Stem cells obtained from adult muscle can multiply as often as stem cells from embryos, indicating that adult-derived cells could be cultivated for treatment purposes.

The findings challenge the notion that embryonic stem cells can be grown in the lab for longer periods than adult stem cells and thus have more therapeutic potential, said lead investigator Johnny Huard, a muscle stem cell expert at Children's Hospital.

"The embryonic stem cell is a very interesting topic of research, but the adult-derived stem cell is not so bad, either," he said. "You can do a lot of things with them."

For the study, published in next month's issue of the journal Molecular Biology of the Cell, stem cells from newborn or young mice were collected and purified.

Co-investigator Bridget Deasy let them grow for 228 days without interference. The cell population doubled more than 300 times, producing an estimated googol, or 1 followed by 100 zeros, of cells.

So "when they say that the embryonic stem cells are the only cells that can grow for a long, long time, this is not true," Huard noted.

In addition to their growth potential, embryonic stem cells may be capable of producing almost any type of specialized cell; whether adult stem cells have the same capability has been a matter of scientific debate.

In the muscle stem cell growth experiment, the first generation of cells didn't start multiplying until a few weeks had passed. After four weeks, the population was doubling in about 52 hours. By six months, doubling time was steady at 14 to 16 hours.

Cells that had doubled 200 times seemed as "fresh," in Huard's words, as those from 50 doublings. They still carried the markers of stem cells and were able to repair muscle fibers in transplanted mice.

This is important because "if you cannot expand them for over 50 or 100 population doublings, then you will never have enough cells" to use for treatments, Huard said.

Dr. Amit Patel, director of cardiac cell therapy centers at the University of Pittsburgh Medical Center and the McGowan Institute for Regenerative Medicine, said Huard's novel stem cell population has potential.

But "we don't know how much self-renewal you really need in order to have clinical benefit down the road," he said. "It might be of great benefit for research purposes."

After the cells underwent 250 doublings, some cells began to lose the classic surface markers and the ability to generate muscle fibers, but they continued to multiply, Huard said.

He added that some studies have shown that embryonic stem cells will undergo about as many doublings, and other kinds of adult stem cells have doubled at most 120 times.

After 300 doublings, a few of the muscle-derived stem cells became cancerous. It's not clear if that's because an already damaged cell was in the initial batch or because the long growth period somehow transformed the cells.

For a follow-up experiment, Huard wants to let a single, muscle-derived stem cell keep multiplying to see whether cancer cells are eventually produced. Some scientists have proposed that cancer actually begins in stem cells.

If so, "perhaps we can learn the biology of those stem cells and try to prevent this or cure it," Huard said.

He also will study the longevity of cells isolated from human muscle that could be stem cells.

In other animal experiments, Huard has found that muscle-derived stem cells can generate not only muscle fibers, but also cardiac, bone and cartilage cells.