This suggests that certain adult stem cells - previously thought to be wedded permanently to their roles in the body - can reset their clock and become other types of cells.

This development, published in the journal Science, will add a new twist to the controversial debate surrounding stem cell research.

Late last year, US scientists announced that they had isolated and maintained in the lab human embryonic stem cells.

Ethical research

These are the "master" cells that give rise to all the tissues in the body - adult stem cells differ in that they operate further down the command chain.

But the embryonic work, as the name suggests, involves the destruction of embryos or the use of aborted foetuses. The research has consequently been attacked by church groups and anti-abortion activists.

However, this new research indicates that adult stem cells can "reinvent" themselves - they can return to a primordial state and begin a new life. This means the use of embryos may be unnecessary.

"We went the other way around and used adult stem cells instead," says Angelo Vescovi, of NeuroSpheres Limited in Canada and the National Neurological Institute in Italy.

Blood cells

Adult stem cells supply new cells to parts of the body with a high cellular turnover rate, such as the hematopoeitic (blood producing) system, the intestines, or the skin.

In their experiment, Vescovi and his colleagues used neural stem cells (NSCs) from the central nervous system of mice. NSCs generate the major cell types found in the adult brain, namely neurons and their support cells.

The NSCs were injected into a second group of mice whose hematopoietic stem cells - which produce the different varieties of blood cells - had been destroyed by radiation.

The injected NSCs appeared to take over the job of the hematopoietic stem cells and produced a fresh supply of blood cells.

Bone marrow transplants

"It took us a while to believe our own data. The tissue of the body has always been seen as unchangeable," Vescovi says.

Vescovi and his colleagues speculate that it may be possible in the future to perform bone marrow transplants using adult stem cells to create a new supply of healthy blood in patients with cancers of the hematopoietic system such as leukemia.

To reach this point, however, researchers will have to confirm that human stem cells behave the same way the mouse stem cells do.

In their Science paper, Vescovi's group say their results are consistent with a key finding by the Roslin Institute in Scotland who announced the cloning of Dolly the sheep in 1997.

In both experiments, adult cells returned to their primordial states and then began the process of division and specialisation again.

The latest development is announced just a day after the National Institutes for Health (NIH) in the US announced that they would fund embryonic stem cell research, despite the controversial nature of the work and a Congressional ban covering the public financing of science in this most sensitive of fields.