WASHINGTON (Reuters) - Batches of human embryonic stem cells available under the strict policies of the U.S. government are contaminated with an animal molecule and are probably no good for using to treat people, scientists reported on Sunday.
Their finding, published in the journal Nature Medicine, supports arguments that federal government policy is holding back research in the promising but controversial field of stem cell research.
The only way around the problem is to start again with new batches of stem cells -- something that cannot be done using federal funds, said the team at the University of California San Diego.
Human embryonic stem cells are taken from embryos at fertility clinics. Many researchers say they can form any cell or tissue if grown correctly, and offer the promise of treating diseases like Parkinson's, Alzheimer's and juvenile diabetes.
Opponents, who include President Bush, say destroying a human embryo for any purpose is immoral and unethical. Bush issued an executive order in 2001 restricting federal funding for stem cell research to only those batches of the cells, called cell lines, that existed at the time.
The idea was to prevent the destruction of any more embryos while allowing the research to continue.
Many scientists argued this was too restrictive and later said it was likely the existing cells would be contaminated by the animal products the cells were grown with.
Dr. Ajit Varki of UCSD, Dr. Fred Gage of the Salk Institute and colleagues said they had confirmed this.
They found batches of available stem cells were contaminated with a non-human molecule called N-glycolylneuraminic acid or Neu5Gc. The molecule is found on the surface of animal cells but not on human cells, and is attacked by the human immune system.
It is a major reason why the human body rejects animal organ transplants.
Neu5Gc is taken up by the stem cells when they are grown or derived from laboratory cultures that contain animal-derived materials, including "feeder layers" from mice and fetal calf serum," Varki's team found.
"The human embryonic stem cells remained contaminated by Neu5Gc even when grown in special culture conditions with commercially available serum replacements, apparently because these are also derived from animal products," Varki said in a statement.
"It would seem best to start over again with newly derived human embryonic stem cells that have never been exposed to any animal products. However, such an approach could not be pursued under existing rules for the use of federal grant dollars."

By Maggie Fox, Health and Science Correspondent