Scientists from a private biotechnology company in the US have for the first time generated stem cells from human embryos without destroying the embryos in the process.
They claim the breakthrough could overcome the principal moral and ethical objection to using human embryonic stem cells for treating a range of incurable conditions, from heart disease to Parkinson's.
Robert Lanza, the chief scientist at Advanced Cell Technology in Massachusetts, said that his team had generated two continuously dividing "lines" of stem cells from human embryos left over from IVF treatment.
Normally the embryos - which at that early stage consist of a microscopic ball of a few dozen cells - would be destroyed in extracting the stem cells, which are capable of developing into any one of the many specialised tissues of the body.
However, the scientists used a special technique for extracting single cells one at a time from the embryo, leaving the rest undisturbed and capable of developing into a full-term baby.
"It is possible to generate stem cells without destroying the embryo and without destroying its potential for life," Dr Lanza said.
The technique used by the company is similar to the one already used to extract single cells from IVF embryos when they are screened for inherited disorders. Dr Lanza and his team believe that it will be possible to adapt the existing technique so that stem cells could be almost routinely extracted from human IVF embryos without harming their future health and well-being.
"The derivation of human embryonic stem cells currently requires the destruction of ex-utero [IVF] embryos," the scientists say in their study, published in the journal Nature. "The ability to create new stem cell lines and therapies without destroying embryos would address the ethical concerns of many, and allow the generation of matched tissue for children and siblings born from transferred preimplantation genetic diagnosis (PGD) embryos."
The scientists have already shown that stem cells can be derived from IVF mouse embryos and that these embryos can develop normally in the womb.
In the latest study, they extracted a total of 91 cells from 16 spare IVF embryos. Two cells developed into stem cells lines that have grown continuously in the laboratory for eight months and have shown potential for developing into specialised tissues.
"These cell lines were genetically normal and retained their potential to form all of the cells of the human body, including nerve, liver, blood, vascular and retinal cells that could potentially be used to treat a range of human diseases," the scientists say.
Ronald Green, head of Dartmouth College Ethics Institute in Hanover, New Hampshire, said the technique could be a way out of the moral impasse exemplified by President George Bush's opposition to human embryonic stem cell research. "It shows it's possible to make any number of lines in future without harming embryos or impairing their development," Dr Green told Nature. However, scientists and ethicists in Britain were less convinced about whether the technique could be used practically, and whether it was capable of satisfying the ethical worries about embryonic stem cells.
"The use of embryonic cells will only become non-controversial when it is accepted that the early embryo is of little or no moral significance," said John Harris, professor of bioethics at the University of Manchester.
Professor Ian Wilmut of the Roslin Institute in Edinburgh, who cloned Dolly the sheep, said that the technique was not very efficient and further research was needed to make it useful.
"It would be very unfortunate if this result was used to discourage embryo donation because the authors make unjustified claims for their techniques," Professor Wilmut said.
Professor Peter Braude, of King's College London, said that the approach was strictly limited.
"We don't undertake embryo biopsy willy-nilly, as it is better not to remove a cell from a developing embryo unless one really has to. We do it for PGD, for good medical reasons, and for carefully calculated risk benefits," he said.