Scientists have turned mouse skin cells into eggs that produced baby mice, a technique that, if successfully applied to humans, could someday allow women to stop worrying about the ticking of their biological clocks.

For technical as well as ethical reasons, nobody expects doctors will be making eggs from women's skin cells any time soon. But some see possibilities and raise questions about its use.

Some experts say it could help millions of women who don't have working eggs of their own, whether because of a medical condition or cancer treatment, or because they are too old.

"It could mean the reproductive clock doesn't tick for women anymore," said Hank Greely, a Stanford University law professor who studies the implications of biomedical technologies.The mice experiments were reported online yesterday in the journal Science by scientists at Kyoto University in Japan.

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They began with genetically reprogrammed skin cells from female fetal mice. The reprogramming technique, discovered several years ago, makes an ordinary cell revert to a kind of blank slate, so it can be chemically prodded to develop into any kind of cell.

The Japanese researchers turned these cells into an early-stage version of eggs. Then they mixed them with mouse ovarian cells and implanted them in mice. Four weeks later they collected immature eggs, matured and fertilized them in the laboratory, and placed them in surrogate mother mice. The result: three baby mice, which grew into fertile adults.