Nerve regeneration through the cryoinjured allogeneic nerve graft in the rabbit

Evaluation of collimated polarized light imaging for real-time intraoperative selective nerve identification in the human hand

Intraoperative peripheral nerve lesions are common complications due to misidentification and limitations of surgical nerve identification. This study validates a real-time non-invasive intraoperative method of nerve identification. Long working distance collimated polarized light imaging (CPLi) was used to identify peripheral radial nerve branches in a human cadaver hand by their nerve specific anisotropic optical reflection. Seven ex situ and six in situ samples were examined for nerves, resulting after histological validation, in a 100% positive correct score (CPLi) versus 77% (surgeon). Nerves were visible during a clinical in vivo observation using CPLi. Therefore CPLi is a promising technique for intraoperative nerve identification.

Cauda equina-derived extracellular matrix for fabrication of nanostructured hybrid scaffolds applied to neural tissue engineering

Extracellular matrix (ECM) components have become important candidate materials for use as neural scaffolds for neural tissue engineering. In the current study, we prepared cauda equina-derived ECM materials for the production of scaffolds. Natural porcine cauda equina was decellularized using Triton X-100 and sodium deoxycholate, shattered physically, and made into a suspension by differential centrifugation. The decellularization procedure resulted in the removal of >94% of the nuclear material and preserved the extracellular collagen and sulfated glycosaminoglycan. Immunofluorescent staining confirmed the presence of collagen type I, laminin, and fibronectin in the ECM. The cauda equine-derived ECM was blended with poly(l-lactide-co-glycolide) (PLGA) to fabricate nanostructured scaffolds using electrospinning. The incorporation of the ECM increased the hydrophilicity of the scaffolds. Fourier transform infrared spectroscopy and multiphoton-induced autofluorescence images showed the presence of the ECM in the scaffolds. ECM/PLGA scaffolds were beneficial for the survival of Schwann cells compared with scaffolds consisting of PLGA alone, and the aligned fibers could regulate cell morphologic features by modulating cellular orientation. Axons in the dorsal root ganglia explants extended to a greater extent along ECM/PLGA compared with PLGA-alone fibers. The cauda equina ECM might be a promising material for forming scaffolds for use in neural tissue engineering.

Experimental and clinical evidence for use of decellularized nerve allografts in peripheral nerve gap reconstruction

Despite the inherent capability for axonal regeneration, recovery following severe peripheral nerve injury remains unpredictable and often very poor. Surgeons typically use autologous nerve grafts taken from the patient's own body to bridge long nerve gaps. However, the amount of suitable nerve available from a given patient is limited, and using autologous grafts leaves the patient with scars, numbness, and other forms of donor-site morbidity. Therefore, surgeons and engineers have sought off-the-shelf alternatives to the current practice of autologous nerve grafting. Decellularized nerve allografts have recently become available as an alternative to traditional nerve autografting. In this review, we provide a critical analysis comparing the advantages and limitations of the three major experimental models of decellularized nerve allografts: cold preserved, freeze-thawed, and chemical detergent based. Current tissue engineering-based techniques to optimize decellularized nerve allografts are discussed. We also evaluate studies that supplement decellularized nerve grafts with exogenous factors such as Schwann cells, stem cells, and growth factors to both support and enhance axonal regeneration through the decellularized allografts. In examining the advantages and disadvantages of the studies of decellularized allografts, we suggest that experimental methods, including the animal model, graft length, follow-up time, and outcome measures of regenerative progress and success be consolidated. Finally, all clinical studies in which decellularized nerve allografts have been used to bridge nerve gaps in patients are reviewed.

Wallerian degeneration and peripheral nerve conditions for both axonal regeneration and neuropathic pain induction

Wallerian degeneration is a cascade of stereotypical events in reaction to injury of nerve fibres. These events consist of cellular and molecular alterations, including macrophage invasion, activation of Schwann cells, as well as neurotrophin and cytokine upregulation. This review focuses on cellular and molecular changes distal to various types of peripheral nerve injury which simultaneously contribute to axonal regeneration and neuropathic pain induction. In addition to the stereotypical events of Wallerian degeneration, various types of nerve damage provide different conditions for both axonal regeneration and neuropathic pain induction. Wallerian degeneration of injured peripheral nerve is associated with an inflammatory response including rapid upregulation of the immune signal molecules like cytokines, chemokines and transcription factors with both beneficial and detrimental effects on nerve regeneration or neuropathic pain induction. A better understanding of the molecular interactions between the immune system and peripheral nerve injury would open the possibility for targeting these inflammatory mediators in therapeutic interventions. Understanding the pleiotropic effects of cytokines/chemokines, however, requires investigating their highly specific pathways and precise points of action.

