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This is ALL THINGS CONSIDERED from NPR News. I'm Melissa Block.

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And I'm Audie Cornish. Scientists in Japan today report they have done something startling - for the first time, they've used stem cells to create eggs. Their experiments involved mice but as NPR's Rob Stein reports, the work is generating a lot of excitement - and questions - about doing the same thing for humans.

ROB STEIN, BYLINE: Ever since scientists discovered human embryonic stem cells, there has been talk these seemingly magical cells could transform society. The latest advance is stirring that kind of talk again. Hank Greely is a bioethicist at Stanford.

HANK GREELY: Wow - that's my general reaction.

STEIN: Greely says the advance goes way beyond all the hopes that stem cells will someday cure diseases.

GREELY: Repairing hearts, repairing brains, repairing kidneys - that's all good and important; and we'd love to be able to do that. But this involves making the next generation.

STEIN: Stem cells can morph into any cell in the body. So there's always been the possibility that they could offer a way to create eggs from anyone, at any age. That could be a game-changer for how humans reproduce.

DR. GEORGE DALEY: They've gotten to what was our holy grail, which is making eggs.

STEIN: George Daley's a leading stem cell scientist at Harvard. He's referring to scientists at Kyoto University. In this week's issue of the journal "Science," the Japanese researchers report they finally achieved that elusive goal - they created eggs from embryonic stem cells from mice. They then used those eggs to breed healthy mice. Next, they did something even more astonishing. They used a different kind of stem cell, to do the same thing; stem cells that look exactly like embryonic stem cells but instead of coming from embryos, these stem cells can be made out of cells like skin, or blood. So they don't come with all the ethical issues swirling around stem cells from embryos. Here's Harvard's George Daley again.

DALEY: It's like cellular alchemy. I mean, they can turn lead into gold here; they can turn skin cells, or blood cells, into eggs.

STEIN: The big question, of course, is whether anyone could do the same thing for people. No one knows, for sure. John Gearhart, a stem cell pioneer at the University of Pennsylvania, says it would surely take a long time; but mice are close enough to humans to think, it's doable.

JOHN GEARHART: I think this'll be worked out, in time. I don't have any doubt about it.

STEIN: And if that's right, then at the very least, it would be a huge breakthrough for women who are infertile for medical reasons, or just because they waited too long to have babies. Here's Stanford bioethicist Hank Greely again.

GREELY: If we can make eggs from stem cells, then the biological clock isn't ticking so much, for women.

STEIN: But that could just be the beginning. This could give scientists an endless supply of eggs to experiment on; create vast egg banks. Greely points out that the same team previously made sperm from stem cells.

GREELY: There are lots of lesbian and gay couples who would be very excited about the possibility, for the first time, of being able to have children who were genetically their own.

STEIN: And Greely goes much, much further. Combined with new genetic techniques, eggs from stem cells could someday make it much easier to pick babies with blue eyes or blonde hair, or a talent for sports or music.

GREELY: It will change the world, if it happens, by giving parents - I hope, parents; potentially, governments or insurers or doctors, or somebody else. But in the U.S., I suspect it will give parents a greater ability to choose the genetic traits of their children.

STEIN: Speculation about the possibilities get even more sci-fi. Ronald Green's a bioethicist at Dartmouth. He says stem cell eggs could even let someone create someone else's baby, without their permission.

RONALD GREEN: Any skin cell that you can find on the edge of a coffee cup, theoretically could be induced back to being an egg; and a baby could be produced. When you think about the commercial possibilities of people selling, to infertile people, babies produced from George Clooney or Jennifer Aniston or whatever, you have to worry about it.

STEIN: Now, it's important to remember that none of this may end up being possible. And even if it is possible, it's probably decades away. But just the possibility is already stirring intense debate about whether scientists should even be trying to unleash the power of stem cells, to manipulate human reproduction in these ways.

Rob Stein, NPR News.

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