The egg decides which sperm fertilizes it

By Dr. Ananya Mandal, MDJun 11 2020

There is ample evidence to show that as millions of human sperm cells swim towards a waiting ovum or egg, only one gets to fertilize it. Now, a new study shows that even though the fastest and most capable sperms reach the ovum first, it is the egg that has the final say on which sperm fertilizes it. The study titled, "Chemical signals from eggs facilitate cryptic female choice in humans," is published in the latest issue of the journal Proceedings of the Royal Society B.

Study: Chemical signals from eggs facilitate cryptic female choice in humans. Image Credit: Nobeastsofierce / Shutterstock

What is the study about?

This new study by researchers from the U.K. and Sweden looked at the dynamics between the waiting ovum and the sperms that swim towards it. The team says that there is a chemical communication that occurs between the female reproductive system that receives the sperm and the incoming sperm cells from the male partner. They explained that the primary mechanism might be known, but the molecular mechanism that forms the basis of choice of sperm cell by the egg is not clear. They write, "there is a growing appreciation that females can bias sperm use and paternity by exerting cryptic female choice for preferred males."

The team from Stockholm University and Manchester University NHS Foundation Trust speculates that there may be chemicals that attract the sperm. These chemo-attractants are released from the eggs, they wrote. These allow the egg to choose between the sperms swimming towards them. This post-mating choice of sperms has been seen in some species. This study looked at similar findings among humans.

What was done?

For this study, the team assayed the mate choice microscopically. They assessed the "gamete mediate mate choice." This essentially means the egg chooses the sperm cell. They explained that once the sperm is released in the female reproductive tract, they start to swim towards the ovum. Only around a few hundred sperms reach the follicles or tubes. There it meets the follicular fluid, which has these chemo-attractants.

They looked at the effects of follicular fluid present in the female reproductive system on the incoming sperms. The team wrote that this fluid in the follicles or tubes is known to have certain chemo-attractants which attract the sperms. They used follicular fluids from the female partner and exposed the sperms to these fluids. Both partners (the male partner of the female whose follicular fluid was tested) and non-partner (another male) sperms were exposed to the follicular fluid of a female. For this study, the researchers used the follicular fluids and sperms from six couples who were undergoing treatment for infertility.

What was found?

This study found that there is a marked differentiation between the attractions from the follicular fluid of a female partner compared to the follicular fluid of a non-partner for the sperm cells. This indicated that there is a distinct mate choice when it comes to eggs and sperms. This, however, did not influence "pre-mating mate choice" in humans, they wrote.

Study author John Fitzpatrick, Associate Professor at Stockholm University, explained, "Human eggs release chemicals called chemo-attractants that attract sperm to unfertilized eggs. We wanted to know if eggs use these chemical signals to pick which sperm they attract."

Fitzpatrick explained that when the sperms they experimented with travel into the follicular fluids of their partner females, "they start to go straighter, and they start to change the way they swim." He added, "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." The sperm slows down if the follicular fluid is not conducive to it.

Conclusions and implications

The researchers wrote, "Our results demonstrate that chemo-attractants facilitate gamete-mediated mate choice in humans, which offers females the opportunity to exert cryptic female choice for sperm from specific males."

Fitzpatrick added, "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…. This shows that interactions between human eggs and sperm depend on the specific identity of the women and men involved."

The researchers believe that this study and the understanding of how the sperms fertilize eggs could help couples with infertility. Choosing sperms that are compatible with the follicular fluid could help. "Eggs attracting around 18 percent 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, said Fitzpatrick. He said that there may be a chemical incompatibility between the sperms and the follicular fluid that made it difficult for couples to conceive. Fitzpatrick said, "We weren't considering how chemical signals might influence egg-sperm interactions before. Our work helps open the door to consider this in the future."

Professor Daniel Brison, the scientific director of the Department of Reproductive Medicine at Saint Marys Hospital, senior author of the study, said, "The idea that eggs are choosing sperm is really novel in human fertility." He added, "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." He said, "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 future." One in ten couples suffer from fertility issues, say researchers.