Harvested human neurons engineered as live nervous tissue constructs: implications for transplantation. Laboratory investigation

Enhanced axon outgrowth of spinal motor neurons in co-culturing with dorsal root ganglions antagonizes the growth inhibitory environment

Introduction

Forming a bridge made of functional axons to span the lesion is essential to reconstruct the motor circuitry following spinal cord injury (SCI). Dorsal root ganglion (DRG) axons are robust in axon growth and have been proved to facilitate the growth of cortical neurons in a process of axon-facilitated axon regeneration. However, whether DRG transplantation affects the axon outgrowth of spinal motor neurons (SMNs) that play crucial roles in motor circuitry remains unclear.

Methods

We investigated the axonal growth patterns of co-cultured DRGs and SMN aggregates (SMNAs) taking advantage of a well-designed 3D-printed in vitro system. Chondroitin sulphate proteoglycans (CSPG) induced inhibitory matrix was introduced to imitate the inhibitory environment following SCI. Axonal lengths of DRG, SMNA or DRG & SMNA cultured on the permissive or CSPG induced inhibitory matrix were measured and compared.

Results

Our results indicated that under the guidance of full axonal connection generated from two opposing populations of DRGs, SMNA axons were growth-enhanced and elongated along the DRG axon bridge to distances that they could not otherwise reach. Quantitatively, the co-culture increased the SMNA axonal length by 32.1 %. Moreover, the CSPG matrix reduced the axonal length of DRGs and SMNAs by 46.2 % and 17.7 %, respectively. This inhibitory effect was antagonized by the co-culture of DRGs and SMNAs. Especially for SMNAs, they extended the axons across the CSPG-coating matrix, reached the lengths close to those of SMNAs cultured on the permissive matrix alone.

Conclusions

This study deepens our understanding of axon-facilitated reconstruction of the motor circuitry. Moreover, the results support SCI treatment utilizing the enhanced outgrowth of axons to restore functional connectivity in SCI patients.

Dorsal root ganglion neurons recapitulate the traumatic axonal injury of CNS neurons in response to a rapid stretch in vitro

Introduction: In vitro models of traumatic brain injury (TBI) commonly use neurons isolated from the central nervous system. Limitations with primary cortical cultures, however, can pose challenges to replicating some aspects of neuronal injury associated with closed head TBI. The known mechanisms of axonal degeneration from mechanical injury in TBI are in many ways similar to degenerative disease, ischemia, and spinal cord injury. It is therefore possible that the mechanisms that result in axonal degeneration in isolated cortical axons after in vitro stretch injury are shared with injured axons from different neuronal types. Dorsal root ganglia neurons (DRGN) are another neuronal source that may overcome some current limitations including remaining healthy in culture for long periods of time, ability to be isolated from adult sources, and myelinated in vitro. Methods: The current study sought to characterize the differential responses between cortical and DRGN axons to mechanical stretch injury associated with TBI. Using an in vitro model of traumatic axonal stretch injury, cortical and DRGN neurons were injured at a moderate (40% strain) and severe stretch (60% strain) and acute alterations in axonal morphology and calcium homeostasis were measured. Results: DRGN and cortical axons immediately form undulations in response to severe injury, experience similar elongation and recovery within 20 min after the initial injury, and had a similar pattern of degeneration over the first 24 h after injury. Additionally, both types of axons experienced comparable degrees of calcium influx after both moderate and severe injury that was prevented through pre-treatment with tetrodotoxin in cortical neurons and lidocaine in DRGNs. Similar to cortical axons, stretch injury also causes calcium activated proteolysis of sodium channel in DRGN axons that is prevented by treatment with lidocaine or protease inhibitors. Discussion: These findings suggest that DRGN axons share the early response of cortical neurons to a rapid stretch injury and the associated secondary injury mechanisms. The utility of a DRGN in vitro TBI model may allow future studies to explore TBI injury progression in myelinated and adult neurons.

