Pregnancy is such a fear for so much of our sexual lives, at times it really does feel as though it might befall us through even the most modest of touches. But when you stop to think about it, the mechanics of fertilization are pretty complex. Sperm make their way through a hostile environment, up into the Fallopian tube where they find the egg and burrow into it. The sperm encounters a lot of other surfaces on the way to the egg — how does it know it’s reached the egg when it gets there?
We actually didn’t know the specifics of this, but we are starting to get a pretty good idea. In a paper published in Nature last week, researchers explained their discovery of a protein lining the human egg, which is compatible to a protein that lines the sperm cell.
In 2005, a protein was discovered on the surface of sperm thought to enable binding to the egg. Researchers named the protein Izumo1, after the Izumo-taisha in Japan, a shrine dedicated to Okuninushi, the Shinto deity of marriage. The search was on to discover a counterpart protein on eggs. What they didn’t realize is that the answer was already before them: a protein known as folate receptor 4, or Folr4, had already been discovered, but its function had been misunderstood by researchers.
Researchers Enrica Bianchi, Brendan Doe, David Goulding and Gavin Wright had their suspicions. They took a closer look by genetically modifying female mice to prevent the production of Folr4 and allowing them to copulate. No pregnancies resulted. The sperm couldn’t locate the egg without the presence of the protein — the mice were effectively sterile. In their paper, researchers ask to rename the Folr4 to reflect its true purpose: the new name is Juno, after the Roman goddess closely associated with marriage.
When the sperm enters the female reproductive system, Izumo and Juno help the cells recognize one another and bring about fertilization — and Juno helps prevent fertilization of an egg by more than one sperm, too. After a sperm cell burrows into the egg, Juno expression on the egg surface decreases. The mammalian body is such a marvelous thing — yes, we’ve subsequently discovered this type of protein interaction in mammals other than mice, humans included.
What’s really exciting is what this information means for couples grappling with infertility. If it is determined that women have been unable to conceive because they lack Juno or their partners lack Izumo, intracytoplasmic sperm injection may be able to solve the problem by enabling sperm to reach the egg directly.
This discovery is equally important to achieve the opposite effect: contraception. Specifically, non-hormonal contraception.
“Without this essential interaction, fertilization just cannot happen,” Gavin Wright, one of the researchers with the Welcome Trust Sanger Institute in England, told the Telegraph. “We may be able to use this discovery to improve fertility treatments and develop new contraceptives.”
Header image via the Welcome Trust Sanger Institute.