Stem cells made from skin have become "grandparents" after generations of life were created in experiments by scientists in Japan. The cells were used to create eggs, which were fertilised to produce baby mice. These later had their own babies.
If the technique could be adapted for people, it could help infertile couples have children and even allow women to overcome the menopause. But experts say many scientific and ethical hurdles must be overcome.
Healthy and fertile
Stem cells are able to become any other type of cell in the body from blood to bone, nerves to skin. Last year the team at Kyoto University managed to make viable sperm from stem cells. Now they have performed a similar feat with eggs.
They used stem cells from two sources: those collected from an embryo and skin-like cells which were reprogrammed into becoming stem cells. The first step, reported in the journal Science, was to turn the stem cells into early versions of eggs.
A "reconstituted ovary" was then built by surrounding the early eggs with other types of supporting cells which are normally found in an ovary. This was transplanted into female mice. Surrounding the eggs in this environment helped them to mature.
IVF techniques were used to collect the eggs, fertilise them with sperm from a male mouse and implant the fertilised egg into a surrogate mother.
Dr Katsuhiko Hayashi, from Kyoto University, told the BBC: "They develop to be healthy and fertile offspring."