Tissue-engineered grafts exploit axon-facilitated axon regeneration and pathway protection to enable recovery after 5-cm nerve defects in pigs

Functional restoration following major peripheral nerve injury (PNI) is challenging, given slow axon growth rates and eventual regenerative pathway degradation in the absence of axons. We are developing tissue-engineered nerve grafts (TENGs) to simultaneously "bridge" missing nerve segments and "babysit" regenerative capacity by providing living axons to guide host axons and maintain the distal pathway. TENGs were biofabricated using porcine neurons and "stretch-grown" axon tracts. TENG neurons survived and elicited axon-facilitated axon regeneration to accelerate regrowth across both short (1 cm) and long (5 cm) segmental nerve defects in pigs. TENG axons also closely interacted with host Schwann cells to maintain proregenerative capacity. TENGs drove regeneration across 5-cm defects in both motor and mixed motor-sensory nerves, resulting in dense axon regeneration and electrophysiological recovery at levels similar to autograft repairs. This approach of accelerating axon regeneration while maintaining the pathway for long-distance regeneration may achieve recovery after currently unrepairable PNIs.

Biomanufacturing of Axon-Based Tissue Engineered Nerve Grafts Using Porcine GalSafe Neurons

Existing strategies for repair of major peripheral nerve injury (PNI) are inefficient at promoting axon regeneration and functional recovery and are generally ineffective for nerve lesions >5 cm. To address this need, we have previously developed tissue engineered nerve grafts (TENGs) through the process of axon stretch growth. TENGs consist of living, centimeter-scale, aligned axon tracts that accelerate axon regeneration at rates equivalent to the gold standard autograft in small and large animal models of PNI, by providing a newfound mechanism-of-action referred to as axon-facilitated axon regeneration (AFAR). To enable clinical-grade biomanufacturing of TENGs, a suitable cell source that is hypoimmunogenic, exhibits low batch-to-batch variability, and able to tolerate axon stretch growth must be utilized. To fulfill these requirements, a genetically engineered, FDA-approved, xenogeneic cell source, GalSafe® neurons, produced by Revivicor, Inc., have been selected to advance TENG biofabrication for eventual clinical use. To this end, sensory and motor neurons were harvested from genetically engineered GalSafe day 40 swine embryos, cultured in custom mechanobioreactors, and axon tracts were successfully stretch-grown to 5 cm within 25 days. Importantly, both sensory and motor GalSafe neurons were observed to tolerate established axon stretch growth regimes of ≥1 mm/day to produce continuous, healthy axon tracts spanning 1, 3, or 5 cm. Once stretch-grown, 1 cm GalSafe TENGs were transplanted into a 1 cm lesion in the sciatic nerve of athymic rats. Regeneration was assessed through histological measures at the terminal time point of 2 and 8 weeks. Neurons from GalSafe TENGs survived and elicited AFAR as observed when using wild-type TENGs. At 8 weeks postrepair, myelinated regenerated axons were observed in the nerve section distal to the injury site, confirming axon regeneration across the lesion. These experiments are the first to demonstrate successful harvest and axon stretch growth of GalSafe neurons for use as starting biomass for bioengineered nerve grafts as well as initial safety and efficacy in an established preclinical model-important steps for the advancement of clinical-grade TENGs for future regulatory testing and eventual clinical trials. Impact statement Biofabrication of tissue engineered medical products requires several steps, one of which is choosing a suitable starting biomass. To this end, we have shown that the clinical-grade, genetically engineered biomass-GalSafe® neurons-is a viable option for biomanufacturing of our tissue engineered nerve grafts (TENGs) to promote regeneration following major peripheral nerve injury. Importantly, this is a first step in clinical-grade TENG biofabrication, proving that GalSafe TENGs recapitulate the mechanism of axon-facilitated axon regeneration seen previously with research-grade TENGs.