Repair of extended peripheral nerve lesions in rhesus monkeys using acellular allogenic nerve grafts implanted with autologous mesenchymal stem cells

Despite intensive efforts in the field of peripheral nerve injury and regeneration, it remains difficult in humans to achieve full functional recovery following extended peripheral nerve lesions. Optimizing repair of peripheral nerve injuries has been hindered by the lack of viable and reliable biologic or artificial nerve conduits for bridging extended gaps. In this study, we utilized chemically extracted acellular allogenic nerve segments implanted with autologous non-hematopoietic mesenchymal stem cells (MSCs) to repair a 40 mm defect in the rhesus monkey ulnar nerve. We found that severely damaged ulnar nerves were structurally and functionally repaired within 6 months following placement of the MSC seeded allografts in all animals studied (6 of 6, 100%). Furthermore, recovery with the MSC seeded allografts was similar to that observed with Schwann cell seeded allografts and autologous nerve grafts. The findings presented here are the first demonstration of the successful use of autologous MSCs, expanded in culture and implanted in a biological conduit, to repair a peripheral nerve gap in primates. Given the difficulty in isolating and purifying sufficient quantities of Schwann cells for peripheral nerve regeneration, the use of MSCs to seed acellular allogenic nerve grafts may prove to be a novel and promising therapeutic approach for repairing severe peripheral nerve injuries in humans.

Morphological Variations in Brachial Plexus of Beagle Dogs: Evaluation of Utility as Sources of Allogeneic Nerve Grafts

Basic studies were carried out to apply frozen allogeneic nerve grafts in dogs after wide-ranging defects of the brachial plexus due to surgical resection of tumor. In this study, morphological variations in branching patterns of the brachial plexus were examined in ten beagle dogs, to evaluate whether the brachial plexus might represent a useful source of allogeneic nerve grafts. Spatial relationships between the axillary lymph node, which had the possibility of carcinomatous metastasis, and the musculocutaneous (MC) nerve, which was important for the function of the forelimbs, were also investigated. In all ten cases examined, the brachial plexus received ventral roots from the fifth cervical nerve to the first thoracic nerve. No significant variation in the branching pattern was found in any nerve except the phrenic, MC and dorsal thoracic nerves. Four communicating branches were observed and had some morphological variations which might be negligible for nerve grafting. Considering previous physiological and anatomical reports, the most important nerve to be reunited in graft operations for functional recovery is the radial nerve. The MC nerve and median or ulnar nerve should also be considered as possibilities for reuniting. Distances between the axillary lymph nodes and the MC nerve ranged from 11.2 mm to 21 mm (mean +/- SD: 16.1 +/- 2.3 mm). In conclusion, it was suggested that morphological variations in the brachial plexus were technically acceptable to apply allogeneic nerve grafts at least in beagle dogs.

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.

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.

Cold preserved nerve allografts: changes in basement membrane, viability, immunogenicity, and regeneration

Rat sciatic nerve graft segments were harvested and pretreated by either placement in the University of Wisconsin Cold Storage Solution at 5 degrees C and storage from 1 to 26 weeks, or repeatedly freezing (-40 degrees C) and thawing (20 degrees C). Following pretreatment, grafts were transplanted as either syngeneic or allogeneic nerve grafts. Storage and freeze-thawing did not affect the Schwann cell basal lamina or laminin distribution of the peripheral nerve. Graft cell viability decreased with increasing time of storage, with some viable cells detectable even after 3 weeks of storage. Freeze-thawed grafts were not viable. Increasing time of storage led to decreasing immune response and graft rejection, but improved regeneration. Freeze-thawed and 26-week stored allografts were nonimmunogenic and rejection was not seen, but regeneration was delayed compared to autografts. Graft storage may become a useful adjunct to clinical nerve allografting to permit elective scheduling of surgery, provide greater time for preoperative tissue testing, and possibly blunt the immune response.

