Internodes can nearly double in length with gradual elongation of the adult rat sciatic nerve

Schwann Cell Development and Myelination

Glial cells in the peripheral nervous system (PNS), which arise from the neural crest, include axon-associated Schwann cells (SCs) in nerves, synapse-associated SCs at the neuromuscular junction, enteric glia, perikaryon-associated satellite cells in ganglia, and boundary cap cells at the border between the central nervous system (CNS) and the PNS. Here, we focus on axon-associated SCs. These SCs progress through a series of formative stages, which culminate in the generation of myelinating SCs that wrap large-caliber axons and of nonmyelinating (Remak) SCs that enclose multiple, small-caliber axons. In this work, we describe SC development, extrinsic signals from the axon and extracellular matrix (ECM) and the intracellular signaling pathways they activate that regulate SC development, and the morphogenesis and organization of myelinating SCs and the myelin sheath. We review the impact of SCs on the biology and integrity of axons and their emerging role in regulating peripheral nerve architecture. Finally, we explain how transcription and epigenetic factors control and fine-tune SC development and myelination.

The role of mechanics in axonal stability and development

Much of the focus of neuronal cell biology has been devoted to growth cone guidance, synaptogenesis, synaptic activity, plasticity, etc. The axonal shaft too has received much attention, mainly for its astounding ability to transmit action potentials and the transport of material over long distances. For these functions, the axonal cytoskeleton and membrane have been often assumed to play static structural roles. Recent experiments have changed this view by revealing an ultrastructure much richer in features than previously perceived and one that seems to be maintained at a dynamic steady state. The role of mechanics in this is only beginning to be broadly appreciated and appears to involve passive and active modes of coupling different biopolymer filaments, filament turnover dynamics and membrane biophysics. Axons, being unique cellular processes in terms of high aspect ratios and often extreme lengths, also exhibit unique passive mechanical properties that might have evolved to stabilize them under mechanical stress. In this review, we summarize the experiments that have exposed some of these features. It is our view that axonal mechanics deserves much more attention not only due to its significance in the development and maintenance of the nervous system but also due to the susceptibility of axons to injury and neurodegeneration.

The structure of sensory afferent compartments in health and disease

Primary sensory neurons are a heterogeneous population of cells able to respond to both innocuous and noxious stimuli. Like most neurons they are highly compartmentalised, allowing them to detect, convey and transfer sensory information. These compartments include specialised sensory endings in the skin, the nodes of Ranvier in myelinated axons, the cell soma and their central terminals in the spinal cord. In this review, we will highlight the importance of these compartments to primary afferent function, describe how these structures are compromised following nerve damage and how this relates to neuropathic pain.

Understanding, engineering, and modulating the growth of neural networks: An interdisciplinary approach

A deeper understanding of the brain and its function remains one of the most significant scientific challenges. It not only is required to find cures for a plethora of brain-related diseases and injuries but also opens up possibilities for achieving technological wonders, such as brain-machine interface and highly energy-efficient computing devices. Central to the brain's function is its basic functioning unit (i.e., the neuron). There has been a tremendous effort to understand the underlying mechanisms of neuronal growth on both biochemical and biophysical levels. In the past decade, this increased understanding has led to the possibility of controlling and modulating neuronal growth in vitro through external chemical and physical methods. We provide a detailed overview of the most fundamental aspects of neuronal growth and discuss how researchers are using interdisciplinary ideas to engineer neuronal networks in vitro. We first discuss the biochemical and biophysical mechanisms of neuronal growth as we stress the fact that the biochemical or biophysical processes during neuronal growth are not independent of each other but, rather, are complementary. Next, we discuss how utilizing these fundamental mechanisms can enable control over neuronal growth for advanced neuroengineering and biomedical applications. At the end of this review, we discuss some of the open questions and our perspectives on the challenges and possibilities related to controlling and engineering the growth of neuronal networks, specifically in relation to the materials, substrates, model systems, modulation techniques, data science, and artificial intelligence.

Chronic effects of muscle and nerve-directed stretching on tissue mechanics

Tissue-directed stretching interventions can preferentially load muscular or nonmuscular structures such as peripheral nerves. How these tissues adapt mechanically to long-term stretching is poorly understood. This randomized, single-blind, controlled study used ultrasonography and dynamometry to compare the effects of 12-wk nerve-directed and muscle-directed stretching programs versus control on maximal ankle dorsiflexion range of motion (ROM) and passive torque, shear wave velocity (SWV; an index of stiffness), and architecture of triceps surae and sciatic nerve. Sixty healthy adults were randomized to receive nerve-directed stretching, muscle-directed stretching, or no intervention (control). The muscle-directed protocol was designed to primarily stretch the plantar flexor muscle group, whereas the nerve-directed intervention targeted the sciatic nerve tract. Compared with the control group [mean; 95% confidence interval (CI)], muscle-directed intervention showed increased ROM (+7.3°; 95% CI: 4.1-10.5), decreased SWV of triceps surae (varied from -0.8 to -2.3 m/s across muscles), decreased passive torque (-6.8 N·m; 95% CI: -11.9 to -1.7), and greater gastrocnemius medialis fascicle length (+0.4 cm; 95% CI: 0.1-0.8). Muscle-directed intervention did not affect the SWV and size of sciatic nerve. Participants in the nerve-directed group showed a significant increase in ROM (+9.9°; 95% CI: 6.2-13.6) and a significant decrease in sciatic nerve SWV (> -1.8 m/s across nerve regions) compared with the control group. Nerve-directed intervention had no effect on the main outcomes at muscle and joint levels. These findings provide new insights into the long-term mechanical effects of stretching interventions and have relevance to clinical conditions where change in mechanical properties has occurred.NEW & NOTEWORTHY This study demonstrates that the mechanical properties of plantar flexor muscles and sciatic nerve can adapt mechanically to long-term stretching programs. Although interventions targeting muscular or nonmuscular structures are both effective at increasing maximal range of motion, the changes in tissue mechanical properties (stiffness) are specific to the structure being preferentially stretched by each program. We provide the first in vivo evidence that stiffness of peripheral nerves adapts to long-term loading stimuli using appropriate nerve-directed stretching.

Growth and elongation of axons through mechanical tension mediated by fluorescent-magnetic bifunctional Fe3O4·Rhodamine 6G@PDA superparticles

Background

The primary strategy to repair peripheral nerve injuries is to bridge the lesions by promoting axon regeneration. Thus, the ability to direct and manipulate neuronal cell axon regeneration has been one of the top priorities in the field of neuroscience. A recent innovative approach for remotely guiding neuronal regeneration is to incorporate magnetic nanoparticles (MNPs) into cells and transfer the resulting MNP-loaded cells into a magnetically sensitive environment to respond to an external magnetic field. To realize this intention, the synthesis and preparation of ideal MNPs is an important challenge to overcome.

