Scientists believe adult stem cells may be more flexible than first thought.

Embryonic stem cells can become any tissue, but adult ones are limited to the part of the body they are in.

US researchers said evidence had shown adult cells could be effectively fused with other cells to work elsewhere, New Scientist magazine reported.

The Oregon Health and Science University team said it meant adult stem cells may be useful in fighting disease, but UK experts were less sure.

Critics of embryonic stem cell research have argued it is not ethical to create human embryos for stem cells only to destroy them.

As a result scientists - particularly in the US where state funds cannot be used to fund embryonic stem cell research - have been exploring ways of treating disease with stem cells via adult cells.

Previous research has shown that adult cells can be fused with cells from other parts of the body.

But because the fused cells contain twice the number of chromosomes they have trouble dividing, and struggle to replenish damaged tissue.

Markus Grompe, who led the Oregon team, told the American Society for Cell Biology this week that he had evidence that fused cells could reduce the number of chromosomes to the normal number.

He said the process was called "reduction divisions" and occurred routinely.

The theory is that cells are pre-programmed to know how many chromosomes they have, and can reduce their number if they have too many.

The researchers have shown that mice with a disease called tyrosinemia type 1, which causes jaundice and cirrhosis of the liver, can be cured by infusing their livers with bone marrow stem cells.

Analysis suggested the cells had reverted to the right number of chromosomes - the first time this had been seen outside of insects.

However, why this happened was not clear.

Dr Grompe is now investigating whether it is possible to use a signalling molecule called insulin-like growth factor 1 to speed up the process, as it is too slow to be effective on many human conditions.

Disease

Arnold Kriegstein, who heads the Institute for Stem Cell and Tissue Biology at the University of California, said: "In much of the stem cell field, the promise is way down the line.

"But this is something that has already shown potential."

But Stephen Minger, director of the Stem Cell Biology Laboratory at King's College London, said fusion had limited potential.

"There are few stem cells that can fuse like this, bone marrow stem cells are one.

"But I am not sure how useful it is to spend time on this, when other sources such as embryonic stem cells have the potential for much more.

"It has to be remembered this is coming out of the US, there is a political agenda."