This is the most compelling support yet for controversial claims that scientists created a cloned hybrid embryo by fusing a man's skin cell with a cow's egg cell.
A team of American scientists from a leading animal cloning laboratory have released preliminary findings of a series of experiments in which they generated early embryos by fusing the cells of four different animal species - including a monkey - with bovine egg cells.
The research, details of which are scheduled to be published early next year in a scientific journal, raises the prospect of using cows' eggs as "incubators" to grow early human embryos, to allow the extraction of vital cells for transplant operations.
Scientists have expressed interest in the possibility of using bovine eggs for therapeutic cloning - where embryos do not survive beyond 14 days - because of the shortage of human eggs and because it may be ethically more acceptable than using fully human embryos.
Earlier this year an American company, Advanced Cell Technology, which is closely linked to the University of Massachusetts, said its scientists had created a hybrid embryo clone by transferring the cell nucleus of a human skin cell into a cow's egg that had its own nucleus removed.
The embryo lived for several days and divided about five times to form a 32-cell embryo. However, the company's claims were not supported by published data, leading other scientists to be sceptical.
Details of the new research, by a team led by Professor Neal First, an authority on animal cloning at the University of Wisconsin-Madison, have emerged at scientific conferences in America. The experiments indirectly support the claims of Advanced Cell Technology.
Professor First and colleagues Tanya Dominko and Maya Mitalipova successfully produced hybrid embryos by fusing bovine egg cells, which had their own nuclei removed, with cell nuclei taken from the skin cells of sheep, pigs, rats and rhesus macaque monkeys.
Dr Dominko, who now works at the Oregon Regional Primate Research Center in Portland, said the monkey cells were the easiest to use of the four species. "If we can do it with rhesus macaque monkeys, there is no reason at all to suppose we cannot do it with human cells," Dr Dominko said.
The scientists did not have a licence to transfer the resulting embryos into the wombs of female monkeys, but they did transfer some back into sheep, Dr Mitalipova said. "We showed that the embryo implanted and the pregnancy lasted for about 30 days but we could not recover the foetus."
Professor First emphasised that the intention of the research was not to create adult animals but to produce embryos old enough to extract "stem cells'', the body's mother cells that develop into the many different types of tissue which can be used for transplant operations.
It is the prospect of extracting human stem cells from early embryos and growing the resulting tissues for transplant operations that has resulted in Britain's Human Fertilisation and Embryology Authority (HFEA) recommending the use of therapeutic cloning.
Dr Anne McLaren, HFEA member and a distinguished embryologist, said it was conceivable that scientists in Britain may one day be given a licence to create human-animal embryonic clones.
"They could certainly apply, but they would be turned down at present because of insufficient animal research. We'd need a body of animal work, fully documented, to be done before agreeing this," Dr McLaren said. Professor First's research, when published in the new year, could mark the starting point for licence applications in Britain.Reuse content