In the mating dance of eggs and sperm, it was long believed that the egg just waited to be fertilized, while the sperm “fought it out” until the strongest one prevailed. Now, research suggests the egg may play a more active role in choosing sperm, based on genetic compatibility and other desirable factors.
Find out how the sperm and egg fertilization process works, obstacles that sperm encounter, and what happens in the body when eggs decide to “swipe right” on sperm.
- A number of obstacles prevent many sperm from reaching the mature egg and the sperm must be healthy enough to withstand these obstacles.
- These obstacles include the acidic pH of the vagina, cervical mucus, the fallopian tube, and protective layers of the egg.
- Chemical signals from the egg may present another barrier, as new research shows that eggs can influence the number of sperm the body retains and/or the sperm’s swimming performance.
Overview of the fertilization process
Fertilization is a complex, multi-step process where two gametes, or reproductive cells (sperm and egg), fuse to become a zygote, or fertilized egg. This zygote contains all the genetic information required to form a new individual organism.
The journey to the egg
When semen is ejaculated or inserted into the vagina, sperm cells encounter an acidic environment that will kill off many initial contenders. Sperm that make it past this acidic environment will then have to swim through cervical mucus, fluid produced at the opening to the uterus.
Surviving sperm move through the uterus to the fallopian tubes. The site of fertilization is typically in the ampullary region of the fallopian tube, where sperm reach the ovulated egg with the help of chemical “attractants” released by the egg itself.
While making its journey, each sperm will be undergoing a process of capacitation. Capacitation is a series of physiological changes spermatozoa must endure in order to have the ability to penetrate and fertilize an egg. Capacitation is prompted by hormones and chemicals released by the egg.
Capacitation includes a process known as “sperm hyperactivation,” in which sperm become extra-motile — AKA, they start moving very quickly — to facilitate fertilization.
If the sperm has begun the capacitation process by the time it reaches the mature egg, it can attempt to penetrate the two outer protective layers of the egg: the corona radiata and by the zona pellucida.
The zona pellucida induces the acrosome reaction, the second important step of capacitation. The acrosome is a small cap at the head of the sperm that contains enzymes that can “digest” the outer layer of the egg. Once they approach the zona pellucida, the acrosome releases these digestive enzymes to create a path for one sperm to fuse with the plasma membrane of the egg.
After the first sperm fuses with the egg, the zona pellucida will become impermeable, preventing polyspermy (penetration by more than one sperm). A zygote has now been successfully created. If it develops properly, this zygote will become an embryo and will implant itself in the uterine lining, resulting in pregnancy.
Obstacles encountered by sperm
The sperm’s journey is not an easy one. A number of obstacles prevent many sperm from reaching the mature egg, and in order to create a pregnancy, enough sperm must be healthy enough to withstand these obstacles. Of the 200 million sperm deposited near the cervix in an average ejaculation, less than 100,000 are likely to make it to the fertilization site.
Here are some of the obstacles encountered by sperm:
Sperm disorders, blockages, and more
Sperm may encounter their first obstacles before they’ve even left their body of origin. Sperm are produced in the seminiferous tubules in the testes, and travel from the testicles, through the vas deferens, all the way to the urethra and out of the end of the penis to get into the vagina (where, of course it will meet a number of other external obstacles).
In order to make this journey successful, the body must be able to make plenty of high quality sperm that is motile (moving) and shaped properly, and deliver this sperm successfully. This delivery process can be impeded by a number of conditions, such as sperm disorders leading to low sperm count or low quality sperm, anatomical blockages, or harmful antibodies within the body that attack sperm.
Research shows that, of all infertility cases, approximately 40–50% involve male factor infertility. Up to 30% of men may have abnormal semen parameters, according to some studies.
The vagina’s acidity
The normal pH of the vagina is low, around 4–5, making it acidic. This acidic environment protects the female body against many sexually transmitted pathogens, since no tissue barrier exists between the vagina and the peritoneal cavity.
The environment of the vagina is considered to be “inhospitable to the survival of sperm.” Some research shows that the pH of the upper vagina increases when sperm is introduced to help it reach the cervix, but the buffering effect lasts only a few minutes in humans.
Not only is the cervical entrance small, but, at many times throughout the menstrual cycle, cervical mucus is incredibly sticky, presenting an almost impenetrable barrier for sperm.
However, during ovulation, cervical mucus becomes more watery and thus more amenable to penetration by sperm. Still, the cervical mucus is capable of filtering out sperm with poor morphology and motility. The cervix also contains “crypts,” small crevices which can trap and store sperm for up to five days.
The fallopian tube
There are two fallopian tubes, but in most cases, only one will contain an egg. If sperm choose the wrong one, they are doomed to fail. Some estimates suggest that 10,000 or fewer sperm cells in the ejaculate end up choosing the correct tube.
These sperm cells then collect in the lower part of the fallopian tube where they may attach to the epithelium, or lining, for about 24 hours.
Once the sperm has entered the fallopian tube, they are drawn toward the egg thanks to a sort of chemical signal “breadcrumb trail” created by the egg. But again, a sperm must be motile and healthy enough to make its way up the tube and toward the egg (a journey that’s roughly equivalent to a human swimming 30 miles).
The two protective layers of the egg
To reach the inside of the egg and successfully fertilize it, the sperm must penetrate two protective layers: the corona radiata and the zona pellucida, some of the last barriers sperm must face before successfully fusing with the egg. In cases where sperm morphology is abnormal, there can be a defective sperm-zona pellucida interaction, which is a major cause of fertilization failure during in vitro fertilization.
And, if capacitation and hyperactivation is not successfully completed, sperm may not have the right speed or enzymes to fertilize the egg.
How the egg chooses the sperm: “cryptic female choice”
Sperm chemoattraction is a form of chemical communication between eggs and sperm that occurs before gamete contact.
While it has been known for decades that sperm are attracted to the egg by chemical signals in ovarian follicular fluid, recent research has shown that these chemical signals may also be selective, allowing eggs to influence which sperm should reach them — providing one last barrier for sperm deemed incompetent.
Referred to as “cryptic female choice,” eggs may actually influence the number of sperm the body retains and/or the sperm’s swimming performance. In an in vitro study of mice gametes, researchers found that eggs preferred sperm from less related males, showing that sperm selection happens at the cellular level.
Cryptic female choice has been noted in human reproduction as well. A 2020 study of in vitro fertilization found that female follicular fluid caused an increased response in specific sperm from specific males. It was also found that different eggs had different preferences. As researchers explained, “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.”
Why? Researchers don’t know exactly what influenced which sperm the eggs chose. The sperm that responded most actively to the follicular fluid was not necessarily more “fit”, and it was not necessarily from the female’s intimate partner. However, they did find that fertilization rates were higher when the sperm responded more actively to the follicular fluid.
Perhaps, similarly to the action found in mice, eggs are choosing the most genetically dissimilar sperm. Or there may be some other qualification that we haven’t yet discovered.
Can eggs reject sperm?
Maybe. This research shows that interactions between human eggs and sperm are more complex than once believed. And interestingly, a female’s egg may not always “agree” with her choice of partner.
Now that you know the obstacles that sperm face on their journey to the egg, it may be time to check if your sperm is equipped for the challenges ahead. Check your sperm health with an at-home semen analysis kit and explore our guide to sperm improvement for more resources.