Results

In this study, we designed and prepared novel fluorescent-magnetic bifunctional Fe₃O₄·Rhodamine 6G@polydopamine superparticles (FMSPs) as neural regeneration therapeutics. With the help of their excellent biocompatibility and ability to interact with neural cells, our in-house fabricated FMSPs can be endocytosed into cells, transported along the axons, and then aggregated in the growth cones. As a result, the mechanical forces generated by FMSPs can promote the growth and elongation of axons and stimulate gene expression associated with neuron growth under external magnetic fields.

Conclusions

Our work demonstrates that FMSPs can be used as a novel stimulator to promote noninvasive neural regeneration through cell magnetic actuation.

Relevant advances in bone lengthening research: a bibliometric analysis of the 100 most-cited articles published from 2001 to 2017

This study aimed to assess the scientific production of bone lengthening research by identifying the most-cited papers. All articles including the term 'bone lengthening' published between 2001 and 2017 were retrieved through the Web of Science database. The 100 most-cited articles on bone lengthening included a total of 4244 citations, with 414 (9.7%) citations in 2017. There was an average of 249.6 citations per year. The articles predominantly addressed biomechanics and bone formation (38). Different surgical techniques, including intramedullary nail (14), Ilizarov (nine), intramedullary skeletal kinetic distractor (ISKD) (six), Taylor spatial frame (6), the PRECICE device (three), and lengthening and submuscular locking plate (three), were the second most-studied topic. Most studies were therapeutic (58), whereas 30 studies were experimental investigations using animal models. Among the clinical studies, case series were predominant (level of evidence IV) (57). This study presents the first bibliometric analysis of the most relevant articles on bone lengthening. The list is relatively comprehensive in terms of identifying the top issues in this field. However, the most influential clinical studies have a poor level of evidence, although a slight tendency toward a better level of evidence has been observed in more recent years.

[Experimental study of the effect of the sciatic nerve elongation on pain in rats]

OBJECTIVE

To investigate the effect of the sciatic nerve elongation on pain in rats.

METHODS

Thirty-six adult male Wistar rats of SPF grade, weighing 250-300 g. Eighteen of them were randomly divided into 3 groups, 6 rats in each group. They were sciatic nerve elongation group (group A), nerve no-elongation group (group B), and nerve ligation group (group C). The model of 10-mm sciatic nerve defect was established in all 3 groups. The sciatic nerve was extended at a speed of 1 mm/d for 14 days in group A. The group B was only installed with external fixation. The nerve stumps were ligated in the group C. At 3, 7, 10, and 14 days after operation, the foot injury was evaluated by the autotomy scoring scale. At 14 days after operation, the dorsal root ganglia (DRG) of L 4-S 1 spinal cord of rats in each group was observed by tumor necrosis factor α (TNF-α) immunohistochemical staining, and the primary antibodies were replaced by pure serum as negative control group. Another 18 rats were randomly divided into 3 groups, 6 rats in each group. They were sciatic nerve elongation group (group A1), nerve no-elongation group (group B1), positive control group (group C1). In groups A1 and B1, the 10-mm long sciatic nerve defect model was established by the same method as groups A and B, and then fixed with external fixation. Nerve elongation was done or not done without anesthesia at 3 days after operation. In group C1, no modeling was done and 20 μL 2.5% formaldehyde was injected into the toes. After 90 minutes, the dorsal horn of spinal cord of L 4-S 1 segment of rats was cutting for c-Fos immunohistochemical staining and the number of positive cells was counted. Primary antibodies were replaced with pure serum as negative control group.

RESULTS

The autotomy scores of rats in groups B and C gradually increased postoperatively, and group A remained stable at 0.25±0.50. The scores of group C were significantly higher than those of group A and group B at each time point postoperatively ( P<0.05). The scores of group A were significantly lower than those of group B at 10 and 14 days postoperatively ( P<0.05). TNF-α immunohistochemical staining showed that the TNF-α expression in group A was weak, slightly positive (+/-); in group B was positive (+); in group C was strongly positive (++); and the negative control group had no TNF-α expression (-). c-Fos immunohistochemical staining showed that the c-Fos expressions in groups A1 and B1 were weak positive, in group C1 was strong positive, and negative control group had no c-Fos positive expression. The number of c-Fos positive cells in groups A1, B1, C1, and negative control group were (21.5±6.6), (19.3±8.1), (95.6±7.4), and 0 cells/field, respectively, and group C1 was significantly higher than groups A1 and B1 ( P<0.05), there was no significant difference between group A1 and group B1 ( P>0.05).

CONCLUSION

Nerve elongation does not cause obvious pain neither during the operation of elongation nor throughout the whole elongation.

Decoupled epineurial and axonal deformation in mouse median and ulnar nerves

INTRODUCTION

Peripheral nerves accommodate mechanical loads during joint movement. Hypothesized protective features include increased nerve compliance near joints and axonal undulation. How axons perceive nerve deformation is poorly understood. We tested whether nerves increase local axonal undulation in regions of high epineurial strain to protect nerve fibers from strain-induced damage.

METHODS

Regional epineurial strain was measured near the elbow in median and ulnar nerves of mice expressing axonal fluorescence before and after decompression. Regional axonal tortuosity was quantified under confocal microscopy.

RESULTS

Nerves showed higher epineurial strain just distal to the medial epicondyle; these differences were eliminated after decompression. Axonal tortuosity also varied regionally; however, unlike in the epineurium, it was greater in proximal regions.

DISCUSSION

In this study we have proposed a neuromechanical model whereby axons can unravel along their entire length due to looser mechanical coupling to the peri/epineurium. Our findings have major implications for understanding nerve biomechanics and dysfunction. Muscle Nerve 59:619-619, 2019.

[Stretch-induced axon growth: a universal, yet poorly explored process]

The growth of axons is a key step in neuronal circuit assembly. The axon starts elongating with the migration of its growth cone in response to molecular signals present in the surrounding embryonic tissues. Following the formation of a synapse between the axon and the target cell, the distance which separates the cell body from the synapse continues to increase to accommodate the growth of the organism. This second phase of elongation, which is universal and crucial since it contributes to an important proportion of the final axon size, has been historically referred to as "stretch-induced axon growth". It is indeed likely to result from a mechanical tension generated by the growth of the body, but the underlying mechanisms remain poorly characterized. This article reviews the experimental studies of this process, mainly analysed on cultured neurons so far. The recent development of in vivo imaging techniques and tools to probe and perturb mechanical forces within embryos will shed new light on this universal mode of axonal growth. This knowledge may inspire the design of novel tissue engineering strategies dedicated to brain and spinal cord repair.

