases is assessed to determine the efficacy of the peripheral nerve graft. In addition, using allografts for research require the use of an immunosuppressant, which creates unwanted side effects and another variable within the experiment that can affect regeneration. The objective of this study was to analyze graft rejection in peripheral nerve grafts and the effects of cyclosporine A (CSA) on axonal regeneration.

METHODS

Peripheral nerve grafts in inbred Lewis rats were compared with Sprague-Dawley (SD) rats to assess graft rejection, CSA side effects, immune responses, and regenerative capability. Macrophages and CD8+ cells were labeled to determine graft rejection, and neurofilaments were labeled to determine axonal regeneration.

RESULTS

SD rats without CSA had significantly more macrophages and CD8+ cells compared to Lewis autografts, Lewis isografts, and SD allografts treated with CSA. Lewis autografts, Lewis isografts, and SD autografts had significantly more regenerated axons than SD rat allografts. Moreover, allografts in immunosuppressed SD rats had significantly less axons than Lewis rat autograft and isografts.

DISCUSSION

Autografts have long been the gold standard for treating major nerve injuries and these data suggest that even though CSA is effective at reducing graft rejection, axon regeneration is still superior in autografts versus immunosuppressed allografts.

Regeneration strategies after the adult mammalian central nervous system injury-biomaterials

The central nervous system (CNS) has very restricted intrinsic regeneration ability under the injury or disease condition. Innovative repair strategies, therefore, are urgently needed to facilitate tissue regeneration and functional recovery. The published tissue repair/regeneration strategies, such as cell and/or drug delivery, has been demonstrated to have some therapeutic effects on experimental animal models, but can hardly find clinical applications due to such methods as the extremely low survival rate of transplanted cells, difficulty in integrating with the host or restriction of blood–brain barriers to administration patterns. Using biomaterials can not only increase the survival rate of grafts and their integration with the host in the injured CNS area, but also sustainably deliver bioproducts to the local injured area, thus improving the microenvironment in that area. This review mainly introduces the advances of various strategies concerning facilitating CNS regeneration.

Clinical translation of autologous Schwann cell transplantation for the treatment of spinal cord injury

PURPOSE OF REVIEW

To describe the current status of testing Schwann cell transplantation as a therapy for human spinal cord injury (SCI).

RECENT FINDINGS

Transplanted Schwann cells have reparative effects in the damaged spinal cord. A few clinical studies have reported that Schwann cell transplantation appears safe. Compared with allogeneic cell transplants, autologous cells do not require immune suppression, but the workload of cell manufacturing is greater. Preclinical Schwann cell transplant studies conducted at the University of Miami in 2009-2012 supported an investigational new drug approved by the Food and Drug Administration. A Phase 1 safety study has been initiated.

SUMMARY

Spinal cord repair after severe SCI requires that axonal regeneration and myelination occur in a context of reduced inhibition, enhanced plasticity, and new circuit formation. Evolving clinical experience with Schwann cell transplantation may provide a basis upon which additionally combined therapeutics can be tested to increase the extent of repair after SCI. Safety is the primary consideration when ex-vivo manipulated cells are introduced into the damaged nervous system. Preclinical studies across several species have not indicated safety concerns regarding Schwann cells. Initial clinical reports from studies in Iran and China are suggestive of clinical safety, although more rigorous characterization of the implanted cells is needed.

Using extracellular matrix for regenerative medicine in the spinal cord

Regeneration within the mammalian central nervous system (CNS) is limited, and traumatic injury often leads to permanent functional motor and sensory loss. The lack of regeneration following spinal cord injury (SCI) is mainly caused by the presence of glial scarring, cystic cavitation and a hostile environment to axonal growth at the lesion site. The more prominent experimental treatment strategies focus mainly on drug and cell therapies, however recent interest in biomaterial-based strategies are increasing in number and breadth. Outside the spinal cord, approaches that utilize the extracellular matrix (ECM) to promote tissue repair show tremendous potential for various application including vascular, skin, bone, cartilage, liver, lung, heart and peripheral nerve tissue engineering (TE). Experimentally, it is unknown if these approaches can be successfully translated to the CNS, either alone or in combination with synthetic biomaterial scaffolds. In this review we outline the first attempts to apply the potential of ECM-based biomaterials and combining cell-derived ECM with synthetic scaffolds.

Spinal cord injury

A spinal cord injury is a devastating, life-changing neurologic event that challenges patients, families, and caregivers. A myriad of neurologic and medical sequelae occur subsequent to the original insult. This article discusses epidemiology, primary and secondary injuries, acute therapy, and neuroprotective agents as well as the exciting areas of spinal cord recovery and regeneration, with an emphasis on cellular transplantation. Neurologic neurorehabilitation techniques and equipment are also reviewed, with a focus on their relation to increasing the independence and functional capacity of the patient. The article concludes with the clinical presentation and management of common spinal cord injury complications.

Rapid repair of rat sciatic nerve injury using a nanosilver-embedded collagen scaffold coated with laminin and fibronectin

AIM

Scaffold with micro-channels has shown great promise in facilitating axonal regeneration after peripheral nerve injury. Significant research has focused on mimicking, in terms of composition and function, the ability of the basement membrane of Schwann cells to both promote and guide axonal regeneration. We aim to investigate the ability of a tissue-engineered scaffold with nanosilver and collagen to adsorb laminin and fibronectin, and the usefulness of this scaffold for repairing and regenerating a 10-mm peripheral nerve gap in rats.

METHODS

In this study, nanosilver-embedded collagen scaffolds were prepared and coated with laminin (LN) or LN plus fibronectin (FN). Scanning electron microscopy of the transverse and longitudinal sections of the scaffold revealed axially oriented microtubules ranging from 20 to 80 µm in diameter, and the internal surface of microtubules was found to be evenly coated with LN and FN. Energy dispersive spectrometry also confirmed an even distribution of nanosilver particles within the scaffold. To test its effectiveness in restoring neuronal connection, the scaffold was used in order to bridge 10 mm gaps in the severed sciatic nerve of rats. The rats were divided into an experimental group (receiving scaffold coated with LN and FN), a control group (receiving scaffold coated with LN only) and an autologous graft group. The functional recovery 40 days after surgery was examined by electrophysiology and sciatic nerve functional index (SFI) evaluation. FluoroGold™ (FG) retrograde tracing, toluidine blue staining and transmission electron microscopy were also used to examine the regenerated nerve fibers and to establish their myelination status.

RESULTS

The experimental group displayed partially restored nerve function. The recovery was comparable to the effect of autologous nerve graft and was better than that observed in the control group. A better functional recovery correlated with more FG-labeled neurons, higher density of toluidine blue stained nerve fibers and thicker myelin sheath.

CONCLUSION

Our results demonstrated that nanosilver-embedded collagen scaffolds with LN and FN coating is effective in aiding axonal regeneration, and recovery is comparable to the effect of an autologous nerve graft.

