Combinatorial Tissue Engineering Partially Restores Function after Spinal Cord Injury

Hydrogel scaffolds provide a beneficial microenvironment in transected rat spinal cord. A combinatorial biomaterials based strategy provided a microenvironment that facilitated regeneration while reducing foreign body reaction to the 3-dimensional spinal cord construct. We used poly lactic-co-glycolic acid microspheres to provide sustained release of rapamycin from Schwann cell (SC)-loaded, positively charged oligo-polyethylene glycol fumarate scaffolds. Three dose formulations of rapamycin were compared to controls in 53 rats. We observed a dose-dependent reduction in the fibrotic reaction to the scaffold and improved functional recovery over 6 weeks. Recovery was replicated in a second cohort of 28 animals that included retransection injury. Immunohistochemical and stereological analysis demonstrated that blood vessel number, surface area, vessel diameter, basement membrane collagen, and microvessel phenotype within the regenerated tissue was dependent on the presence of SCs and rapamycin. TRITC-dextran injection demonstrated enhanced perfusion into scaffold channels. Rapamycin also increased the number of descending regenerated axons, as assessed by Fast Blue retrograde axonal tracing. These results demonstrate that normalization of the neovasculature was associated with enhanced axonal regeneration and improved function after spinal cord transection.

The spatial relationships among cutaneous, muscle sensory and motoneuron axons during development of the chick hindlimb

Previous studies have suggested that interactions with other axons are important in sensory axon pathfinding in the developing chick hindlimb. Yet the nature of these interactions remains unknown, in part because information about the spatial relationships among the different kinds of axons is lacking. To obtain this information, we combined retrograde axonal tracing with an immunofluorescent labelling approach that distinguishes between sensory and motoneuron axons. This allowed us to follow the trajectories of sensory axons having a known destination, while also identifying their neighbors. We found that as sensory and motoneuron axons meet in the spinal nerves and travel into the limb, sensory axons remain bundled together. The large bundles that are present proximally gradually split into smaller bundles as the axons course distally in the spinal nerves; more distally, some bundles join to again form large bundles. Younger, later-growing sensory axons appear to grow primarily along bundles of older sensory axons that grew out earlier. Starting from very proximal levels, axons projecting along an individual cutaneous nerve are found together in bundles that are situated in characteristic regions of each spinal nerve. Some of these bundles are initially interspersed with bundles of axons projecting along other nerves, thereby indicating that the initial position of a cutaneous axon in the spinal nerves does not strictly determine its subsequent trajectory. As they travel distally, bundles of axons projecting along one cutaneous nerve gradually join one another, becoming increasingly separated from axons having different destinations. In contrast, muscle sensory axons are situated adjacent to motoneuron axons innervating the same muscle for much of their course. This suggests that muscle sensory axons may be guided to the appropriate muscles by fasciculating along motoneuron axons. Taken together, the results show that sensory axons projecting along different nerves are different from one another and respond to cues in their environment to navigate through the spinal nerves and plexus. Thus, sensory neurons must be intrinsically specified with respect to their peripheral targets. Sensory axons appear to respond differentially to the axons they encounter, segregating from axons that project along different nerves and often growing with axons destined for the same nerve, suggesting that fasciculation may aid pathfinding.