Adult stem cell are used more often in clinical trials than embryonic cells. They have been injected into the hearts of patients with heart disease to see if they are an effective treatment. A specific kind of bone marrow stem cell is being tested to see if it helps kidney patients with acute renal failure.

Another clinical trial is trying to treat an "orphan" neurodegenerative disease with neuro stem cells. Recently, Oregon doctors injected stem cells to try to stop seizures, while doctors elsewhere hope stem cells will restore vision to a man who lost it more than 20 years ago.

Results of most clinical trials are not yet available. And it will likely be years before any resulting treatment is readily available, experts agree. But the number of clinical trials being proposed continues to grow, and the FDA is approving more of them, Kelley said, so "more patients are starting to get stem cells as a possible treatment."

Embryonic stem-cell research is "going on at a rapid speed," Kelley said, describing "a lot of progress and still a lot of hurdles to overcome."

Many researchers believe it is possible to take immature hESC and, in the lab, turn them into pancreatic islet cells or kidney cells or heart cells, for instance. Researchers are still trying to learn the right signals to make them become specific cells, although the National Institutes of Health reports some "recipes" have been developed.

NIH lists advantages and disadvantages for the types of stem cells: While embryonic cells can become all cell types, adult stem cells are limited to changing into different cell types of their tissue of origin, although there's emerging evidence they have more flexibility than originally believed.

Embryonic cells can be grown and expanded in culture, but adult stem cells are rare, and the methods for expanding their numbers in culture haven't yet panned out. That's important, NIH says, because of the large number of cells needed for therapies. Adult stem cells may have a large advantage in that those cells taken from an individual would not be rejected by his immune system, which could be a problem with embryonic stem cells derived from a genetically different background.

Researchers hope to get around that by learning how to genetically engineer cells to make them identical to an individual's. Unfortunately, Kelley said, early claims of success turned out to be fraudulent.

Another problem: When researchers have been able to get a stem cell to turn into the type of cell they want and have injected the cells into animals, malignant germ-cell tumors have resulted.

Bone-marrow transplant is the only use of stem cells now an actual standard of care, Kelley said. It's the gold standard to cure leukemia and lymphoma. But it has taken more than 50 years of research and fine-tuning.

Other considerations include long-term effects, how many treatments might be needed, cost-effectiveness and who pays for it.

"My sense is that scientists and clinicians think research should go on with both (adult and embryonic stem cells)," Kelley said. "It's not going to be a single magic bullet. Maybe adults cells will work in certain cases, and you have to have embryonic stem cells to have an effective treatment for something else. There's great interest in continuing to pursue both avenues."

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E-mail: lois@desnews.com