Scientists may have discovered the key to mammalian fertilization: the interaction between two proteins, Izumo and Juno, located on the surface of sperm and eggs, respectively. The finding, published in a mouse study today in the journal Nature, closes a gap in our understanding of how mammals conceive, and could help researchers develop novel fertilization techniques and contraceptives.

Scientists didn't know what the protein on sperm recognized, or to what it might bind

In 2005, scientists discovered a protein named Izumo, which allows sperm to recognize the egg during fertilization. But until now, scientists didn't know what Izumo recognized, or to what it might bind. So researchers at the Wellcome Trust Sanger Institute in the UK created an artificial version of Izumo that they introduced to the surface of mouse eggs. This experiment allowed the scientists to identify Izumo's binding partner on the egg's surface — a partner they named "Juno," after the Roman goddess of fertility. "We believe that we have identified the molecules displayed on the surface of our father's sperm and our mother's egg that must have interacted at the moment we were conceived," said Gavin Wright, a cell biologist at the Sanger Institute and senior author of the paper, in an email to The Verge.

To validate their findings, researchers bred mice that didn't produce Juno on eggs or Izumo on sperm. In both cases, these mice were unable to reproduce. Moreover, researchers realized that the Juno disappears from the surface of the egg moments after fertilization — an event they think reveals why eggs aren't usually fertilized by more than one sperm cell at a time. "This explains a 50-year-old mystery as to how eggs fuse with one — and only one — sperm so that there aren't too many chromosome contributed by the male which would result in a nonviable embryo," Wright said.

"Infertile women could be tested to see if they have a correct Juno gene."

The weak binding force involved in the Izumo-Juno interaction might explain why scientists hadn't been able to identify this essential step until recently. But now that scientists have pinpointed this pivotal fertilization event, researchers plan to start screening infertile women to determine if defects in Juno's ability to detect Izumo plays a role in human infertility. "Our results would suggest that by using a simple and non-invasive genetic screening test, infertile women could be tested to see if they have a correct Juno gene," Wright said. If doctors find defects in the Juno receptor, he said, then women could proceed directly to the most effective fertilization methods, instead of going through more common, time-consuming, and costly fertility treatments first.