The prevailing dogma in regeneration holds that "axons grow where the [growth promoting] cells go," said Hans Keirstead, assistant professor of anatomy and neurobiology at the University of California in Irvine. But researchers have not been able to take advantage of that concept because growth-promoting cells generally do not migrate, Keirstead explained.

Embryonic stem cells are one exception to that rule; so are olfactory ensheathing cells (OECs), which enable the sense of smell. OECs grow rapidly, are non-tumorigenic, and unlike most cells of the central nervous system, can replace themselves when injured. When they are transplanted, the cells can also migrate. "That's a rare, rare trait," Keirstead said.

In the past few years, several teams have investigated the ability of OECs to promote axon regeneration and generate new myelin sheaths, primarily with cells derived from canine and rodent olfactory bulbs. The researchers have met with mixed results, ranging from excellent recovery to none at all.

When Keirstead purified OECs from human nasal mucosa and transplanted them into a rat model of severe spinal injury, there was some regrowth of neurons in the injured spinal cords, even in areas with scarring, Keirstead reported. The rats also quickly regained bladder function and regained some of their ability to walk.

"This is the first time human OECs prepared in this high-purity manner have been used to investigate their ability to treat the injured spinal cord," says Keirstead. "Because our study used human cells, it has direct significance for clinical use."

Using human OECs has many advantages. Extraction of the cells is an easy five- minute procedure, and subjects experience no side-effects, apart from temporary hallucinations of smell, Keirstead said. They can also be used in autologous transplants, minimizing most of the complications associated with transplantation.

OECs are only one of several new promising techniques in regeneration. At the University of Utah, researcher Ray Lund and his colleagues have transplanted a human pigment epithelial cell line and improved vision in a rat model of retinal degeneration.

Yale University researchers, led by neurobiologist Stephen Strittmatter, have developed a synthetic peptide that blocks the Nogo gene, which inhibits regeneration. The peptide promotes new nerve fiber growth in the damaged spinal cords of laboratory rats, allowing them to walk better, Strittmatter said.

Before any of these therapies can be moved to human trials, however, they must first undergo extensive testing for safety and efficacy in human disease.