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Spinal Cord Injury: Progress, Promise, and Priorities
tors for a specific neurotrophic factor known as NT-3 (McMahon et al., 1994).
In one of the first experiments of its kind, researchers applied NT-3 directly onto the spinal cords of rats (intrathecally) whose sensory fibers had been cut near the entry point into the spinal cord. The cut end of the nerve regrew into the spinal cord and reconnected with target cells at the appropriate level. Not only were the new synapses anatomically correct, but proprioceptive functioning was restored behaviorally and physiologically (Ramer et al., 2002). In a separate set of experiments, patients with a disease that causes demyelination in the peripheral nervous system were given NT-3. This treatment led to improved sensation, a return of the reflexes, and peripheral axon regeneration (Sahenk, 2003). Thus, there is a need to explore the use of neurotrophic factors for promotion of the regrowth of the sensory fibers.
STEM CELLS AND OTHER CELL-BASED THERAPIES
Cell-based therapies hold great potential as a means of replacing cells and restoring function that has been lost because of a disease or an injury. The application of cell-based therapies to spinal cord injuries is a natural outgrowth of research in other fields, such as cancer, diabetes, and heart disease. Hematopoietic stem cell-based therapies are now being used routinely to treat certain cancers and are being tested for use in regenerative medicine, for example, to replace insulin-secreting cells destroyed by juvenile diabetes or muscle cells destroyed by heart attacks. Therapies are being developed to restore function in individuals with spinal cord injuries by transplanting many different types of cells, including Schwann cells and OECs to restore nerve conduction, genetically engineered cells to restore trophic support and support regrowth, and stem cells that have the capacity to improve function through a number of mechanisms (Hulsebosch, 2002).
The Promise of Schwann Cells and Olfactory Ensheathing Cells
For more than a decade, researchers have known that Schwann cells, the ensheathing cells ordinarily found only in the peripheral nervous system, migrate into the spinal cord after it is injured. Thus, Schwann cells may be used in potential therapies for spinal cord injuries whether they are endogenous or transplanted (Bunge and Wood, 2004). There they may help stimulate axonal growth and myelinate the newly grown axons. The use of Schwann cells is attractive because they are readily accessible and proliferate rapidly in cell culture—up to 100,000 times—and do not trigger an immune response, as long as the individual’s own Schwann cells are used. One problem, however, is that regrowing axons do not exit and grow