Japanese stem cell scientists raised hopes of a cure for infertility in humans Friday when they announced they had created viable eggs using normal cells from adult mice.
The breakthrough raises the possibility that women who are unable to produce eggs naturally could have them created in a test tube from their own cells and then implanted in their body.
A team at Kyoto University harvested stem cells from mice and altered a number of genes to create cells very similar to the primordial germ cells that generate sperm in men and oocytes -- or eggs -- in women.
They then nurtured these with cells that would become ovaries and transplanted the mixture into living mice, where the cells matured into fully-grown oocytes.
They extracted the matured oocytes, fertilised them in vitro -- in a test tube -- and implanted them into surrogate mother mice.
The resulting mice pups were born healthy and were even able to reproduce once they matured.
Researchers said the findings, published in the US journal Science, provided a promising basis for hope in reproductive medicine.
"This achievement is expected to help us understand further the egg-producing mechanism and contribute to clarifying the causes of infertility," professor Michinori Saito told reporters ahead of publication.
"We intend to continue this research with monkeys and humans," he said.
But Saito cautioned that this was not a ready-made cure for people with fertility problems, adding that a lot of work remained.
Stem cells -- infant cells that develop into the specialised tissues of the body -- have sparked great excitement because they offer the chance of rebuilding organs damaged by disease or accident.
Until fairly recently, the only way to obtain stem cells was to harvest them from embryos.
Religious conservatives, amongst others, have objected to research on human embryonic stem cells because they hold that the destruction of a foetus -- necessary for the harvest -- is wrong.
But pioneering work done in 2006 by Shinya Yamanaka, also at Kyoto University, succeeded in generating "induced pluripotent stem (iPS) cells", from skin tissue.
Like embryonic stemcells, iPS cells are also capable of developing into any cell in the body, but crucially their source material is readily available.
Yamanaka told the daily Asahi Shimbun the researchers had made "great strides towards clarifying the causes of infertility and the development of treatment".
Researchers said in addition to the many practical difficulties that remained, the latest stem cell science also poses a number of ethical problems.
"This procedure cannot be directly applied to humans," said Saito's partner and Kyoto University associate professor Katsuhiko Hayashi.
Hayashi noted in the latest study with mice that the primordial germ cells needed to be nurtured with cells taken from a separate foetus, which is destroyed in the process.
"Theoretically it would be possible to create a foetus from iPS cells, but even this does not set aside the ethical dilemmas," he said, noting questions would be raised over the creation and destruction of even an artificial foetus.
The findings on egg development published this week come just a year after scientists in Kyoto successfully coaxed sperm cells from mouse stem cells.
In that work, researchers took mice that were unable to produce normal sperm and injected them with the stemcell-derived primordial germ cells, or PGCs.
These PGCs "produced normal-looking sperm, which were then used to successfully fertilize eggs", the study said last year.