Schwann cell basal lamina and nerve regeneration

Metabosensitive Afferent Fiber Responses after Peripheral Nerve Injury and Transplantation of an Acellular Muscle Graft in Association with Schwann Cells

Studies dedicated to the repair of peripheral nerve focused almost exclusively on motor or mechanosensitive fiber regeneration. Poor attention has been paid to the metabosensitive fibers from group III and IV (also called ergoreceptor). Previously, we demonstrated that the metabosensitive response from the tibialis anterior muscle was partially restored when the transected nerve was immediately sutured. In the present study, we assessed motor and metabosensitive responses of the regenerated axons in a rat model in which 1 cm segment of the peroneal nerve was removed and immediately replaced by an autologous nerve graft or an acellular muscle graft. Four groups of animals were included: control animals (C, no graft), transected animals grafted with either an autologous nerve graft (Gold Standard-GS) or an acellular muscle filled with Schwann Cells (MSC) or Culture Medium (MCM). We observed that (1) the tibialis anterior muscle was atrophied in GS, M(SC) and M(CM) groups, with no significant difference between grafted groups; (2) the contractile properties of the reinnervated muscles after nerve stimulation were similar in all groups; (3) the metabosensitive afferent responses to electrically induced fatigue was smaller in M(SC) and MCM groups; and (4) the metabosensitive afferent responses to two chemical agents (KCl and lactic acid) was decreased in GS, M(SC) and M(CM) groups. Altogether, these data indicate a motor axonal regeneration and an immature metabosensitive afferent fiber regrowth through acellular muscle grafts. Similarities between the two groups grafted with acellular muscles suggest that, in our conditions, implanted Schwann cells do not improve nerve regeneration. Future studies could include engineered conduits that mimic as closely as possible the internal organization of uninjured nerve.

Focal adhesion kinase signaling at sites of integrin-mediated adhesion controls axon pathfinding

Extracellular matrix (ECM) components regulate neurite outgrowth in tissue culture and in vivo. Live imaging of phosphotyrosine (PY) signals revealed that Xenopus laevis growth cones extending on permissive ECM substrata assemble adhesive point contacts containing enriched levels of tyrosine-phosphorylated proteins. Whereas focal adhesion kinase (FAK) signaling is dispensable for the assembly of focal adhesions in non-neuronal cells, FAK activity is required for the formation of growth cone point contacts. FAK-dependent point contacts promote rapid neurite outgrowth by stabilizing lamellipodial protrusions on permissive ECM substrata. Moreover, local FAK activity is required for ECM-dependent growth cone turning in vitro, suggesting that FAK may control axon pathfinding in vivo. Consistent with this possibility, proper growth and guidance of Rohon-Beard sensory neurons and spinal commissural interneurons requires FAK activity. These findings identify FAK as a key regulator of axon growth and guidance downstream of growth cone-ECM interactions.

Lysophospholipid receptors are differentially expressed in rat terminal Schwann cells, as revealed by a single cell rt-PCR and in situ hybridization

Terminal Schwann cells (TSCs) that cover motor neuron terminals, are known to play an important role in maintaining neuromuscular junctions, as well as in the repair process after nerve injury. However, the molecular characteristics of TSCs remain unknown, because of the difficulties in analyzing them due to their paucity. By using our previously reported method of selectively and efficiently collecting TSCs, we have analyzed the difference in expression patterns of lysophospholipid (LPL) receptor genes (LPA1, LPA2, LPA3, S1P1, S1P2, S1P3, S1P4, and S1P5) between TSCs and myelinating Schwann cells (MSCs). LPL, which includes lysophosphatidic acid (LPA) and sphingosine 1-phosphate (S1P), is the bioactive lipid that induces a myriad of cellular responses through specific members of G-protein coupled receptors for LPA. It turned out that LPA3 was expressed only in TSCs, whereas S1P1 was expressed in TSCs and skeletal muscle, but not in MSCs. Other types of LPL receptor genes, including LPA1, S1P2, S1P3, S1P4, were expressed in both types of Schwann cells. None of the LPL receptor gene family showed MSCs-specific expression.

The role of neuroinflammation in neuropathic pain: mechanisms and therapeutic targets

Neuroinflammation is a proinflammatory cytokine-mediated process that can be provoked by systemic tissue injury but it is most often associated with direct injury to the nervous system. It involves neural-immune interactions that activate immune cells, glial cells and neurons and can lead to the debilitating pain state known as neuropathic pain. It occurs most commonly with injury to peripheral nerves and involves axonal injury with Wallerian degeneration mediated by hematogenous macrophages. Therapy is problematic but new trials with anti-cytokine agents, cytokine receptor antibodies, cytokine-signaling inhibitors, and glial and neuron stabilizers provide hope for future success in treating neuropathic pain.

Possibility of using nerve segments dissected from human cadavers for grafting: preliminary report

An intercostal nerve obtained from a human cadaver 6 h post-mortem was transplanted into the rat sciatic nerve and nerve regeneration was observed 4 and 8 weeks after surgery. Sciatic nerves from deceased rats up to 2 days post-mortem were also transplanted for comparison. Good nerve regeneration was observed through the human cadaver-derived graft to the distal segment at the medial plantal nerve 8 weeks after surgery. The results of the present study indicate the possibility that nerves from human cadavers can be used for nerve grafting in clinical applications.

New therapeutic target for CNS injury? The role of retinoic acid signaling after nerve lesions

Experiments with sciatic nerve lesions and spinal cord contusion injury demonstrate that the retinoic acid (RA) signaling cascade is activated by these traumatic events. In both cases the RA-synthesizing enzyme is RALDH-2. In the PNS, lesions cause RA-induced gene transcription, intracellular translocation of retinoid receptors, and increased transcription of CRBP-I, CRABP-II, and retinoid receptors. The activation of RARbeta appears to be responsible for neurotrophic and neuritogenic effects of RA on dorsal root ganglia and embryonic spinal cord. While the physiological role of RA in the injured nervous system is still under investigation three domains of functions are suggested: (1) neuroprotection and support of axonal growth, (2) modulation of the inflammatory reaction by microglia/macrophages, and (3) regulation of glial differentiation. Few studies have been performed to support nerve regeneration with RA signals in vivo, but a large number of experiments with neuronal and glial cell cultures and spinal cord explants point to beneficial effects of RA, so that future therapeutic approaches will likely focus on the activation of RA signaling.

Functional evaluation of regenerated and misrouted low threshold mechanoreceptors and polymodal nociceptors in the skin of rat hindfeet after crush lesions to the sciatic nerve

Tracer studies on motor axons after nerve crush injuries have indicated that misrouting may occur even when the endoneurium is intact. Misrouting of regenerated polymodal nociceptive C-fibres and low threshold mechanoreceptive axons have been studied functionally in 50 rats three months after unilateral crush lesions to the sciatic nerve. Two weeks before evaluation the tibial fascicle (or the peroneal fascicle) above the lesion was cut and tied off. In this way only functional regeneration of misrouted axons was tested. Misrouted low threshold mechanoreceptive axons and polymodal nociceptor C-fibres were found after regeneration in both glabrous and hairy skin. We conclude that functional misdirection of both myelinated and unmyelinated sensory axons innervating either glabrous or hairy skin can occur after a crush lesion to a peripheral nerve in rats.

Expression of a chemorepellent factor, Slit2, in peripheral nerve regeneration

The expression of mRNA for chemorepellent factors slit1 and slit2 in rat peripheral nerve regeneration was examined. The mRNA of slit2 increased when the continuity of basal lamina tubes was disrupted, not when it remained and the Slit2 protein was located in Schwann cells. These results suggest that disruption of the continuity of basal lamina tubes induces the expression of slit2 in Schwann cells during peripheral nerve regeneration.

