The use of adult stem cells to treat the body, which would eliminate the ethical dilemma raised by the use of embryonic stem cells, is theoretically appealing. The larger question, however, is can adult stem cells grow into any other cell type.

The answer appeared to be "yes" when the journal Nature published research results from Catherine Verfaillie of the University of Minnesota Medical School in Minneapolis and her team in January 2002.

The researchers had isolated a rare cell in bone marrow from a mouse, rat and human, and when they injected the mouse cells into mouse embryos, they discovered that the cells' descendants spread throughout the body, appearing in blood as well as brain, muscle, lung and liver tissues, according to Nature.

The ability for a stem cell to grow into any type of tissue, a adaptability originally only attributed to early embryos, would mean cells from the patients' own body could be used as a perfect match for tissue and even organ repairs.

So in addition to clearing the ethical hurdle, adult stem cells have the potential to avoid another obstacle posed by embryonic stem cell use -- the possibility that a patient's body could reject the foreign cells.

But the apparent breakthrough did not put to rest the question of whether adult stem cells could become other cells. Some subsequent studies began to call into question the 2002 findings. As research continued, for every study that said the stem cells could grow into other functioning cells, another study seemed to indicate the opposite.

Dr. Amit Patel of the University of Pittsburgh Medical Center, who plans to perform his own clinical trials involving cardiac stem cell therapy in humans, said even trials that purport to use the exact same techniques in animals have produced contradictory results.

However, human patient tests so far have all showed positive data no matter what type of stem cells are used, said Patel. Heart patients receiving stem cell therapy showed improvement in the contraction of their hearts, less chest pain, etc., but those "soft results" didn't explain the mechanism for the improvements.

"Thus far, no one has actually been able to show exactly what these cells are becoming or why these patients are becoming better," he said.

The problem in human trials -- unlike animal ones, in which the animals can be killed -- is once you inject the cells, you can't get them out again.

But Patel said he is hoping to rectify that with patients undergoing heart transplants. He is awaiting approval from the Food and Drug Administration to conduct human clinical trials as early as the fall in which he would inject stem cells into the patient's heart muscle prior to the transplant, and then inspect the results after the heart is replaced with a healthy one.

The FDA has approved such trials on a case-by-case basis, but now will allow scientists in the United States to make their case for further study using data from other countries, such as Argentina, Italy and Uruguay which allow stem cell therapy on humans, said Patel.

In April 2004, the FDA approved the first clinical trial in the United States of using stem cells from adult bone marrow to treat heart disease.

Although the use of bone marrow-derived stem cells is new, myoblast or muscle-derived stem cells have been studied for years. And according to Patel, the use of stem cell therapies may become more routine within the next three to five years.

Some lingering questions include what type of stem cells should be used and what kinds of patients are the best candidates for the therapy.

"Usually you do the basic science and slowly evolve to human trials. In cardiac stem cell therapy, it's happening at the same time," Patel said.

Although the results from adult stem cell therapies appears positive, clinicians still need to be cautious when doing the trials, Patel said, to reduce the possibility of negative side effects.

-- By Larisa Epatko, Online NewsHour