The researchers also want to make sure the transplanted cells don't develop into teratomas, a type of tumor associated with the injection of embryonic stem cells. Okarma says this is an unlikely side effect, since the Geron treatment is made up of differentiated cells rather than stem cells. Still, the researchers have searched for teratomas under various experimental conditions and seen no signs of the tumors, he says. The company plans to continue safety testing for another year, at the request of the Food and Drug Administration, and will then file for permission to start human tests.

Plans for the trials are already underway, though. According to Okarma, researchers have almost finished the protocol for the experiments and are in discussions with spinal cord injury centers throughout the country that will run the actual tests.

Initial clinical trials will focus on patients with newly acquired, severe spinal cord injuries -- those with damage in the thoracic region (between the neck and lower back) who cannot move or feel anything below the site of injury. These patients usually undergo surgery two weeks after the injury to stabilize the spine. For those participating in the trial, surgeons will inject oligodendrocyte cells into the site of the injury. Patients will then be followed for signs of improvement.

The first phase of the human testing will assess mostly safety, which is the conventional procedure in testing new drugs. But rehabilitation therapists will also look for signs of motor improvement, comparing the results with control patients who do not receive the injections. If initial tests are successful, Geron plans to test the therapy in patients with less-severe spinal cord injuries. The company is also developing stem-cell-based treatments for heart disease and diabetes. "We hope to test a new cell type each year," says Okarma.

Geron's pilot trial will likely become a test case for future trials of embryonic stem cells treatments. As stem cell research moves from the research lab to human trials, hospitals, doctors, and scientists conducting the tests will face many of the same regulatory and ethical issues faced by scientists using stem cells for research purposes. For example, stem cell research requires complex approval procedures from institutional review boards and an array of ethical oversight boards. "I was shocked to find out many institutions outside California don't have ESCRO [embryonic stem cell research oversight] committees," says David Magnus, an bioethicist at Stanford University. "That's where I think one of the big challenges is going to be."

Still, scientists are confident they will be able to overcome the obstacles. "Stem cell research is like a plant creeping out of crack in the desert -- there's no way to keep it down," says Keirstead. "Where it's allowed to flourish, it blooms."