A new way of growing human embryonic stem cells in the laboratory will reduce the risk that their use in therapy could go wrong, say scientists.
Stem cells are cultured in animal-derived material
At present the cells are cultured using live animal cells which carries the risk of contamination with viruses and other harmful agents.
Researchers at Advanced Cell Technology in Boston have developed a method which avoids the use of animal cells.
Details are published online by The Lancet medical journal.
Stem cells derived from early stage human embryos have the ability to become any kind of body tissue.
Scientists hope in future they will be used to treat a range of diseases, including cardiovascular conditions, Parkinson's and diabetes.
Once isolated from the embryo, embryonic stem (ES) cells are cultivated in "feeder layers" consisting of a nutrient material derived from live animal cells.
But there is a theoretical risk of viruses and other harmful agents being transmitted from the animal cells to the stem cells, and thus on to patients who receive stem cell therapy.
The threat was underlined by a warning from a team from the University of California in January that use of feeder layers had led to stem cells becoming contaminated with an animal molecule.
The researchers warned this rendered the stem cells useless, as the presence of the molecule would lead the human immune system to attack the cells.
They concluded that the only solution was to come up with a new form of culture which was completely free of animal products.
The Advanced Cell Technology team has developed a way to replace the use of animal cells with a sterile protein matrix.
Tests on stem cells cultured using the new technique for six months showed that they retained their ability to form different tissues.
Lead researcher Dr Robert Lanza said: "Experience with organ and tissue transplantation has shown that disease such as HIV infection and Creutzfeldt-Jakob disease, hepatitis B or C viruses, and other infectious agents can be transmitted from human donor cells to the recipient.
"Similarly, co-culture of human embryos with live animal cells causes concern for infection with recognised or as yet unrecognised infectious agents.
"We derived and established new human embryonic stem cell lines in completely feeder-layer free conditions.
"Our findings help solve one of the major problems associated with the use of human embryonic stem cell therapy in the treatment of human medical conditions."
In an accompanying article, Dr Outi Hovatta, from Huddinge University Hospital, Sweden, said it was vital that stem cells were cultured in a safe way.
"There are other challenges: how to avoid immune reactions in transplantation, and how to prevent genetic changes during long-term culture.
"And before clinical treatment, the controlled differentiation of the cells still requires research."
Dr Stephen Minger of King's College London, who created the first embryonic stem (ES) cell line in the UK, said the new work was a step forward.
However, he said that the culture medium was still derived from animal sources.
"This might be cell free and serum free, but it is not animal free."
And Roger Pedersen, Professor of Regenerative Medicine at the University of Cambridge, said: "The study provides evidence that it is possible to derive human embryonic stem cell lines in the absence of living feeder cells."