Peripheral nerve grafts support regeneration after spinal cord injury

Traumatic insults to the spinal cord induce both immediate mechanical damage and subsequent tissue degeneration leading to a substantial physiological, biochemical, and functional reorganization of the spinal cord. Various spinal cord injury (SCI) models have shown the adaptive potential of the spinal cord and its limitations in the case of total or partial absence of supraspinal influence. Meaningful recovery of function after SCI will most likely result from a combination of therapeutic strategies, including neural tissue transplants, exogenous neurotrophic factors, elimination of inhibitory molecules, functional sensorimotor training, and/or electrical stimulation of paralyzed muscles or spinal circuits. Peripheral nerve grafts provide a growth-permissive substratum and local neurotrophic factors to enhance the regenerative effort of axotomized neurons when grafted into the site of injury. Regenerating axons can be directed via the peripheral nerve graft toward an appropriate target, but they fail to extend beyond the distal graft-host interface because of the deposition of growth inhibitors at the site of SCI. One method to facilitate the emergence of axons from a graft into the spinal cord is to digest the chondroitin sulfate proteoglycans that are associated with a glial scar. Importantly, regenerating axons that do exit the graft are capable of forming functional synaptic contacts. These results have been demonstrated in acute injury models in rats and cats and after a chronic injury in rats and have important implications for our continuing efforts to promote structural and functional repair after SCI.

Axon regeneration through scaffold into distal spinal cord after transection

We employed Fast Blue (FB) axonal tracing to determine the origin of regenerating axons after thoracic spinal cord transection injury in rats. Schwann cell (SC)-loaded, biodegradable, poly(lactic-co-glycolic acid) (PLGA) scaffolds were implanted after transection. Scaffolds loaded with solubilized basement membrane preparation (without SCs) were used for negative controls, and nontransected cords were positive controls. One or 2 months after injury and scaffold implantation, FB was injected 0-15 mm caudal or about 5 mm rostral to the scaffold. One week later, tissue was harvested and the scaffold and cord sectioned longitudinally (30 microm) on a cryostat. Trans-scaffold labeling of neuron cell bodies was identified with confocal microscopy in all cell-transplanted groups. Large (30-50 microm diameter) neuron cell bodies were predominantly labeled in the ventral horn region. Most labeled neurons were seen 1-10 mm rostral to the scaffold, although some neurons were also labeled in the cervical cord. Axonal growth occurred bidirectionally after cord transection, and axons regenerated up to 14 mm beyond the PLGA scaffolds and into distal cord. The extent of FB labeling was negatively correlated with distance from the injection site to the scaffold. Electron microscopy showed myelinated axons in the transverse sections of the implanted scaffold 2 months after implantation. The pattern of myelination, with extracellular collagen and basal lamina, was characteristic of SC myelination. Our results show that FB labeling is an effective way to measure the origin of regenerating axons.

Pluripotent hair follicle neural crest stem-cell-derived neurons and schwann cells functionally repair sciatic nerves in rats

In this paper, we constructed a novel acellular nerve xenograft (ANX) seeded with neurons and Schwann cells to bridge long-distance gaps in rat sciatic nerves. The neurons and Schwann cells were induced from Sprague Dawley (SD) rat hair follicle neural crest stem cells with sonic hedgehog/retinoic acid and neuregulin 1, respectively. Fifty male SD rats were randomly divided into two groups (n = 25): ANX + cells group and ANX group. A 4-cm-long sciatic nerve defect was created on the right hind limb and bridged with cell-seeded ANX in ANX + cells group or ANX alone in ANX group. We found that the implanted neurons and Schwann cells could survive by 4 weeks and as far as 52 weeks posttransplantation. In implanted grafts, chemical synaptic structures were also found under transmission electron microscope and confirmed with immunostaining of synapsin 1, a synaptic marker. The number of regenerated axons in ANX + cells group was higher than that in ANX group (P < 0.01). This novel implantation of neurons and Schwann cells via acellular nerve graft may provide an alternative way for repairing peripheral nerve defect.

Transplantation of preconditioned Schwann cells following hemisection spinal cord injury

STUDY DESIGN

Chronically compressed sciatic nerve segments were transplanted to hemisected spinal cord injured rats. Histologic evaluation and behavior functional outcomes were tested after 6 weeks following surgery.

OBJECTIVE

To evaluate the outcome of preconditioned peripheral nerves as a permissive environment in axonal regeneration of the injured spinal cord.

SUMMARY OF BACKGROUND DATA

Schwann cells have been used to facilitate a permissive environment for the injured spinal cord to regenerate. Previous experiments have shown compressive mechanical stress to be important in stimulating the regenerative behavior of Schwann cells. Transplantation of highly permissive Schwann cell-enriched peripheral nerve grafts may enhance regeneration in spinal cord injury.

METHODS

Adult Sprague-Dawley rats (n = 24) were used to create a hemisection injury of the spinal cord. At 1-week postinjury creation, the spinal cords were reexposed for all animals. Peripheral nerve grafts were obtained from rat sciatic nerve, either untreated or subjected to mechanical compression for 2 weeks with nonconstrictive tubing. Transplantation of grafts was performed after a resection of the glial scar. Functional outcome was measured using the Basso, Beattie, Bresnahan Locomotor Rating Score and footprint analysis. Tract tracing of descending and ascending spinal cord tracts was performed at 6 weeks after surgery for histologic evaluation of axonal regeneration.

RESULTS

Preconditioned transplants had significantly higher Basso, Beattie, Bresnahan Scores versus hemisection alone in the late postoperative period (P < 0.05). They also had significantly less foot exorotation and base of support when compared to nonconditioned transplants. Histologic analysis showed increased regeneration at lesional sites for preconditioned transplants versus control group (P < 0.05).

CONCLUSIONS

Functional recovery after hemisection injury improved significantly in the late postoperative period with transplantation of preconditioned peripheral nerve. Preconditioned grafts also exhibit sustained axonal regeneration at and past the lesional site in histologic analysis. Further investigation with later time points is warranted.

Building In Vitro Models of Organs

Tissue-engineering techniques are being used to build in vitro models of organs as substitutes for human donor organs for transplantation as well as in vitro toxicology testing (as alternatives to use of animals). Tissue engineering involves the fabrication of scaffolds from materials that are biologically compatible to serve as cellular supports and microhabitats in order to reconstitute a desired tissue or organ. Three organ systems that are currently the foci of tissue engineering efforts for both transplantation and in vitro toxicology testing purposes are discussed. These are models of the cornea, nerves (peripheral nerves specifically), and cardiovascular components. In each of these organ systems, a variety of techniques and materials are being used to achieve the same end results. In general, models that are designed with consideration of the developmental and cellular biology of the target tissues or organs have tended to result in morphologically and physiologically accurate models. Many of the models, with further development and refinement, have the potential to be useful as functional substitute tissues and organs for transplantation or for in vitro toxicology testing.

