While people usually put a lot of effort into finding a partner, recent research suggests that our bodies are choosy in their own ways. Chemical signals that are released by human eggs can attract sperm, and new work has shown that eggs also use these molecules to 'select' the sperm they want. The findings, reported in Proceedings of the Royal Society B, showed that eggs also don't always pick the same sperm; eggs from different women can opt for sperm from different men.
"Human eggs release chemicals called chemoattractants that attract sperm to unfertilized eggs. We wanted to know if eggs use these chemical signals to pick which sperm they attract," explained study author John Fitzpatrick, an Associate Professor at Stockholm University.
This study assessed how the chemoattractant-containing follicular fluid that surrounds eggs affected sperm. The researchers aimed to determine whether follicular fluid from certain women was more or less attractive to sperm than others. Their work showed that the intensity of attraction did vary, but it depended on the sperm the egg was exposed to.
"Follicular fluid from one female was better at attracting sperm from one male, while follicular fluid from another female was better at attracting sperm from a different male," explained Professor Fitzpatrick. "This shows that interactions between human eggs and sperm depend on the specific identity of the women and men involved."
The egg didn't always pick the sperm of a woman's partner; sperm from another man was sometimes more attractive to the egg than that of their partner. Since sperm function only to fertilize eggs, Fitzpacktrick thinks that the choice is entirely up to the egg, which benefits from selecting sperm that is higher quality or somehow more compatible.
"The idea that eggs are choosing sperm is really novel in human fertility," said senior study author Professor Daniel Brison, the scientific director of the Department of Reproductive Medicine at Saint Marys' Hospital, part of Manchester University NHS Foundation Trust (MFT). "Research on the way eggs and sperm interact will advance fertility treatments and may eventually help us understand some of the currently unexplained causes of infertility in couples."
"I'd like to thank every person who took part in this study and contributed to these findings, which may benefit couples struggling with infertility in [the] future," Brison added.