A study led by Jonathan Tilly of the Massachusetts General Hospital overturns the decades-long idea that women are born with all the eggs they will ever have. It reports that women of reproductive age carry ovarian stem cells, meaning that they can produce new eggs. Tilly's team, which made a similar finding in mice in 2004, also discovered that mouse eggs derived from such stem cells can indeed be fertilized.

Our colleagues at Nature Medicine, which is publishing the paper today online, created this four-minute video explaining the results. (Scientific American is part of Nature Publishing Group.)