By Layal Liverpool
Alexey Kotelnikov / Alamy
A person’s eggs may influence how likely they are to conceive with a particular partner by releasing chemicals that attract more sperm from some individuals than others.
“This is the first time this has been described in humans, or in any other species with internal fertilisation,” says John Fitzpatrick at Stockholm University in Sweden.
He and his colleagues studied samples of sperm and follicular fluid – the nutrient-rich fluid that surrounds an egg while it develops and when it is released – that were collected from 16 couples undergoing fertility treatments. Sperm swim through follicular fluid on their way to reach an unfertilised egg.
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The researchers found that each woman’s follicular fluid attracted more sperm from some men than others. There was no obvious pattern to explain which man’s sperm would be attracted to a woman’s follicular fluid; it appeared to be random and didn’t necessarily correlate with a woman’s chosen partner. “It was a real surprise,” says Fitzpatrick.
The researchers measured the number of sperm that were able to move into each follicular fluid sample. They found that the average difference in sperm count between the fluid that attracted the most and the least sperm was approximately 18 per cent.
“Eggs attracting around 18 per cent more sperm from specific males would likely be pretty important during fertilisations inside the female reproductive tract”, since only a small fraction of sperm reach the egg after sex, says Fitzpatrick.
It is possible that eggs are more attracted to genetically compatible sperm, which may increase the chance that they are fertilised, says Fitzpatrick. The chemical interactions between eggs and sperm after sex may also play a role in why some people have difficulty conceiving. In around one in three couples who have fertility problems, there is no clear cause, says Fitzpatrick.
This study may suggest a previously unknown role for sperm attraction by eggs in mammals, says Michael Eisenbach at the Weizmann Institute of Science in Rehovot, Israel. Future studies should test whether these results also occur in chemicals produced by mature eggs released after ovulation, since follicular fluid contains chemicals that are produced before an egg has fully matured, he says.
Journal reference: Proceedings of the Royal Society B, DOI: 10.1098/rspb.2020.0805
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