Myelinating Schwann Cell Polarity and Mechanically-Driven Myelin Sheath Elongation

Myelin sheath geometry, encompassing myelin sheath thickness relative to internodal length, is critical to optimize nerve conduction velocity and these parameters are carefully adjusted by the myelinating cells in mammals. In the central nervous system these adjustments could regulate neuronal activities while in the peripheral nervous system they lead to the optimization and the reliability of the nerve conduction velocity. However, the physiological and cellular mechanisms that underlie myelin sheath geometry regulation are not yet fully elucidated. In peripheral nerves the myelinating Schwann cell uses several molecular mechanisms to reach and maintain the correct myelin sheath geometry, such that myelin sheath thickness and internodal length are regulated independently. One of these mechanisms is the epithelial-like cell polarization process that occurs during the early phases of the myelin biogenesis. Epithelial cell polarization factors are known to control cell size and morphology in invertebrates and mammals making these processes critical in the organogenesis. Correlative data indicate that internodal length is regulated by postnatal body growth that elongates peripheral nerves in mammals. In addition, the mechanical stretching of peripheral nerves in adult animals shows that myelin sheath length can be increased by mechanical cues. Recent results describe the important role of YAP/TAZ co-transcription factors during Schwann cell myelination and their functions have linked to the mechanotransduction through the HIPPO pathway and the epithelial polarity factor Crb3. In this review the molecular mechanisms that govern mechanically-driven myelin sheath elongation and how a Schwann cell can modulate internodal myelin sheath length, independent of internodal thickness, will be discussed regarding these recent data. In addition, the potential relevance of these mechanosensitive mechanisms in peripheral pathologies will be highlighted.

Influence of Mechanical Stimuli on Schwann Cell Biology

Schwann cells are the glial cells of the peripheral nervous system (PNS). They insulate axons by forming a specialized extension of plasma membrane called the myelin sheath. The formation of myelin is essential for the rapid saltatory propagation of action potentials and to maintain the integrity of axons. Although both axonal and extracellular matrix (ECM) signals are necessary for myelination to occur, the cellular and molecular mechanisms regulating myelination continue to be elucidated. Schwann cells in peripheral nerves are physiologically exposed to mechanical stresses (i.e., tensile, compressive and shear strains), occurring during development, adulthood and injuries. In addition, there is a growing body of evidences that Schwann cells are sensitive to the stiffness of their environment. In this review, we detail the mechanical constraints of Schwann cells and peripheral nerves. We explore the regulation of Schwann cell signaling pathways in response to mechanical stimulation. Finally, we provide a comprehensive overview of the experimental studies addressing the mechanobiology of Schwann cells. Understanding which mechanical properties can interfere with the cellular and molecular biology of Schwann cell during development, myelination and following injuries opens new insights in the regulation of PNS development and treatment approaches in peripheral neuropathies.

mTOR regulates peripheral nerve response to tensile strain

While excessive tensile strain can be detrimental to nerve function, strain can be a positive regulator of neuronal outgrowth. We used an in vivo rat model of sciatic nerve strain to investigate signaling mechanisms underlying peripheral nerve response to deformation. Nerves were deformed by 11% and did not demonstrate deficits in compound action potential latency or amplitude during or after 6 h of strain. As revealed by Western blotting, application of strain resulted in significant upregulation of mammalian target of rapamycin (mTOR) and S6 signaling in nerves, increased myelin basic protein (MBP) and β-actin levels, and increased phosphorylation of neurofilament subunit H (NF-H) compared with unstrained (sham) contralateral nerves (P < 0.05 for all comparisons, paired two-tailed t-test). Strain did not alter neuron-specific β3-tubulin or overall nerve tubulin levels compared with unstrained controls. Systemic rapamycin treatment, thought to selectively target mTOR complex 1 (mTORC1), suppressed mTOR/S6 signaling, reduced levels of MBP and overall tubulin, and decreased NF-H phosphorylation in nerves strained for 6 h, revealing a role for mTOR in increasing MBP expression and NF-H phosphorylation, and maintaining tubulin levels. Consistent with stretch-induced increases in MBP, immunolabeling revealed increased S6 signaling in Schwann cells of stretched nerves compared with unstretched nerves. In addition, application of strain to cultured adult dorsal root ganglion neurons showed an increase in axonal protein synthesis based on a puromycin incorporation assay, suggesting that neuronal translational pathways also respond to strain. This work has important implications for understanding mechanisms underlying nerve response to strain during development and regeneration.NEW & NOTEWORTHY Peripheral nerves experience tensile strain (stretch) during development and movement. Excessive strain impairs neuronal function, but moderate strains are accommodated by nerves and can promote neuronal growth; mechanisms underlying these phenomena are not well understood. We demonstrated that levels of several structural proteins increase following physiological levels of nerve strain and that expression of a subset of these proteins is regulated by mTOR. Our work has important implications for understanding nerve development and strain-based regenerative strategies.

Low-intensity pulsed ultrasound upregulates pro-myelination indicators of Schwann cells enhanced by co-culture with adipose-derived stem cells

OBJECTIVES

Peripheral nerve injuries are a common occurrence, resulting in considerable patient suffering; it also represents a major economic burden on society. To improve treatment options following peripheral nerve injuries, scientists aim to find a way to promote Schwann cell (SC) myelination to help nerves to carry out their functions effectively. In this study, we investigated myelination ability of SCs, regulated by co-culture with adipose-derived stem cells (ASCs) or low-intensity pulsed ultrasound (LIPUS), and synergistic effects of combined treatments.

MATERIALS AND METHODS

Schwann cells were co-cultured with or without ASCs, and either left untreated or treated with LIPUS for 10 min/d for 1, 4 or 7 days. Effects of LIPUS and ASC co-culture on pro-myelination indicators of SCs were analysed by real-time PCR (RT-PCR), Western blotting and immunofluorescence staining (IF).

RESULTS

Our results indicate that ASC-SC co-culture and LIPUS, together or individually, promoted mRNA levels of epidermal growth factor receptor 3 (EGFR3/ErbB3), neuregulin1 (NRG1), early growth response protein 2 (Egr2/Krox20) and myelin basic protein (MBP), with corresponding increases in protein levels of ErbB3, NRG1 and Krox20. Interestingly, combination of ASC-SC co-culture and LIPUS displayed the most remarkable effects.

CONCLUSION

We demonstrated that ASCs upregulated pro-myelination indicators of SCs by indirect contact (through co-culture) and that effects could be potentiated by LIPUS. We conclude that LIPUS, as a mechanical stress, may have potential in nerve regeneration with potential clinical relevance.