Review: peripheral nerve regeneration using non-tubular alginate gel crosslinked with covalent bonds

We have developed a nerve regeneration material consisting of alginate gel crosslinked with covalent bonds. in the first part of this study, we attempted to analyze nerve regeneration through alginate gel in the early stages within 2 weeks. in the second part, we tried to regenerate cat peripheral nerve by using alginate tubular or non-tubular nerve regeneration devices, and compared their efficacies. Four days after surgery, regenerating axons grew without Schwann cell investment through the partially degraded alginate gel, being in direct contact with the alginate without a basal lamina covering. One to 2 weeks after surgery, regenerating axons were surrounded by common Schwann cells, forming small bundles, with some axons at the periphery being partly in direct contact with alginate. At the distal stump, numerous Schwann cells had migrated into the alginate 8-14 days after surgery. Remarkable restorations of the 50-mm gap in cat sciatic nerve were obtained after a long term by using tubular or non-tubular nerve regeneration material consisting mainly of alginate gel. However, there was no significant difference between both groups at electrophysiological and morphological evaluation. Although, nowadays, nerve regeneration materials being marketed mostly have a tubular structure, our results suggest that the tubular structure is not indispensable for peripheral nerve regeneration.

Type grouping in skeletal muscles after experimental reinnervation: another explanation

Type grouping signifies clustering of muscle fibres of the same metabolic type, and is a frequent finding in reinnervated muscles. To elucidate the mechanism behind it, the rat sciatic nerve was either autografted or grafted with hollow synthetic nerve grafts. Twelve weeks later the number and fibre area of the type I and type II muscle fibres in the gastrocnemic and anterior tibial muscles were determined after ATP-ase staining. The number and diameter of peroneal nerve fibres distal to the grafts were measured, and the number of Aalpha-nerve fibres was derived. Nearly all nerve and muscle morphometrical parameters changed equally in both experimental groups. However, type grouping occurred frequently only after autografting, whereas the number of nerve fibres and the number of Aalpha-nerve fibres increased in this group. Hence type grouping cannot be explained by increased intramuscular sprouting subsequent to a decrease in the number of innervating nerve fibres, as previously presumed. Regenerating axons branch along their course through the peripheral nerve. We propose that the probability of the occurrence of type grouping is related to the dispersion of sibling branches in the nerve. In the autograft, emerging branches are kept together by Schwann cell basal lamina scaffolds, in contrast to the hollow synthetic nerve grafts where the emerging branches become dispersed. Thus, in muscles reinnervated after autografting, the probability that nerve branches that arrive at a specific muscle territory are sibling branches is greater than after hollow tube grafting. Consequently, the probability that type grouping will occur is greater.

Experimental study on the regeneration of peripheral nerve gaps through a polyglycolic acid-collagen (PGA-collagen) tube

We have developed a bioabsorbable polyglycolic acid (PGA) tube filled with collagen sponge (PGA-collagen tube) as a nerve connective guide, and compared its effectiveness with that of autograft in terms of nerve regeneration across a gap. The PGA-collagen tube was implanted into 24 beagle dogs across a 15-mm gap in the left peroneal nerve. The right peroneal nerve was reconstructed with the autograft harvested from the left side, as a control. After the surgery, the connective tissue extended from both cut ends in the PGA-collagen tube and connected again at the center. Pathologically, the collagen sponge in the tube provided adequate scaffolding for nerve tissue extension, and the nerve tissue reconnected within 3 weeks. Electrophysiologically, muscle-evoked potentials (MEPs) and compound nerve action potentials (CNAPs) were detected 18 days after the surgery. For up to 6 months postsurgery, CNAPs and somatosensory-evoked potentials (SEPs) on the PGA-collagen side had a shorter latency and larger peak voltage than those on the autograft side. The myelinated axons on the PGA side were larger in diameter than those on the autograft side. It is suggested that the PGA-collagen tube has the potential to be an effective alternative to conventional autografting for the repair of some peripheral nerve defects.

Schwann cells and endoneurial extracellular matrix molecules as potential cues for sorting of regenerated axons: a review

Besides very well elaborated microsurgical management of severed peripheral nerves, the clinical results of functional recovery following surgical repair of mixed nerves are disappointing. An improvement of functional recovery after peripheral nerve lesion requires the accurate regeneration of axons to their original target tissues and structures. Therefore, better clinical results could be obtained by a greater understanding of the cellular and molecular biology of selective nerve regeneration. The studies concerning Schwann cells and their endoneurial extracellular matrix as potent cues for selective promotion and influence of regenerating motor and sensory axons are reviewed. Knowledge of the sorting mechanisms of regenerated motor and sensory axons is needed not only for improvement of functional recovery, but also for the development of biocompatible nerve prostheses.

Transmigration of donor cells involved in the sciatic nerve graft

INTRODUCTION

Recently, human hand transplantation in Europe has shown that motor function may be recovered in some cases. However, little is known about cell trafficking involved the graft nerve. We have succeeded to use green fluorescent protein transgenic (GFP-Tg) rats with various cells strongly expressing GFP in a model a long-term survival of limb graft. In this model, we found retrograde migration of GFP-positive donor cells through the sclatic nerve anastomosis. It is well known that cellular components in the peripheral nerve graft especially Schwann cells, play an important role in the axonal regeneration promoted by nerve grafting. However, it was difficult to distinguish the cellular component of the nerve graft from recipient cells. The purpose of this study was to evaluate the migration of donor origin cells to the recipient's nerve and to examine the contribution of these cells in axonal regeneration using a simplified model of sciatic grafting.

METHODS

Nerve defects were created in recipient rats, using three experimental combinations: group 1: wild-type rats from GFP Tg rats; group 2: GFP Tg rats from wild-type rats; group 3: wild-type rats from GFP Tg rats whose nerve grafts had been pretreated by freeze-thawing cycles (representing an acellular graft). The sciatic nerve specimens were examined under excitation light at 1, 2, and 3 weeks after transplantation.

RESULTS

GFP-positive area expanded clearly beyond the anastomosis both proximally and distally in group 1 and infiltrated into the middle of the null graft in group 2. On the contrary, freeze-thawing grafts donated GFP Tg rats lost GFP expression completely. Columns of GFP-positive cells were formed in the degenerated graft migrated into the recipient's nerve both ante- and retrograde. The S100-positive GFP-positive cells were considered to be graft-origin Schwann cells. The regenerating axons were accompanied with these double-positive cells in the recipient nerve. In conclusion, we have visualized the contribution of graft cells to axonal regeneration beyond a peripheral nerve anastomosis.

Matrix metalloproteinase-9 promotes nerve growth factor-induced neurite elongation but not new sprout formation in vitro

Matrix metalloproteinase-9 (MMP-9) is a basal-lamina-degrading protease that we have recently shown to be localized in regenerating sciatic nerve. We now demonstrate that MMP-9 colocalizes with growth-associated protein GAP-43 in regenerating nerves in vivo and is involved in vitro in axonal sprouting. By using a PC12 cell model for neuronal sprouting, we analyzed the effects of recombinant MMP-9, MMP-9-neutralizing antibody, and a broad-spectrum MMP inhibitor (Ro 31-9790) on sprout formation, elongation, and branching. Quantitative phase-contrast microscopy showed that MMP-9 elongated neuronal sprouts by 67% and increased their branching by 14% but did not change the number of sprouts relative to nerve growth factor (NGF) treatment. Double immunofluorescence for GAP-43, a marker for growth cones, and alpha-tubulin, a marker for axonal microtubules, showed that MMP-9-treated cells had increased distribution of alpha-tubulin but no effect on GAP-43. Western blot analyses of cell lysates demonstrated that the NGF-induced increase in GAP-43 was unchanged with MMP-9 treatment or inhibition, confirming that MMP-9 had no effect on new sprout formation. However, Ro 31-9790 reduced GAP-43 levels to those seen in untreated cells, suggesting that an MMP other than MMP-9 is important for sprout formation. Finally, phosphorylated neurofilament M (NFM-p), a marker for regenerative elongation, was induced with MMP-9 treatment and was inhibited by the anti-MMP-9 antibody treatment, confirming the role of MMP-9 in axonal elongation. NFM-p colocalized with MMP-9 in regenerating sciatic nerve fibers. These findings suggest that MMP-9 regulates neurite extension in regenerating peripheral nerve fibers and, therefore, might be of therapeutic value in promoting regeneration in vivo.