Biomaterials for Neural Tissue Engineering

The therapy of neural nerve injuries that involve the disruption of axonal pathways or axonal tracts has taken a new dimension with the development of tissue engineering techniques. When peripheral nerve injury (PNI), spinal cord injury (SCI), traumatic brain injury (TBI), or neurodegenerative disease occur, the intricate architecture undergoes alterations leading to growth inhibition and loss of guidance through large distance. To improve the limitations of purely cell-based therapies, the neural tissue engineering philosophy has emerged. Efforts are being made to produce an ideal scaffold based on synthetic and natural polymers that match the exact biological and mechanical properties of the tissue. Furthermore, through combining several components (biomaterials, cells, molecules), axonal regrowth is facilitated to obtain a functional recovery of the neural nerve diseases. The main objective of this review is to investigate the recent approaches and applications of neural tissue engineering approaches.

Tissue Engineered Axon Tracts Serve as Living Scaffolds to Accelerate Axonal Regeneration and Functional Recovery Following Peripheral Nerve Injury in Rats

Strategies to accelerate the rate of axon regeneration would improve functional recovery following peripheral nerve injury, in particular for cases involving segmental nerve defects. We are advancing tissue engineered nerve grafts (TENGs) comprised of long, aligned, centimeter-scale axon tracts developed by the controlled process of axon "stretch-growth" in custom mechanobioreactors. The current study used a rat sciatic nerve model to investigate the mechanisms of axon regeneration across nerve gaps bridged by TENGs as well as the extent of functional recovery compared to nerve guidance tubes (NGT) or autografts. We established that host axon growth occurred directly along TENG axons, which mimicked the action of "pioneer" axons during development by providing directed cues for accelerated outgrowth. Indeed, axon regeneration rates across TENGs were 3-4 fold faster than NGTs and equivalent to autografts. The infiltration of host Schwann cells - traditional drivers of peripheral axon regeneration - was also accelerated and progressed directly along TENG axons. Moreover, TENG repairs resulted in functional recovery levels equivalent to autografts, with both several-fold superior to NGTs. These findings demonstrate that engineered axon tracts serve as "living scaffolds" to guide host axon outgrowth by a new mechanism - which we term "axon-facilitated axon regeneration" - that leads to enhanced functional recovery.

Stretch growth of motor axons in custom mechanobioreactors to generate long-projecting axonal constructs

The central feature of peripheral motor axons is their remarkable lengths as they project from a motor neuron residing in the spinal cord to distant target muscle. However, current in vitro models have not replicated this feature owing to challenges in generating motor axon tracts beyond a few millimeters in length. To address this, we have developed a novel combination of microtissue engineering and mechanically assisted growth techniques to create long-projecting centimeter-scale motor axon tracts. Here, primary motor neurons were isolated from rat spinal cords and induced to form engineered microspheres via forced aggregation in custom microwells. This technique yielded healthy motor neurons projecting dense, fasciculated axonal tracts. Within our custom-built mechanobioreactors, motor neuron culture conditions, neuronal/axonal architecture, and mechanical growth conditions were optimized to generate parameters for robust and efficient stretch growth of motor axons. We found that axons projecting from motor neuron aggregates were able to tolerate displacement rates at least 10 times greater than those by axons projecting from dissociated motor neurons. The growth and structural characteristics of these stretch-grown motor axons were compared with that of benchmark stretch-grown sensory axons, revealing increased motor axon fasciculation. Finally, motor axons were integrated with myocytes and stretch grown to create novel long-projecting axonal-myocyte constructs that recreate characteristic dimensions of native nerve-muscle anatomy. This is the first demonstration of mechanical elongation of spinal motor axons and may have applications as anatomically inspired in vitro testbeds or as tissue-engineered living scaffolds for targeted axon tract reconstruction following nervous system injury or disease.

An update-tissue engineered nerve grafts for the repair of peripheral nerve injuries