Optimal myelin elongation relies on YAP activation by axonal growth and inhibition by Crb3/Hippo pathway

Fast nerve conduction relies on successive myelin segments that electrically isolate axons. Segment geometry—diameter and length—is critical for the optimization of nerve conduction and the molecular mechanisms allowing this optimized geometry are partially known. We show here that peripheral myelin elongation is dynamically regulated by stimulation of YAP (Yes-associated protein) transcription cofactor activity during axonal elongation and limited by inhibition of YAP activity via the Hippo pathway. YAP promotes myelin and non-myelin genes transcription while the polarity protein Crb3, localized at the tips of the myelin sheath, activates the Hippo pathway to temper YAP activity, therefore allowing for optimal myelin growth. Dystrophic Dy^2j/2j mice mimicking human peripheral neuropathy with reduced internodal lengths have decreased nuclear YAP which, when corrected, leads to longer internodes. These data show a novel mechanism controlling myelin growth and nerve conduction, and provide a molecular ground for disease with short myelin segments.

Molecular mechanisms regulating optimal myelin geometry are only partially understood. Here authors show that peripheral myelin growth is orchestrated by the Crb3/Hippo/YAP pathway, and that defects in YAP activation may underlie peripheral neuropathies caused by shorter myelin.

Schwann cell myelination

Myelinated nerve fibers are essential for the rapid propagation of action potentials by saltatory conduction. They form as the result of reciprocal interactions between axons and Schwann cells. Extrinsic signals from the axon, and the extracellular matrix, drive Schwann cells to adopt a myelinating fate, whereas myelination reorganizes the axon for its role in conduction and is essential for its integrity. Here, we review our current understanding of the development, molecular organization, and function of myelinating Schwann cells. Recent findings into the extrinsic signals that drive Schwann cell myelination, their cognate receptors, and the downstream intracellular signaling pathways they activate will be described. Together, these studies provide important new insights into how these pathways converge to activate the transcriptional cascade of myelination and remodel the actin cytoskeleton that is critical for morphogenesis of the myelin sheath.

Use of nerve elongator to repair short-distance peripheral nerve defects: a prospective randomized study

Repair techniques for short-distance peripheral nerve defects, including adjacent joint flexion to reduce the distance between the nerve stump defects, "nerve splint" suturing, and nerve sleeve connection, have some disadvantages. Therefore, we designed a repair technique involving intraoperative tension-free application of a nerve elongator and obtained good outcomes in the repair of short-distance peripheral nerve defects in a previous animal study. The present study compared the clinical outcomes between the use of this nerve elongator and performance of the conventional method in the repair of short-distance transection injuries in human elbows. The 3-, 6-, and 12-month postoperative follow-up results demonstrated that early neurological function recovery was better in the nerve elongation group than in the conventional group, but no significant difference in long-term neurological function recovery was detected between the two groups. In the nerve elongation group, the nerves were sutured without tension, and the duration of postoperative immobilization of the elbow was decreased. Elbow function rehabilitation was significantly better in the nerve elongation group than in the control group. Moreover, there were no security risks. The results of this study confirm that the use of this nerve elongator for repair of short-distance peripheral nerve defects is safe and effective.

The relationship of nerve fibre pathology to sensory function in entrapment neuropathy

The impact of peripheral entrapment neuropathies on target innervation remains unknown. Using quantitative sensory testing, neurophysiology and skin biopsies, Schmid et al. demonstrate that carpal tunnel syndrome affects large fibres and their nodal complexes, but is also associated with a reduction in the number and functioning of small sensory axons.

Surprisingly little is known about the impact of entrapment neuropathy on target innervation and the relationship of nerve fibre pathology to sensory symptoms and signs. Carpal tunnel syndrome is the most common entrapment neuropathy; the aim of this study was to investigate its effect on the morphology of small unmyelinated as well as myelinated sensory axons and relate such changes to somatosensory function and clinical symptoms. Thirty patients with a clinical and electrophysiological diagnosis of carpal tunnel syndrome [17 females, mean age (standard deviation) 56.4 (15.3)] and 26 age and gender matched healthy volunteers [18 females, mean age (standard deviation) 51.0 (17.3)] participated in the study. Small and large fibre function was examined with quantitative sensory testing in the median nerve territory of the hand. Vibration and mechanical detection thresholds were significantly elevated in patients with carpal tunnel syndrome (P < 0.007) confirming large fibre dysfunction and patients also presented with increased thermal detection thresholds (P < 0.0001) indicative of C and Aδ-fibre dysfunction. Mechanical and thermal pain thresholds were comparable between groups (P > 0.13). A skin biopsy was taken from a median nerve innervated area of the proximal phalanx of the index finger. Immunohistochemical staining for protein gene product 9.5 and myelin basic protein was used to evaluate morphological features of unmyelinated and myelinated axons. Evaluation of intraepidermal nerve fibre density showed a striking loss in patients (P < 0.0001) confirming a significant compromise of small fibres. The extent of Meissner corpuscles and dermal nerve bundles were comparable between groups (P > 0.07). However, patients displayed a significant increase in the percentage of elongated nodes (P < 0.0001), with altered architecture of voltage-gated sodium channel distribution. Whereas neither neurophysiology nor quantitative sensory testing correlated with patients’ symptoms or function deficits, the presence of elongated nodes was inversely correlated with a number of functional and symptom related scores (P < 0.023). Our findings suggest that carpal tunnel syndrome does not exclusively affect large fibres but is associated with loss of function in modalities mediated by both unmyelinated and myelinated sensory axons. We also document for the first time that entrapment neuropathies lead to a clear reduction in intraepidermal nerve fibre density, which was independent of electrodiagnostic test severity. The presence of elongated nodes in the target tissue further suggests that entrapment neuropathies affect nodal structure/myelin well beyond the focal compression site. Interestingly, nodal lengthening may be an adaptive phenomenon as it inversely correlates with symptom severity.

Limb lengthening and peripheral nerve function-factors associated with deterioration of conduction

BACKGROUND

and purpose Limb lengthening is performed for a diverse range of orthopedic problems. A high rate of complications has been reported in these patients, which include motor and sensory loss as a result of nerve damage. We investigated the effect of limb lengthening on peripheral nerve function.

PATIENTS AND METHODS

36 patients underwent electrophysiological testing at 3 points: (1) preoperatively, (2) after application of external fixator/corticotomy but before lengthening, and (3) after lengthening. The limb-length discrepancy was due to a congenital etiology (n = 19), a growth disturbance (n = 9), or a traumatic etiology (n = 8).

RESULTS

2 of the traumatic etiology patients had significant changes evident on electrophysiological testing preoperatively. They both deteriorated further with lengthening. 7 of the 21 patients studied showed deterioration in nerve function after lengthening, but not postoperatively, indicating that this was due to the lengthening process and not to the surgical procedure. All of these patients had a congenital etiology for their leg-length discrepancy.