The perineurium modifies the effects of phenol and glycerol in rat sciatic nerve

Endoneurial cell response and type of nerve fibre damage were studied after perineural injections of 7% phenol-aqua and pure glycerol. Our previous studies have shown that phenol and glycerol induce different types of nerve fibre degeneration after intraneural injections: phenol dissolves axons and Schwann cells inside the basal lamina tubes but glycerol breaks them down into cellular flakes. The current study investigated whether the difference in type of endoneurial damage also appears after perineural application and how the perineurium affects the effect of these neurolytic agents. Rat sciatic nerves were treated with perineural injections of 7% phenol-aqua or pure glycerol and were followed up to 6 months. The results support the previous findings that perineural phenol injection induces damage that covers almost the whole endoneurium, but glycerol injection results in minor subperineurial damage. An ultrastructural study showed that the endoneurial effects are much milder after perineural injection than after intraneural injections. Phenol-induced nerve fibre dissolving was only rarely seen and the nerve fibre damage appeared similar to that after regular Wallerian degeneration in both groups. Axonal regeneration began within 2 weeks of the injections. Endoneurial macrophages were numerous in the damaged area in many individual nerves even at 3-6 months in both groups, which may indicate impaired phagocytotic activity. Regenerating axonal sprouts were seen first at 1 week post injection and Schwann cells proliferated within 2 weeks in both groups. However, the number of axonal sprouts was higher (P=0.002) and the size of the sprouts appeared larger after glycerol injection at 4 weeks post injection. The present study shows that the effects of extraneurally applied neurolytic agents phenol and glycerol are modified by the perineurium. Phenol readily penetrates the perineurium, but glycerol causes only subperineurial damage. The type of damage is rather similar to regular Wallerian degeneration in both groups and the endoneurial effects differ from those seen after intraneural injections.

Synergistic effects of micropatterned biodegradable conduits and Schwann cells on sciatic nerve regeneration

This paper describes a novel biodegradable conduit that provides a combination of physical, chemical and biological cues at the cellular level to facilitate peripheral nerve regeneration. The conduit consists of a porous poly(D,L-lactic acid) (PDLLA) tubular support structure with a micropatterned inner lumen. Schwann cells were pre-seeded into the lumen to provide additional trophic support. Conduits with micropatterned inner lumens pre-seeded with Schwann cells (MS) were fabricated and compared with three types of conduits used as controls: M (conduits with micropatterned inner lumens without pre-seeded Schwann cells), NS (conduits without micropatterned inner lumens pre-seeded with Schwann cells) and N (conduits without micropatterned inner lumens, without pre-seeded Schwann cells). The conduits were implanted in rats with 1 cm sciatic nerve transections and the regeneration and functional recovery were compared in the four different cases. The number or size of regenerated axons did not vary significantly among the different conduits. The time of recovery, and the sciatic function index, however, were significantly enhanced using the MS conduits, based on qualitative observations as well as quantitative measurements using walking track analysis. This demonstrates that biodegradable micropatterned conduits pre-seeded with Schwann cells that provide a combination of physical, chemical and biological guidance cues for regenerating axons at the cellular level offer a better alternative for repairing sciatic nerve transactions than conventional biodegradable conduits.

Modulation of sciatic nerve expression of class 3 semaphorins by nerve injury

Beginning with the unexpected finding by cDNA array analysis that neuropilin-2 is induced in sciatic nerve distal to a transection, we document, for the first time, up-regulation in the axotomized adult peripheral nervous system of class 3 semaphorins and their receptors, which are known to play prominent roles in axonal guidance during neural development. Previously, we described the use of cDNA arrays to screen for novel peripheral nervous system axotomy-induced candidate neurotrophic proteins. A novel finding of that prior study was substantial induction of neuropilin 2 (NP2) mRNA in the axotomized nerve segments. Following up on that initial observation, we have now used real-time quantitative reverse transcription-polymerase chain reaction to demonstrate induction of genes encoding neuropilin 1 (NP1), which, like NP2, serves as a coreceptor for members of the class 3 semaphorin family of axonal guidance molecules and of five of the six known class 3 semaphorins (Sema3A, Sema3B, Sema3C, Sema3E, and Sema3F, but not Sema3D) in crushed or transected sciatic nerves.

Comparison of continuous and discontinuous FK506 administration on autograft or allograft repair of sciatic nerve resection

An immunosuppressant drug that also possesses neuroregenerative properties, FK506 enhances the rate of axonal regeneration and improves recovery after nerve lesions. Nevertheless, prolonged immunosuppression may not be justified to assure the success of nerve regeneration. In this study, we compare the effects of continuous and discontinuous FK506 treatment on regeneration and reinnervation after sciatic nerve resection repaired with autologous or allogenic grafts in the mouse. For each type of repair, one group received FK506 (5 mg/kg) for 4 months, whereas a second group was treated with FK506 at 5 mg/kg for 5 weeks followed by 3 mg/kg for 4 weeks; a control group received saline only. Functional reinnervation was assessed by noninvasive methods to determine recovery of motor, sensory, and autonomic functions in the hind paw over 4 months after operation. Morphological analysis of the regenerated nerves was performed at the termination of the study. Autografts and allografts treated with sustained FK506 (5 mg/kg) reached high levels of reinnervation and followed a course of recovery faster than controls. The numbers of myelinated fibers also were similar. Allografts without immunosuppression demonstrated a slower rate of regeneration, exhibiting lower final levels of recovery compared with other groups and containing fewer numbers of regenerating myelinated fibers. Withdrawal of immunosuppressant therapy resulted in a decline in the degree of reinnervation in all functions tested during the third month, with stabilization between the third and fourth months. The number of regenerated myelinated fibers in the group was significantly lower than in autografts. Thus, continuous or discontinuous FK506 administration slightly accelerated the rate of reinnervation in autografts. In allograft repair, FK506 significantly enhanced both the rate and degree of regeneration and recovery, but its withdrawal resulted in graft rejection, a marked deterioration in function, and loss of regenerating fibers.

Use of Skeletal Muscle Tissue in Peripheral Nerve Repair: Review of the Literature

The management of peripheral nerve injury continues to be a major clinical challenge. The most widely used technique for bridging defects in peripheral nerves is the use of autologous nerve grafts. This technique, however, necessitates a donor nerve and corresponding deficit. Many alternative techniques have thus been developed. The use of skeletal muscle tissue as graft material for nerve repair is one example. The rationale regarding the use of the skeletal muscle tissue technique is the availability of a longitudinally oriented basal lamina and extracellular matrix components that direct and enhance regenerating nerve fibers. These factors provide superiority over other bridging methods as vein grafts or (non)degradable nerve conduits. The main disadvantages of this technique are the risk that nerve fibers can grow out of the muscle tissue during nerve regeneration, and that a donor site is necessary to harvest the muscle tissue. Despite publications on nerve conduits as an alternative for peripheral nerve repair, autologous nerve grafting is still the standard care for treatment of a nerve gap in the clinical situation; however, the use of the skeletal muscle tissue technique can be added to the surgeon's arsenal of peripheral nerve repair tools, especially for bridging short nerve defects or when traditional nerve autografts cannot be employed. This technique has been investigated both experimentally and clinically and, in this article, an overview of the literature on skeletal muscle grafts for bridging peripheral nerve defects is presented.

