The medical potential of creating embryos that are part-human and part-animal is considered so significant that two British teams have already applied for permission to use the technique, and a third is expected to submit similar proposals shortly.

The goal is to use animal eggs, from either cows or rabbits, as a short cut to the production of cloned embryonic stem (ES) cells for the investigation of the progression and treatment of disease.

A group led by Stephen Minger at King’s College, London, has applied for a licence to use the technique to study degenerative neurological diseases, including Parkinson’s, Alzheimer’s and spinal muscular atrophy.

Lyle Armstrong, of the University of Newcastle-upon-Tyne, has also submitted a proposal to use it to study the development of ES cells to understand how they can be grown into replacement tissues for treating conditions such as diabetes and spinal paralysis.

A third team led by Ian Wilmut, of the University of Edinburgh, and Chris Shaw, of King’s, wants to create human-animal cells to investigate motor neurone disease, although it has yet to apply for a licence.

Cloned ES cells are valuable to medical science for two reasons. In the short term, their chief value is as laboratory models of disease. Cells cloned from a patient suffering from motor neuron disease, for example, will carry the genetic defects that cause it, allowing researchers to study how the condition progresses and to test potential therapies.

ES cells also have the ability to grow into any type of tissue in the body. This means that in the longer term it might be possible to transplant cells cloned from individual patients to cure diseases. As they would be genetically identical to the recipient, there would be no risk of rejection by the body’s immune system.

In each case, there is no technical reason why animal eggs are needed for the research. It would be perfectly possible to create cloned embryos by injecting the nucleus of an adult cell into a human egg from which the DNA has been removed, and then splitting them into ES cells.

The problem is that human eggs are in very short supply, and cloning is an inefficient process. Many more eggs are needed than are left over from IVF treatment, or are available from donors, and animal eggs can offer a solution.

When human DNA is injected into a rabbit or cow egg from which the nucleus has been removed, the result is an embryo that is more than 99.5 per cent genetically human.

Experiments have shown that the stem cells that can be derived from such embryos behave identically to human ones. They are to all intents and purposes human, and while they would not be suitable for transplant because of the danger of introducing animal diseases, they are just as good as fully human stem cells for laboratory research.

“To us it’s a logical step — to use animal eggs to preserve human eggs, a resource that’s in very short supply,” Dr Armstrong said. “I can easily get 200 eggs from a slaughterhouse in one day, which are going to be useful. With human eggs I am lucky to get two or three viable ones in a month.”

The animal eggs are used as empty incubators for human DNA and contribute virtually nothing to the resulting stem cells.

There is no intention to implant any embryos into human or animal wombs, and in any case this is already illegal.

While the scientists accept that there are valid questions to be asked about creating hybrids or chimeras, which carry substantial quantities of animal DNA, these should not apply to their work because the embryos they want to make are to all intents and purposes human.

The value of the research, they say, should trump ethical concerns that arise from misunderstandings of what they are trying to do.

Dr Minger said: “We want to use non-human eggs as surrogate eggs for this procedure. The resulting embryos would be human embryos with a small residual element of cow protein and cow mitochondria, which would be slowly replaced by their human counterparts over time.”

Charities yesterday joined the scientists in emphasising how stem cell research could be vital in understanding many incurable conditions. Susanne Sorensen, head of research at the Alzheimer’s Society, said: “The animal-human hybrid stem cells have the potential to be an excellent laboratory tool in the search to understand how Alzheimer’s disease pro- gresses.

“The method should allow us a window onto the earliest molecular changes in cells, revealing what goes wrong as they begin to develop signs of Alzheimer’s disease.

“We welcome continued public debate on this issue along with increased awareness about the facts versus the fiction in this type of research.”

Richard Green, of the Jennifer Trust, a charity for sufferers of spinal muscular atrophy, said: “We appreciate that this is a sensitive issue but believe that stem cell research offers real potential benefit for people affected by spinal muscular atrophy.

“It would be extremely disappointing if scientists were denied the opportunity to undertake this world leading research for the benefit of people with life-threatening and debilitating conditions.”

Chimera

biology an organism consisting of at least two genetically different kinds of tissue as a result of mutation, grafting etc (C16 from Latin chimaera, from Greek khimaira, she-goat, from khimaros, he-goat)