INTERPRETATION

As detailed electrophysiological tests were carried out before surgery, after surgery but before lengthening, and finally after completion of lengthening, it was possible to distinguish between the effects of the operation and the effects of lengthening on nerve function. The results indicate that the etiology, site (femur or tibia), and nerve (common peroneal or tibial) had a bearing on the risk of nerve injury and that these factors had a far greater effect than the total amount of lengthening.

Cell biology in neuroscience: Cellular and molecular mechanisms underlying axon formation, growth, and branching

Proper brain wiring during development is pivotal for adult brain function. Neurons display a high degree of polarization both morphologically and functionally, and this polarization requires the segregation of mRNA, proteins, and lipids into the axonal or somatodendritic domains. Recent discoveries have provided insight into many aspects of the cell biology of axonal development including axon specification during neuronal polarization, axon growth, and terminal axon branching during synaptogenesis.

Effect of limb lengthening on internodal length and conduction velocity of peripheral nerve

The influences of axon diameter, myelin thickness, and internodal length on the velocity of conduction of peripheral nerve action potentials are unclear. Previous studies have demonstrated a strong dependence of conduction velocity on internodal length. However, a theoretical analysis has suggested that this relationship may be lost above a nodal separation of ∼0.6 mm. Here we measured nerve conduction velocities in a rabbit model of limb lengthening that produced compensatory increases in peripheral nerve growth. Divided tibial bones in one hindlimb were gradually lengthened at 0.7 mm per day using an external frame attached to the bone. This was associated with a significant increase (33%) of internodal length (0.95-1.3 mm) in axons of the tibial nerve that varied in proportion to the mechanical strain in the nerve of the lengthened limb. Axonal diameter, myelin thickness, and g-ratios were not significantly altered by limb lengthening. Despite the substantial increase in internodal length, no significant change was detected in conduction velocity (∼43 m/s) measured either in vivo or in isolated tibial nerves. The results demonstrate that the internode remains plastic in the adult but that increases in internodal length of myelinated adult nerve axons do not result in either deficiency or proportionate increases in their conduction velocity and support the view that the internodal lengths of nerves reach a plateau beyond which their conduction velocities are no longer sensitive to increases in internodal length.

A novel internal fixator device for peripheral nerve regeneration

Recovery from peripheral nerve damage, especially for a transected nerve, is rarely complete, resulting in impaired motor function, sensory loss, and chronic pain with inappropriate autonomic responses that seriously impair quality of life. In consequence, strategies for enhancing peripheral nerve repair are of high clinical importance. Tension is a key determinant of neuronal growth and function. In vitro and in vivo experiments have shown that moderate levels of imposed tension (strain) can encourage axonal outgrowth; however, few strategies of peripheral nerve repair emphasize the mechanical environment of the injured nerve. Toward the development of more effective nerve regeneration strategies, we demonstrate the design, fabrication, and implementation of a novel, modular nerve-lengthening device, which allows the imposition of moderate tensile loads in parallel with existing scaffold-based tissue engineering strategies for nerve repair. This concept would enable nerve regeneration in two superposed regimes of nerve extension--traditional extension through axonal outgrowth into a scaffold and extension in intact regions of the proximal nerve, such as that occurring during growth or limb-lengthening. Self-sizing silicone nerve cuffs were fabricated to grip nerve stumps without slippage, and nerves were deformed by actuating a telescoping internal fixator. Poly(lactic co-glycolic) acid (PLGA) constructs mounted on the telescoping rods were apposed to the nerve stumps to guide axonal outgrowth. Neuronal cells were exposed to PLGA using direct contact and extract methods, and they exhibited no signs of cytotoxic effects in terms of cell morphology and viability. We confirmed the feasibility of implanting and actuating our device within a sciatic nerve gap and observed axonal outgrowth following device implantation. The successful fabrication and implementation of our device provides a novel method for examining mechanical influences on nerve regeneration.

Simultaneous gradual lengthening of proximal and distal nerve stumps for repair of chronic peripheral nerve defect in rats

We investigated nerve regeneration of rat sciatic nerves after chronic injury of 15 mm-defect by the gradual lengthening of proximal and distal nerve stumps. Thirty days after the primal injury, both stumps were grasped and lengthened at a rate of 1 mm per day using external nerve-lengthening devices for 15 days. Then end-to-end neurorrhaphy was performed. After the lengthening, both stumps were evaluated by immunohistochemical analysis. Nerve regeneration was evaluated by electrophysiological and histological studies at 12 weeks after the repair. In the lengthened proximal stump, Schwann cells and axons existed along the whole nerve stump. In the lengthened distal stump, Schwann cells exist along the overall length. The whole nerve trunk was lengthened. The nerve regeneration was comparable with the delayed end-to-end suture without nerve defect. We showed the feasibility of direct gradual lengthening of proximal and distal nerve stumps for the treatment of chronic segmental nerve defect.

FUNCTIONAL AND MORPHOLOGICAL EFFECTS OF INDIRECT GRADUAL ELONGATION OF PERIPHERAL NERVE: ELECTROPHYSIOLOGICAL AND MORPHOLOGICAL CHANGES AT DIFFERENT ELONGATION RATES

We investigated the neuropathy induced by leg lengthening histological evaluation using teased nerve fiber specimens and electrophysiological evaluation. Indirect elongation of the sciatic nerve associated with leg lengthening was performed at 1 and 3 mm/day over 30 mm in rats. Electrophysiological evaluation was performed immediately and 60 days after the end of elongation, teased nerve fiber specimens were prepared, and the mean axonal diameter was calculated. The electrophysiological results were more wrong, and the recovery was poorer, in the 3-mm than in the 1-mm group. In the 1-mm group, the nerve conduction velocity (NCV) and the duration of the compound nerve action potential (C-NAP) recovered to a level close to the intact side, but the decrease in the amplitude of the C-NAP persisted. In the teased fiber study, while paranodal demyelination was observed in both groups immediately after elongation, demyelination was decreased in the 1-mm group indicationg recovery compared to the 3-mm group. Paranodal demyelination caused by indirect nerve elongation is considered to have induced electrophysiological disorders.

Electrophysiological and morphological damages appeared to be more severe according to elongation speed. The nerve disorder were remained even at 1 mm per day in 60 days.

