Scientists have found a way to generate blood-producing stem cells in the laboratory, in an advance that may lead to better treatments for cancer and leukaemia.

Stem cells are the "master cells" from which the body's tissues develop. When a patient undergoes intense chemotherapy or irradiation, it is the stem cells that are responsible for regrowing the blood cell system.

A bone marrow transplant, which contains new blood stem cells, will replace those that have been damaged or destroyed. But large numbers of the stem cells are required for the treatment to be effective. This is because only one in every 100,000 bone marrow cells is a stem cell that can produce blood.

Doctors would prefer to be able to culture large numbers of blood stem cells in the lab before transplantation. But this has so far proved impossible - until now.

Mouse model

Reporting in the US Proceedings of the National Academy of Sciences, University of Washington scientists say they have generated large numbers of mouse blood stem cells and kept them alive for up to four months.

Stem cells are regarded as the "master cells" in the body

To prove that the blood stem cells were still viable, they were injected into mice whose own stem cells had been destroyed. The new cells gave the mice a new blood system.

"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," says Dr Stephen Bartelmez of the University of Washington School of Public Health and Community Medicine.

"This is the first time that blood-producing stem cells have survived and reproduced successfully outside a body for any length of time," he added.

In the future, it may also be possible to remove blood stem cells from a patient and repair any genetic defect in them before multiplying them and returning the repaired cells back to the bone marrow.

As a next step, the scientists plan to study the technique in baboons. Human culture studies are also underway. If the studies are successful, trials in humans may follow.