Enhanced peripheral nerve regeneration through a poled bioresorbable poly(lactic-co-glycolic acid) guidance channel

In this study we investigated the effects of materials prepared with electrical poling on neurite outgrowth in vitro and nerve regeneration in vivo. Neuro-2a cells were seeded on poled and unpoled poly(lactic-co-glycolic) (PLGA) films and observed at time periods 24, 48 and 72 h post-seeding. The percentage of cells with neurites and the neurites per cell were quantified using light microscopy. At 48 and 72 h post-seeding, both the number of cells with neurites and the neurites per cell were significantly increased on the poled films compared to those on unpoled films. An established rat sciatic nerve model was used for in vivo studies to assess the effects of PLGA guides, poled for two different periods, on peripheral nerve regeneration. Guides were inserted in rats to bridge a 1.0 cm gap created in the right sciatic nerve. After four weeks, nerves regenerated through poled guides displayed a significant increase in conduction velocity and significantly increased numbers of axons across the guides, as compared to nerves regenerating through an unpoled guidance channel. Electrical poling was shown to promote neurite growth, axon regeneration and the conduction rate of the repaired nerve. We concluded that guides prepared with electrical poling enhance peripheral nerve regeneration.

Peripheral nerve regeneration through guidance tubes

Biological nerve grafts have been extensively utilized in the past to repair peripheral nerve injuries. More recently, the use of synthetic guidance tubes in repairing these injuries has gained in popularity. This review focuses on artificial conduits, nerve regeneration through them, and an account of various synthetic materials that comprise these tubes in experimental animal and clinical trials. It also lists and describes several biomaterial considerations one should regard when designing, developing, and manufacturing potential guidance channel candidates. In the future, it it likely that the most successful synthetic nerve conduit will be one that has been fabricated with some of these strategies in mind.

Alginate Enhances Elongation of Early Regenerating Axons in Spinal Cord of Young Rats

Freeze-dried alginate sponge cross-linked with covalent bonds has been demonstrated to enhance nerve regeneration in peripheral nerves and spinal cords. The present study examined, at early stages after surgery, the outgrowth of regenerating axons and reactions of astrocytes at the stump of transected spinal cord in young rats. Two segments (Th7-8) were resected, and alginate was implanted in the lesion. As controls, collagen gel was implanted in place of alginate or the lesion was left without implantation. Two and 4 weeks after surgery, nerve outgrowth and astrocyte reactions were examined. Many regenerating axons, some of which were accompanied by astrocytic processes, were found to extend from the stump into the alginate-implanted lesion. In the all nonimplanted animals, large cystic cavities were formed at both interfaces with no definite axonal outgrowth into the lesion. In collagen-implanted animals, cavity formation was found in some rats, and regenerating axons once formed at the stumps did not extend further into the lesion. Astrocytic processes extending into alginate-implanted lesion had no basal laminae, whereas those found in control experiments were covered by basal laminae. These findings suggest that alginate contributed to reducing the barrier composed of connective tissues and reactive astrocytic processes, and served as a scaffold for the outgrowth of regenerating axons and elongation of astrocytic processes.

Functional assessment of sciatic nerve recovery: biodegradable poly (DLLA-epsilon-CL) nerve guide filled with fresh skeletal muscle

The aim of this study was to compare functional peripheral nerve recovery in the rat sciatic nerve model after reconstruction of a 10-mm gap with a biodegradable poly (DLLA-epsilon-CL) nerve guide, as filled with either fresh skeletal muscle or phosphate-buffered saline (PBS). During 24 weeks of recovery, motor and sensory functional evaluation was tested by extensor postural thrust (EPT) and withdrawal reflex latency (WRL), respectively. At the end of the experiment, anesthetized animals were prepared for motor nerve conduction velocity (MNCV) studies, followed by gastrocnemius and soleus muscle weight measurement. Motor functional recovery was greater in the muscle-grafted group, and reached a significant difference from weeks 8-12 (P < 0.05). The results of this investigation suggest that filling a nerve guide with fresh skeletal muscle induces faster maturation of regenerated nerve fibers in comparison with traditional tubular repair.

Inside-out vein graft and inside-out artery graft in rat sciatic nerve repair

Although veins and arteries present similar wall structures, there are differences which may be relevant in peripheral nerve reconstruction. Inside-out vein grafts (IOVG) have been satisfactorily used to repair both motor and sensitive nerves. However, the inside-out artery graft (IOAG) is a new technique and not fully investigated. Our study presents comparative morphological data on nerve regeneration achieved with IOVG and IOAG in the repair of Wistar rat sciatic nerves. Jugular veins and aorta arteries were harvested from donor animals and used "inside-out" to bridge a 10-mm gap. Animals were sacrificed at 10 weeks to evaluate nerve regeneration. Both techniques presented great variability in nervous tissue, though some animals showed satisfactory results. Different intensities of scarring processes might have interfered with nerve regeneration. Although IOVG and IOAG techniques showed similar morphometric results, in general, IOVG presented a closer-to-normal nerve organization than IOAG.

Active Src expression is induced after rat peripheral nerve injury

The non-receptor-type Src tyrosine kinases are key components of intracellular signal transduction that are expressed at high levels in the nervous system. To improve understanding of the cascades of molecular events underlying peripheral nerve regeneration, we analyzed active Src expression in the crushed or cut rat sciatic nerves using a monoclonal antibody (clone 28) that recognizes the active form of Src tyrosine kinases, including c-Src and c-Fyn. Western blots showed that active Src expressed in the normal sciatic nerve transiently increased up to threefolds after both types of injury. Immunohistochemistry using clone 28 showed that axonal components are the primary sites of active Src expression in the normal sciatic nerve. Soon after both types of injury, active Src was abundantly expressed in Schwann cells of the segments distal to the injury site. The expression of active Src in the cells decreased with restoration of the axon-Schwann cell relationship and eventually became depleted to very low levels after crushing, but was sustained at high levels in the cut model until the end of the experiment. Regenerated axons consistently expressed active Src throughout nerve regeneration and these eventually became the major sites of active Src expression in the crushed nerve. Among the Src tyrosine kinases, active c-Src selectively increased after crushing according to immunoprecipitation and immunoblotting analyses. Due to its potent biological activity, the increased amounts of the active form of Src probably enhance axonal regrowth, the Schwann cell response, and axon-Schwann cell contact for peripheral nerve regeneration.

