Researchers looking for ways to make safer stem cells for use in medical therapies said on Monday they had grown human cells without the use of contaminating animal cells.

They said their work, done outside U.S. federal restraints, could bypass problems with existing stem cell batches, which scientists complain are contaminated by animal products and thus of no use in treating people.

“The science now exists to produce new lines that will be safe,” said Dr. Robert Lanza of Massachusetts-based Advanced Cell Technology, whose company conducted the study along with a team at Harvard Medical School and the Howard Hughes Medical Institute.

The finding, published in the Lancet medical journal, follows similar research done by a team at the University of Wisconsin that is also working with human embryonic stem cells.

These cells, taken from human embryos, have the potential to become any type of cell or tissue in the body and are being studied as possible treatments for a range of diseases or injuries.

Opponents of their use, including the current U.S. administration, say it involves the destruction of a human embryo and is thus unethical. President Bush has restricted federal funding of this research to a few batches, or lines, of cells that already existed as of August 2001.

But scientists complained these cell lines are contaminated by the mouse cells used to nurture them and therefore can never be used to treat a human patient.

In February a team at the University of Wisconsin reported in Nature Materials that they had weaned stem cells off some of the mouse feeder material.

Irina Klimanskaya and colleagues at ACT took this a step further, growing stem cells from the beginning on a cell- and serum-free mixture called extracellular matrix. They used embryos left over from in-vitro fertilization of IVF clinics.

“The importance of this work, of course, is that by eliminating contact with animal and human cells, you minimize the risk of contamination with pathogens that could be transmitted to patients and the population at large,” Lanza said.

“Experience with organ transplantation has shown that AIDS, Hepatitis, and dozens of other diseases can be transmitted from the donor cells to the recipient. Similarly, exposure of human embryos to live animal cells poses concern for infection with recognized as well as unknown pathogens.”

Outi Hovatta of Sweden’s Karolinska Institutet and Heli Skottman of the Institute for Regenerative Medicine at Finland’s University of Tampere pointed out that the materials used by the researchers still contained some animal products that may trigger an immune response.

But, they wrote in a commentary, the risk of transmitting a virus or other pathogen had been eliminated. “Klimanskaya’s derivation procedure is a real step forward,” they wrote.