Peripheral nerve injuries (PNI) are caused by a range of etiologies and result in a broad spectrum of disability. While nerve autografts are the current gold standard for the reconstruction of extensive nerve damage, the limited supply of autologous nerve and complications associated with harvesting nerve from a second surgical site has driven groups from multiple disciplines, including biomedical engineering, neurosurgery, plastic surgery, and orthopedic surgery, to develop a suitable or superior alternative to autografting. Over the last couple of decades, various types of scaffolds, such as acellular nerve grafts (ANGs), nerve guidance conduits, and non-nervous tissues, have been filled with Schwann cells, stem cells, and/or neurotrophic factors to develop tissue engineered nerve grafts (TENGs). Although these have shown promising effects on peripheral nerve regeneration in experimental models, the autograft has remained the gold standard for large nerve gaps. This review provides a discussion of recent advances in the development of TENGs and their efficacy in experimental models. Specifically, TENGs have been enhanced via incorporation of genetically engineered cells, methods to improve stem cell survival and differentiation, optimized delivery of neurotrophic factors via drug delivery systems (DDS), co-administration of platelet-rich plasma (PRP), and pretreatment with chondroitinase ABC (Ch-ABC). Other notable advancements include conduits that have been bioengineered to mimic native nerve structure via cell-derived extracellular matrix (ECM) deposition, and the development of transplantable living nervous tissue constructs from rat and human dorsal root ganglia (DRG) neurons. Grafts composed of non-nervous tissues, such as vein, artery, and muscle, will be briefly discussed.

[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.

An update-tissue engineered nerve grafts for the repair of peripheral nerve injuries

Peripheral nerve injuries (PNI) are caused by a range of etiologies and result in a broad spectrum of disability. While nerve autografts are the current gold standard for the reconstruction of extensive nerve damage, the limited supply of autologous nerve and complications associated with harvesting nerve from a second surgical site has driven groups from multiple disciplines, including biomedical engineering, neurosurgery, plastic surgery, and orthopedic surgery, to develop a suitable or superior alternative to autografting. Over the last couple of decades, various types of scaffolds, such as acellular nerve grafts (ANGs), nerve guidance conduits, and non-nervous tissues, have been filled with Schwann cells, stem cells, and/or neurotrophic factors to develop tissue engineered nerve grafts (TENGs). Although these have shown promising effects on peripheral nerve regeneration in experimental models, the autograft has remained the gold standard for large nerve gaps. This review provides a discussion of recent advances in the development of TENGs and their efficacy in experimental models. Specifically, TENGs have been enhanced via incorporation of genetically engineered cells, methods to improve stem cell survival and differentiation, optimized delivery of neurotrophic factors via drug delivery systems (DDS), co-administration of platelet-rich plasma (PRP), and pretreatment with chondroitinase ABC (Ch-ABC). Other notable advancements include conduits that have been bioengineered to mimic native nerve structure via cell-derived extracellular matrix (ECM) deposition, and the development of transplantable living nervous tissue constructs from rat and human dorsal root ganglia (DRG) neurons. Grafts composed of non-nervous tissues, such as vein, artery, and muscle, will be briefly discussed.

A model for stretch growth of neurons

In the first phase of axon growth, axons sprout from neuron bodies and are extended by the pull of the migrating growth cones towards their targets. Thereafter, once the target is reached, a lesser known second phase of axon growth ensues as the mechanical forces from the growth of the animal induce extension of the integrated axons in the process of forming tracts and nerves. Although there are several microscopic physics based models of the first phase of axon growth, to date, there are no models of the very different second phase. Here we propose a mathematical model for stretch growth of axon tracts in which the rate of production of proteins required for growth is dependent on the membrane tension. We assume that growth occurs all along the axon, and are able to predict the increase in axon cross-sectional area after they are rapidly stretched and held at a constant length for several hours. We show that there is a length dependent maximum stretching rate that an axon can sustain without disconnection in steady state when the axon length is primarily increased near the cell body. Our results could inform better design of stretch growth protocols to create transplantable axon tracts to repair the nervous system.

Functional, electrophysiological recoveries of rats with sciatic nerve lesions following transplantation of elongated DRG cells

OBJECTIVES

Functional data are essential when confirming the efficacy of elongated dorsal root ganglia (DRG) cells as a substitute for autografting. We present the quantitative functional motor, electrophysiological findings of engineered DRG recipients for the first time.

METHODS

Elongated DRG neurons and autografts were transplanted to bridge 1-cm sciatic nerve lesions of Sprague Dawley (SD) rats. Motor recoveries of elongated DRG recipients (n=9), autograft recipients (n=9), unrepaired rats (n=9) and intact rats (n=6) were investigated using the angle board challenge test following 16 weeks of recovery. Electrophysiology studies were conducted to assess the functional recovery at 16 weeks. In addition, elongated DRGs were subjected to histology assessments.