Characterization of cortical neuron outgrowth in two- and three-dimensional culture systems

To improve the ability of regeneration by grafting living cells or by adding growth factor to a lesion site, it is important to find good biomaterials for neuron survival and regeneration. This study focused on two- and three-dimensional cultures in a matrix using biomaterials such as agarose, collagen, fibrin, and their mixtures, because these are considered to be suitable biomaterials for neuron outgrowth. Cortical neurons were dissected from E17 rat embryos and cultured in agarose gel, collagen gel, fibrin glue, and mixtures of collagen and fibrin. Results showed that neurons cultured in collagen gel and fibrin glue had longer periods of survival (more than 3 weeks) and better neurite extension than those observed in agarose gels. As to the survival rate according to the MTT and lactate dehydrogenase assays, fibrin glue was the most suitable biomaterial for neuron survival among the biomaterials examined. With two-dimensional fibrin plating, neuron cells exhibited cell aggregation and stress fibers, but the same results were not observed with collagen gel. There were no differences in neurite extension and survival in the mixtures of collagen and fibrin. The results suggest that collagen and fibrin can provide a suitable substrate for a three-dimensional culture matrix for neuronal survival and differentiation.

Intravitreal transplants of Schwann cells and fibroblasts promote the survival of axotomized retinal ganglion cells in rats

Schwann cells (SCs) are considered one of the major cellular components to maintain the integrity of the peripheral nervous system (PNS) neurons after injury. Intravitreal transplant of peripheral nerves or Schwann cells has been shown to enhance the regenerative ability of retinal ganglion cells (RGCs). In the present study, we compared the effects of intravitreal transplants of Schwann cells and fibroblasts, two major components of peripheral nerves, on the survival of retinal ganglion cells in adult rats after optic nerve (ON) transection. Purified Schwann cells and fibroblasts from neonatal sciatic nerves were injected into the vitreous body of adult rats. Three days after the injection, the optic nerves were transected intraorbitally. After 1 week or 1 month, surviving retinal ganglion cells were retrogradely labelled with Fluoro-Gold (FG) and the number of surviving retinal ganglion cells was counted. The retinas were further processed for 200-kDa neurofilament RT-97 immunohistochemistry. It was found that intravitreally injected- Schwann cells and -fibroblasts delayed the death of axotomized retinal ganglion cells for 1 week. In addition, in the animal group with 1 month survival time after optic nerve transection, those received a larger number of Schwann cells had more surviving retinal ganglion cells and more profusely ramified axonal processes near the optic disc. These findings reveal that both Schwann cells and fibroblasts isolated from the peripheral nerve can promote retinal ganglion cell survival after optic nerve transection, presumably by secreting neurotrophic factors. In addition, the data also demonstrate that Schwann cells could promote intraretinal axonal sprouting. Our findings demonstrate a remarkable glial source of neurotrophic factors with potential clinical applications, as autologous Schwann cells and fibroblasts can be feasibly obtained from peripheral nerves.

Addition of fibronectin to alginate matrix improves peripheral nerve regeneration in tissue-engineered conduits

Schwann cell (SC) transplantation has been proposed to encourage peripheral nerve regeneration, but an optimal SC-carrying matrix would be needed. The aim of this study was to characterize how the addition of fibronectin to alginate would affect the outcome of nerve regeneration promoted by Schwann cells embedded in this matrix. Genetically labeled rat SCs were obtained by lacZ gene transduction. SCs were suspended in alginate hydrogel matrix with/without addition of liquid fibronectin, and their viability and growth in the different types of matrices were assessed in vitro by AlamarBlue assay. In vivo assessment of SC transplantation in the matrix was carried out with poly-3-hydroxybutyrate (PHB) conduits to bridge a sciatic nerve gap. The grafted conduits were harvested at 2, 3, and 6 weeks and assessed for the presence of labeled SCs in relation to regrowing axons. The amount and rate of axonal regeneration were assessed by quantitative immunohistochemistry. Addition of fibronectin to alginate hydrogel improved SC viability and growth profile in vitro. X-Gal staining confirmed that SCs transplanted in PHB conduits were viable throughout the time course, and that the labeled SCs were clearly associated with regenerating axons. The regeneration rate was enhanced when liquid fibronectin was added to the alginate matrix. Furthermore, the presence of SCs also enhanced regeneration and there was an additive effect when both SCs and fibronectin were combined with alginate. In conclusion, the addition of fibronectin to alginate hydrogel matrix contributed to improve nerve regeneration, supporting SC viability and augmenting their effect on axonal growth when transplanted in a bioengineered nerve conduit.

Regeneration of acutely and chronically injured descending respiratory pathways within post-traumatic nerve grafts

Central respiratory neurons, which are acutely axotomized by peripheral nerve grafts implanted at the level of the descending respiratory pathways within the C2 spinal cord, can regenerate their axons within the grafts and still transmit normal physiological messages [Decherchi et al., 1996. Exp. Neurol. 137, 1-14]. The present work investigated the extent to which mature central neurons, acutely or chronically axotomized by a spinal lesion, still maintain the potential to regenerate an axon following post-traumatic nerve grafting within supra-lesional spinal structures and remained functional. This study is an extension of earlier work employing the more chronic lesions, that investigated whether respiratory neurons chronically axotomized by a spinal cord injury can retain the ability to regenerate their axonal process within a post-traumatic peripheral nerve graft. Here implantation was performed into the supra-lesional ventrolateral part of the ipsilateral C2 spinal cord (at the level of the descending respiratory pathways) previously hemisected at the C3 level. In the present study, these post-traumatic peripheral nerve grafts were performed either acutely (group I, n=15, 2.5 h post-injury: acute conditions) or chronically (group II, n=17, 3 weeks; group III, n=6, 3 months: chronic conditions) after the injury.Electrophysiological recording of teased filaments (n=2362) within the post-traumatic peripheral nerve grafts revealed the presence of regenerated nerve fibers with spontaneous unitary impulse traffic (graft units, n=954) in all animals. These graft units were respiratory (n=247) and non-respiratory (n=707). Respiratory discharges originated from central respiratory neurons which remained functional with preserved afferent connections. Except for the group III, post-traumatic C2 peripheral nerve grafts of the groups I and II contained a significantly higher occurrence rate (13.2+/-2% and 11.6+/-1.9%) of respiratory units than C2 spinal peripheral nerve grafts (5.9+/-1.6%) realized without previous CNS injury. The main conclusion of our study is that for a prolonged period of 3 weeks following a spinal cord injury, central respiratory neurons have the potential to remain functional and to regenerate their axonal process within post-traumatic peripheral nerve grafts inserted rostrally to the spinal damage. This indicates that supra-lesional post-traumatic nerve grafts may constitute an efficient delayed strategy for inducing axonal regrowth of chronically axotomized adult central neurons. This suggests that surgical intervention which is not always possible immediately after a spinal cord injury may be satisfactorily carried out after an appropriate delay.