Extracellular matrix of human amnion manufactured into tubes as conduits for peripheral nerve regeneration

The human amnion consists of the epithelial cell layer and underlying connective tissue. After removing the epithelial cells, the resulting acellular connective tissue matrix was manufactured into thin dry sheets called amnion matrix sheets. The sheets were further processed into tubes, amnion matrix tubes (AMTs), of varying diameters, with the walls of varying numbers of amnion matrix sheets with or without a gelatin coating. The AMTs were implanted into rat sciatic nerves. Regenerating nerves extended in bundles through tubes of 1-2 mm in diameter and further elongated into host distal nerves 1-3 weeks after implantation. Morphometrical analysis of the regenerated nerve cable at the middle of each amnion matrix tube 3 weeks after implantation was performed. The average numbers of myelinated axons were almost the same (ca. 80-112/10(4) microm(2)) in AMTs of 1-2 mm in diameter, as in the normal sciatic nerve (ca. 95/10(4) microm(2)). No myelinated fibers were found in AMTs composed of multiple thin tubes of 0.2 mm in diameter. The myelinated axons were thinner in implanted tubes than those in the normal sciatic nerve. The rate of occurrences of myelinated axons less than 4 microm in diameter was significantly higher in the AMTs, whereas axons in the normal sciatic nerve were diverse in distribution, with the highest population at 8-12 microm in diameter. Reinnervation to the gastrocnemius muscle was demonstrated electrophysiologically 9 months after implantation. It was concluded that the extracellular matrix sheet from the human amnion is an effective conduit material for peripheral nerve regeneration.

A novel technique for peripheral nerve repair

OBJECTIVE

To evaluate a novel technique for the repair of neural deficits using a single fascicle to bridge an injury in the rat sciatic nerve.

STUDY DESIGN

Twenty-four male Lewis rats were divided into four groups as follows: group 1 (control group), 1.5-cm deficit without repair; group 2, conventional epineural repair with autografts (100% diameter); group 3, nerve repair with large single autograft fascicle (50% diameter); and group 4, nerve repair with small single autograft fascicle (25% diameter).

METHODS

Nerve regeneration was evaluated at 3, 6, and 12 weeks by somatosensory evoked potential (SSEP) evaluation and standardized pin-prick and toe-spread tests. Nerve samples were harvested at 12 weeks and stained with toluidine blue to assess the total number of myelinated axons, axon area, and myelin sheath thickness.

RESULTS

In group I, the pin-prick and toe-spread tests showed no response at 3, 6, and 12 weeks. Rats in groups 3 and 4 demonstrated significantly better pin-prick test results and a trend toward better toe-spread test responses compared with conventional-repair animals. The SSEP evaluations displayed nondiagnostic waves in rats in group 1 rats. There was no evidence that the other surgery groups differed significantly in median SSEP latencies. Histological evaluation revealed fibrosis in rats in group 1 rats and a significantly higher median number of axons and myelin thickness in the small single fascicle (1296 axons and 4.22 microm, respectively) and large fascicle (2682 axons and 4.62 microm, respectively) groups compared with the conventional autograft group (630 axons and 2.93 microm, respectively). The small fascicle group had a significantly greater mean axon area (58.59 micro m2) than the large fascicle (29.66 micro m2) and conventional autograft (25.35 micro m2) groups.

CONCLUSIONS

Peripheral nerve repair using a single fascicle graft resulted in better functional recovery and morphometric outcome without a significant difference in electrophysiological status compared with conventional nerve repair. This technique may provide expanded sources of nerve autografts and alleviate the morbidity of harvesting peripheral nerves from multiple sites for individuals with extensive peripheral nerve injuries.

Experimental study on hepatic reinnervation after orthotopic liver transplantation in rats

BACKGROUND/AIMS

The present study examined whether extrinsic hepatic reinnervation occurred after orthotopic liver transplantation (OLT) in rats.

METHODS

Inbred male Lewis rats were the recipients and females the donors. Tissue specimens were obtained postoperatively from the stump of a recipient's hepatoduodenal ligament (A), and the hepatic hilus (B) and peripheral parenchyma (C) of liver allografts, up to 6 months post-operation. Specimens were subjected to immunohistochemical examination using growth-associated protein (GAP)-43 as an axonal marker and transmission electron microscopy (TEM) for observing regenerating axons, as well as the polymerase chain reaction assay to detect the rat sex-determining region Y (SRY) protein gene of the regenerating nerves.

RESULTS

At site A, GAP-43-positive nerve axons were identified from day 1 to 1 month post-OLT and SRY protein genes were expressed at and after 3 days post-OLT. At site B, GAP-43-positive axons were observed between 3 days and 1 month, and SRY protein genes were detected at 1 month post-OLT and thereafter. TEM confirmed the presence of regenerating axons at and after 3 days post-OLT.

CONCLUSIONS

The results demonstrated that regenerating nerve fibers originating from the recipients reinnervated liver allografts. This extrinsic innervation occurred shortly after OLT, and most likely terminated after about 3 months.

Age differences in morphological patterns of axonal sprouting and multiple innervation of neuromuscular junctions during muscle reinnervation following nerve crush injury

During the first 4-20 weeks after sciatic nerve crushing injury regrowing axons return to the neuromuscular junction and its reformation is in progress. During this time period age differences in patterns of axonal reinnervation from Wistar rats, with special reference to multiple axonal innervation and sprouting, was morphologically investigated using a neuronal marker (protein gene product 9.5). In young (4 months old) and aged (24 months old) animals, terminal outgrowth at the junction consisted of offshoots extending out from the junctional zone (extraterminal sprouts), and an extraterminal sprout extending to an adjacent endplate (endplate-to-endplate connections). Endplate-to-endplate connections and a nodal sprout served as partners of multiple axonal innervation. Large and complex junctions were formed by multiple innervation and elaboration of terminal branching. The most obvious changes in aged animals were as follows. (1) There were consistently more frequent numbers of extraterminal sprouting, endplate-to-endplate connections, and multiple innervation. The rates of process extension in extraterminal sprouting, however, displayed a significant drop at 4 and 8 weeks post-crush. (2) Late in reinnervation (12, 20 weeks), persistent aberrant changes in axonal reinnervation were more frequently observed, such as clumping of poorly organized nerve bundles, aggregates of multiple extensions, and poorly developed endplate-to-endplate connections, along with disorderly development of nerve terminals. Thus, age affects the reinnervating and sprouting capabilities of axons giving rise to persistent compensatory (though impaired) growth, extension, and branching in the formation of motor pathways during muscle reinnervation and endplate regeneration. The spatiotemporal relationship of these axonal changes to that of the postsynaptic receptor region is discussed.

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.

Absence of Wallerian Degeneration does not Hinder Regeneration in Peripheral Nerve

Wallerian degeneration of the distal stump of a severed peripheral nerve involves invasion by myelomonocytic cells, whose presence is necessary for destruction of myelin and for initiating mitosis in Schwann cells (Beuche and Friede, 1984). Degeneration of the distal ends of the axons themselves is assumed to occur by autolytic mechanisms. We describe a strain of mice (C57BL/6/Ola) in which leucocyte invasion is slow and sparse. In these mice, confirming Beuche and Friede, myelin removal is extremely slow. A new finding is that axon degeneration is also very slow. This is a consequence of lack of recruitment of myelomonocytic cells for if such recruitment is prevented in other mouse strains by a monoclonal antibody against the complement type 3 receptor (Rosen and Gordon, 1987) axon degeneration is again slowed. We have also, surprisingly, found that nerve regeneration in the C57BL/6/Ola mice is not impeded by the presence of largely intact axons in the distal stump and absence of recruited cells, myelin debris and the absence of Schwann cell mitosis.