RESULTS

At threshold levels (35° angle) of the angle board challenge test, the autograft recipients', DRG recipients' and unrepaired group's performances were equal to each other and were less than the intact group (p<0.05). However, during the subthreshold (30°) angle board challenge test, the elongated DRG recipients' performance was similar to both the intact group and the autograft nerve recipients, and was better (p<0.05) than the unrepaired group. The autograft recipients' performance was similar to the unrepaired group and was significantly different (p<0.05) compared with the performance of the intact group. During electrophysiological testing, the rats with transplanted engineered DRG constructs had intact signal transmission when recorded over the lesion, while the unrepaired rats did not. It was observed that elongated DRG neurons closely resembled an autograft during histological assessments.

CONCLUSION

Performances of autograft and elongated DRG construct recipients were similar. Elongated DRG neurons should be further investigated as a substitute for autografting.

Effect of nano-hydroxyapatite on the axonal guidance growth of rat cortical neurons

Nanomaterials such as carbon nanotubes (CNT) can improve axonal connecting in a target direction during regeneration, however, it is limited by the neurotoxicity of CNT. Here we investigate the possible protective effect of nano-hydroxyapatite (n-HA) against nerve injury, as well as CNT in cultured rat cortical neurons. In this study the nanomaterials were characterized by X-Ray diffractometry (XRD) and atomic force microscopy (AFM) analysis. Our results showed that axonal migration and extension were increased significantly after n-HA treatment by immunocytochemistry assay. The patch clamp assay results showed that n-HA acts protectively after nerve injury, which inhibited the average amplitude and frequency of excitatory postsynaptic currents (EPSCs). n-HA is not neurotoxic for the electrophysiology activity of cells. To find the effect of n-HA on axonal guidance growth in the cultured cortical neurons, Netrin 1, one of the axonal guidance cues, was determined by RT-PCR and western blot assay. Compared to the control group, n-HA down-regulated the mRNA level of netrin 1, and moreover, the expression of netrin 1 decreased significantly in the cells. n-HA caused the axonal guidance growth to be mediated by netrin 1 during nerve regeneration. Therefore, the data from the present study provided a new approach for the therapy or prevention of nerve injury.

Experimental in vivo and in vitro models of multiple sclerosis: EAE and beyond

Although the primary cause of multiple sclerosis (MS) is unknown, the widely accepted view is that aberrant (auto)immune responses possibly arising following infection(s) are responsible for the destructive inflammatory demyelination and neurodegeneration in the central nervous system (CNS). This notion, and the limited access of human brain tissue early in the course of MS, has led to the development of autoimmune, viral and toxin-induced demyelination animal models as well as the development of human CNS cell and organotypic brain slice cultures in an attempt to understand events in MS. The autoimmune models, collectively known as experimental autoimmune encephalomyelitis (EAE), and viral models have shaped ideas of how environmental factors may trigger inflammation, demyelination and neurodegeneration in the CNS. Understandably, these models have also heavily influenced the development of therapies targeting the inflammatory aspect of MS. Demyelination and remyelination in the absence of overt inflammation are better studied in toxin-induced demyelination models using cuprizone and lysolecithin. The paradigm shift of MS as an autoimmune disease of myelin to a neurodegenerative disease has required more appropriate models reflecting the axonal and neuronal damage. Thus, secondary progressive EAE and spastic models have been crucial to develop neuroprotective approaches. In this review the current in vivo and in vitro experimental models to examine pathological mechanisms involved in inflammation, demyelination and neuronal degeneration, as well as remyelination and repair in MS are discussed. Since this knowledge is the basis for the development of new therapeutic approaches for MS, we particularly address whether the currently available models truly reflect the human disease, and discuss perspectives to further optimise and develop more suitable experimental models to study MS.

