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WASHINGTON -- Injecting human embryonic stem cells into the brains of Parkinson's disease patients may cause tumors to form, U.S. researchers reported on Sunday.

Steven Goldman and colleagues at the University of Rochester Medical Center in New York said human stem cells injected into rat brains turned into cells that looked like early tumors.

Writing in the journal Nature Medicine, the researchers said the transplants clearly helped the rats, but some of the cells started growing in a way that could eventually lead to a tumor.

Various types of cell transplants are being tried to treat Parkinson's disease, caused when dopamine-releasing cells die in the brain.

This key neurotransmitter, or message-carrying chemical, is involved in movement and Parkinson's patients suffer muscle dysfunction that can often lead to paralysis. Drugs can slow the process for a while but there is no cure.

The idea behind brain cell transplants is to replace the dead cells . Stem cells are considered particularly promising as they can be directed to form the precise desired tissue and do not trigger an immune response.

Goldman's team used human embryonic stem cells . Taken from days-old embryos, these cells can form any kind of cell in the body. This batch had been cultured in substances aimed at making them become brain cells .

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Previous groups have tried to coax stem cells into becoming dopamine-releasing cells .

Goldman's team apparently succeeded and transplanted them into the rats with an equivalent of Parkinson's damage. The animals did get better.

But the grafted cells started to show areas that no longer consisted of dopamine-releasing neurons, but of dividing cells that had the potential to give rise to tumors.

The researchers killed the animals before they could know for sure, and said any experiments in humans would have to be done very cautiously.

Scientists have long feared that human embryonic stem cells could turn into tumors, because of their pliability.

Opponents of embryonic stem cell research cite such threats. Many opponents, including President George W. Bush and some members of Congress, believe it is immoral to destroy human embryos to obtain their stem cells.

Copyright Reuters 2006. All rights reserved. Republication or redistribution of Reuters content, including by caching, framing or similar means, is expressly prohibited without the prior written consent of Reuters.

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