Histological and electrophysiological analysis of the peripheral nerve allografts using an immunosuppressive agent

In peripheral nerve allografts, use of an immunosuppressive agent is one of the ways of reducing nerve rejection. FK506 is a newly discovered substance, extracted from Streptomyces tsukubaensis, and has strong immunosuppressive effects. In the present study, immunosuppressive effects of FK 506 were examined using allografts of rat sciatic nerves. Good nerve regeneration was demonstrated through 12 weeks in this model. The immunosuppressed group gained weight over the course of the experiment. Another study was performed to observe the histological changes caused by ceasing the administration of FK506. Administration of FK506 was terminated 12 weeks after grafting. At 8 weeks after cessation, cellular infiltration and large unmyelinated axons were observed in the extended subperineurial space of grafts. At 12 weeks, histological characteristics of rejection were not observed. In the electrophysiological study, the temporal dispersions were recorded at 4 and 8 weeks. However, the normal electrophysiological waves were recorded at 12 weeks after cessation. It was concluded that FK506 is effective for preventing rejection of nerve allografts without any serious side effects on rats, and findings of total rejection of grafts were not recognized after ceasing the administration of FK 506. In peripheral nerve allografts, short-term administration of an immunosuppressive agent is sufficient to lead to good nerve regeneration.

Endoneurial remodeling by TNFalph- and TNFalpha-releasing proteases. A spatial and temporal co-localization study in painful neuropathy

Peripheral nerve injury causing Wallerian degeneration results in endoneurial remodeling initiated by an increase in tumor necrosis factor-alpha (TNF), which is activated from its precursor by extracellular proteases of the matrix metalloproteinase (MMP) family. We used immunohistochemistry to analyze the distribution of TNF, TNF-releasing MMPs, including gelatinases (MMP-2 and MMP-9), and TNF-alpha converting enzyme (TACE) in painful neuropathy caused by chronic constriction injury of rat sciatic nerve. Tissue was analyzed at the injury site and in the corresponding L4 and L5 dorsal root ganglia (DRG) throughout the time-course of the neuropathy. Using confocal laser scanning microscopy, we co-localized TNF with each MMP, and observed spatial and temporal distinction in their distribution. TNF co-localized in vessel endothelium with MMP-2 and in macrophages with MMP-9 and TACE at the period of active immune cell migration. TNF co-localized with myelin degrading MMP-9 within Schwann cells during demyelination, and intraaxonally during remyelination. These studies were performed to explore the role of basal-lamina degrading gelatinases and other TNF-releasing MMPs in TNF-mediated Wallerian degeneration. The data provided in this study may be useful in designing selective therapy for painful neuropathy using synthetic hydroxamate MMP inhibitors.

Muscle autografts in nerve gaps. Pattern of regeneration and myelination in various lengths of graft: an experimental study in guinea pigs

Denatured muscle autograft contains large inner basement membrane tubes, which are anatomically and chemically similar to peripheral nerve basement membrane tubes. These autografts can be used for bridging nerve gaps. In 30 Duncan-Hartley guinea pig sciatic nerves, experimentally induced gaps of 5 mm, 10 mm, and 15 mm were bridged with freeze-thawed gluteus maximus muscle autografts. The results were studied for up to 12 weeks. Functional and histological criteria, morphometry, and electron microscopy were used for the evaluation of regeneration and myelination. Functional recovery was seen by 12 weeks in 5-mm grafts. Morphometric study of 1-microm semithin sections was carried out, and the number of axons in each representative field was studied, as the percentage of myelinated and unmyelinated fibers; the thickness of myelin was also measured. The percentage of myelinated and unmyelinated fibers was estimated by transmission electron microscopy (TEM). Axonal repopulation in the distal segment was seen by the third week in the 5-mm graft, and myelination was more extensive in this group in comparison to the others. On morphometric analysis, the mean fiber diameter in the distal part of the graft was 3.81 microm in the 5-mm graft and 2.9 microm in the 15-mm graft at 12 weeks. The results of this experiment prove that the length of the graft is an important consideration for muscle autografts when they are used for mixed peripheral nerve repair.

Deposition of the NG2 proteoglycan at nodes of Ranvier in the peripheral nervous system

The node of Ranvier is a complex macromolecular assembly of ion channels and other proteins that is specialized for the rapid propagation of the action potential. A full understanding of the processes responsible for the assembly and maintenance of the node requires first the identification and characterization of the proteins found there. Here we show that NG2, a structurally unique chondroitin sulfate proteoglycan, is a molecular component of the node of Ranvier in the peripheral nervous system. In adult sciatic nerve, NG2 is (1) associated with thin, elongated fibroblast-like cells, (2) on some but not all basal laminae, and (3) at nodes of Ranvier. At the nodes, NG2 is restricted to the nodal gap and is absent from the paranodal or juxtaparanodal region. In dissociated cell cultures of adult sciatic nerve, perineurial fibroblasts but not Schwann cells express NG2 on their surfaces. Approximately 45% of the total NG2 in peripheral nerves is in a soluble, rather than particulate, subcellular compartment. NG2 is also present in membrane fractions that also contain high levels of voltage-dependent sodium channels, caspr, and neuron-glia related cell adhesion molecule. These medium-density membranes likely correspond to the nodal and paranodal region of the axon-Schwann cell unit. These results suggest a model in which perineurial fibroblasts secrete or shed NG2, which subsequently associates with nodes of Ranvier. The growth-inhibitory and anti-adhesive properties of NG2 may limit the lateral extension of myelinating Schwann cells as nodes mature. NG2 may also participate in the barrier functions of the perineurial linings of the nerve.

The spatiotemporal relationship among Schwann cells, axons and postsynaptic acetylcholine receptor regions during muscle reinnervation in aged rats

To morphologically define the aging-related features during muscle reinnervation the spatiotemporal relationships among the major components of the neuromuscular junctions (NMJs) were investigated. A total of 64 rats, 30 adults (4 months old) and 34 aged adults (24 months old), were used. Between 1 and 12 weeks after sciatic nerve-crushing injury, cryosections of skeletal muscle were single or double labeled for S100, a marker of Schwann cells (SCs), for protein gene product 9.5, a neuronal marker, and for alpha-bungarotoxin (alpha-BT), a marker of the acetylcholine receptor site (AChR site), and then observed by confocal laser microscopy. The most obvious age changes were noted: (1) the regenerating SCs and axons were delayed in their arrival at the NMJ, (2) the dimensions of terminal SCs and AChR sites displayed a drastic and long-lasting drop (for terminal SCs, during 1-8 weeks; for AChR sites, during 1-12 weeks); (3) the degree of spatial overlap between AChR sites and terminal SCs was markedly low until 8 weeks post-crush; (4) damage and poor formation in the SCs, terminal axons and AChR sites, together with poor process extension from the terminal SC or terminal axon, were pronounced; (5) persistent aberrant changes, such as multiple innervation and terminal axon sprouting, together with poorly formed collateral innervation, nerve bundles, and NMJs, more frequently occurred in the later reinnervation period. Thus, with aging, regeneration is impaired during the period in which regenerating SC strands and axons extend into NMJs and the subsequent establishment of nerve-muscle contact is in progress. A complex set of morphological abnormalities between or among the TSCs, terminal axons, and AChR sites may be important in slowing of regeneration and reinnervation in aged motor endplates.

Axonal regeneration into acellular nerve grafts is enhanced by degradation of chondroitin sulfate proteoglycan

Although the peripheral nerve has the potential to regenerate after injury, degenerative processes may be essential to promote axonal growth into the denervated nerve. One hypothesis is that the nerve contains growth inhibitors that must be neutralized after injury for optimal regeneration. In the present study, we tested whether degradation of chondroitin sulfate proteoglycan, a known inhibitor of axon growth, enhances the growth-promoting properties of grafts prepared from normal donor nerves. Excised segments of rat sciatic nerve were made acellular by freeze-killing before treatment with chondroitinase ABC. Chondroitinase-dependent neoepitope immunolabeling showed that chondroitin sulfate proteoglycan was thoroughly degraded throughout the treated nerve segments. In addition, neuronal cryoculture assays revealed that the neurite-promoting activity of acellular nerves was significantly increased by chondroitinase treatment. Control and chondroitinase-treated acellular nerves were then used as interpositional grafts in a rat nerve injury model. Axonal regeneration into the grafts was assessed 4 and 8 d after implantation by growth-associated protein-43 immunolabeling. At both time points, the number of axons regenerating into acellular grafts treated with chondroitinase was severalfold greater than in control grafts. Growth into the chondroitinase-treated grafts was pronounced after only 4 d, suggesting that the delay of axonal growth normally associated with acellular grafts was attenuated as well. These findings indicate that chondroitinase treatment significantly enhanced the growth-promoting properties of freeze-killed donor nerve grafts. Combined with the low immunogenicity of acellular grafts, the ability to improve axonal penetration into interpositional grafts by preoperative treatment with chondroitinase may be a significant advancement for clinical nerve allografting.

