How does fertilization take place? Science’s new view on the race from sperm to egg – 10/19/2022

Sperm do not swim vigorously, nor does the egg passively wait for its arrival for fertilization to take place. How does this process happen then?

The process of human fertilization has been taught to many of us as if it were a fable or a fairy tale. A story in which millions of tadpoles with giant heads and thin tails swim frantically in a hostile environment with a single objective: to reach the egg that patiently waits for their arrival.

The fastest and most agile sperm to complete this marathon wins the prize. She conquers the egg, penetrates it, and thus an embryo begins to take shape.

With a few words more or a few less, this is the narrative that usually accompanies the fertilization process.

However, this story that presents the sperm as an active agent as opposed to the egg, whose role is considered to be passive, does not correctly reflect how this event unfolds.

Both — and especially the female reproductive system — play a crucial role in reproduction.


The story begins with ejaculation. Once produced, tens of millions of sperm are deposited in the vagina (it is estimated that an average ejaculate can contain around 250 million sperm).

Once there, they must first overcome the barrier of the cervix, explains Kristin Hook, an evolutionary biologist on the Science, Technology Assessment and Analysis team at the US Government Accountability Office, to BBC News Mundo (the BBC’s Spanish-language news service). ).

“Within the reproductive system there are a number of what I would call, from a female point of view, ‘checkpoints’ that sperm must pass through to get to the site of fertilization, which is a long way from the point of entry.”

Unless the sperm are in good shape (most of them have DNA damage or other defects), they won’t be able to get through this barrier.

“This is a very important selection process,” says Daniel Brison, scientific director of the Department of Reproductive Medicine at the University of Manchester, UK.

“Of the several million sperm that are produced in ejaculation, only a few hundred make it to the egg.”

Contractions and secretions

However, sperm cannot reach the end of the fallopian tubes (also called the oviducts) on their own, where conception takes place, because they are not strong enough.

The movement that the sperm tail makes sideways has a force ten times greater than that made forward.

“Sperm don’t swim, but are mainly driven by contractions of the uterus,” explains Brison.

“Swimming is just a small part and that only happens when they reach the egg,” adds the researcher.

On the other hand, secretions within the uterus and oviducts can also modify the trajectory of sperm, promoting or preventing their movement and altering their consistency.

In short, “it is the mechanical action of the oviduct [o canal], as well as its chemistry — whether it has a salty or viscous fluid, or a certain type of pH — both controlled by the female reproductive system, which will regulate how conception will occur. In other words, which sperm will be able to reach the egg,” Virgina Hayssen, a biology professor at Smith College, told BBC Mundo.

“The pH of the vaginal environment is lower than ideal for sperm, but this acidity is essential,” says Filippo Zambelli, a researcher at the Eugin Group in Spain, dedicated to assisted reproduction.

Some researchers maintain that female orgasm may also contribute to this upward journey of sperm, causing internal muscle contractions. But others point out that more studies are needed to confirm this hypothesis.

shorter trip

The egg, meanwhile, is not passively waiting for the winning sperm to arrive.

Although the egg does not have the ability to move on its own, the cilia (a kind of hair) inside the fallopian tubes help it to move downwards on a short journey that starts in the ovary.

“The egg moves along the fallopian tube towards the uterus, secreting so-called chemoattractants, chemical molecules that attract sperm and actively guide them to it,” says Filippo Zambelli.

The egg can use these molecules “to attract or repel, as well as to modify where each sperm goes,” adds Hayssen.

Describing the encounter between the sperm and the egg as an act of penetration also does not accurately portray what happens, since the egg is what attracts the sperm and controls — in most cases — a single sperm fertilizes it.

Bonding is actually a process of mutual interactions in which both parties play an active role and in which a number of receptors and chemicals are involved.

hostile environment

Is the environment in which sperm develop really hostile, as they say?

In Hayssen’s opinion, this is again an unfortunate adjective, because it describes the event from a male point of view.

“It’s hostile if you see it as a competition and not a collaboration,” he says.

“The environment is promoting the generation of viable offspring, so it’s not hostile in terms of the goal, which is ‘we’ll have a baby at the end of all this’.”

“The environment is trying to produce the best baby possible, one that can produce as many offspring as possible, so the environment can never be hostile to that.”

If you look at “from a female perspective, the uterus is doing exactly what it needs to do to benefit the mother in getting the best offspring.”

New technologies, old ideas

Although some details of the conception process have been discovered relatively recently, thanks to the advancement of scientific research, much of the information – such as the minor importance of sperm motility – has been known for decades.

American anthropologist Emily Martin was one of the first to draw attention to the language used to talk about fertilization and the fact that cultural values ​​in general infiltrate the way scientists describe their findings about the natural world.

Her academic text, published in the early 1990s, which analyzes in detail the gender stereotypes hidden in scientific texts on the subject, has become a reference for feminism.

The experts consulted by BBC News Mundo also point to the lack of representation of women in science and in decision-making areas in education.

“The continuing lack of representation in science of people with different perspectives will influence the types of questions you ask and the explanations you find,” says Kristin Hook.

Hayssen emphasizes the need to question the terminology we use and use neutral language to talk about it: we should use “conception not fertilization”, for example.

But removing gender bias from our understanding of the mechanism of conception is not just necessary to correct scientific misinformation and accurately describe biological processes.

It is also a key step towards improving existing assisted fertility treatments and paving the way for new technologies.

– This text was published at

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