US scientists have fused a human embryo stem cell and a scrap of adult skin to make cells that could become almost any tissue in the human body.
In doing so, they claim to have created a new tool for research, confirmed hopes of new ways of treating hitherto intractable human diseases - and sidestepped a bitter controversy.
Embryo stem cells are tiny biological agents that in just 40 weeks turn a single fertilised egg into a complete human composed of 100 trillion cells of 200 different kinds. The technique could lead to a new way of making human tissue without the creation of any further human embryos. This is a contentious matter in the US, where the Bush administration will not back such research, and even in Britain, where such techniques were authorised in 2001.
But the latest embryonic stem cells could never be used to treat patients. They have twice the usual numbers of chromosomes. In effect, each cell received DNA from four parents rather than the usual two. But even so, these cells could answer profound questions about how life develops.
The research, to be published in the journal Science on Friday, once again raises hopes of new treatments for diabetes, Parkinson's disease, liver failure and so on.
The dream is of laboratory dishes of "personalised" stem cells that could be used to replace failing nerve cells, or heart muscle, or pancreatic tissue. But the latest step also highlights a huge ideological and ethical divide. Many scientists in the US and Europe argue that adult stem cells taken from a developed human - for instance from umbilical cord blood, or from bone marrow - could be reprogrammed to make a range of different tissue.
Many in Britain believe that the technology that led to Dolly the Sheep should be used to grow embryo stem cells from embryos left over from fertility treatments that would otherwise be destroyed.
In May, British scientists announced that they had created a cloned human embryo. On the same day, a South Korean team announced that it had gone further: it had created human embryo stem cell lines from 11 patients in a Seoul hospital.
Last week a team from Kingston University announced that it had worked with researchers in Texas to "grow" foetal stem cell lines from umbilical cord blood, using technology developed for the International Space Station.
Other researchers, using fat stem cells from liposuction, and samples from bone marrow, have also claimed to be able to reverse the developing process.
Kevin Eggan of Harvard University and colleagues will report in Science that they used a detergent-like chemical to force samples from a line of already existing embryo stem cells to fuse with skin cells. The effect was to "turn back the biological clock" and make tissue that seemed to have the characteristics of an embryo.
They could then guide this experimental tissue to grow into nerve cells, hair follicles, muscle cells and intestinal lining.
But the Harvard researchers stress that the technique pioneered in Britain that led to Dolly the Sheep in 1996 is still a powerful and necessary tool for research.
"These cells are almost identical in every way to normal stem embryonic cells. By every method that we can test, these cells have the properties of embryonic stem cells, with the exception that they have twice as much DNA as regular cells. This simple point renders the cells that we use useless for therapy," said Dr Eggan. "It could be 10 years before we get to where we want to go."
Ian Wilmut, of the University of Edinburgh, the first scientist to clone a mammal from an adult cell and an egg with the mother's DNA scooped out- Dolly - said the latest US research could be exploited to help scientists understand one of the great mysteries: the way a cell can "reprogramme" itself and start life again.
But he warned: "Let us not waste time. There are methods of deriving embryo stem cells from cloned embryos that could be used to study and in time treat human disease. Let's get on with this, for the sake of thousands of patients."