Long acellular nerve transplants for allogeneic grafting and the effects of basic fibroblast growth factor on the growth of regenerating axons in dogs: a preliminary report

Sciatic nerves were excised from 3 beagle dogs about 5 h after their sacrifice, treated three times by freezing and thawing, and stored in physiological saline for 3 months at -20 degrees C until used. Nerve segments 5 cm in length prepared from these stored nerves were transplanted to the common peroneal nerve in the right hindlimb of beagle dogs. Sixteen beagle dogs in total were used, in four treatment groups of two pairs each studied at 1 and 3 months. Five-hundred microliters basic fibroblast growth factor (bFGF) of two different concentrations (10 micrograms/300 microliters and 100 micrograms/300 microliters) which were impregnated in 0.5 ml gelatin hydrogels was applied around the sutured allografts. Autografting was also done in 4 beagle dogs, with no bFGF application. One month after the grafting, no regenerating nerves extended beyond the middle of the transplant in any of the allografts, except in the autografts in which a number of regenerated (myelinated) axons were present. Three months after the grafting, an abundance of myelinated axons was found at the middle of the graft: the numbers of axons per 10(4) micron 2 were 22.6 in the autografts and 10.6, 10.4 and 19.2 in the allografts treated with no bFGF, low-dose bFGF, and high-dose bFGF, respectively. Regenerating axons extended into the host nerve: the numbers of myelinated axons at the level 1.5 cm distal to the distal suture were 35.7, 0.9, 3.8, and 12.1 per 10(4) micron 2 in the above respective order. Although it was inferior in quality to the autograft, peripheral nerve regeneration was extensive in the distal nerve using freeze-thawed and bFGF-treated allografts at 3 months. Electromyography showed that the peroneus longus muscle responded to the electrical stimuli given at the site proximal to the transplant in all four groups. These data indicate that a 5-cm acellular nerve segment containing Schwann cell basal laminae can be used successfully as an allograft without any immunosuppressants and that exogenously applied bFGF can improve nerve regeneration by enhancing the growth of regenerating axons.

Axonal elongation through acellular nerve segments of the cat tibial nerve: importance of the near-nerve environment

Peripheral nerve regeneration is considered to be influenced by structural, cellular and humoral factors in the distal nerve stump. Axonal elongation was, however, not affected by the presence of a 20 mm acellular nerve segment (ANS) distal to a crush lesion in a cat tibial nerve which was shielded from the environment by a silicone cuff [K. Fugleholm, H. Schmalbruch, C. Krarup, Early peripheral nerve regeneration after crushing, sectioning, and freeze studied by implanted electrodes in the cat, J. Neurosci., 14 (1994) 2659-2673]. In the present study axons were challenged to regenerate through crush lesions combined with 30-, 40-, 50-, 60- and 70-mm ANSs. For 30- and 40-mm ANSs, the nerves were shielded by impermeable silicone cuffs containing electrodes for electrophysiological evaluation of axonal elongation. All nerves were examined histologically by light microscopy 9 weeks after the lesion. The elongation through the shielded 30-mm ANS was slower than through a shielded nerve segment with viable cells. In the isolated 40-mm ANS, incomplete Wallerian degeneration and lack of blood vessels were observed, and axonal elongation was severely impaired. Regeneration across 40-70 mm non-shielded ANSs was intact and there was no relation between the number of regenerated fibers and the length of the ANS. There was no reduction in the number of blood vessels in the non-isolated ANSs. The results suggest that regeneration through an isolated acellular nerve segment exceeding 30 mm depends on cellular and humoral support from the near-nerve environment. Thus, the near-nerve environment is crucial for regeneration through long ANSs, and the importance of humoral, cellular and vascular support is discussed.

Peripheral nerve regeneration

Peripheral nerve regeneration comprises the formation of axonal sprouts, their outgrowth as regenerating axons and the reinnervation of original targets. This review focuses on the morphological features of axonal sprouts at the node of Ranvier and their subsequent outgrowth guided by Schwann cells or by Schwann cell basal laminae. Adhesion molecules such as N-CAM, L1 and N-cadherin are involved in the axon-to-axon and axon-to-Schwann cell attachment, and it is suggested that integrins such as alpha 1 beta 1 and alpha 6 beta 1 mediate the attachment between axons and Schwann cell basal laminae. The presence of synaptic vesicle-associated proteins such as synaptophysin, synaptotagmin and synapsin I in the growth cones of regenerating axons indicates the possibility that exocytotic fusion of vesicles with the surface axolemma supplies the membranous components for the extension of regenerating axons. Almost all the subtypes of protein kinase C have been localized in growth cones both in vivo and in vitro. Protein kinase C and GAP-43 are implicated to be involved in at least some part of the adhesion of growth cones to the substrate and their growth activity. The significance of tyrosine kinase in growth cones is emphasized. Tyrosine kinase plays an important role in intracellular signal transduction of the growth of regenerating axons mediated by both nerve trophic factors and adhesion molecules. Growth factors such as NGF, BDNF, CNTF and bFGF are also discussed mainly in terms of the influence of Schwann cells on regenerating axons.

