What is a human-animal embryo?

True hybrid embryos are the result of fertilising an egg with the sperm of another closely related species; for example, a mule is the hybrid offspring of a donkey and a horse. Chimeric embryos are made by injecting cells or genetic material from one species into the embryo of another. Scientists at Stanford University in California plan to use this technique to create a mouse with 10% human brain cells. The third type are human transgenic embryos, made by injecting a segment of animal DNA into a human egg.

The fourth type of hybrid, the kind being developed in British universities, is called cytoplasmic. It is created by transferring the nuclei of human cells, such as skin cells, into animal eggs from which almost all the genetic information has been removed. The resulting embryo would contain only a tiny amount of animal DNA – around 0.1% – and the rest would be human. The embryo would be grown in a lab to a size of around 200 cells.

Why create human-animal embryos?

Scientists developing these embryos say they will provide a plentiful source of stem cells – immature cells that can develop into many different types of tissue – for use in medical research. Researchers believe that, by producing stem cells carrying the genetic defects of diseases, they will be able to work out how a cell's molecular machinery goes awry and perhaps find new cures for diseases.

The research has been hampered by the severe shortage of "spare" human eggs donated by couples undergoing fertility treatment. By using animal eggs, which are far more readily available, British research teams hope to make more rapid progress. Their experiments have shown that the stem cells harvested from hybrid and chimeric embryos behave identically to human ones.

Who are the scientists?

A team led by Professor Stephen Minger, director of the stem cell biology laboratory at King's College London, has been offered a licence by the Human Fertilisation and Embryology Authority (HFEA) to use human-bovine embryos to study degenerative neurological diseases such as Parkinson's and Alzheimer's. Dr Lyle Armstrong, of the North-east England Stem Cell Institute, Newcastle University, has a licence to use cow eggs to research replacement tissues for use in treating conditions such as diabetes and spinal paralysis. His team has successfully created such embryos: one developed for three days until it had 32 cells. The research has not yet been published, but the team plans to submit the work for peer-review in the next few months.

A third team, led by Professor Ian Wilmut, the Edinburgh-based creator of the first cloned mammal, Dolly the sheep, wants to create human-rabbit embryos to develop treatments for motor neurone disease, although the team has yet to apply for a licence.

What are the objections?

There were 300 responses to the government consultation, with 277 opposed to the research, many of them from pro-life groups opposed to any intrusive study of embryos. Many Catholics object to the idea of mixing human and animal DNA within the same entity and to the notion of creating what they regard as a life for the purposes of research – a life that will then be destroyed. Cardinal Keith O'Brien denounced the research as experiments of "Frankenstein proportion".

But some scientists are also sceptical. Professor Sir John Gurdon, a Cambridge University researcher who has injected human DNA into frogs' eggs, told the Guardian: "Scientifically ... I'm not persuaded it will work. If you put cells from one species into the egg of another, the egg may divide, but you could get a lot of genetic abnormality that won't lead to good-quality stem cells."

What does the law say?

The Human Fertilisation and Embryology Act 1990 does not clearly set out the parameters of human-animal embryo research. The HFEA approved such research after it decided that the embryos being created were in effect human and thus fell under the remit of the current law. However, if the HFEA were to decide hybrid embryos were not human, the research would fall outside the law and escape regulation. The human fertilisation and embryology bill will define the parameters of embryo research. It proposes that hybrid embryos could not legally be implanted into a woman's womb, or an animal's.

What is the situation in other countries?

Chinese scientists were reportedly the first to successfully create human-animal embryos. In 2003 a team at the Shanghai Second Medical University fused human cells with rabbit eggs. The embryos were allowed to develop for several days in a laboratory dish before being destroyed to harvest their stem cells. Later that same year, a US scientist, Panayiotis Zavos, announced he had created "human-cow" embryos that lived for around a fortnight and could theoretically have been implanted into a woman's womb.

In 2004 researchers at the Mayo Clinic in Minnesota produced pigs with hybrid pig-human blood cells. In 2005 Parkinson's disease researchers at the Salk Institute in San Diego reported they had created mice with 0.01% human cells by injecting about 100,000 human embryonic stem cells per mouse. Last year a Yale researcher, Eugene Redmond, led a project injecting millions of human neural stem cells into the brains of monkeys afflicted with Parkinson's disease. Many countries have banned this human-animal embryo research, including Australia, Canada, France, Germany and Italy.