Transection of the sciatic nerve and reinnervation in adult rats: muscle and endplate morphology

Micro-nanofiber composite biomimetic conduits promote long-gap peripheral nerve regeneration in canine models

Peripheral nerve injuries may result in severe long-gap interruptions that are challenging to repair. Autografting is the gold standard surgical approach for repairing long-gap nerve injuries but can result in prominent donor-site complications. Instead, imitating the native neural microarchitecture using synthetic conduits is expected to offer an alternative strategy for improving nerve regeneration. Here, we designed nerve conduits composed of high-resolution anisotropic microfiber grid-cordes with randomly organized nanofiber sheaths to interrogate the positive effects of these biomimetic structures on peripheral nerve regeneration. Anisotropic microfiber-grids demonstrated the capacity to directionally guide Schwann cells and neurites. Nanofiber sheaths conveyed adequate elasticity and permeability, whilst exhibiting a barrier function against the infiltration of fibroblasts. We then used the composite nerve conduits bridge 30-mm long sciatic nerve defects in canine models. At 12 months post-implant, the morphometric and histological recovery, gait recovery, electrophysiological function, and degree of muscle atrophy were assessed. The newly regenerated nerve tissue that formed within the composite nerve conduits showed restored neurological functions that were superior compared to sheaths-only scaffolds and Neurolac nerve conduit controls. Our findings demonstrate the feasibility of using synthetic biophysical cues to effectively bridge long-gap peripheral nerve injuries and indicates the promising clinical application prospects of biomimetic composite nerve conduits.

Tissue-engineered spiral nerve guidance conduit for peripheral nerve regeneration

Recently in peripheral nerve regeneration, preclinical studies have shown that the use of nerve guidance conduits (NGCs) with multiple longitudinally channels and intra-luminal topography enhance the functional outcomes when bridging a nerve gap caused by traumatic injury. These features not only provide guidance cues for regenerating nerve, but also become the essential approaches for developing a novel NGC. In this study, a novel spiral NGC with aligned nanofibers and wrapped with an outer nanofibrous tube was first developed and investigated. Using the common rat sciatic 10-mm nerve defect model, the in vivo study showed that a novel spiral NGC (with and without inner nanofibers) increased the successful rate of nerve regeneration after 6 weeks recovery. Substantial improvements in nerve regeneration were achieved by combining the spiral NGC with inner nanofibers and outer nanofibrous tube, based on the results of walking track analysis, electrophysiology, nerve histological assessment, and gastrocnemius muscle measurement. This demonstrated that the novel spiral NGC with inner aligned nanofibers and wrapped with an outer nanofibrous tube provided a better environment for peripheral nerve regeneration than standard tubular NGCs. Results from this study will benefit for future NGC design to optimize tissue-engineering strategies for peripheral nerve regeneration.

STATEMENT OF SIGNIFICANCE

We developed a novel spiral nerve guidance conduit (NGC) with coated aligned nanofibers. The spiral structure increases surface area by 4.5 fold relative to a tubular NGC. Furthermore, the aligned nanofibers was coated on the spiral walls, providing cues for guiding neurite extension. Finally, the outside of spiral NGC was wrapped with randomly nanofibers to enhance mechanical strength that can stabilize the spiral NGC. Our nerve histological data have shown that the spiral NGC had 50% more myelinated axons than a tubular structure for nerve regeneration across a 10 mm gap in a rat sciatic nerve. Results from this study can help further optimize tissue engineering strategies for peripheral nerve repair.

Enhanced Peripheral Nerve Regeneration by a High Surface Area to Volume Ratio of Nerve Conduits Fabricated from Hydroxyethyl Cellulose/Soy Protein Composite Sponges

Multichannel nerve guide conduits (MCNGCs) have been widely studied and exhibited outstanding nerve repair function. However, the effect of the geometric structure of MCNGCs on the nerve repair function was still not clear. Herein, we postulated that MCNGCs with different inner surface area-to-volume ratios (ISA/V) of the channels inside the nerve guide conduits (NGCs) would show different nerve repair functions. Therefore, in current work, we constructed a series of hydroxyethyl cellulose/soy protein sponge-based nerve conduit (HSSN) with low, medium, and high ISA/V from hydroxyethyl cellulose (HEC)/soy protein isolate (SPI) composite sponges, which were abbreviated as HSSN-L, HSSN-M and HSSN-H, respectively. These NGCs were applied to bridge and repair a 10 mm long sciatic nerve defect in a rat model. Finally, the influence of ISA/V on nerve repair function was evaluated by electrophysiological assessment, histological investigation, and in vivo biodegradability testing. The results of electrophysiological assessment and histological investigation showed that the regenerative nerve tissues bridged with HSSN-H and HSSN-M had higher compound muscle action potential amplitude ratio, higher percentage of positive NF200 and S100 staining, larger axon diameter, lower G-ratio, and greater myelination thickness. Furthermore, the regenerative nerve tissues bridged with HSSN-H also showed higher density of regenerated myelinated nerve fibers and more number of myelin sheath layers. On the whole, the repair efficiency of the peripheral nerve in HSSN-H and HSSN-M groups might be better than that in HSSN-L. These results indicated that higher ISA/V based on HEC/SPI composite sponge may result in greater nerve repair functions. The conclusion provided a probable guiding principle for the structural designs of NGCs in the future.

