Implantation of Engineered Axon Tracts to Bridge Spinal Cord Injury Beyond the Glial Scar in Rats.

Regeneration after spinal cord injury (SCI) is limited by the presence of a glial scar and inhibitory cell signaling pathways that favor scar formation over regrowth of endogenous neurons. Tissue engineering techniques, including the use of allografted neural networks, have shown promise for nervous system repair in prior studies. Through the use of a minimally invasive injury model in rats, we describe the implantation of micro-tissue engineered neural networks (micro-TENNs) across a region of SCI, spanning the glial scar to promote axonal regeneration. Forty-three female Sprague-Dawley rats were included in this study. Micro-TENNs were preformed in vitro before implant, and comprised rat sensory dorsal root ganglion (DRG) neurons projecting long bundled axonal tracts within the lumen of a biocompatible hydrogel columnar encasement (1.2 cm long; 701 μm outer diameter × 300 μm inner diameter). Animals were injured using a 2F embolectomy catheter inflated within the epidural space. After a 2-week recovery period, micro-TENNs were stereotactically implanted across the injury. Animals were euthanized at 1 week and 1 month after implantation, and the tissue was interrogated for the survival of graft DRG neurons and outgrowth of axons. No intraoperative deaths were noted with implantation of the micro-TENNs to span the injury cavity. Graft DRG axons were found to survive at 1 week postimplant within the hydrogel encasement. Graft-derived axonal outgrowth was observed within the spinal cord up to 4.5 mm from the implant site at 1 month postinjury. Limited astroglial response was noted within the host, suggesting minimal trauma and scar formation in response to the graft. Micro-TENN sensory neurons survive and extend axons into the host spinal cord following a minimally invasive SCI in rats. This work serves as the foundation for future studies investigating the use of micro-TENNs as a living bridge to promote recovery following SCI. Impact statement As spinal cord injury pathology develops, the establishment of a glial scar puts an end to the hope of regeneration and recovery from the consequent neurological deficits. Therefore, growing attention is given to bioengineered scaffolds that can bridge the lesions bordered by this scar tissue. The utilization of longitudinally aligned preformed neural networks-referred to as micro-tissue engineered neural networks (TENNs)-presents a promising opportunity to provide a multipurpose bridging strategy that may take advantage of several potential mechanisms of host regeneration. In addition to providing physical support for regenerating spinal cord axons, micro-TENNs may serve as a functional "cable" that restores lost connections within the spinal cord.