The L2/HNK-1 Carbohydrate Epitope is Involved in the Preferential Outgrowth of Motor Neurons on Ventral Roots and Motor Nerves

Based on the observation that in adult mice the carbohydrate epitope L2/HNK-1 is detectable on Schwann cells in ventral spinal roots, but only scarcely in dorsal roots (Martini et al., Dev. Biol., 129, 330 - 338, 1988), the possibility was investigated that the carbohydrate is involved in the outgrowth of regenerating motor neuron axons on peripheral nerve substrates expressing the epitope. To monitor whether the L2 carbohydrate remains present during the time periods in which regenerating axons penetrate the denervated distal nerve stumps, the expression of L2 in motor and sensory branches of the femoral nerve was investigated in normal animals and after a crush lesion. During the first two postoperative weeks, L2 immunoreactivity remained high in the myelinating Schwann cells of the motor branch, whereas L2 immunoreactivity was virtually absent in the sensory branch. In a first experimental approach, cryosections of ventral and dorsal spinal roots and of motor and sensory nerves of adult rats and mice were used as substrates for neurite outgrowth. Neurites of motor neurons from chicken embryos were approximately 35% longer after 30 h of maintenance on ventral roots than on dorsal roots. Neurites from sensory neurons had the same length on dorsal as on ventral motors and were as long as neurites from motor neurons grown on dorsal roots. L2 antibodies reduced neurite outgrowth of motor neurons on ventral roots but not on dorsal roots. Neurite outgrowth of sensory neurons on both roots was not altered by the antibodies. Neurite outgrowth of motor neurons on a mixture of the extracellular matrix glycoprotein laminin and the L2 carbohydrate-carrying glycolipid was significantly higher than on the laminin substrate mixture with GD1b ganglioside or sulphatide. L2 antibodies reduced neurite outgrowth of motor neurons by 50% on the L2 glycolipid, but not on GD1b or sulphatide. These observations indicate that the L2 carbohydrate promotes neurite outgrowth of motor neurons in vitro and may thus contribute to the preferential reinnervation of motor nerves by regenerating motor axons in vivo.

Substrate-bound human recombinant L1 selectively promotes neuronal attachment and outgrowth in the presence of astrocytes and fibroblasts

Axonal pathfinding is a complex process that is mediated through cell-matrix and cell-cell interactions. A large number of studies have demonstrated that ECM and ECM-derived proteins and peptides are potent promoters of neurite outgrowth, however much less attention is given to the fact that these same ligands also elicit responses in a wide variety of non-neuronal cell types. We examined the use of a substrate-bound recombinant form of human L1, an integral membrane protein, as a ligand for bridging materials for repairing the CNS by studying its effectiveness in promoting specific responses of neuronal cells in the presence of astrocytes and fibroblasts. L1, a cell adhesion molecule expressed in the developing CNS and PNS, has strong neurite promoting activity, and contributes to axonal guidance and axonal fasciculation during development. In this study, substrates treated with L1-Fc were compared to subtrates treated with fibronectin and poly-lysine (PDL) with respect to their interaction with a variety of cell types, including three types of neurons (DRG neurons, cerebellar granule neurons, and hippocampal neurons), astrocytes, dermal fibroblasts, and meningeal cells. L1-Fc-treated substrates supported significantly higher levels of neurite outgrowth relative to fibronectin and PDL, while inhibiting the attachment of astrocytes, meningeal cells, and fibroblasts. We also show that neuronal cells attach to and extend neurites on 30 microm diameter L1-Fc-treated filaments as an example of a potentially useful bridging substrate. The high level of biological specificity displayed by surface-bound L1, along with the fact that it is a potent promoter of neurite outgrowth, is normally expressed on axons and regulates axonal fasciculation during normal development bodes well for its use on bridging materials for the repair of the CNS, and suggests that cell adhesion molecules, in general, may be useful for biomaterial modification. Moreover, small diameter filaments coated with L1-Fc may function in an analogous way to pioneering axons that guide the growth of axons to distal targets during development.

Columns of Schwann cells extruded into the CNS induce in-growth of astrocytes to form organized new glial pathways

Our previous work showed that stereotaxic microextrusion of columns of purified peripheral nerve-derived Schwann cells into the thalamus of syngeneic adult rats induces host axons to grow into the column and form a new fiber tract. Here we describe the time course of cellular events that lead to the formation of this new tract. At 2 h postoperation, numerous OX42-positive microglia accumulated at the graft-host interface, after which donor columns became progressively and heavily infiltrated by microglia/macrophages that took on an elongated morphology in parallel with the highly orientated processes of the donor Schwann cells. The penetration of host astrocytic processes into the Schwann cell columns was substantially slower in onset, being first detected at 4 days postoperation. This event was contemporaneous with the in-growth of host thalamic axons. Between 7 and 14 days postoperation, GFAP-positive astrocytes became fully incorporated into the transplants, where they too adopted an elongated form, orientated in parallel with the longitudinal axis of the graft. Thus, the columns became a mosaic of elongated and highly orientated donor Schwann cells intimately mingled with host microglia, astrocytes, and numerous, largely unbranched 200-kDa neurofilament-positive axons from the adjacent thalamus. Electron microscopy demonstrated that the processes of donor Schwann cells and host astrocytes within the column formed tightly packed bundles that were surrounded by a partial or complete basal lamina. Control columns, formed by extruding freeze-thaw-killed Schwann cells or purified peripheral nerve fibroblasts induced a reactive injury response by the adjacent host microglia and astrocytes, but neither host astrocytes nor neurofilament-positive axons were incorporated into the columns. A better understanding of the mechanisms that regulate the interactions between donor and host glia should facilitate improved integration of such grafts and enhance their potential for inducing tissue repair.

The cell recognition molecule CHL1 is strongly upregulated by injured and regenerating thalamic neurons

Close homologue of L1 (CHL1) is a cell recognition molecule known to promote axonal growth in vitro. We have investigated the expression of CHL1 mRNA by regenerating central nervous system (CNS) neurons, by using in situ hybridisation 3 days to 10 weeks following the implantation of living and freeze-killed peripheral nerve autografts into the thalamus of adult rats. At all survival times after implantation of living grafts, neurons of the thalamic reticular nucleus (TRN), close to the graft tip and up to 1 mm away from it, displayed strong signal for CHL1 mRNA, even though TRN neurons show very low levels of CHL1 mRNA expression in unoperated animals. When the cell bodies of regenerating neurons were identified by retrograde labelling from the distal portion of the grafts, 4-6 weeks after operation, most of the labelled cells were found in the TRN and could be shown to haveupregulated CHL1 mRNA. In addition, some neurons in dorsal thalamic nuclei near the graft tip transiently upregulated CHL1 mRNA during the first 3 weeks after graft implantation, and glial cells showing CHL1 mRNA expression were present at the brain/graft interface 3 days to 2 weeks after operation. Freeze-killed grafts, into which axons do not regenerate, caused a transient upregulation of CHL1 in very few TRN neurons near the graft tip and in glial cells at the brain/graft interface but did not produce prolonged CHL1 mRNA expression. CHL1 can therefore be added to the list of molecules (including GAP-43, L1, and c-jun) strongly expressed by CNS neurons that regenerate their axons into nerve grafts, but not by those neurons that fail to regenerate their axons.

