Scientists behind research into hybrid embryos are aware of concerns that their work may produce half-human, half-animal beings. However they are adamant that these worries are far removed from the reality.

The creation of hybrid, or human admixed embryos, which combine animal eggs and human DNA, was given the green light by the Human Fertilisation and Embryology Authority (HFEA) in September 2007 after a lengthy public consultation exercise. The authority said at the time that there was “no fundamental reason” to prevent the research but admitted that public opinion was very finely divided with people “generally opposed to this research unless it is tightly regulated and is likely to lead to scientific or medical advancements”.

In January two research projects – at the University of Newcastle and at King’s College London – received one-year licences. Both are subject to rigorous conditions, one of which stipulates that any embryos created must be destroyed within 14 days.

To create hybrid embryos, scientists take unfertilised animal eggs and surgically remove the nucleus, the central bundle containing genetic information about the animal. The researchers then take a human cell, such as a skin cell, and again remove its nucleus, transplanting this ball of human genetic information into the animal egg. The resulting egg cell contains more than 99 per cent human DNA and a small amount of animal DNA.

By hitting the egg with electricity, the cell starts to divide, creating multiple new copies from which a human admixed embryo eventually develops.

International law forbids the implantation of this embryo into a womb – whether human or animal – so the embryo is kept alive in a cell incubator. In five to six days stem cells, which can develop into a multitude of different body cells, such as skin, brain and blood cells have appeared in the embryo and can be isolated and removed for further research. The scientists then destroy the human admixed embryo before the 14-day limit, imposed because it signifies the appearance in the embryo of something called the primitive streak. This groove in the embryo is believed by bioethicists to indicate the moment that a unique potential human being starts to take shape.

Both research groups are clear that their work is aimed at combatting human disease. Dr Lyle Armstrong, who leads the Newcastle team, says: “The thing I would see out of the work is the ability to produce cells to treat diseases like Parkinson’s or muscular dystrophy. We do not want to produce some weird clone with horns, a black and white coat and a tail.”

Armstrong’s team uses cow eggs obtained from local abattoirs in its research because of the scarcity of human eggs. Behind the research is a desire for greater understanding into how nature directs embryonic stem cells into developing into a specific type of body cell. Research in this area has become increasingly active after scientists in America and Japan last year showed that human cells such as skin cells could be reprogrammed and made to act as though they were embryonic stem cells.

Scientists hope that the techniques can be used to treat diseases by replacing damaged cells with newly grown clone cells. Armstrong says the thrust of the research is to find out the chemical processes at work inside the egg. “If we can work out what the egg does to the incoming genes then we can build a test tube technique for reprogramming stem cells. That should allow us to produce embryonic stem cells from human skin without having to bother with animal or human eggs.”

The King’s College team, meanwhile, led by Dr Stephen Minger, aims to farm embryonic stem cells to study the effects of neuro-degenerative diseases, such as Alzheimer’s.