Distribution of protein kinase C (alpha, beta, gamma subtypes) in normal nerve fibers and in regenerating growth cones of the rat peripheral nervous system

The distribution of protein kinase C (alpha, beta, gamma subtypes) was studied using immunocytochemical techniques in normal nerve fibers and in regenerating sprouts (growth cones) from the nodes of Ranvier following crush injuries to the rat peripheral nervous system. In normal nerves, for each protein kinase C subtype, immunoreactivity was present in both myelinated and unmyelinated axons. In myelinated axons, immunoreactivity for all three subtypes was patchy in the axoplasm and diffuse in the subaxolemmal peripheral zones. No immunoreactivity was found in the microtubule and neurofilament (cytoskeletal) domain. In contrast, in unmyelinated axons, immunoreactivity was distributed diffusely in the axoplasm. Schwann cells of myelinated fibers exhibited protein kinase C immunoreactivity, but those of unmyelinated fibers did not. In regenerating nerves, early sprouts and growth cones extending through the crushed site along Schwann cell basal laminae exhibited intense immunoreactivity for all three subtypes. Immunoreactivity was distributed diffusely throughout the axoplasm of the regenerating sprouts (growth cones), in which microtubules and neurofilaments were very rare. Thus, the subcellular localization of the protein kinase C immunoreactivity in growth cones of early regenerating nerves differed from that of normal parent axons. These findings suggest that protein kinase C (alpha, beta and gamma subtypes), whose subcellular distribution becomes more extensive in regenerating axons, may have important functional roles in axonal sprouting and in the regulation of growth cone activity in the peripheral nervous system.

Nerve repair using freezing and fibrin glue: immediate histologic improvement of axonal coaptation

Despite the fine microsurgical techniques available, injuries to peripheral nerves are still a surgical problem. Sutures placed in the epineurium or perineurium cause compression, brushing, and misdirection of endoneural tissue. A technique of nerve repair using freezing to trim the nerve and fibrin glue to coat it before thawing is described. The entire surgical repair procedure is carried out with the nerve stumps frozen. The observed axonal alignment with this technique was much better than that obtained by microsuture alone.

Schwann cell migration through freeze-killed peripheral nerve grafts without accompanying axons

Freeze-dried tibial nerve grafts were anastomosed to either the proximal stump or the distal stump of severed tibial nerves in adult inbred Fischer rats. In the case of grafts attached to the proximal stump the tibial nerve was ligated three times, the most distal ligature from the spinal cord being 1 cm from the site of anastomosis. In both types of experiment Schwann cells were, therefore, free to enter the initially acellular grafts without accompanying axons. The grafts were examined 17 days to 12 weeks after operation. Immunofluorescence for S-100 protein was used to evaluate the distance migrated by the Schwann cells and electron microscopy was used to examine the morphology of the cells which invaded the grafts. Schwann cell migration was similar from the proximal and distal stumps. The migrating Schwann cells formed columns which resembled bands of Bungner. They were found mainly, but not exclusively, inside the pre-existing basal lamina tubes left behind by the killed nerve fibres. Some Schwann cells secreted a thin, patchy basal lamina even though they lacked axonal contact. Schwann cell columns became partially compartmentalized by fibroblast processes. Myelin and other debris were removed most rapidly in those parts of the grafts penetrated by large numbers of Schwann cells. The maximum distance the Schwann cells penetrated into the grafts was 8.5 mm and this was achieved by 6 to 8 weeks after operation. This is about half the maximum distance migrated by Schwann cells accompanying regenerating axons through similar grafts.(ABSTRACT TRUNCATED AT 250 WORDS)

Peripheral nerve regeneration

Schwann cell basal laminae were demonstrated to serve as efficient conduits for the growth of regenerating axons in frozen nerve grafts, and in in situ freezing experiments. Regenerating axonal sprouts usually emanated from the first node of Ranvier proximal to the site of damage, and grew out along the inner surface of the basal lamina. Early growth cones contained numerous clear vesicles of about 50 nm in diameter.