Transplantation of Embryonic Spinal Cord Derived Cells Helps to Prevent Muscle Atrophy after Peripheral Nerve Injury

Injuries to peripheral nerves are frequent in serious traumas and spinal cord injuries. In addition to surgical approaches, other interventions, such as cell transplantation, should be considered to keep the muscles in good condition until the axons regenerate. In this study, E14.5 rat embryonic spinal cord fetal cells and cultured neural progenitor cells from different spinal cord segments were injected into transected musculocutaneous nerve of 200–300 g female Sprague Dawley (SD) rats, and atrophy in biceps brachii was assessed. Both kinds of cells were able to survive, extend their axons towards the muscle and form neuromuscular junctions that were functional in electromyographic studies. As a result, muscle endplates were preserved and atrophy was reduced. Furthermore, we observed that the fetal cells had a better effect in reducing the muscle atrophy compared to the pure neural progenitor cells, whereas lumbar cells were more beneficial compared to thoracic and cervical cells. In addition, fetal lumbar cells were used to supplement six weeks delayed surgical repair after the nerve transection. Cell transplantation helped to preserve the muscle endplates, which in turn lead to earlier functional recovery seen in behavioral test and electromyography. In conclusion, we were able to show that embryonic spinal cord derived cells, especially the lumbar fetal cells, are beneficial in the treatment of peripheral nerve injuries due to their ability to prevent the muscle atrophy.

Electrical stimulation of transplanted motoneurons improves motor unit formation

Motoneurons die following spinal cord trauma and with neurological disease. Intact axons reinnervate nearby muscle fibers to compensate for the death of motoneurons, but when an entire motoneuron pool dies, there is complete denervation. To reduce denervation atrophy, we have reinnervated muscles in Fisher rats from local transplants of embryonic motoneurons in peripheral nerve. Since growth of axons from embryonic neurons is activity dependent, our aim was to test whether brief electrical stimulation of the neurons immediately after transplantation altered motor unit numbers and muscle properties 10 wk later. All surgical procedures and recordings were done in anesthetized animals. The muscle consequences of motoneuron death were mimicked by unilateral sciatic nerve section. One week later, 200,000 embryonic day 14 and 15 ventral spinal cord cells, purified for motoneurons, were injected into the tibial nerve 10-15 mm from the gastrocnemii muscles as the only neuron source for muscle reinnervation. The cells were stimulated immediately after transplantation for up to 1 h using protocols designed to examine differential effects due to pulse number, stimulation frequency, pattern, and duration. Electrical stimulation that included short rests and lasted for 1 h resulted in higher motor unit counts. Muscles with higher motor unit counts had more reinnervated fibers and were stronger. Denervated muscles had to be stimulated directly to evoke contractions. These results show that brief electrical stimulation of embryonic neurons, in vivo, has long-term effects on motor unit formation and muscle force. This muscle reinnervation provides the opportunity to use patterned electrical stimulation to produce functional movements.

Development and evaluation of silk fibroin-based nerve grafts used for peripheral nerve regeneration

Silk fibroin (SF), derived from natural silk long used as a textile material, has recently become an important biomaterial for tissue engineering applications. We have previously reported on good in vitro biocompatibility of SF fibers with peripheral nerve tissues and cells. In the present study, we developed a novel biomimetic design of the SF-based nerve graft (SF graft) which was composed of a SF-nerve guidance conduit (NGC) inserted with oriented SF filaments. The SF-NGC prepared via well-established procedures exhibits an eggshell-like microstructure that is responsible for its superior mechanical and permeable properties beneficial to nerve regeneration. The SF graft was used for bridge implantation across a 10-mm long sciatic nerve defect in rats, and the outcome of peripheral nerve repair at 6 months post-implantation was evaluated by a combination of electrophysiological assessment, FluoroGold retrograde tracing and histological investigation. The examined functional and morphological parameters show that SF grafts could promote peripheral nerve regeneration with effects approaching those elicited by nerve autografts which are generally considered as the gold standard for treating large peripheral nerve defects, thus raising a potential possibility of using these newly developed nerve grafts as a promising alternative to nerve autografts.

