“We really want the best trial to be done for this first trial, and this might not be it,” he said.

Dr. Okarma of Geron emphasized that the purpose of the first trial was safety, so that lack of efficacy should not be a problem. While researchers will also look for signs the treatment works, he said, the best that could be hoped for would be some slight restoration of function that could then be enhanced through physical therapy.

“We don’t expect to take someone who is completely paralyzed from the waist down and have them dance six months later,” he said. If the first trial shows safety, the company would then hope to test higher doses of cells and treat patients with less severe injuries, he said.

Geron’s therapy involves using various growth factors to turn embryonic stem cells into precursors of neural support cells called oligodendrocytes, which are then injected into the spinal cord at the site of the injury.

The hope is that the injected cells will help repair the insulation, known as myelin, around nerve cells, restoring the ability of some nerve cells to carry signals. There is also some hope that growth factors produced by the injected cells will spur damaged nerve cells to regenerate.

The therapy was developed in collaboration with Hans Keirstead of the University of California, Irvine. He has shown videos of paralyzed rats that were able to walk again, albeit imperfectly, after receiving the therapy. Those videos helped persuade California voters to approve the $3 billion stem cell research program in 2004.

The main safety concern is that if raw embryonic cells are put into the body, they can form tumors. Even though most such tumors do not spread like other cancers, any unwanted growth in the spinal cord can further damage nerves.