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For the first time, scientists have created viable mammalian eggs from scratch in the lab - and used them to produce healthy offspring.

Experts say the breakthrough could one day offer new hope to women who have lost their fertility - as a result of cancer treatment, for example.

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.

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.