Using gene therapy, spinal cord injury researchers have succeeded in introducing growth factors that have led to some recovery of function in rodent models (Blesch and Tuszynski, 2004; Hendriks et al., 2004). The experiments have thus far established the potential value of gene therapy, which can be used alone or in combination with other therapies.
Spinal cord injury not only leaves a glial cell scar but also leaves a physical gap. As early as 1906, a peripheral nerve was transplanted into the brain to see if CNS axons would regrow in an environment that was known to be supportive of axonal growth in the peripheral nervous system. Seven decades later, Richardson and colleagues (1980) found that months after they inserted a segment of a peripheral nerve into a gap in the spinal cord, the cut axons had regrown into the implanted nerve from both stumps of the severed spinal cord. This technique has been validated in studies with optic nerve neurons, which travel long distances between the eye and the brain. When peripheral nerve grafts were attached to the optic nerve stump, retinal axons were induced to regenerate long distances within the grafts and were capable of making functional connections when the grafts ended near their correct targets in the brain (Carter et al., 1989). Similar techniques have been used in the spinal cord. For example, researchers have induced some neuronal regeneration by transplanting peripheral nerve and Schwann cells inside a polymer tube to fill a complete or partial gap in the spinal cords (Bunge, 2001). Today, scientists are continuing to develop a number of different types of bridges that consist not only of peripheral nerves or Schwann cells, but also olfactory ensheathing cells (OECs), stem cells, marrow stromal cells, trophic factors, biomaterials, or some combination thereof.
A new generation of scaffolds is being developed for the broad field of tissue engineering (Holmes, 2002). The ideal scaffold for use in the repair of a spinal cord injury would be attractive to regenerating axons, a physical conduit for entry and exit, nontoxic and nonimmunogenic, versatile enough to house a wide range of drugs or cell types, and degradable over a time window sufficient for regrowth (Geller and Fawcett, 2002). The types of materials that may potentially be used as scaffolds include naturally occurring materials (e.g., collagen), organic polymers, and inorganic materials. Even more innovative scaffolds are materials that are injected as liquids and that then self-assemble into fibers with diameters of less than 1 micrometer (Silva et al., 2004).