Doctors have long been trying to find a way to simply grow large numbers of the cells, which give rise to the immune system's red and white blood cells and are used to replace bone marrow after intense chemotherapy or radiotherapy. The cells are very difficult to find and filter out of the blood, and usually die after about a month in a laboratory dish.

But Mayumi Yagi, Stephen Bartelmez and colleagues at the University of Washington in Seattle and the Veterans Affairs Puget Sound Health Care System said they had coaxed stem cells into living in the lab for up to four months.

Writing in the Proceedings of the National Academy of Sciences, they said they had transplanted their 4-month-old cells into mice whose bone marrow was destroyed by radiation, and saved the lives of the mice.

"It's now almost a year later, and these animals are walking around as healthy as they can possibly be. We can't find anything wrong with them," Bartelmez said in a statement.

"The implications of this are huge," he added. "It was the first demonstration that stem cells could be substantially expanded outside of an animal."

One of the secrets was thrombopoietin, a hormone the team tested to try to encourage the growth of platelet cells involved in blood clotting. Bartelmez, who studies stem cells, found them flourishing in the cultures.

"I looked at this culture and I just about fell over. It was totally unheard of," he said.

He said this might offer a new source of stem cells for patients whose bone marrow has been destroyed by cancer treatments. The harsh treatments often kill off the cancer, but they also leave a person without an immune system or a way to produce new blood cells.

Infusions of stem cells can replace their bone marrow in days. The problem has been getting enough of the elusive cells.

"Using the new method, doctors might remove stem cells from a patient, and multiply them in the laboratory while the patient is receiving chemotherapy," Bartelmez said.

He said the research also has implications for the more experimental use of stem cells -- delivering gene therapy.

The idea would be to introduce normal genes into the blood stem cells of patients suffering from genetic diseases and try to recolonize the patient's bone marrow with these genetically engineered genes.

"If we can insert a normal gene into these dividing stem cells, we may in many cases cure the disease," Bartelmez said.

"One reason nobody has been able to do this up until now is because no one can get stem cells to divide outside the animal and just keep dividing. This discovery may be an important step for gene therapy to become a practical reality."

Blood stem cells are just one of several types of stem cells that are used to treat people. Researchers have also isolated neural stem cells from the brain, and stem cells that form muscle, cartilage, fat and other tissue.