With patches of stem cells on their broken spinal cords, partially paralyzed rats once again reached out and grabbed distant treats, researchers report in Nature Medicine.

While previous studies have shown progress in regenerating certain types of nerve cells in injured spinal cords, the study is the first to coax the regrowth of a specific set of nerve cells, called corticospinal axons. These bundles of biological wiring carry signals from the brain to the spinal cord and are critical for voluntary movement. In the study, researchers were able to use stem cells from rats and humans to mend the injured rodents.

“The corticospinal projection is the most important motor system in humans,” senior author Mark Tuszynski at the University of California, San Diego said. “It has not been successfully regenerated before. Many have tried, many have failed—including us, in previous efforts.”

For the study, the researchers used rat and human neural progenitor cells, which can produce several different types of cells found in the central nervous system. The researchers coaxed the cells into forming spinal cord tissue using specific chemical signals. When injected into the damaged spinal cords of rats, the cells took root, filling lesions with new tissue and corticospinal axons. And the new nerve cells linked up with the severed connections left hanging from the injury, allowing signals to traverse the patch.

In mobility tests, injured rats that got the spinal patch could better stretch out their front legs to grab hard-to-reach treats compared with injured rats without the stem-cell grafts.

Still, the cord-patching method is far from clinical use in humans, the authors caution. Researchers will need to follow the rats to look at long-term safety and effectiveness of the patches. Then, they'll have to try out the patches in other animal models before optimizing the method for humans.

But,Tuszynski said, "now that we can regenerate the most important motor system for humans, I think that the potential for translation is more promising."

Nature Medicine, 2015. DOI: 10.1038/nm.4066 (About DOIs).