Electrophysiological and histological investigation on the gradual elongation of rabbit sciatic nerve

A basic study using animal models was performed to investigate whether the sciatic nerve retains physiological functions and normal morphology after the gradual elongation associated with adjacent bone elongation. Electrophysiological and histological studies were performed on the elongated sciatic nerve of rabbit accompanied by the femur bone elongation. Compound action potentials evoked by electrical stimulation of the sciatic nerve were recorded and histological specimens of elongated nerve fibers were obtained immediately after final bone elongation from 4 rabbits (immediate group). Three rabbits were allowed to recover for 8 weeks after the bone elongation (maintained group). Three rabbits without bone elongation were used as controls of the immediate and maintained groups (control group). In the immediate group, the average amplitude of evoked nerve potentials were 30.38 ± 1.58 mV before elongation and diminished significantly to 18.35 ± 1.25 mV immediately after elongation (P<0.01). The amplitude of evoked potentials was not significantly different between before (30.30 ± 0.61 mV) elongation and after elongation (27.47 ± 1.63 mV) in the maintained group. The axonal area of the myelinated nerve fibers of the proximal region of the sciatic nerve in the immediate group was significantly decreased after elongation (P<0.01). The decrease in the area of the distal region was greatest in the control group and was followed by that in the maintained group and the immediate group (P<0.05, 0.01). These results suggest that the sciatic nerve shows dysfunction immediately after elongation, but can recover electrophysiologically and histologically several weeks after elongation.

The emerging role of forces in axonal elongation

An understanding of how axons elongate is needed to develop rational strategies to treat neurological diseases and nerve injury. Growth cone-mediated neuronal elongation is currently viewed as occurring through cytoskeletal dynamics involving the polymerization of actin and tubulin subunits at the tip of the axon. However, recent work suggests that axons and growth cones also generate forces (through cytoskeletal dynamics, kinesin, dynein, and myosin), forces induce axonal elongation, and axons lengthen by stretching. This review highlights results from various model systems (Drosophila, Aplysia, Xenopus, chicken, mouse, rat, and PC12 cells), supporting a role for forces, bulk microtubule movements, and intercalated mass addition in the process of axonal elongation. We think that a satisfying answer to the question, "How do axons grow?" will come by integrating the best aspects of biophysics, genetics, and cell biology.

The role of stretching in slow axonal transport

Axonal stretching is linked to rapid rates of axonal elongation. Yet the impact of stretching on elongation and slow axonal transport is unclear. Here, we develop a mathematical model of slow axonal transport that incorporates the rate of axonal elongation, protein half-life, protein density, adhesion strength, and axonal viscosity to quantify the effects of axonal stretching. We find that under conditions where the axon (or nerve) is free of a substrate and lengthens at rapid rates (>4 mm day⁻¹), stretching can account for almost 50% of total anterograde axonal transport. These results suggest that it is possible to accelerate elongation and transport simultaneously by increasing either the axon's susceptibility to stretching or the forces that induce stretching. To our knowledge, this work is the first to incorporate the effects of stretching in a model of slow axonal transport. It has relevance to our understanding of neurite outgrowth during development and peripheral nerve regeneration after trauma, and hence to the development of treatments for spinal cord injury.

Growth and elongation within and along the axon

Mechanical tension is a particularly effective stimulus for axonal elongation, but little is known about how it leads to the formation of new axon. To better understand this process, we examined the movement of axonal branch points, beads bound to the axon, and docked mitochondria while monitoring axonal width. We found these markers moved in a pattern that suggests elongation occurs by viscoelastic stretching and volume addition along the axon. To test the coupling between "lengthening" and "growth," we measured axonal width while forcing axons to grow and then pause by controlling the tension applied to the growth cone or to the cell body. We found axons thinned during high rates of elongation and thickened when the growth cones were stationary. These findings suggest that forces cause lengthening because they stretch the axon and that growth occurs, in a loosely coupled step, by volume addition along the axon.

Stretch growth of integrated axon tracts: extremes and exploitations

Although virtually ignored in the literature until recently, the process of 'stretch growth of integrated axon tracts' is perhaps the most remarkable axon growth mechanism of all. This process can extend axons at seemingly impossible rates without the aid of chemical cues or even growth cones. As animals grow, the organization and extremely rapid expansion of the nervous system appears to be directed purely by mechanical forces on axon tracts. This review provides the first glimpse of the astonishing features of axon tracts undergoing stretch growth and how this natural process can be exploited to facilitate repair of the damaged nervous system.

The morphologic characteristics of nerve shortening following traumatic bone loss

Little is known about peripheral nerve shortening secondary to joint contracture or traumatic bone loss. We used the rat sciatic nerve as a model to study nerve shortening secondary to leg shortening. Nerve shortening was induced by surgically removing 16 mm of the femur. The histology of the ipsilateral and contralateral (control) sciatic nerves were compared at 1 h, 3 weeks, and 6 weeks. Transverse semithin sections of sciatic nerve were prepared and examined; single fibers also were teased from the nerve for study. The epineurium was shortened about 25% at 6 weeks. Axonal diameter was unchanged at 1 h, but increased over time, and was 0.68 microm larger than controls at 6 weeks (p < 0.05). In teased-fiber preparations, internodal length decreased 2.3% at 6 weeks, but not significantly. Peripheral nerve shortening secondary to leg shortening shortens the epineurium, but does not effect on internodal length.

Molecular domains of myelinated axons in the peripheral nervous system

Myelinated axons are organized into a series of specialized domains with distinct molecular compositions and functions. These domains, which include the node of Ranvier, the flanking paranodal junctions, the juxtaparanodes, and the internode, form as the result of interactions with myelinating Schwann cells. This domain organization is essential for action potential propagation by saltatory conduction and for the overall function and integrity of the axon.

Myelination and regional domain differentiation of the axon

During evolution, as organisms increased in complexity and function, the need for the ensheathment and insulation of axons by glia became vital for faster conductance of action potentials in nerves. Myelination, as the process is termed, facilitates the formation of discrete domains within the axolemma that are enriched in ion channels, and macromolecular complexes consisting of cell adhesion molecules and cytoskeletal regulators. While it is known that glia play a substantial role in the coordination and organization of these domains, the mechanisms involved and signals transduced between the axon and glia, as well as the proteins regulating axo-glial junction formation remain elusive. Emerging evidence has shed light on the processes regulating myelination and domain differentiation, and key molecules have been identified that are required for their assembly and maintenance. This review highlights these recent findings, and relates their significance to domain disorganization as seen in several demyelinating disorders and other neuropathies.

