The human embryonic stem (search) cells available for research are contaminated with nonhuman molecules from the culture medium used to grow the cells, researchers report.

The nonhuman cell-surface sialic acid (search) can compromise the potential uses of the stem cells in humans, say scientists at the University of California, San Diego. Their study was published Sunday in the online edition of Nature Medicine.

Stem cells form very early in an embryo's development. They can develop into numerous types of cells to form organs and other parts of the body. Researchers hope to use these cells to repair damaged organs and cure diseases.

The work is controversial because the cells are taken from days-old embryos, which then die.

Opponents say this is unethical. President Bush has limited federal funding to cell lines already in use, but not to newly developed ones.

Currently available stem cell lines have been grown in materials derived from animals. Such materials include connective tissue cells (search), called feeder layers, from mice and fetal calf serum.

That has raised concerns about potential contamination. Last summer, more than half the members of the Senate urged easing limits on new cell lines, noting that potential contamination could make available lines use for humans uncertain.

"People have always been concerned about the possibility that something deleterious might be transferred from feeder cells to stem cells. This puts a face on that substance," Dr. James Battey, chairman of the stem cell task force at the National Institutes of Health, said about the new report.

This is a safety consideration, along with many others that will need to be addressed, Battey said in a telephone interview.

The paper suggests that growing new stem cell lines in ways that avoid animal contaminants. Battey also suggested that existing cells might be isolated from animal products for a time, allowing the acid to be diluted.

"We eat animal products and drink milk all the time and get this acid into our cells, and yet we are not always suffering from raging autoimmune disease," said Battey, who was not part of the research team.

The study reports that the cell lines currently approved for study under federal funding contain a sialic acid called N-glycolylneuraminic acid (search), or Neu5Gc.

Human embryonic stem cells are contaminated by this acid "even when grown in special culture conditions with commercially available serum replacements, apparently because these are also derived from animal products," said the lead researcher, Dr. Ajit Varki.

Human cells are unable to make the acid, Varki said. In earlier research he had found that humans have antibodies directed against Neu5Gc.

"We considered that one partial solution to the problem was to use human serum in the growth medium," Varki said in a statement.

The researchers said that when the team grew stem cells in human serum specially selected for low amounts of anti-Neu5Gc antibodies, the immune response was reduced, but not completely eliminated.

The study was funded by the National Institute of General Medical Sciences of the National Institutes of Health, the Lookout Fund and the G. Harold and Leila Y. Mathers Charitable Foundation.