Scientists have made a major advance in overcoming the principal ethical objection to the medical use of stem cells derived from human embryos. They have generated embryonic stem cells - which might eventually be used to treat chronically ill patients - from a person's skin without creating or destroying human embryos in the process.
Although the breakthrough is still in its early stages, the researchers believe they may one day be able to generate embryonic repair cells for a range of incurable illnesses from Parkinson's and heart disease to diabetes and spinal injuries.
The findings will reignite the vitriolic debate over the need to experiment with human embryos, although the scientists still believe such research is necessary, despite their discovery.
Until now, it was assumed that the only way of generating embryonic stem cells from a patient's skin cell was by fusing it with an unfertilised egg, then destroying the resulting cloned embryo to harvest its cells. But a study by Harvard University scientists has shown that embryonic stem cells can be generated through another route by simply fusing a patient's skin cell with a pre-existing "line" of continuously dividing embryonic stem cells.
Kevin Eggan, an assistant professor of molecular and cellular biology at Harvard, said it raises the possibility of bypassing the need to create and then to destroy human embryos each time scientists want to generate stem cells for future treatment.
"If this could be done, this would prevent the destruction of further embryos," Dr Eggan said yesterday. But he also emphasised that the technique is still too preliminary to be of any immediate practical benefit because the resulting embryonic stem cells have double the normal number of chromosomes.
He said that until such problems are resolved it is important to continue with research into cloned human embryos despite the ethical objections of many religious and political leaders.
"Some people - and I think President [George] Bush is one of them - would be pleased if there were no further destruction of embryos," Dr Eggan said. "The great danger is that our results can be over-extended or over-interpreted. It could be misconstrued that our research could be a viable alternative to cell nuclear replacement [cloning] and that is simply not true.
"Our technology is not ready for prime-time yet. Our results do not offer an alternative now. It could be 10 years before we get to where we want to go, but someone could use this information tomorrow, although that's extremely unlikely."
Stem cells derived from human embryos can be stimulated in the laboratory to develop into any one of the many dozens of specialised tissues of the body, from brain cells to kidney tissue.
If cloning is used to generate embryonic stem cells patients suffering from incurable diseases could be treated with their own genetically matched cells, avoiding the need to take tissue-rejection drugs. Scientists in South Korea announced in May that they have produced lines of embryonic stem cells from cloned human embryos, proving that the technique is a viable method of generating natural repair cells. Dr Eggan said: "Our work cannot be used to replace what the Korean group has so skilfully done; it suggests a route to an alternative method, but it does not deliver that route."
The cloning approach has been condemned by anti-abortion groups, the Catholic church and the religious right in America, who find it morally repugnant that cloned human embryos are deliberately created in order to be destroyed for their stem cells.
President Bush has banned use of federal government funds on research into human embryonic stem cells and cloning, and has forbidden his scientists to generate any more embryonic cell lines, although the restriction does not apply to privately funded researchers.
The Harvard study is to be published in the journal Science this week.