Degeneration and regeneration of the lip mucosal epithelium after cryo treatment in mice

The process of degeneration and regeneration of the lip mucosal epithelium after cryo treatment was observed by transmission electron microscopy. The epithelial cells were degenerated by the formation of ice crystals and subsequently detached from the basement membrane, forming a blister cavity. The separation occurred between the epithelial cells and the lamina densa, leaving a small amount of cell debris on the lamina densa. The surviving cells at the periphery of the blister cavity, especially the cells in the basal half of the epithelium, provided the regeneration cells. They migrated over the cell debris, attached to the lamina densa and gradually phagocytozed it. Finally, they formed hemidesmosomes with the old lamina densa. The connections between the epithelial cells by desmosomes were so tight that desmosomes were preserved even between dead cells and between dead and living cells. Regenerating cells were moving in a multilayered form, remaining connected to each other by the dosmosomes. They were seen to divide by mitosis and thereby increase the number of the cell layer, whilst maintaining their connections with the neighbouring cells.

Axonal elongation through long acellular nerve segments depends on recruitment of phagocytic cells from the near-nerve environment. Electrophysiological and morphological studies in the cat

The distal nerve stump plays a central role in the regeneration of peripheral nerve but the relative importance of cellular and humoral factors is not clear. We have studied this question by freezing the tibial nerve distal to a crush lesion in cat. The importance of constituents from the near-nerve environment was assessed by modification of the contact between the tibial nerve and the environment. Silicone cuffs, containing electrodes for electrophysiological assessment of nerve regeneration, were placed around the tibial nerve distal to the crush site. The interaction between long acellular frozen nerve segments (ANS) and the near-nerve environment was ascertained by breaching the silicone cuff to allow access of cellular or humoral components. Tibial nerves were crushed and frozen for 40 mm and enclosed in nerve cuffs with 0.45-microm holes or 2.0-mm holes to allow access of humoral factors or tissue ingrowth, respectively. In a second set of experiments, tibial nerves were crushed and either frozen for 20+20 mm, leaving a 10 mm segment with viable cells in the center (stepping-stone segment) or frozen for 50 mm. These nerves were enclosed in cuffs with 2.0 mm holes corresponding to the viable nerve segment. The regeneration was monitored electrophysiologically by implanted electrodes and after 2 months the nerves were investigated by light and electron microscopy. The results indicate that soluble substances in the near-nerve environment, such as nutrients, oxygen or tropic substances did not exert any independent beneficial effect on the outgrowing axons. However, phagocytic cells entering the acellular segment from the near-nerve environment were crucial for axonal outgrowth in long ANS.

Grafting of detergent-denatured skeletal muscles provides effective conduits for extension of regenerating axons in the rat sciatic nerve

The basal laminae of muscle fibers, when treated by denaturing methods including freeze thawing, have been used as conduits for regenerating nerves. In this study, we developed a new method for denaturing skeletal muscle fibers through treatment with a biological detergent, sodium dodecyl sulfate. Laminin and type IV collagen proteins of muscle fiber basal laminae were preserved after the detergent treatment. A segment of detergent-denatured muscle was grafted to a 1-cm defect of the rat sciatic nerve. One week after grafting, regenerating axons immunostained for neurofilaments were seen extending within laminin-positive muscle fiber basal lamina tubes. Four weeks after grafting, numerous myelinated axons at a much higher level than the control unoperated sciatic nerve, were found in the middle of the graft. They were smaller in diameter than those in the control nerve. Distal host nerves were well reinnervated 4 weeks after grafting. These findings suggest that the basal laminae of detergent-denatured muscle fibers provide effective conduits for regenerating axons.

Differential response of sensory and motor axons in nerve allografts after withdrawal of immunosuppressive therapy

OBJECT

Rejection of nerve allografts and loss of regenerated host axons after withdrawal of immunosuppressive therapy poses an ongoing challenge in peripheral nerve repair. The present report is of a blinded prospective controlled study in which an established rat model of nerve allotransplantation is used to examine the effect of fiber type on survival and degeneration of nerve allografts after discontinuation of immunosuppression. The authors hypothesized that sensory axons will selectively resist a rejection response, whereas motor axons will degenerate.

METHODS

Four-centimeter nerve segments from ACI rats were grafted into peroneal and sural (mixed) or saphenous (sensory) nerve gaps in Lewis rats. In some rats, L4-6 dorsal root ganglia were ablated before grafting, creating pure motor sural and peroneal nerves. All rats received 12 weeks of immunosuppressive therapy to support nerve regeneration into allografts. Immunosuppression with cyclosporin was then withdrawn. At planned death (12-18 weeks postsurgery), graft tissue was subjected to histomorphometric analysis for evaluation of axon survival and loss. Graft rejection led to loss of all axons in approximately 60% of the allograft segments. The mixed nerve group was most prone to complete rejection, with significantly lowered axon counts at Weeks 16 and 18 compared with the Week 12 baseline. Axons from the sensory nerve were least likely to degenerate. The pure motor nerve group axons demonstrated intermediate sensitivity, with a selective loss of larger axons at Week 16 and a significant decrease in axon counts from the Week 12 baseline at Week 18.

CONCLUSIONS

Whereas the majority of axons are lost after withdrawal of immunosuppressive therapy from nerve allografts, there is a selective survival of axons from cutaneous sensory nerves and smaller-diameter motor fibers. The biological and molecular mechanisms that make some axons impervious to injury remain to be determined.

The use of conventional and invaginated autologous vein grafts for nerve repair by means of entubulation

Nerve repair by entubulation has re-emerged recently as a possible means of enhancing the microenvironment at the site of repair by inclusion within the tube of various trophic factors. To this end, a modification of the vein-graft technique has been used by turning it inside out before repair, to expose the adventitial surface to the regenerating axons. A comparative study of standard vein grafting versus the inside-out technique was carried out in two equal-sized groups of inbred Lewis rats. Jugular vein isografts were derived from litter mates. The sciatic nerve was transected and repaired by entubulation using the standard vein graft in one group and the inside-out graft in the other group. Morpho-metric and electrophysiological assessment were carried out 3 months after repair. When the animals were assessed it was found that both the standard-vein-graft group and the inside-out group exhibited a reduction in all of the morphometric and electrophysiological variables when compared to normal nerves. The mean axon diameter, fibre diameter and myelin sheath thickness were, however, found to be greater in the group that underwent the inside-out repair. The superior morphometric results seen in the inside-out group were not matched by improved electrophysiological performance. It is concluded that the use of the inside-out technique confers no functional benefit over standard vein grafting.

