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Eggs, at least the human kind, have always been a precious commodity. Women are born with all the eggs they will ever have in a lifetime, experts say — a biological handicap that helps explain why older mothers are more likely to miscarry and to bear children with birth defects: as women age, their dormant egg cells build up genetic mutations that make a healthy pregnancy less likely. But as it turns out, that thinking may be wrong, and infertility treatments may be the better for it.

Eight years ago, Jonathan Tilly, a reproductive biologist at Massachusetts General Hospital, and his team were the first people to contradict the notion that women's egg supplies are finite, showing that female mice harbor egg stem cells in their ovaries that could generate new oocytes, or early eggs. The idea was so controversial at the time that the evidence was dismissed as a likely mistake: the researchers had probably confused immature eggs for stem cells. But those results have since been replicated, and now Tilly's group reports that human ovaries contain similar egg cells.

Working with ovarian tissue from patients who had their ovaries removed during gender-reassignment surgery, the researchers extracted stem cells using the same technique they used with mice. They then grew the stem cells in a dish, transplanted them back into the original ovarian tissue, tucked the entire system under the skin on the back of a mouse and watched as the stem cells gave rise to brand-new eggs.

More studies will be needed to confirm whether those eggs can be fertilized to form viable embryos. That work will be done in collaboration with scientists in Scotland, where, unlike in the U.S., regulations allow scientists to fertilize human eggs for research purposes.

If the results hold, they promise to deepen our understanding of how oocytes develop and perhaps someday improve the treatment of infertility. For example, if women are continuously making new eggs, hormones or growth factors could help enhance their number or quality, increasing women's chances of getting pregnant. Lab-grown egg cells could also make in vitro fertilization easier: if it's possible to generate many eggs from a relatively simple biopsy of ovarian tissue, it could spare women the repeated painful cycles of hormone treatments and surgery now needed to obtain eggs. "It's impossible to say if and when these therapeutics will reach the clinic," says Tilly. "We have a lot of work to do in developing new hormones for testing in women. But we are now encouraged that it's feasible."

• Alice Park
• Study shows women's ovary stem cells can generate new eggs