• A woman’s eggs may be able to choose which sperm to accept
• Chemical signals released by the eggs ‘attract’ certain sperm
• This means that egg-sperm chemical dynamics may make particular egg-sperm combinations work better

People have certain qualities they look for in a mate, and now a new study finds that a woman’s eggs may be choosy about sperm, too.

Researchers said the findings offer new insight into human reproduction – showing that eggs will not accept just any sperm, and actually have more say in the union than previously recognised.

In the moments just before fertilisation, there is a chemical “conversation” between an egg and sperm, explained John Fitzpatrick, an associate professor at Stockholm University, in Sweden, and leader of the new study.

“The chemical signals released from eggs allow sperm to change their swimming behaviour,” Fitzpatrick said. “Sperm swim straighter and move towards the egg when they are exposed to the chemical signals. In other words, eggs are releasing sperm-guidance chemicals, like a trail of bread crumbs leading the sperm to the egg, and this can influence fertility.”

Chemicals draw sperm in

Based on the new findings, a woman’s eggs use those signals preferentially, to better attract sperm from certain men.

The researchers arrived at that conclusion after studying samples of sperm and follicular fluid from couples undergoing infertility treatment. Follicular fluid surrounds eggs and contains the chemicals that draw sperm in.

It turned out that follicular fluid from any one woman was better at attracting sperm from certain men, versus others. And the egg’s preferences did not always match with the woman’s choice of mate.

It’s not clear whether there could be any practical implications for addressing infertility, Fitzpatrick said.

“But,” he said, “about one-third of cases of infertility don’t have a clear cause. We weren’t considering how chemical signals might influence egg-sperm interactions and infertility before. Our work helps open the door to consider this in the future.”

The findings were published online on 10 June in the journal Proceedings of the Royal Society B.

Vying for the fittest sperm

Dr Natan Bar-Chama is president of the Society for Male Reproduction and Urology, in Birmingham, Alabama. He cautioned that the findings are based on a small number of couples, and a statistical finding in a study does not always translate into actual physiology.

That said, the existence of such a process makes sense, according to Bar-Chama. The implication, he said, is that right up to the last moment, the egg is vying for the most fit sperm.

But it’s not clear how important that “final filter” is in a couple’s chances of having a baby, Bar-Chama said.

“This is one step, and its impact has yet to be determined,” he said.

Fitzpatrick said that in the context of infertility treatment, the effects of the egg’s chemical signals may, in fact, be minimal. “This is because eggs and sperm are treated very well under clinical in-vitro fertilisation protocols and given every chance to produce viable embryos,” he explained.

Few sperm ‘make it’

Plus, Bar-Chama pointed out, during infertility treatment sperm are often injected directly into eggs – in a process called intracytoplasmic sperm injection (ICSI).

But could bypassing that final chemical interaction between eggs and sperm have consequences? Again, Bar-Chama pointed to all the steps that are taken to ensure the most viable embryo is implanted.

“I think all the data we have on (the health of) ICSI babies speaks for itself,” he said.

But eggs’ chemical attractants may be more critical during natural fertilisation, according to Fitzpatrick. On average, the study found, when follicular fluid was being “more attractive” in its chemical signals, about 18% more sperm swam for their goal. And that could be “pretty important”, Fitzpatrick said.

“Our best estimates are that only a few hundred sperm make it to the egg,” he noted. “And only around 10% of these sperm are able to fertilise an egg at any given time.”

Of course, couples presumably get together without a clue of their egg-sperm compatibility. And no one is saying these egg-sperm chemical dynamics are likely to make or break the chances of conceiving.

“Instead, it may make particular egg-sperm combinations easier to fertilise,” Fitzpatrick said. “We still have a long way to go before we can say this definitely, but it is certainly a possibility.”

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