Prevention of death of axotomized hypoglossal neurones and promotion of regeneration by chitin grafting

1. Chitin is known to promote skin wound healing. In this study, chitin, prepared from Zuwai crab shell, was used as a bridge between the proximal and distal stumps of cut hypoglossal nerves in shrews. We compared the effects of chitin on the regeneration of transected right hypoglossal nerve axons, with those of porcine dermis, bovine dermal aterocollagen, and autologous nerve bundles. 2. To assess the survival of neurones, the size of neuronal cell body, and number of motoneurones were determined in the absence of any bridged material and in the presence of porcine dermis, bovine dermal aterocollagen, chitin, or autologous nerve bundles as a bridge. 3. Our results revealed a significantly better outcome in chitin and autologous nerve bridged groups; the size of neuronal cell body and number of hypoglossal neurones were higher than in the other groups. Chitin also enhanced the regeneration of neurones; the number of horseradish peroxide positive neurones indicative of repaired axonal processes was significantly higher in chitin and autologous nerve-bridged groups than in other groups. 4. Our results demonstrated that the use of chitin sheet or autograft successfully prevented the death of severed neurones and promoted the regeneration of the lesioned nerve. Although the mechanisms underlying the effects of chitin are still unknown, chitin seems to be a potentially useful biocompatible material for nerve repair and regeneration.

Enhancement of rat hypoglossal nerve regeneration by chitin sheet plus gangliosides

The effects of chitin sheet interposition with and without brain gangliosides on the regeneration of hypoglossal nerve fibres was studied in the rat following resection of a 5mm length of the nerve. At 10 weeks after operation, the number of horseradish peroxidase (HRP)-labelled motor neurones, indicative of the axonal repair process, on the side treated with chitin and gangliosides was higher than on the control side (where 5mm of the nerve was simply resected). The ratios of HRP-positive neurones in the right hypoglossal nucleus (treated side)/left hypoglossal nucleus (intact side) was 0 in the 5mm-resected group, 53% in the chitin-grafted group, 88% in the ganglioside (0.2 microg)-injected group, 90% in the ganglioside (2 microg)-injected group, 91% in the chitin with ganglioside (0.2mg)-injected group, 91% in the chitin with ganglioside (2 microg)-injected group and 85% in the autograft group, respectively. There were significant differences between the 5 mm-resected group and chitin-grafted group, ganglioside-injected group, chitin with ganglioside group and autograft group, and between the chitin-grafted group and ganglioside-injected, chitin with ganglioside and autograft groups (P< 0.005, respectively). Our results indicated that the use of chitin and gangliosides stimulated the regeneration of severed motor nerve fibres. These findings suggest that chitin and gangliosides might be therapeutically useful for treatment of neuronal degeneration.

Regeneration of mouse lip epidermis after cryo treatment. Hemidesmosome formation and HSPG (heparan sulfate proteoglycan) distribution in basement membrane

The processes of degeneration and the regeneration of the lip epidermal cells was observed by electron microscopy, focussing on the substance and the structure of the lamina lucida, on which regenerating cells migrated. After the repetitive freezing and thawing treatment, epidermal cells degenerated and detached from the dermis. The separation occurred between the epidermal cells and the basement membrane, leaving a small amount of cell debris on the lamina densa. After the separation of the epidermis, there were some thick parts in the lamina densa which appeared to be the part below hemidesmosomes. Regenerating epidermal cells migrated from the nondegenerated area along the cellular surface of the old lamina densa. They migrated over the cell debris which was gradually phagocitized, and formed new hemidesmosomes with the old lamina densa. Regenerating epidermal cells did not make close contact with the old lamina densa during their migration, but there was a clear space in between, indicating that some of the materials and the structure of the lamina lucida of the old basement membrane was preserved. By immunoelectron microscopy using anti-HSPG (heparan sulfate proteoglycan) antibody, it became clear that after the epidermal separation, HSPG was preserved in the basement membrane to some extent, especially in the thick parts of the lamina densa located below. The immunoelectron micrographs support the view that hemidesmosomes may reform at the previous locations at the old lamina densa.

Experimental median nerve repair by fresh or frozen nerve autografts and xenografts

The authors described the reconstruction of a terminal branch of the brachial plexus (the median nerve) by different kinds of peripheral nerve grafts, in rats. Fresh or frozen autografts from Sprague-Dawley rats and fresh or frozen xenografts from Beagle dogs were used. Three, six, nine and twelve months after grafting, rats underwent histological assessment (muscle, nerve and spinal cord) and simple functional assessment by the grasping test. The immune reaction was prevented by the freezing and thawing method that had rendered xenografts acellular. This process allowed a satisfactory reinnervation of the flexor carpi radialis muscle (FCR) and a function recovery about 75% of control value. Nevertheless, the force recovery in rats that received frozen grafts was slower than those received fresh autografts. Probably, the destruction of cellular elements by freezing produced a deficient environment for nerve regeneration. However, this gap was partially compensated at twelve months after surgery by the maturation and the secondary adaptation of regenerated nerve fibers. Theses results showed that the force recovery is directly correlated to the capability of the nerve fibers to reproduce, histologically, a next to normal nerve pattern.

Experimental study on nerve regeneration through the basement membrane tubes of the nerve, muscle, and artery

We evaluated neurotization after transplantation with lyophilized nerves, muscles, and arteries, and examined the possibility of practical application of long bridging grafts. Grafts of 10 mm and 25 mm of lyophilized nerves, muscles, and arteries harvested from Fisher rats were transplanted to the sciatic nerves of recipient Lewis rats. The histological changes undergone by short grafts were observed at weekly intervals. The sham-operated and isograft groups were used to compare the results of long grafts. In both the nerve and muscle-graft group, regenerated axons grew out through the residual basement membrane tube. But in the muscle graft group, phagocytosis of myofibril debris took longer than that of degenerated axons. No statistical differences were found between results of TSI, induced EMG, and quantitative analysis of myelinated axons in the nerve and muscle graft groups. No neurotization was noted in the long artery graft. In long grafts, laminin found on the basement membrane may not be sufficient to accelerate neurotization, and arteries should not be used for tubulization.

Regeneration across cold preserved peripheral nerve allografts

The feasibility of peripheral nerve allograft pretreatment utilizing cold storage (5 degrees C in the University of Wisconsin Cold Storage Solution) or freeze-thawing to prevent rejection was investigated. Regeneration across cold-stored (3 or 5 weeks) or freeze-thawed (FT), 3.0-cm sciatic nerve allografts were compared to fresh auto- and allografts in an inbred rat model. At 16-week post-engraftment, only FT allografts appeared similar to autografts on gross inspection; FT grafts were neither shrunken nor adherent to the surrounding tissue as seen in the other allograft groups. Qualitatively, the pattern of regeneration in the graft segments of the fresh allograft and to a lesser extent of pretreated allografts was inferior to that of autografts as evidenced by a disruption in the perineurium, more extrafascicular axons, smaller and fewer myelinated axons, increased intrafascicular collagen deposition, and the persistence of perineurial cell compartmentation and perivascular infiltrates. Distal to these grafts, the regeneration became more homogenous between groups, although areas of ongoing Wallerian degeneration, new regeneration as well as compartmentation, were more prevalent in fresh and pretreated allografts. Although the number of myelinated fibres was equivalent to autografts, the fibre diameters, the number of large diameter fibres, and the G-ratio were significantly decreased in the allograft groups, which, in part, accounted for the significant decrease in conduction velocity in the 3-week stored and fresh allograft, and the slight decrease in the 5-week stored and FT allograft groups. There was a small return in the Sciatic Function Index towards normal, but no consistent differences between groups were found. Prolonged cold storage and freeze-thawing of nerve allografts resulted in regeneration that was better than fresh allografts, but inferior to autografts. With the concomitant use of host immunosuppression or other immunotherapies, these storage techniques can provide a means of transporting nerve allografts between medical centres and for converting urgent into elective procedures.