Live imaging of axon stretch growth in embryonic and adult neurons

Strategies for nervous system repair arise from knowledge of growth mechanisms via a growth cone. The distinctive process of axon stretch growth is a robust, long-term growth that may reveal new pathways to accelerate nerve repair. Here, a live imaging bioreactor was engineered to closely explore cellular events initiated by applied tension. The stretch growth potential between adult and embryonic dorsal root ganglion (DRG) neurons was investigated, an important difference in nerve repair. Embryonic axons were capable of unidirectional stretch growth rates of 4?mm/d and reliably reached 4?cm in length within 2 weeks. Adult axons could only reach 2?mm/d and took over 3 weeks to reach 4?cm. Utilizing time-lapse imaging, we observed growth cone motility in coordination with stretch growth. Upon initiation of stretching, growth cones retracted. However, within 10?h of continuous stretching, growth cones extended at a rate of 0.2?mm/d opposite the direction of applied tension, contributing to overall axon elongation. We analyzed fast mitochondrial transport under increasing levels of strain to determine the effect of stretch on axonal transport. Transport began to diminish at 24% strain, and was almost completely absent at 39% strain. Surprisingly, axons recovered and were capable of subsequent stretch growth. When tension was completely released (?5% strain), stretch grown axons retracted at rates up to 6.1??m/sec and slowed as resting tension was restored. This ability to assess the process of axon stretch growth in real time will allow detailed study of how tension can be used to drive axonal growth and retraction.

Hydrogel for Cell Housing in the Brain and in the Spinal Cord

Purpose

Neurons in the adult mammalian central nervous system do not proliferate or renew themselves and consequently strong interest in cell replacement therapies to repair brain and spinal cord damages has emerged in the last decade.

Methods

An injectable resorbable hydrogel with a controlled nanostructure, specifically designed for neural cell housing, was developed together with a new protocol for building three-dimensional (3D) biohybrid cell/hydrogel constructs: cells are housed within the polymeric matrix which is directly built with a specific cell culture media. This matrix was tested with standard glial populations, primary astrocytes and mesenchymal stem cells.

Results

Physico-chemical characterization of the hydrogel matrix confirmed a 2- week (+ 2 days) stability before massive degradation; mean mesh size of about 5 nm and thixotropic behavior with transition yield stress at 60+5 Pa. Cell survival within the hydrogel resulted in about 55±5% (minimum value) survivals, data also confirmed by optical assessments. Cell viability also remained high after extraction from the gel, indicating survival to inclusion latency period.

Conclusions

Since the intimate structure of the gel mimics extracellular matrix cells as would be expected to be found in an in vivo context, this polymeric formulation is a promising base for building 3D constructs for neural cell housing, in which cells are embedded and kept alive directly from the time of polycondensation.

Long-term survival and integration of transplanted engineered nervous tissue constructs promotes peripheral nerve regeneration

Although peripheral nerve injury is a common consequence of trauma or surgery, there are insufficient means for repair. In particular, there is a critical need for improved methods to facilitate regeneration of axons across major nerve lesions. Here, we engineered transplantable living nervous tissue constructs to provide a labeled pathway to guide host axonal regeneration. These constructs consisted of stretch-grown, longitudinally aligned living axonal tracts inserted into poly(glycolic acid) tubes. The constructs (allogenic) were transplanted to bridge an excised segment of sciatic nerve in the rat, and histological analyses were performed at 6 and 16 weeks posttransplantation to determine graft survival, integration, and host regeneration. At both time points, the transplanted constructs were found to have maintained their pretransplant geometry, with surviving clusters of graft neuronal somata at the extremities of the constructs spanned by tracts of axons. Throughout the transplanted region, there was an intertwining plexus of host and graft axons, suggesting that the transplanted axons mediated host axonal regeneration across the lesion. By 16 weeks posttransplant, extensive myelination of axons was observed throughout the transplant region. Further, graft neurons had extended axons beyond the margins of the transplanted region, penetrating into the host nerve. Notably, this survival and integration of the allogenic constructs occurred in the absence of immunosuppression therapy. These findings demonstrate the promise of living tissue-engineered axonal constructs to bridge major nerve lesions and promote host regeneration, potentially by providing axon-mediated axonal outgrowth and guidance.

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.