Axonal regeneration of retinal ganglion cells: effect of trophic factors

A variety of neurotrophic factors can influence the cell functions of the developing, mature and injured retinal ganglion cells. The discovery that retinal ganglion cell loss can be alleviated by neurotrophic factors has generated a great deal of interest in the therapeutic potential of these molecules. Recently, evidence has provided valuable information on the receptors that mediate these events and the intracellular signaling cascades after the binding of these ligands. Signaling by neurotrophic factors does not seem to restrict to retrograde messenger from the target but also includes local interactions with neighbouring cells along the axonal pathways, anterograde signaling from the afferents and autocrine signaling. More insight into the mechanisms of action of neurotrophic factors and the signal transduction pathway leading to the protection and regeneration of retinal ganglion cells may allow the design of new therapeutic strategies.

Schwann cells, neurotrophic factors, and peripheral nerve regeneration

The peripheral nervous system retains a considerable capacity for regeneration. However, functional recovery rarely returns to the preinjury level no matter how accurate the nerve repair is, and the more proximal the injury the worse the recovery. Among a variety of approaches being used to enhance peripheral nerve regeneration are the manipulation of Schwann cells and the use of neurotrophic factors. Such factors include, first, nerve growth factor (NGF) and the other recently identified members of the neurotrophin family, namely, brain-derived neurotrophic factor (BDNF), neurotrophin-3 (NT-3), neurotrophin-4/5 (NT-4/5); second, the neurokines ciliary neurotrophic factor (CNTF) and leukemia inhibitory factor (LIF); and third, the transforming growth factors (TGFs)-beta and their distant relative, glial cell line-derived neurotrophic factor (GDNF). In this review article we focus on the roles in peripheral nerve regeneration of Schwann cells and of the neurotrophin family, CNTF and GDNF, and the relationship between these. Finally, we discuss what remains to be understood about the possible clinical use of neurotrophic factors.

Acellular nerve graft re-seeded by Schwann cells migrating from the nerve stump can stimulate spinal motoneurons for functional reinnervation of the rat muscle

The acellular nerve graft was utilised to restore a functional reinnervation of the biceps brachii muscle from the motoneuron pool of the cervical spinal cord. The musculocutaneous nerve stump was sutured to an acellular nerve graft, the opposite end of which was inserted into the cervical spinal cord cranial to the avulsed C5 ventral root. The acellular nerve graft was repopulated by Schwann cells heavily immunostained for NGFr within 90 days. The Schwann cells migrating from the nerve stump reached the spinal cord grey matter, where they stimulated the motoneurons to send axonal sprouts. The functional reinnervation of the biceps brachii muscle was assessed by means of the behavioural (grooming) test and EMG, the presence of myelinated and unmyelinated axons was demonstrated by light and electron microscopy. The axonal reconnection of the musculocutaneous nerve stump was verified by horseradish peroxidase retrograde labelling of the spinal motoneurons. Moreover, the motoneurons on the operated side of the C5 spinal segment displayed increased immunostaining for GAP-43 in comparison to the contralateral side, whereas the pattern of AChE histochemical reaction was similar on both the operated and contralateral side, of the C5 segment 150 days after acellular graft implantation. The regenerated axons bridged a 4-cm long originally acellular nerve graft to reach and reinnervate the biceps brachii muscle. The reinnervation of the neuromuscular junctions was morphologically determined by immunofluorescence for neurofilaments. The number of myelinated axons in the acellular nerve graft was significantly higher than those growing over the cellular graft, but their diameter was smaller. The results of experiments presented here demonstrate functional recovery of the biceps muscle reinnervation through the acellular nerve graft repopulated by migrating Schwann cells. The process of reinnervation by acellular nerve graft is however delayed and worse in comparison with the cellular graft.

Enhanced axonal regeneration following combined demyelination plus schwann cell transplantation therapy in the injured adult spinal cord

We have treated spinal cord injured rats with demyelination plus Schwann cell transplantation and assessed neurite outgrowth in a quantifiable model of axonal regeneration. Axonal injuries of differing severity were induced in the dorsal funiculus of adult rats using a micromanipulator-controlled Scouten knife. Demyelinated regions were produced so as to overlap with the injury site by the injection of galactocerebroside antibodies plus complement one segment cranial to the axonal injury site. Schwann cells were isolated from the sciatic nerve, expanded in vitro, and transplanted into the injury site 1 day later. Animals were killed after an additional 7 days. Schwann cells were evenly distributed throughout the region of demyelination, which extended 6-7 mm cranial to the axonal injury site. The severity of axonal injury was quantified by counting degenerate axons in transverse resin sections. The degree of axonal regeneration was assessed by an electron microscopic analysis of growth cone frequency and distribution relative to the site of axonal injury. Quantification of growth cones at a distance from the site of axonal injury indicated a strong linear relationship (P < 0.001) between the number of growth cones and the number of severed axons; the ratio of growth cones to severed axons was increased by 26.5% in demyelinated plus transplanted animals compared to demyelinated animals without a transplant. Furthermore, only the demyelinated plus transplanted animals contained growth cones associated with myelin in white matter immediately outside of the region of complete demyelination. Growth cones were absent in transplanted-only animals at a distance from the site of axonal injury. These findings indicate that combined demyelination plus Schwann cell transplantation therapy enhances axonal regeneration following injury and suggests that growth cones are able to overcome myelin-associated inhibitors of neurite outgrowth in the presence of trophic support.

Regrowth of axons into the distal spinal cord through a Schwann-cell-seeded mini-channel implanted into hemisected adult rat spinal cord

Schwann cells (SCs) have been shown to be a key element in promoting axonal regeneration after being grafted into the central nervous system (CNS). In the present study, SC-supported axonal regrowth was tested in an adult rat spinal cord implantation model. This model is characterized by a right spinal cord hemisection at the eighth thoracic segment, implantation of a SC-containing mini-channel and restoration of cerebrospinal fluid circulation by suturing the dura. We demonstrate that a tissue cable containing grafted SCs formed an effective bridge between the two stumps of the hemicord 1 month after transplantation. Approximately 10 000 myelinated and unmyelinated axons (1 : 9) per cable were found at its midpoint. In addition to propriospinal axons and axons of peripheral nervous system (PNS) origin, axons from as many as 19 brainstem regions also grew into the graft without additional treatments. Most significantly, some regenerating axons in the SC grafts were able to penetrate through the distal graft-host interface to re-enter the host environment, as demonstrated by anterograde axonal labelling. These axons coursed toward, and then entered the grey matter where terminal bouton-like structures were observed. In channels containing no SCs, limited axonal growth was seen within the graft and no axons penetrated the distal interface. These findings further support the notion that SCs are strong promotors of axonal regeneration and that the mini-channel model may be appropriate for further investigation of axonal re-entry, synaptic reconnection and functional recovery following spinal cord injury.

