By Steve Johnson
Mercury News

Scientists have made stunning progress helping paralyzed rats and mice walk again by injecting them with stem cells. Now researchers at Geron of Menlo Park want to take the next step -- in people.

They hope to get federal permission to inject those cells into damaged spinal cords. The procedure -- which Geron intends to do next year -- would be the first human tests of a treatment derived from human embryonic stem cells, the highly versatile body cells that can be coaxed into becoming almost any tissue in the body.

If the U.S. Food and Drug Administration gives its OK and the injections help, it could bring hope to many of the more than 250,000 people in this country with spinal disabilities.

``We don't have our hopes sky-high,'' said Don Reed of Fremont, whose son Roman Reed was left a quadriplegic after breaking his neck in a college football game 11 years ago. He also hopes the FDA and Geron don't rush into human tests until all safety concerns are resolved.

But if Geron's procedure helps patients, ``it would be fantastic,'' Reed added. ``Nobody should go through the hell that paralyzed people have to.''

Nonetheless, the plan has aroused concerns among some stem-cell researchers who claim Geron is moving too fast. They have urged the company to do more tests in non-human animals before trying it in people, because they fear cells could cause tumors or other health problems in those receiving the injections.

``Frankly, I cannot conceive of a human trial with the use of human embryonic stem cells following immediately from experiments in rodents only,'' said Jerry Silver, a neuroscience professor and stem-cell researcher at Case Western Reserve University in Cleveland. ``Many treatments that work in rodents to alleviate disease fail miserably in humans.''

Stem cells are the body's basic building blocks and they essentially fall into two major categories: embryonic stem cells, which can grow into any tissue, and adult stem cells, which are more specialized and less flexible.

Treatments derived from adult stem cells already are being used to help people with everything from cancer to various blood disorders. But adult stem cells can be difficult to locate for study, are limited in the body and sometimes develop abnormalities that can hinder their medical usefulness.

Working with embryonic stem cells, first isolated in 1998, can pose problems, too. For one thing, they are highly controversial because the only way to obtain them for research is to destroy an embryo. As a result, President Bush in 2001 severely limited federal money for studies involving human embryonic stem cells.