Modeling mitochondrial dynamics during in vivo axonal elongation

Many models of axonal elongation are based on the assumption that the rate of lengthening is driven by the production of cellular materials in the soma. These models make specific predictions about transport and concentration gradients of proteins both over time and along the length of the axon. In vivo, it is well accepted that for a particular neuron the length and rate of growth are controlled by the body size and rate of growth of the animal. In terms of modeling axonal elongation this radically changes the relationships between key variables. It raises fundamental questions. For example, during in vivo lengthening is the production of material constant or does it change over time? What is the density profile of material along the nerve during in vivo elongation? Does density change over time or vary along the nerve? To answer these questions we measured the length, mitochondrial density, and estimated the half-life of mitochondria in the axons of the medial segmental nerves of 1st, 2nd, and 3rd instar Drosophila larvae. The nerves were found to linearly increase in length at an average rate of 9.24 microm h(-1) over the 96 h period of larval life. Further, mitochondrial density increases over this period at an average rate of 4.49x10(-3) (mitochondria microm(-1))h(-1). Mitochondria in the nerves had a half-life of 35.2h. To account for the distribution of the mitochondria we observe, we derived a mathematical model which suggests that cellular production of mitochondria increases quadratically over time and that a homeostatic mechanism maintains a constant density of mitochondria along the nerve. These data suggest a complex relationship between axonal length and mass production and that the neuron may have an "axonal length sensor."

Gradual stretching of the proximal nerve stump induces the growth of regenerating sprouts in rats

We investigated the effect of direct gradual stretching on the proximal nerve stump morphologically. A 10-mm-long nerve segment was resected from the sciatic nerve of a rat. The end of the proximal nerve stump was fixed to a small ring and the marking suture was placed at a point 1 mm proximal to the ring. Then, the nerve stump was lengthened at a rate of 1 mm/day via a stretching of the ring using an original external device. After a stretching of 20 days, the distance from the ring to the marking suture became 12 mm. Whereas large mature myelinated axons were observed in the proximal part of the marking, only small axons with thin myelin sheath were observed in the distal part, and the mean axonal diameter showed a significant difference between the two parts. Moreover, the mean internodal length was 172.4 +/- 13.4 microm in the distal part of the marking and 1019.0 +/- 56.2 microm in the proximal part. The internodal length also showed a significant difference between the two parts. Thus, the axonal diameter and internodal length were consistent with the characteristics of regenerating axons in the distal part. Furthermore, ultrastructural analysis also showed the histological characteristics of axonal regeneration. Thus, a transected proximal nerve stump may be lengthened by axonal regeneration during gradual stretching, and the stimulus of mechanical stretching may induce the growth of regenerating axons.

A physical model of axonal elongation: force, viscosity, and adhesions govern the mode of outgrowth

Whether the axonal framework is stationary or moves is a central debate in cell biology. To better understand this problem, we developed a mathematical model that incorporates force generation at the growth cone, the viscoelastic properties of the axon, and adhesions between the axon and substrate. Using force-calibrated needles to apply and measure forces at the growth cone, we used docked mitochondria as markers to monitor movement of the axonal framework. We found coherent axonal transport that decreased away from the growth cone. Based on the velocity profiles of movement and the force applied at the growth cone, and by varying the attachment of the axonal shaft to the coverslip, we estimate values for the axial viscosity of the axon (3 x 10(6) +/- 2.4 x 10(6) Pa.s) and the friction coefficient for laminin/polyornithine-based adhesions along the axon (9.6 x 10(3) +/- 7.5 x 10(3) Pa.s). Our model suggests that whether axons elongate by tip growth or stretching depends on the level of force generation at the growth cone, the viscosity of the axon, and the level of adhesions along the axon.

Axon kinematics change during growth and development

The microkinematic response of axons to mechanical stretch was examined in the developing chick embryo spinal cord during a period of rapid growth and myelination. Spinal cords were isolated at different days of embryonic (E) development post-fertilization (E12, E14, E16, and E18) and stretched 0%, 5%, 10%, 15%, and 20%, respectively. During this period, the spinal cord grew approximately 55% in length, and white matter tracts were myelinated significantly. The spinal cords were fixed with paraformaldehyde at the stretched length, sectioned, stained immunohistochemically for neurofilament proteins, and imaged with epifluorescence microscopy. Axons in unstretched spinal cords were undulated, or tortuous, to varying degrees, and appeared to straighten with stretch. The degree of tortuosity (ratio of the segment's pathlength to its end-to-end length) was quantified in each spinal cord by tracing several hundred randomly selected axons. The change in tortuosity distributions with stretch indicated that axons switched from non-affine, uncoupled behavior at low stretch levels to affine, coupled behavior at high stretch levels, which was consistent with previous reports of axon behavior in the adult guinea pig optic nerve (Bain, Shreiber, and Meaney, J. Biomech. Eng., 125(6), pp. 798-804). A mathematical model previously proposed by Bain et al. was applied to quantify the transition in kinematic behavior. The results indicated that significant percentages of axons demonstrated purely non-affine behavior at each stage, but that this percentage decreased from 64% at E12 to 30% at E18. The decrease correlated negatively to increases in both length and myelination with development, but the change in axon kinematics could not be explained by stretch applied during physical growth of the spinal cord. The relationship between tissue-level and axonal-level deformation changes with development, which can have important implications in the response to physiological forces experienced during growth and trauma.

Effect of progesterone on recovery from nerve injury during leg lengthening in rats

We investigated the effect of progesterone on the nerve during lengthening of the limb in rats. The sciatic nerves of rats were elongated by leg lengthening for ten days at 3 mm per day. On alternate days between the day after the operation and nerve dissection, the progesterone-treated group received subcutaneous injections of 1 mg progesterone in sesame oil and the control group received oil only. On the fifth, tenth and 17th day, the sciatic nerves were excised at the midpoint of the femur and the mRNA expression level of myelin protein P0 was analysed by quantitative real time polymerase chain reaction. On day 52 nodal length was examined by electron microscopy, followed by an examination of the compound muscle action potential (C-MAP) amplitude and the motor conduction velocity (MCV) of the tibial nerve on days 17 and 52. The P0 (a major myelin glycoprotein) mRNA expression level in the progesterone-treated group increased by 46.6% and 38.7% on days five and ten, respectively. On day 52, the nodal length in the progesterone-treated group was smaller than that in the control group, and the MCV of the progesterone-treated group had been restored to normal. Progesterone might accelerate the restoration of demyelination caused by nerve elongation by activating myelin synthesis.

Internodal function in normal and regenerated mammalian axons

AIM

Following Wallerian degeneration, peripheral myelinated axons have the ability to regenerate and, given a proper pathway, establish functional connections with targets. In spite of this capacity, the clinical outcome of nerve regeneration remains unsatisfactory. Early studies have found that regenerated internodes remain persistently short though this abnormality did not seem to influence recovery in conduction. It remains unclear to which extent abnormalities in axonal function itself may contribute to the poor outcome of nerve regeneration.

METHODS

We review experimental evidence indicating that internodes play an active role in axonal function.

