Human eggs able to 'choose' which sperm cell reaches it, study finds

"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."

Pregnant woman (Illustrative) (photo credit: PIXABAY)
Pregnant woman (Illustrative)
(photo credit: PIXABAY)
Human eggs may have the ability to choose which sperm cell will succeed in fertilizing it to conceive a baby, according to a new study, CNN reported.
The study was published last Tuesday in the journal Proceedings of the Royal Society, and used samples from 16 couples going through assisted reproductive treatment at St. Mary's Hospital in Manchester, England.
The researchers carried out an experiment by exposing sperm from each male to the follicular fluid from their partner and a non-partner.
The results found that eggs attract between 18%-40% more sperm from preferred males.
According to the study author John Fitzpatrick, who is an assistant professor at Stockholm University's zoology department in Sweden, the process is done through a metaphorical trail of breadcrumbs to help the sperm cell find its way.
"Human eggs release chemicals called chemoattractants, which leave a sort of chemical breadcrumb trail that sperm use to find unfertilized eggs," Fitzpatrick explained, according to CNN.
"What we didn't know until this study is those chemical breadcrumbs act differently on sperm from different males, in effect choosing which sperm is successful.
"What this is suggesting is that these fluids are giving females one extra chance – long after she's picked her partner – to bias the number of sperm that are going to be coming towards the eggs."
Elaborating on the exact process, Fitzpatrick explained that sperm cells have odor receptors, and they can respond to these metaphorical breadcrumbs by adjusting how fast they swim and in which direction.
"So when sperm go into the follicular fluid, they start to go straighter and they start to change the way they swim," he said, according to CNN. "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."
It's interesting to note that this is purely a genetic instinct, and has little to do with emotional influence on the part of the woman. This is shown by the fact that a woman's choice of partner does not seem to have an impact on this process.
"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 explained.
"The most likely explanation for this is that these chemical signals allow females to choose males who are more genetically compatible."
Determining genetic compatibility is helped by a set of genes called the "major histocomplatibility complex" (MHC), Fitzpatrick explained.
"Basically what these genes are about is fighting infection, fighting diseases and helping our immune system do really well. 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."
Humans, like all mammals, reproduce with an evolutionary instinct programmed into us to give our children a better chance for survival and a possible jump up the evolutionary ladder. As a result, the human body has multiple means and filters to secure a mate that would provide offspring with the best traits and advantages. These mates are typically stronger, compatible and (genetically speaking) diverse.
The reproductive tract is one example of this filter. Fitzpatrick explained that the reproductive tract fluids force sperm to swim upstream against it, similar to how salmon need to in order to reproduce. This is compounded by the female's immune system treating sperm like a foreign invader and attacking it. The sperm cells need to overcome the body's various obstacles, including cervix contractions and choosing a fallopian tube, in order to finally reach the egg.
And this filter is very effective, Fitzpatrick explained, CNN reported.
"Our best estimate is that only about 250 total sperm get to the site of fertilization where the egg is."
For context, a male might deposit tens of millions of sperm cells.
"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," he added. "So of that 250, it's more like 20 or 30 cells that can actually fertilize an egg at any different time." 
It is therefore especially interesting that the egg can help choose which sperm cell makes it through. However, it only does so in the follicular fluid surrounding 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," Fitzpatrick said, according to CNN. "We're talking some real numbers that could have an important impact on fertility."
"The idea that eggs are choosing sperm is really novel in human fertility," senior author Daniel Brison, the scientific director of the department of reproductive medicine at Saint Mary's Hospital, which is part of the Manchester University NHS Foundation Trust, said in a statement, according to CNN.
"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."
Indeed, with a third of all infertility cases having unknown causes, FItzpatrick believes that 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."
According to Allan Pacey, a professor of andrology at the University of Sheffield, who was not involved in the study, the findings are fascinating, but may not be relevant for most couples struggling with infertility.
"We've known for a long time that one or more components of follicular fluid may be important in helping the sperm find the egg, but to my knowledge it's the first time that anyone has suggested that there might be some sperm-follicular fluid combinations that are better than others. That's intriguing," he said, according to CNN.
"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 [in vitro fertilization]. But they may well give researchers a better handle on how human sperm and eggs meet, and why that sometimes it goes wrong."