Think girls are born with all the eggs they'll ever have? New research in mice suggests that long-held notion may be false.

Chinese researchers, reporting in Nature Cell Biology, say they found stem cells in the rodents' ovaries that could be nudged into becoming eggs that produced offspring. Physicians — not to mention would-be moms wondering when it's too late to get pregnant — have operated on the assumption that the supply of eggs a girl is born with is all she'll ever have and that it depletes with age. If the new finding is  confirmed in humans, though, it could broaden infertility  treatments to include the extraction and stockpile of the stem cells for future use, or drugs that would stimulate the cells to become eggs, the Washington Post notes.

"If you are looking to disprove that females cannot make new eggs, this paper proves it. It's a really significant paper," Jonathan Tilly, a professor of obstetrics, gynecology and reproductive biology at Harvard Medical School, told the newspaper. "This is the smoking gun."

The scientists, from Shanghai Jiao Tong University, write in the journal that they were able to find germline (or precursor stem cells) of mouse eggs in the ovaries of newborn and adult mice. After growing the cells in Petri dishes for six to 15 months, they tagged the cells with a fluorescent green marker and implanted them in mice engineered to be infertile. Some of those mice produced babies that carried the green tag, suggesting that the transplanted cells had made the infertile mice fertile, the researchers say.        

"These results indicate that the [germ] lines developed during this study can be used … to produce offspring from previously sterile recipients," the authors write.

Some scientists, however, are cautious about whether the research shows what the researchers say it does. "It's really hard to know what's going on here," David Albertini, a professor who specializes in reproduction at the University of Kansas Medical Center in Kansas City, told Nature News.

Image of lab mouse/Rama via Wikimedia Commons