RESULTS

By investigating internodal contribution to axonal excitability we have found evidence that axonal function may be permanently compromised in regenerated nerves. Furthermore, we illustrate that internodal function is also abnormal in regenerated human nerves.

CONCLUSION

The data suggest that persistently shorter regenerated internodes lead to increased Na+/K+-pump activity in response to increased Na+ entry during conduction. This may impair axonal function during prolonged repetitive activity and drain the energy reserves of the axons.

Repair of peripheral nerve defect with direct gradual lengthening of the proximal nerve stump in rats

We investigated the effect of direct gradual lengthening on the proximal nerve stump and subsequent nerve regeneration in rats. A 10-mm-long nerve segment was resected from the sciatic nerve of each rat. The proximal nerve stump was directly lengthened at a rate of 1 mm/day using an original external nerve distraction device. Experiment I: After distraction periods of 10, 15, and 20 days, the length of each nerve was evaluated, and the lengthened nerve stump was also examined by immunohistochemical analysis. Experiment II: After a distraction period of 20 days, both nerve stumps were refreshed and direct end-to-end neurorrhaphy was performed. For control, 10-mm nerve grafting was immediately performed after nerve resection. Nerve regeneration was evaluated electrophysiologically and histologically 7, 9, and 15 weeks after nerve resection in both groups. The whole proximal nerve stump, including the endoneurium and the axon, could be lengthened in proportion to the distraction period. There were no significant differences in motor nerve conduction velocity and tetanic muscle contraction force between both groups. Histologically, the total number of myelinated fibers was significantly greater in the nerve lengthening group than in the autografting group. This study demonstrated that the whole proximal nerve stump including the endoneurium and the axon could be lengthened by direct gradual distraction, and that this method might have potential application in the repair of peripheral nerve defects.

Direct evidence for coherent low velocity axonal transport of mitochondria

Axonal growth depends on axonal transport. We report the first global analysis of mitochondrial transport during axonal growth and pauses. In the proximal axon, we found that docked mitochondria attached to the cytoskeletal framework that were stationary relative to the substrate and fast axonal transport fully accounted for mitochondrial transport. In the distal axon, we found both fast mitochondrial transport and a coherent slow transport of the mitochondria docked to the axonal framework (low velocity transport [LVT]). LVT was distinct from previously described transport processes; it was coupled with stretching of the axonal framework and, surprisingly, was independent of growth cone advance. Fast mitochondrial transport decreased and LVT increased in a proximodistal gradient along the axon, but together they generated a constant mitochondrial flux. These findings suggest that the viscoelastic stretching/creep of axons caused by tension exerted by the growth cone, with or without advance, is seen as LVT that is followed by compensatory intercalated addition of new mitochondria by fast axonal transport.

Distribution of sodium channels during nerve elongation in rat peripheral nerve

A number of studies have investigated electrophysiological and morphological changes of peripheral nerves during gradual elongation. There has been, however, no report on the distribution of sodium channels at Ranvier's nodes during peripheral nerve elongation. We investigated peripheral nerve injury after the gradual elongation of rat sciatic nerves. Indirect nerve elongation was induced by leg lengthening at a rate of 3 mm/day by 15 or 30 mm. At 7 days after the leg lengthening, the electrophysiological properties of sciatic nerves, the ultrastructures of the Ranvier's nodes and axons, and the distribution of voltage-dependent sodium channels were examined. In the control nerves, most sodium channels were localized at Ranvier's nodes in myelinated axons, providing the physiological basis of saltatory conduction. In the elongated nerves, both the amplitude and conduction velocity of compound nerve action potential decreased following leg lengthening. The elongated nerves also showed paranodal demyelination in Ranvier's nodes longer than those in the control group. In addition, the distribution of sodium channels became diffuse or disappeared at Ranvier's nodes of elongated nerves. The diffuse distribution and/or disappearance of sodium channels may underlie the electrophysiological changes in compound nerve action potential induced by nerve elongation.

Structure and biomechanics of peripheral nerves: nerve responses to physical stresses and implications for physical therapist practice

The structural organization of peripheral nerves enables them to function while tolerating and adapting to stresses placed upon them by postures and movements of the trunk, head, and limbs. They are exposed to combinations of tensile, shear, and compressive stresses that result in nerve excursion, strain, and transverse contraction. The purpose of this appraisal is to review the structural and biomechanical modifications seen in peripheral nerves exposed to various levels of physical stress. We have followed the primary tenet of the Physical Stress Theory presented by Mueller and Maluf (2002), specifically, that the level of physical stress placed upon biological tissue determines the adaptive response of the tissue. A thorough understanding of the biomechanical properties of normal and injured nerves and the stresses placed upon them in daily activities will help guide physical therapists in making diagnoses and decisions regarding interventions.

Induction of tumor necrosis factor-alpha in Schwann cells after gradual elongation of rat sciatic nerve

BACKGROUND

Although limb lengthening has become a common treatment, the biochemical responses underlying the adaptation of elongated nerves are unclear. The purpose of this study was to clarify whether expression of cytokines and neurotrophins is altered in gradually elongated peripheral nerves.

METHODS

Left sciatic nerves of adult rats were elongated by lengthening the femur up to 20 mm at a rate of 1, 2, or 20 mm/day. The ipsilateral and contralateral sciatic nerves of each group were resected 1, 4, 8, and 16 days after 20 mm of lengthening. mRNAs for interleukin-1beta, interleukin-6, tumor necrosis factor-alpha (TNFalpha), nerve growth factor, brain-derived neurotrophic factor, neurotrophin-3, and neurotrophin-4/5 were semiquantified by reverse transcription-polymerase chain reaction. Histological changes were assessed by immunoperoxidase and immunofluorescence staining.

RESULTS

Expression of TNFalpha mRNA was markedly induced in the ipsilateral sciatic nerves of the gradually elongated, 1 mm/day and 2 mm/day groups, although to a lesser extent than in the acutely elongated, 20 mm/day group. In contrast, mRNAs for other factors remained undetectable. The mRNA level for TNFalpha in each group was highest 1 day after 20 mm of lengthening. The highly up-regulated level in the acute group declined rapidly within 4 days and slowly thereafter; in contrast, the decrease in the gradual groups was always slow. Even 16 days later, the levels in all groups remained significantly elevated. Unexpectedly, TNFalpha mRNA expression was also induced in the contralateral side of all groups. Immunohistochemical staining showed that TNFalpha-immunoreactive cells in gradually elongated nerves were also positive for S-100 protein but negative for proliferating nuclear cell antigen, indicating that TNFalpha was produced by nonproliferating Schwann cells.

CONCLUSIONS

Gradual nerve elongation by limb lengthening induces production of TNFalpha in Schwann cells. Presumably, TNFalpha plays a critical role in the adaptation of peripheral nerves to elongation.