FOR the first time, scientists have created viable mammalian eggs from scratch in the lab - and used them to produce healthy offspring.
However, it is likely to be many years before the technique - so far performed in mice - is reliable and safe enough for humans.
The scientists behind the discovery say the process could also shed light on the complexities of reproduction, and aiding the conservation of endangered species.
In the experiments, the Japanese team - led by Professor Katsuhiko Hayashi, from Kyushu University - used stem cells both obtained from embryos and generated from mature cells taken from the tips of mouse tails.
One day, this approach might be useful for women who have lost their fertility at an early age, as well as for improvements in more conventional infertility treatmentsProfessor Richard Anderson, from the University of Edinburgh
The latter were used to create induced pluripotent stem (iPS) cells which have the properties of embryonic stem cells, including the ability to transform into a multitude of different tissues.
Both kinds of stem cell were exposed to specific cocktails of chemicals and biological signals to coax them to develop into eggs.
A key part of the process was mingling the stem cells with "gonadal somatic cells" taken from 12-day-old mouse embryos.
These play an important supporting role in egg development.
Writing in the online edition of Nature journal, the scientists describe how follicles formed spontaneously and surrounded the early stage eggs.
The sac-like structures house maturing eggs in the ovaries.
A number of the eggs were eventually fertilised using a standard IVF technique and the resulting embryos produced healthy, fertile offspring.
The success rate was low - just 11 out of 316 two-cell embryos ended up delivering live births.
Nevertheless, British scientists working in the same field praised the Japanese achievement.
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Professor Richard Anderson, from the University of Edinburgh, said: "This is the first report of anyone being able to develop fully mature and fertilisable eggs in a laboratory setting right through from the earliest stages of oocyte (egg) development ..
"Although we are a long way from making artificial eggs for women at the moment, this study also provides us with a basis for experimental models to explore how eggs develop from other species, including in women.
"This is extremely challenging at the moment due to the difficulties of getting eggs to study."
He added: "One day, this approach might be useful for women who have lost their fertility at an early age, as well as for improvements in more conventional infertility treatments.
"But the very careful analyses in this paper show the complexity of the process and how it is a long way from being optimised."
Professor Robin Lovell-Badge, from The Francis Crick Institute in London, said the work "should be considered as a beginning, albeit a very promising one, and not an end".
He added: "Clearly, if applied to humans, being able to get functional eggs via reprogramming skin cells to iPS cells, would have importance in overcoming female infertility, e.g. due to cancer treatment as a child, but it also opens up many other uses in research, in regenerative medicine, and potentially in avoiding genetic disease."
But he pointed out that there were "still many practical and ethical challenges to be resolved".
Martin Johnson, professor of reproductive sciences at Cambridge University - who described the research as "remarkable", said: "The data are primarily of interest to scientists, although potentially of clinical interest to those patients who lack eggs of their own."