Regeneration of neurosecretory axons into various types of intrahypothalamic grafts is promoted by the absence of blood brain barrier: fine structural analysis

Isogenous grafts of neural lobe and optic nerve and autologous grafts of sciatic nerve were placed into contact with the intrahypothalamically transected hypothalamo-neurohypophysial tract, and their fine structural characteristics examined at various time periods thereafter. The vascular bed of neural lobe grafts is composed primarily of fenestrated capillaries, that are permeable to blood-borne HRP throughout the entire experimental period. The microvasculature of sciatic nerve grafts consists of continuous, as well as fenestrated capillaries, which are similarly permeable to HRP. Fenestrated capillaries and HRP leakage in optic nerve grafts are observed at 10 days, but only in grafts located ventrally in the hypothalamus at 30 days. Neurosecretory axon regeneration is seen only in grafts or adjacent hypothalamus where the blood-brain barrier is breached. Regenerating axons are closely associated with the specific glial cells of the respective graft. Based on these observations, we conclude that blood-borne factors are necessary to initiate and sustain regeneration of transected neurosecretory axons, and that such regeneration occurs only in the presence of glial cells.

Integration of transplanted cultured Schwann cells into the long myelinated fiber tracts of the adult spinal cord

A suspension of about 10,000 purified Schwann cells cultured from the neonatal rat sciatic nerve was transplanted into a discrete site in the upper cervical level of the corticospinal tract of one side in adult rats. From 4 days after transplantation immunostaining for p75 (low-affinity neurotrophin receptor) showed that the transplants consisted of a central mass of Schwann cells and cuffs of elongated Schwann cells along the perivascular space of curving blood vessels (most of which had been formed in response to the transplantation). Schwann cells leaving the central mass and perivascular cuffs migrated in strictly linear orientation along the rostrocaudal axis of the host corticospinal tract. According to the territory through which they migrated, the transplanted Schwann cells adopted two quite different forms: (1) The row Schwann cells, which migrated singly or in groups within the rows of host oligodendrocytic and astrocytic cell bodies, were non-process-bearing, rather cuboidal, brick-like cells (about 8 x 12 microm in size). (2) In contrast, the interfascicular Schwann cells, which migrated singly or intertwined in rope-like small groups interspersed among the axons of the host corticospinal tract, were larger, symmetrically bipolar cells, with processes about 100-120 microm long and 2 microm wide and bulging, ovoid nuclei, located in centrally placed cell bodies about 10 microm across. After about 6 weeks, the p75 immunoreactivity of the interfascicular Schwann cells had become down-regulated. However, from as early as 10 days after transplantation, immunostaining for the peripheral myelin protein, P0, semithin sections, and electron microscopy showed that these Schwann cells were not lost, but that they had myelinated the segments of the host corticospinal axons in the region of the transplant. In contrast, the row Schwann cells did not express P0 or form myelin. They retained their p75 immunoreactivity at long survivals (presumably because they were secluded from contacting the tract axons). The row Schwann cells also migrated farther than the interfascicular Schwann cells (possibly a function of their maintained p75 expression), becoming dispersed singly for at least 8 mm from the original transplant site. Our previous study of corticospinal tract lesions had shown the formation of a "closed" scar formed by hypertrophic astrocytic processes, which walled off a central astrocyte-free region and totally disrupted the normal longitudinal alignment of the tract astrocytic processes. In contrast, while the present Schwann cell transplants induced a comparable astrocytic hypertrophy over the same time course, the astrocytic processes remained able to penetrate the transplant site, which was not walled off, so that the longitudinal arrangement of the host corticospinal tract astrocytic skeleton was preserved intact across the region of the transplant. These observations show that Schwann cells can be intimately integrated into the cytoarchitecture of the myelinated adult host corticospinal tract. This integration is not a random dispersal in damaged areas: it involves direct interaction with the cell elements present in the host tract, it respects the complex and regular organization of the host tract glial cells, and it results in the formation of a precisely arranged mosaic of central and peripheral tissue.

CNS axonal regeneration with peripheral nerve grafts cryopreserved by vitrification: cytological and functional aspects

To test cool-warm protocols for storing peripheral nerves, 4-cm-long-nerve segments were removed from the hindleg of adult rats and cryopreserved using a vitrification solution (or cryoprotective mixture) containing a mixture of polyalcohols (2,3-butanediol, 1,2-propanediol, polyethylene glycol, and Belzer U.W. medium). Schwann cell viability and morphology were studied with regard to the effect of (i) cryoprotective mixture concentration (100, 50, and 30% diluted in human serum albumin at 4%), (ii) duration of exposure (10, 15, or 30 min in a single step) of nerves to the cryoprotective mixture, (iii) cooling rate (F1/F2, F3, and F4: 3, 12, and 231 degrees C/min, respectively), and (iv) type of replacement of cryoprotectant (T1, one step; or T2, perfusion) after warming. Nerves exposed 10 min to cryoprotective mixture 50% (2,3-butanediol, 1.926 mol.liter-1; 1,2-propanediol, 3.063 mol.liter-1; polyethylene glycol, 0.084 mol.liter-1; and Belzer U.W., 22.4 mosm-1) and cooled-warmed with the F2/F3/F4-T2 protocols contained live and correctly cryopreserved Schwann cells. The capacity of these cryopreserved nerve segments (n = 6) to be subsequently repopulated by regenerating axons from central neurons was compared to that of fresh nerves when used as peripheral nerve autografts implanted within the spinal cord at the level of the descending respiratory pathways. All cryopreserved nerve grafts were successfully reinnervated by regenerated central axons. Unitary spontaneous action potentials propagated along these axons were assessed by recording the discharge of tested nervous filaments (T) from the grafts in artificially ventilated and paralyzed animals. Out of 535 T, 32 (6 +/- 1.2%) presented spontaneous unitary activity with respiratory (R, n = 2) and nonrespiratory (NR, n = 30) pattern of discharge. The T mean number, the occurrence rate referenced to the total number of T (R/T, NR/T, and R + NR/T) and the mean number of spontaneous units (R, NR, R + NR) were compared to those of fresh spinal peripheral nerve grafts. Except for T, cryopreserved peripheral nerve grafts contained statistically significantly (P < 0.05) less spontaneous R and NR unitary activity, which represented, respectively, 6.2 +/- 6.2 and 26.8 +/- 5.7% of that found in the control group. These data indicate that nerves cryopreserved with the protocols described above contain viable Schwann cells which constitute a suitable support to induce regeneration of central fibers. The effectiveness of nerve cryopreservation by vitrification is discussed with regard to Schwann cell viability following cool-warm protocols and to subsequent reinnervation of the cryopreserved peripheral nerve grafts.

Axonal regeneration

Axons damaged in a peripheral nerve are often able to regenerate from the site of injury along the degenerate distal segment of the nerve to reform functional synapses. Schwann cells play a central role in this process. However, in the adult mammalian central nervous system, from which Schwann cells are absent, axonal regeneration does not progress to allow functional recovery. This is due to inhibitors of axonal growth produced by both oligodendrocytes and astrocytes and also to the decreased ability of adult neurons to extend axons during regeneration compared to embryonic neurons during development. However once provided with a substrate conducive to axonal growth, such as a peripheral nerve graft, many central neurons are able to regenerate axons over long distances. Over the past year this response has been utilised in experimental models to produce a degree of behavioural recovery.

