SCIENTISTS have grown five-day-old human embryos with a new technique that uses genes from three adults.
To create each embryo, genetic material from one woman's egg was inserted into another donor egg before being fertilised with male sperm.
Researchers hope the technique will eventually help women with diseases of the mitochondria - the energy powerhouses inside cells - avoid passing the disorder onto their children.
Illustration: Cathy WIlcox
But the controversial procedure, first demonstrated in human embryos in Britain in 2008, has raised ethical concerns among some people who believe it would be inappropriate to produce offspring with genetic material from three adults.
The new fertility method, developed by scientists at the Oregon Health and Science University, involved transplanting the nucleus, which contains most of an organism's DNA including the instructions for characteristics such as sex, height and eye colour, from one woman's egg into a healthy donor egg whose own nucleus had been removed. The donor egg was then fertilised with donor sperm.
If the technique, known as spindle transfer, was used in a woman with a mitochondrial disease, the new egg would contain the mother's chromosomes but the donor's healthy mitochondria.
''The technique works pretty well - technically mitochondrial DNA can be fully replaced in human oocytes,'' said lead researcher Shoukhrat Mitalipov.
Just under half of the eggs that underwent spindle transfer completed normal fertilisation and developed into embryos.
From those, the team was able to produce stem cell lines to demonstrate the gene replacement had been successful.
While the research shows the procedure is technically possible, it would be years before it could be offered as a fertility therapy for humans. The creation of embryos with more than two genetic parents is banned in Australia.
While mitochondrial DNA play no role in the development of a person's characteristics, defects in this type of DNA can cause a range of diseases including blindness, epilepsy and mental retardation.
According to the Australian Mitochondrial Disease Foundation the disorders likely affect more than 100,000 Australians. In 2009, Professor Mitalipov conducted spindle transfer in female macaque monkey eggs, which were then reinserted into the mother's wombs and grew into healthy babies.
Follow-up studies of the monkeys, now three years old, showed they were developing normally, he said.
In 2010, a group of British scientists published research on human embryos they had grown by transplanting the genetic material from a fertilised egg into a donor egg, a process known as pronuclear transfer.
''At this point it is hard to say which of these techniques is better,'' said Professor Mitalipov, whose findings are published in the journal Nature.
The British research prompted the government to launch a public consultation on the ethical issues associated with the technique. A report is due next year.