Researchers have uncovered some of the steps that enable human stem cells to develop into egg cells. Image: Clouds Hill Imaging Ltd./Corbis The causes of infertility, which affects around 10% of couples, are often unknown, but may in some cases result from the body's inability to produce viable gametes — also known as sperm and egg cells. The first study of the development of such 'germ cells' from humans could help scientists to learn how to create them in the laboratory instead. Even though the reproductive age for humans is around 15–45 years old, the precursor cells that go on to produce human eggs or sperm are formed much earlier, when