The influence of cultured Schwann cells on regeneration through acellular basal lamina grafts

Acellular basal lamina grafts have been shown to be less immunogenic in comparison to cellular grafts, but possess a limited potential for supporting axonal regeneration through them. The present study describes the effect of cultured Schwann cells on enhancing regeneration through acellular grafts. 2 cm long acellular grafts, and in vitro Schwann cell populated acellular grafts were used to repair a surgically created gap in the host peroneal nerve. The transplants were analyzed at 1, 2, 4 and 8 weeks to determine their ability to support axonal regeneration. Host axonal regeneration through Schwann cell cocultured acellular grafts occurred rapidly and was significantly better as compared to non-cultured acellular grafts. The results demonstrate a beneficial effect of Schwann cell culture pretreatment on regeneration through acellular grafts and an improved recovery of the target muscle. The procedure of first preparing acellular grafts with subsequent coculture with Schwann cells offers a novel approach for the repair of injured nervous tissue.

Molecular basis of interactions between regenerating adult rat thalamic axons and Schwann cells in peripheral nerve grafts I. Neural cell adhesion molecules

To gain insight into the possible molecular mechanisms underlying axonal regeneration of neurons of the adult central nervous system (CNS), we have investigated, by in situ hybridization and by immunocytochemistry, the localization and sites of synthesis of the neurite outgrowth-promoting cell surface molecules L1, N-CAM and its highly sialylated form, N-CAM-PSA, in and around peripheral nerve grafts implanted into the thalamus of adult rats. Normal unoperated adult rat thalamus contains N-CAM and L1 but no N-CAM-PSA immunoreactive axons. Between 7 days and 13 weeks after graft implantation, L1, N-CAM and N-CAM-PSA were all present at the surface of axonal sprouts in the brain parenchyma close to grafts and in the central parts of Schwann cell columns within grafts. Schwann cell membranes were L1 and N-CAM positive at all postgraft survival times, more strongly at 2-4 weeks than other times, but were associated with N-CAM-PSA reaction product only where they abutted N-CAM-PSA positive axons. Schwann cell membranes apposed to basal laminae (which were avoided by regenerating CNS axons) were L1, N-CAM and N-CAM-PSA negative. Between 3 days and 8 weeks after grafting, N-CAM and L1 mRNA were generally weakly upregulated in neurons of the ipsilateral thalamus, but, most conspicuously, L1 mRNA was strongly upregulated in the neurons of the thalamic reticular nucleus; these neurons are known to regenerate axons very effectively into peripheral nerve grafts and are the probable source of most of the axons which enter thalamic grafts. N-CAM and L1 mRNA were also strongly upregulated in presumptive Schwann cells in the graft. These results show that regenerating CNS axons (re)express N-CAM-PSA and upregulate L1 and N-CAM, suggesting that all of these molecules may play a role in cellular interactions during the regeneration of CNS axons. Furthermore L1 synthesis appears to be particularly well correlated with the ability of CNS neurons to regenerate axons into peripheral nerve grafts.

Molecular basis of interactions between regenerating adult rat thalamic axons and Schwann cells in peripheral nerve grafts. II. Tenascin-C

Tenascin-C is a developmentally regulated extracellular matrix component. There is evidence that it may be involved in axon growth and regeneration in peripheral nerves. We have used in situ hybridization and immunocytochemistry to investigate the association of tenascin-C with central nervous system axons regenerating through a peripheral nerve autograft inserted into the thalamus of adult rats. Between 3 days and 4 weeks after implantation, tenascin-C immunoreactivity was increased in the grafts, first at the graft/brain interface, then in the endoneurium of the graft, and finally within the Schwann cell columns of the graft. By electron microscopy, reaction product was present around collagen fibrils and basal laminae in the endoneurium, but the heaviest deposits were found at the surface of regenerating thalamic axons within Schwann cell columns. Schwann cell surfaces were not associated with tenascin-C reaction product except where they faced the tenascin-rich basal lamina or were immediately opposite axons surrounded by tenascin-C. By 8 weeks after graft implantation tenascin-C in the endoneurium and around axons of the graft was decreased. In the brain parenchyma around the proximal part of the graft, axonal sprouts associated with tenascin-C could not be identified earlier than 2 weeks after grafting and were sparse at this stage. Larger numbers of such axons were present at 8-13 weeks after grafting and were located predominantly where the glia limitans between brain and graft appeared to be incomplete, suggesting that the tenascin-C may have penetrated the brain parenchyma from the graft. By in situ hybridization, cells expressing tenascin-C mRNA (probably Schwann cells) appeared first at the brain/graft interface 3 days after grafting and thereafter were mainly located within the grafts. Lightly labelled cells containing tenascin-C mRNA (probably glial cells) were scattered in the thalamic parenchyma both ipsilateral and contralateral to the graft and a few heavily labelled cells were located very close to the tip of the graft. These results show that regenerating adult thalamic axons, unlike regenerating peripheral axons, become intimately associated with peripheral nerve graft-derived tenascin-C, suggesting that they express a tenascin-C receptor, as many neurons do during development, and that tenascin-C derived from Schwann cells may play a role in the regenerative growth of such axons through the grafts.

Human Schwann cells in vitro. I. Failure to differentiate and support neuronal health under co-culture conditions that promote full function of rodent cells

Schwann cells (SCs) play critical roles in regeneration after injury to the peripheral nervous system and can also induce axonal regeneration in the central nervous system. Transplantation of purified SCs into sites of neural injury in rodents has confirmed the remarkable ability of these cells to promote axonal regrowth, suggesting that human application of SC transplantation could be valuable. In this report, we have compared the functional capacities of SCs derived from adult human and rodent nerves by of SCs derived from adult human and rodent nerves by maintaining SCs from these two sources in culture with sensory neurons. We noted that techniques commonly in use for maintaining pure rat SC populations are not sufficient to sustain populations of human SCs free of fibroblasts. In these co-cultures, human SCs express a limited profile of characteristic behaviors and they proliferate more slowly than rat SCs in response to axonal contact. Slow SC proliferation, relative to that of contaminating fibroblasts, leads to a high proportion of fibroblasts in the cultures. After 3 to 4 weeks of co-culture with neurons, human SCs express extracellular matrix molecules, but only partially ensheathe axons, whereas rat SCs differentiate, form basal lamina, and ensheathe or myelinate axons. Co-culture of sensory neurons with human (but not rat) SC preparations (or conditioned medium therefrom) leads to a progressive neuronal atrophy characterized by shrinking neuronal cell bodies and a decrease in the density of the neurite network in the culture dish. As the divergent effects of human and rat SCs on neuronal health were also observed in co-cultures with human sensory neurons, these effects reflect differences between the rat and human-derived SC populations, rather than a species mismatch between SCs and neurons. The marked differences in behavior observed between rat and human SCs derived by the same methods requires further exploration if human-derived SCs are to be considered in the treatment of disease. In a companion article we report experiments that define culture conditions more effective in promoting human SC function in vitro.

