More flexible

But because those cells are so versatile and easy to grow in large numbers in laboratories, they can be turned into treatments more easily and at less cost than is the case with adult stem cells, many scientists maintain. And given what has been learned from tests of human embryonic stem cells in rodents with spinal cord injuries, Geron executives believe it is time to see if the cells can be used on people, too.

Spinal cord injuries frequently damage one part of nerve cells called axons, the long wire-like extensions of the nerve cells. That damage disrupts the Axon's ability to transmit information, resulting in paralysis.

For its test, Geron proposes to turn human embryonic stem cells into the precursors for specialized nerve cells, called Oligodendrocyte progenitor cells. Surgeons then would inject the cells into the spinal injury with the help of a special stabilizing frame the company has developed.

If everything goes as planned, the progenitor cells would help form new axons and also turn into oligodendrocytes, which help form an insulating sheath for the axons, called Myelin.

The test probably would involve a few dozen patients, all of whom would have irreversible spinal injuries. Initially, the idea would be to merely determine if the injections were safe. But Geron executives hope additional tests would demonstrate the procedure's effectiveness in repairing damaged axons and restoring Motor function in less severely injured people.

Geron has been collaborating with researchers at the University of California-Irvine, who reported in October last year that paralyzed rats could walk nine weeks after being injected with oligodendrocyte progenitor cells derived from human embryonic stem cells.

The FDA did not return phone calls seeking comment on Geron's planned test.

Dr. Irving Weissman, director of the Stanford Institute for Stem Cell Biology and Regenerative Medicine, said he saw no reason the agency should turn down the company or demand that additional non-human tests be done first.

``They better not,'' Weissman said, ``because then they're putting themselves right in the way of treating humans for disease.'' Weissman is also a board member and co-founder of StemCells, a Palo Alto company that won federal approval in October to transplant adult stem cells into the brains of children to correct a fatal disorder.