The state of the contralateral neuromotor apparatus of the rat in conditions of unilateral tenotomy

The effects of unilateral transection of the Achilles tendon (tenotomy) on the electromyographic characteristics of the gastrocnemius muscle on the contralateral side were studied in rats. Motor responses, reflex responses, and motor unit spike activity in the contralateral gastrocnemius muscle were studied in baseline conditions and on afferent stimulation before and 10 days after tenotomy. Changes in the nature of motor unit spike activity and responses to afferent stimulation were observed. The results obtained here demonstrate increases in the excitability of individual motoneurons in the contralateral motor center of the gastrocnemius muscle after exclusion of the symmetrical gastrocnemius muscle from the overall pattern of movement activity.

Dog sciatic nerve regeneration across a 30-mm defect bridged by a chitosan/PGA artificial nerve graft

We have developed a dual-component artificial nerve graft comprising an outer microporous conduit of chitosan and internal oriented filaments of polyglycolic acid (PGA). The novel graft was used for bridging sciatic nerve across a 30-mm defect in six Beagle dogs, which were used as a chitosan/PGA graft group. The other Beagle dogs were divided into an autograft group (n = 6) as the positive control and a non-grafted group (n = 5) as the negative control. All animals of three groups were monitored for changes in their appearance and locomotion activities after surgery. Their posture and gait were recorded regularly with the aid of photographs and videotapes for each dog. Six months post-operatively, a combination of electrophysiological examination, FluoroGold retrograde tracing, histological assessment including light microscopy and transmission electron microscopy, immunohistochemistry as well as morphometric analyses to both regenerated nerves and target muscles was utilized to investigate the nerve repair effects of our artificial nerve graft. The results demonstrated that, in the chitosan/PGA graft group, the dog sciatic nerve trunk had been reconstructed with restoration of nerve continuity and functional recovery, and its target skeletal muscle had been re-innervated, improving locomotion activities of the operated limb. This study proves the feasibility of the chitosan/PGA artificial nerve graft for peripheral nerve regeneration by bridging a longer defect in a large animal model.

Transection of peripheral nerves, bridging strategies and effect evaluation

Disruption of peripheral nerves due to trauma is a frequently occurring clinical problem. Gaps in the nerve are bridged by guiding the regenerating nerves along autologous grafts or artificial guides. This review gives an overview on the different methods of nerve repair techniques. Conventional suturing techniques are discussed as well as the use of e.g. biological, synthetic, non-degradable or degradable nerve guides. Functional assessment showed that repair of a gap with a bio-degradable guide is superior to that with autologous grafts. But still, long lasting changes were observed in the Sciatic Function Index (SFI), abnormal walking patterns, disturbed Electro Myo Graphic (EMG) patterns, next to shifts in the histochemical properties of the muscles and longlasting abnormalities in neuromuscular contacts. These phenomena are explained by an at-random reinnervation. When transecting the nerve at young ages, this did not lead to enhanced recovery. Rearing rats operated at adult age in an enriched environment, also had no beneficial effect. Future research should aim at developing longer guides, possibly lined with Schwann cells, or additives, improving specific reinnervation of the former target areas.

Muscle differentiation after sciatic nerve transection and reinnervation in adult rats

Reinnervation after peripheral nerve transections generally leads to poor functional recovery. In order to study whether changes in muscles might be a contributing factor in this phenomenon we studied muscle morphology and fibre type distributions after sciatic nerve transection in the rat hind limb. Proximally, before the bifurcation in the tibial and common peroneal nerve, a 12 mm segment of the sciatic nerve was resected, reversed and re-implanted as an autologous nerve graft. After survival periods of 7, 15 and 21 weeks the lateral gastrocnemius, tibialis anterior and soleus muscles were dissected, stained with mATP-ase, and fibre type distributions were studied. In addition, numbers of muscle fibres were counted, and cross sectional areas were calculated. After 7 weeks, cross sectional areas were decreased in all muscles. In the gastrocnemius and tibialis anterior muscles the fibre number remained unaltered but the hypotrophy had been reversed at later ages. The number of muscle fibres in the soleus muscle remained decreased over the entire period of observation. The percentages of type II fibres in the gastrocnemius and tibialis anterior muscles were decreased at 7 and 15 weeks but these again approached normal values at 21 weeks. The type I fibres, however, remained arranged in groups. In the soleus muscle a large increase in the percentage of type II muscle fibres was observed and this remained until 21 weeks. We conclude that a non-selective reinnervation and later readjustments by regression of polyneural innervation may in part explain the changes in distributions of various fibre types.