So, according to a recent study from Stockholm University and Manchester University NHS Foundation Trust, female eggs use different chemical signals to attract different male sperm. These signals are supposedly released to choose a specific man’s sperm.
“Human eggs release chemicals called chemoattractants that attract sperm to unfertilised eggs. We wanted to know if eggs use these chemical signals to pick which sperm they attract,” said John Fitzpatrick, an Associate Professor at Stockholm University.
The researchers were examining how sperm cells respond to follicular fluid, which surrounds the female eggs and contains sperm chemoattractants. The researchers wanted to find out if follicular fluids from different females attract sperm from some males more than others.
“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,” said Professor Fitzpatrick.
“This shows that interactions between human eggs and sperm depend on the specific identity of the women and men involved.”
The egg can sometimes disagree with the women’s choice of partner. The researchers found out that eggs did not always attract more sperm from their partner compared to sperm from another male.
Is this egg or sperm choice? Professor Fitzpatrick explained that sperm only has one job – to fertilise an egg. Eggs on the other hand can benefit by picking higher quality or more genetically compatible sperm.
"We expected to see some sort of partner effect, but in half of the cases the eggs were attracting more sperm from a random male," Fitzpatrick said. "The most likely explanation for this is that these chemical signals allow females to choose males who are more genetically compatible."
“What makes a partner genetically compatible? One of the driving factors for that, is a complex set of genes called the "major histocompatibility complex" or MHC for short," Fitzpatrick said.
"Basically, what these genes are about, is fighting infection, fighting diseases and helping our immune system to do really well," he said. "The more diverse those genes are, the more diverse are the kinds of infections you can fight. And if your partner has a slightly different combination of these genes than you do, then you're going to produce offspring that can fight an even broader array of pathogens and diseases."
Our bodies have evolved and created many different methods to reward the strongest and most compatible mate. Even the female reproductive tract is an obstacle course designed to weed out weaker and less acceptable suitors.
“Reproductive tract fluids flow downwards, forcing sperm to swim upstream. The female's immune system views sperm as a foreign invader, attacking those swimmers as if they were germs.” - Fitzpatrick said.
"The journey is so arduous that of the tens of millions of sperm a male might deposit, our best estimate is that only about 250 total sperm get to the site of fertilization where the egg is," he also mentioned.
"On top of all of that, only about 10% of the 250 sperm are able to fertilize at any given time — they sort of blink on and off in their capacity to fertilize eggs, so, of that 250, it's more like 20 or 30 cells that can actually fertilize an egg at any different time."
Finally, the egg can affect which sperm wins the race with the chemicals it releases in the follicular fluid that surrounds the egg.
"And it's only in the last two centimeters between a sperm and the egg that these chemical signals matter since it's the final phase of this long journey where females continue weeding out less acceptable sperm, we're talking some real numbers that could have an important impact on fertility." – Fitzpatrick said.
Sperm cells, have odor receptors in their heads that respond to the chemoattractants in the egg's follicular fluid, thus influencing how robust the sperm swims.
"So, when sperm go into the follicular fluid, they start to go straighter and they start to change the way they swim, so, depending on the strength of that signal, you can get different responses in how the sperm are responding to these female chemical signals within their follicular field."
"In some cases, the female's follicular fluid is going to make one male sperm do the speedy front crawl and then another male sperm does the backstroke."
"The idea that eggs are choosing sperm is really novel in human fertility," said senior author Daniel Brison, the scientific director of the department of reproductive medicine at Saint Mary's Hospital in the UK, which is part of the Manchester University NHS Foundation Trust.
"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," Brison said in a statement.
“About a third of all cases of clinical infertility have unexplained causes. Now that we know that eggs are exerting some sort of control on sperm, we can start looking at that and start to narrow down that 30% of unexplained cases." – said Fitzpatrick according to the interviews on CNN.com
Couples who are currently experiencing infertility, however, should not worry about whether or not they have sperm and follicular fluid compatibility problems, said Allan Pacey, a professor of andrology at the University of Sheffield in the UK, who was not involved in the study –
"Whilst the results of this study are very interesting and the experiments elegantly performed, I am not sure whether they currently have any clinical relevance for anyone concerned about their fertility or undergoing IVF. But they may well give researchers a better handle on how human sperm and eggs meet, and why that sometimes it goes wrong."
Original research: John L. Fitzpatrick, Charlotte Willis, Alessandro Devigili, Amy Young, Michael Carroll, Helen R. Hunter and Daniel R. Brison 2020 Chemical signals from eggs facilitate cryptic female choice in humans Proc. R. Soc. B.28720200805, http://doi.org/10.1098/rspb.2020.0805
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