Axonal regeneration into Schwann cell-seeded guidance channels grafted into transected adult rat spinal cord

Schwann cells (SC) have been shown to promote regeneration in both the peripheral and central nervous systems. In this study we tested the ability of SC to enhance axonal regeneration in adult rat spinal cord by grafting SC-seeded guidance channels into transected cords. SC were purified in culture from adult inbred rat sciatic nerves, suspended in Matrigel, and seeded into semipermeable PAN/PVC channels (2.6 mm I.D. x 10 mm long) at a final density of 120 x 10(6) cells/ml. Channels filled with Matrigel alone served as controls. Adult isologous rat spinal cords were transected at the T8 level, and segments T9-T11 were removed. The rostral stump was inserted 1 mm into channels with capped distal ends. One month after grafting, a vascularized tissue cable was present within the channel in all animals. In SC-seeded channels (n = 14), a mean of 501 myelinated axons was found in the cable, and many axons extended 9-10 mm. Electron microscopy revealed typical SC ensheathment and myelination of axons with four times more unmyelinated than myelinated axons. Control channels (n = 8) contained fewer myelinated axons (mean = 71). When SC were prelabeled in culture with a nuclear dye, labeled nuclei were observed at 30 days, confirming SC survival. Astrocytes identified by glial fibrillary acidic protein staining did not migrate far into the cable, and prelabeled SC did not enter the cord. Lack of immunostaining for serotonin and dopamine beta-hydroxylase indicated that supraspinal axons did not regenerate into the cable. Fast Blue injections into the middle of the cable (n = 3) marked spinal cord interneurons (mean = 306) as far as nine segments rostral (25 mm, C7) extending axons into the graft; fewer dorsal root ganglion neurons were retrogradely labeled. In conclusion, purified populations of SC transplanted within channels promote both propriospinal and sensory axonal regeneration in the adult rat thoracic spinal cord.

Schwann cell processes guide regeneration of peripheral axons

Terminal Schwann cells overlying the neuromuscular junction sprout elaborate processes upon muscle denervation. We show here that motor axons use these processes as guides/substrates during regeneration; in so doing, they escape the confines of endplates and grow between endplates to generate polyneuronal innervation. We also show that Schwann cells in the nerve provide similar guidance. Axons extend from the cut end of a nerve in association with Schwann cell processes and appear to navigate along them. The processes extend from axotomized nerves at the same rate and in the same manner as they do from axon-containing nerves. The rate of process extension limits the rate at which axons regenerate. Thus, Schwann cell processes lead and guide peripheral regeneration.

Regrowth of axons in lesioned adult rat spinal cord: promotion by implants of cultured Schwann cells

Highly purified populations of Schwann cells were grafted into lesioned adult rat spinal cord to determine if they promote axonal regeneration. Dorsal spinal cord lesions were created by a photochemical lesioning technique. Schwann cells derived from E16 rat dorsal root ganglia, either elongated and associated with their extracellular matrix or dissociated and without matrix, were rolled in polymerized collagen to form an implant 4-6 mm long which was grafted at 5 or 28 days after lesioning. No immunosuppression was used. Acellular collagen rolls served as controls. At 14, 28 and 90 days and 4 and 6 months after grafting, animals were analysed histologically with silver and Toluidine Blue stains and EM. The grafts often filled the lesion and the host borders they apposed exhibited only limited astrogliosis. By 14 days, bundles of unmyelinated and occasional thinly myelinated axons populated the periphery of Schwann cell implants. By 28 days and thereafter, numerous unmyelinated and myelinated axons were present in most grafts. Silver staining revealed sprouted axons at the implant border at 28 days and long bundles of axons within the implant at 90 days. Photographs of entire 1 micron plastic cross-sections of nine grafted areas were assembled into montages to count the number of myelinated axons at the graft midpoint; the number of myelinated axons ranged from 517-3214. Electron microscopy of implants showed typical Schwann cell ensheathment and myelination, increased myelin thickness by 90 days, and a preponderance of unmyelinated over myelinated axons. Random EM sampling of five Schwann cell grafts showed that the ratio of unmyelinated to myelinated axons was highest (20:1) at 28 days. These ratios implied that axons numbered in the thousands at the graft midpoint. Dissociated Schwann cells without matrix promoted axonal ingrowth and longitudinal orientation as effectively as did elongated Schwann cells accompanied by matrix. There was a suggestion that axonal ingrowth was at least as successful, if not more so, when the delay between lesioning and grafting was 28 rather than 5 days. Acellular collagen grafts did not contain axons at 28 days, the only interval assessed. In sum, grafts of Schwann cells in a rolled collagen layer filled the lesion and were well tolerated by the host. The Schwann cells stimulated rapid and abundant growth of axons into grafts and they ensheathed and myelinated these axons in the normal manner.

Median nerve neurotization by peripheral nerve grafts directly implanted into the spinal cord: anatomical, behavioural and electrophysiological evidences of sensorimotor recovery

Over the years, peripheral nerve grafts, a favorable environment to support axonal elongation, have given rise to increasing interest as a possible solution for promoting spinal cord repair. In the experiments described here, following an avulsion injury of the rat brachial plexus, the median nerve was repaired by a peripheral nerve graft (PN) inserted directly into the dorsal side of the spinal cord. Eight months later the animals were submitted to behavioral tests, electrophysiological and histological studies. Regrowth of axons from both motoneurons and ganglionic neurons was demonstrated following a single superficial dorsal implantation of a PN. Sensorimotor peripheral reinnervation allowed most of the studied animals to recover enough flexor activity for grasping. Reinnervation was achieved even without prior root avulsion suggesting that the presence of a PN is sufficient to induce sprouting in the spinal cord from axotomized and non-axotomized neurons.

Contacts between regenerating axons and the Schwann cells of sciatic nerve segments grafted to the optic nerve of adult rats

The relation between Schwann cells, basal laminae and axons during retinal ganglion cell regeneration was studied by using cellular, acellular and partially acellular sciatic nerve autografts into the optic nerve. Acellular grafts were achieved by temporary compression which eliminates living Schwann cells and axons. The compressed sciatic nerve together with the intact portion was used as a partially acellular graft. The compressed portion was anastomosed to the optic nerve and the intact portion was situated distally. After 3-21 days post-operation, the grafts were studied by thin sectioning and freeze-fracture. Axons were seen to regenerate into cellular grafts in contact with Schwann cells after one week, but not into acellular grafts for the entire period. In the partially acellular grafts, regenerating axons were first observed after two weeks and were always in contact with Schwann cells migrating from the intact portion. Moreover, membrane specializations, fuzzy materials in the space between apposed membranes, and putative tight junctions, were found between regenerated axons including growth cone and Schwann cells, and between adjoining Schwann cells. An extensive meshwork of putative tight junctions was displayed between reforming perineurial cells surrounding the groups of Schwann cells and associated axons. Gap junctions were seen between adjoining Schwann cells, and between reforming perineurial cells. These results suggest that the axonal contact with Schwann cell surfaces plays an important role in retinal ganglion cell regeneration.