Image caption Safety studies are still needed

US scientists say a human and animal trial of a controversial new IVF treatment has yielded promising results.

The findings in Nature magazine show healthy-looking embryos can be created from a mix of three adult donors.

Human embryos were grown in the lab and some appeared normal, while monkeys born using the same technique remained fit and well, now aged three.

A public consultation on the ethics of using this IVF in the UK is under way.

Its findings will be reported to the health secretary in spring 2013.

The technique is designed to prevent debilitating and fatal "mitochondrial" diseases, which are passed down only from mother to child and which cause muscle weakness, blindness and heart failure.

It is still a long way off ready for human use Prof Peter Braude, King's College London

By using two female egg donors, these DNA errors could be cancelled out, scientists believe.

Three-person IVF uses the core genetic information from mother and father as usual, but puts it into a donor egg which contains healthy mitochondria.

Mitochondria sit in the cystoplasm of the egg - akin to the white of a hen's egg. They contain only a tiny fraction of our genetic material, with the bulk that determines things like our hair and eye colour housed in the nucleus - a speck in the yolk if you use the hen's egg analogy.

Scientists have been studying two ways of creating three person embryos.

One way is to take the nucleus from the mother's egg and put it into a donor egg that has healthy mitochondria and has had its own nucleus removed. This new egg can then be fertilised with the father's sperm.

Another way is to fertilise the mother's egg first before removing the nucleus and putting it into the donor egg.

The latest study looked at the first method.

Testing

The Oregon Health team took eggs from seven women who had volunteered to take part in the research.

The question is which method will work best in humans? Neither the US nor Newcastle researchers has allowed any human embryos to be implanted - all were destroyed after just a few days when still smaller than a pinhead.

The scientists were able to replace the mitochondrial DNA in 65 of the eggs and then looked to see how well these fared over the next week or so.

The fertilisation rate afterwards was similar to the 33 control eggs that had not been manipulated, although over half had abnormal qualities.

Those that did fertilise normally developed to blastocyst stage - five or six days later, the point at which IVF embryos are normally transferred into the mother's womb - at a similar rate to the controls.

Dr Masahito Tachibana and colleagues say their research shows that the technique can work, at least in the lab. It is still not clear if it could lead to a healthy baby.

The scientists now want to be able to do more studies to ensure the treatment is also safe.

UK expert Prof Mary Herbert, of Newcastle University, has also been studying three person IVF, but using the other method which takes the nucleus out of an already fertilised egg.

She called the latest findings "encouraging" and said they were further proof that the concept was sound, although she believes the technique she uses will provide better results.

Review

Before either method could be used to help couples have a healthy baby in the UK, the government would need to give its approval and any clinic would need to obtain a license from the regulator - the Human Fertilisation and Embryology Authority (HFEA). Similar approval would be needed in other countries.

Last year the HFEA was asked to review the scientific effectiveness of both techniques. That review panel decided that the two methods might be useful in preventing mitochondrial disease, but asked for further experiments to assess their safety.

Prof Peter Braude, Emeritus Professor of Obstetrics and Gynaecology, King's College London, said: "It is exactly the sort of science that the HFEA expert committee recommended needed doing, and demonstrates further the feasibility of this technique.

"However it is still a long way off ready for human use."

He said: "Only one in five of the original eggs obtained fertilised normally and made it through to the implantation stage.

"This would mean that in order to be certain of getting embryos that might be suitable for transfer, around 12 eggs might be needed, not always possible in an IVF procedure."

How do you make a baby from three people?