UPDATE 2-UK, Japan scientists win Nobel for stem cell breakthroughs

* Scientists find adult cells can turn back into stem cells

* Stem cells turn into any tissue, may help repair injury

* No need to harvest embryos, less risk of rejection

STOCKHOLM, Oct 8 (Reuters) - Scientists from Britain andJapan shared the Nobel Prize in Medicine on Monday for thediscovery that adult cells can be reprogrammed back into stemcells which can turn into any kind of tissue and may one dayrepair damaged organs.

John Gurdon, 79, of the Gurdon Institute in Cambridge,Britain and Shinya Yamanaka, 50, of Kyoto University in Japan,discovered ways to create tissue that would act like embryoniccells, without the need to harvest embryos. They share the $1.2million prize equally.

"These groundbreaking discoveries have completely changedour view of the development and specialisation of cells," theNobel Assembly at Stockholm's Karolinska Institute said in astatement.

The big hope for stem cells is that they can be used toreplace damaged tissues in everything from spinal cord injuriesto Parkinson's disease.

All of the tissue in the body starts as stem cells, beforedeveloping into mature skin, blood, nerves, muscle and bone.

Scientists once thought it was impossible to turn adulttissue back into stem cells, which meant that new stem cellscould only be created by harvesting embryos. But Yamanaka andGurdon showed that development can be reversed, turning adultcells back into cells that behave like embryos.

With "induced pluripotency stem cells", or iPS cells,ordinary skin or blood cells from adults are transformed backinto stem cells which doctors hope will be able to repairdamaged organs without being rejected by the immune system.

There are concerns, however, that iPS cells could grow outof control and develop into tumours.

"The eventual aim is to provide replacement cells of allkinds," Gurdon's Institute explains on its website.

"We would like to be able to find a way of obtaining spareheart or brain cells from skin or blood cells. The importantpoint is that the replacement cells need to be from the sameindividual, to avoid problems of rejection and hence of the needfor immunosuppression."

Gurdon discovered in 1962 that the specialisation of cellscould be reversed. In what the prize committee called "a classicexperiment", he replaced the immature cell nucleus in an eggcell of a frog with the nucleus from a mature intestinal cell.

This modified egg cell developed into a normal tadpole,proving that the mature cell still had all the informationneeded to develop all cells in the frog.

More than 40 years later, in 2006, Yamanaka discovered howintact mature cells in mice could be reprogrammed to become stemcells by adding just a few genes.

"Thanks to these two scientists, we know now thatdevelopment is not strictly a one-way street," said ThomasPerlmann, Nobel Committee member and professor of MolecularDevelopment Biology at the Karolinska Institute.

"There is lot of promise and excitement, and difficultdisorders such as neurodegenerative disorders, like perhapsAlzheimer's and, more likely, Parkinson's disease, are veryinteresting targets."

(Reporting by Patrick Lannin, Alistair Scrutton, Ben Hirschler,Kate Helland, Kiyoshi Takenaka and Peter Graff; writing by PeterGraff; editing by Philippa Fletcher)