Britain has allowed its scientists to create human-animal embryos for research purposes. This was announced by the British Fertility regulators said on Thursday.
The Human Fertilisation and Embryology Authority (HFEA) approved applications from two universities to create "cytoplasmic" embryos, which merge human cells with eggs from animals such as cattle or rabbits.
Scientists argue the research could pave the way for therapies for diseases such as Parkinson's and Alzheimer's, although opponents condemned the HFEA's decision Thursday as a "disastrous setback for human dignity".
"An HFEA licence committee has considered two applications, from King's College London and Newcastle University, to carry out research using human-animal cytoplasmic hybrid embryos," the authority said in a statement.
"The HFEA licence committee determined that the two applications satisfied all the requirements of the law and has now offered one-year research licences to the two applicants, subject to a series of detailed conditions in each case."
Researchers want to produce hybrids that are 99.9 percent human and 0.1 percent animal.
Doctor Lyle Armstrong, from the Newcastle University team, hailed the decision and hoped to make rapid progress.
"Finding better ways to make human embryonic stem cells is the long term objective of our work and understanding reprogramming is central to this," said Armstrong, who helped to create the world's first cloned human embryo in 2005.
"Cow eggs seem to be every bit as good at doing this job as human eggs so it makes sense to use them since they are much more readily available."
The research involves transferring nuclei containing DNA from human cells to animal eggs that have had nearly all their genetic information removed.
The resulting embryos are therefore mostly human, with a small animal component.
Stem cells, which can grow into different kinds of tissue, are then formed.
Once their own nuclear DNA is removed and replaced with DNA from a human cell, they become effectively human. The only "animal" element left is the tiny amount of DNA housed in the mitochondria, rod-like power plants outside the cell nucleus that generate energy.
The embryos could give researchers a large supply of stem cells to work with.
Scientists have had to rely on human eggs left over from fertility treatment, which are in short supply and often poor quality.
An HFEA consultation out in November found people were "at ease" with the proposals, once the possible implications had been explained.
Professor Robin Lovell-Badge, head of stem cell biology at the Medical Research Council's national institute for medical research, said: "The HFEA's decision is excellent as it adds to the arsenal of techniques UK scientists can use to provide understanding and eventually develop therapies for a wide range of devastating genetic diseases.
"It is logical to use animal eggs to refine techniques, provide knowledge of reprogramming and early development, and understanding of disease mechanisms."
However, John Smeaton, national director of the Society for the Protection of Unborn Children (SPUC), said the decision was a "disastrous setback for human dignity", creating sub-human "slaves" used as raw materials.
"Of those embryos with a smaller proportion of human material, greater uncertainty arises... as to whether such an embryo is a human being with due rights," he said.
The use and destruction of embryos in research is a highly sensitive subject in the United States, for political and religious reasons.
US President George W. Bush has twice vetoed a bill seeking to allow federal funds for stem cell research as it would involve human embryo destruction.