In the January 2013 issue of Scientific American, D. Kacy Cullen and Douglas H. Smith of the University of Pennsylvania reported on their work using stretch-grown axons (the long thin "arm" of a nerve cell) to some day connect prosthetic devices to the peripheral nervous systems of people who had to have part of their arm amputated. There wasn't enough room to talk about it in the article, but there is another way that these "living bridges" could be used to help people with devastating injuries.

The stretch-grown axons could also be used to treat people with major nerve damage that does not necessarily require amputation. The biohybrid bridge provides a conduit for the undamaged part of the peripheral nervous system to bypass the injured nerve and regrow its own axons all the way to the end of the affected limb. If such bridges could be implanted within a few days to weeks of the injury, they would benefit from the fact that neural support cells are still active throughout the length of the limb (these cells usually take a few months to disappear after nerve death) and could guide the regrowing nerve fiber to its final destination. 

Cullen and Smith hope to begin testing their stretch-grown axons soon in a few U.S. soldiers who were injured while fighting overseas.

Cullen described their efforts in a recent email:

Peripheral nerve injury (PNI) is a major source of warfighter morbidity. Indeed, only 50% of patients achieve good to normal restoration of function following surgical repair, regardless of the strategy. Moreover, failure of nerve regeneration may necessitate amputation of an otherwise salvaged limb. This stems from the inadequacy of current PNI repair strategies, where even the “gold-standard“ treatment – the nerve autograft – is largely ineffective for major nerve trauma. Despite significant efforts, PNI repair has not progressed past nerve guidance tubes (NGTs) to bridge small gaps or autografts for larger defects. We have developed novel tissue-engineered nerve grafts (TENGs) that not only have the potential to surpass the performance of autografts, but also to repair currently untreatable PNI.

TENGs are lab-grown nervous tissue, comprised of long axonal tracts spanning two populations of neurons. The ability to generate these nerve grafts is based upon seminal discoveries regarding the process of axon growth via continuous mechanical tension or “stretch growth”. Stretch growth is a natural axon growth mechanism that can extend axons at unprecedented rates without the aid of chemical cues, physical guides or growth cones. We are able to replicate this process in a culture system through the controlled separation of two integrated populations of neurons. During stretch growth, individual axons gradually coalesce with neighboring axons to form large axonal tracts, called fascicles, taking on a highly organized parallel orientation resembling harp strings. TENGS are subsequently created by embedding these living axonal tracts in a three-dimensional matrix and removing them en masse for transplantation. Whereas other technologies and approaches have only been able to achieve axon lengths of 1-5 mm in culture, our platform can generate axons of unprecedented lengths in a very short time frame (5-10 cm in 14-21 days, with no theoretical limit as to the final axon length). To our knowledge, no other approach is capable of generating such long axonal constructs.

Current PNI repair options, whether an autograft or NGT, are unable to create a suitable environment for axonal regeneration in cases of major nerve trauma. The key failing of these strategies is the inability to coax a sufficient number of axons to grow a substantial distance to reinnervate distant targets (e.g., hand) and restore function. As a unique mechanism of action, transplanted TENGs recapitulate one of the most favorable environments for robust and long-distance axon growth: other axons.

We have demonstrated in the rat sciatic nerve model of PNI that regenerating axons have an intrinsic preference to grow along TENG axons, thus facilitating vigorous host axon regeneration. For these studies, allogeneic TENGs (encased in FDA-approved NGTs) were used to repair 1.2-1.5cm PNI lesions. By several weeks post-implant, dense host axon growth was visualized throughout the TENG and beyond. Conversely, host axon growth was limited and substantially delayed in control groups receiving an empty NGT or a NGT seeded with unstretched DRG neurons.

Importantly, host axons closely intertwined with the TENG axons demonstrating axon-induced axon growth, which promoted regeneration through the lesion. Moreover, axons from each end of the transplanted TENG penetrated longitudinally into host tissue providing an extended guidance.

Complete recapitulation of lost nerve anatomy using a TENG was demonstrated at 4 months, including re-vascularization and myelination. Moreover, functional recovery was demonstrated at this time-point via electrophysiological conduction, hindlimb contraflexion and angle board tasks. Remarkably, TENG neurons/axons survived over several months and no immunological response to allogeneic TENGs was observed. These data and others suggest that neurons are immunologically inert, thereby paving the way for the potential use of allogeneic TENGs without immunosuppressive therapy.

Another important aspect of the mechanism of action for TENGs is their ability to prolong the pro-regenerative environment of the distal nerve structure, which is necessary to facilitate and guide axon regeneration to an appropriate target. In particular, degeneration of the axon segments distal to an injury site is an inevitable consequence of transection; however, the supporting Schwann cells (SCs) survive and switch to a pro-regenerative phenotype to support axon growth. 

Unfortunately, this natural pro-regenerative environment degrades after several months without the presence of axons, thus depriving regenerating axons of their “road map” to an end target. This occurs when the time it takes regenerating axons to infiltrate is greater than the time this distal pathway will remain – which is often the case following long or proximal PNI – and is primarily responsible for incomplete functional recovery  (e.g., regaining elbow but not hand function following upper-arm PNI). The effectiveness of viable axons in maintaining this distal pathway is also evidenced by greater recovery following nerve crush injury where only a portion of the axons at the site of injury degenerate.

We have shown that TENGs have the unique ability, via their axons, to “babysit” the distal pathway following nerve transection. In particular, resident SCs in the distal sheath after TENG implantation maintained their pro-regenerative phenotype and alignment over extended time periods compared to an NGT alone. Therefore, beyond creating a suitable environment for axon regeneration, TENGs maintain the efficacy of the distal pathway to provide a complete guide for regenerating host axons to reach long-distance targets.