Waiting for Eggs
The Egg Was Never Waiting
We were taught that the fastest sperm wins and the egg sits at the finish line. Almost every part of that is wrong, and it was wrong for reasons that had nothing to do with the biology.
Everyone gets shown the same picture.
A crowd of sperm, tails going, all pointed the same direction. Ahead of them a single round thing, pale and enormous and entirely still.
The story attached to it never varies either. Millions set off. Only one arrives. The strongest, the fastest, the most determined. He breaks through, and she is fertilised, which is a sentence in the passive voice about the only cell in the story doing nothing.
It is a good story. It has a hero and an obstacle and a prize.
It is also, in almost every particular, not what happens.
I have been trying to work out when I stopped believing it, and I do not think there was a moment. The picture just sat there, unexamined, the way a thing does when nobody has ever presented it as a claim. It was not taught to me as one interpretation among several. It was taught as the diagram.
What follows is not a feminist reading of fertilisation. It is the mechanics, measured by people with instruments, most of it published decades ago and never quite allowed to replace the picture.
There is no race
Start with the swimming, because the whole image rests on it.
Sperm are not strong. In 1988 a team at Johns Hopkins measured the forces involved directly, and then calculated them independently from the beat of the tail. The numbers are tiny. A single molecular bond between the sperm and the outer coat of the egg is enough to hold a fully motile sperm in place.[1]
One bond. Not a swarm of them. One.
So when a sperm reaches the zona pellucida, the glycoprotein shell around the egg, it does not tunnel in. It gets caught. The team's own description is that the zona captures the sperm and tethers it, and that a tethered sperm cannot get through unless those bonds are released in the right order, at the right moment.[1]
Then comes the line I keep going back to. If the bonds were released too early, the sperm would simply swim away.[1]
Read that against the picture you were shown. The sperm is not battering down a door. It is stuck to a surface it cannot pull free from, being held, while the surface decides the sequence.
And the journey before that is not a race either. Most sperm never get near the egg. They are lost, stranded, filtered out, or swim in the wrong direction entirely, and the ones that make progress are largely moved by the contractions and currents of the female tract rather than their own effort. Of the number that set off, the number arriving in the neighbourhood of an egg is measured in dozens.
Nobody wins a race that most of the runners are carried through.
The hero arrives unfinished
There is a further problem with the picture, and it is the one that undoes it completely.
A sperm that has just arrived cannot fertilise anything. Not a weak sperm. Any sperm. Freshly ejaculated sperm are incapable of fertilising an egg, and they stay incapable for hours.
What changes them is the female reproductive tract. Over several hours inside it, the sperm undergo a process called capacitation: cholesterol is stripped from the membrane, the surface chemistry is remodelled, the way calcium moves through the cell changes, and the beat of the tail switches into a different mode entirely, whipping harder and less regularly. Only after all of that can a sperm bind an egg at all.
This is not a minor footnote. It is why IVF laboratories have to capacitate sperm artificially before adding them to an egg. Skip it and nothing happens at all.
So the sequence we were taught has the hero arriving fully formed and doing the work. What actually happens is that he arrives incomplete, gets carried most of the distance, is chemically finished by the environment he has arrived in, and is then held in place by the cell he was supposedly conquering.
The lock is on the inside
In 2014 a team at the Wellcome Trust Sanger Institute went looking for the missing half of a handshake.
A protein on the sperm surface, Izumo1, had been known since 2005 to be essential for fusion. Sperm without it cannot fuse with an egg. What nobody had found was what it binds to on the other side.
Enrica Bianchi and colleagues found it on the egg membrane and named it Juno, after the Roman goddess of marriage and fertility.[2]
Female mice without Juno are infertile. Their eggs will not fuse with normal sperm, however healthy the sperm are. The interaction is conserved across mammals including humans.[2]
No Juno → no fusion, no matter what arrives.
And then the detail that made me put the paper down and stare at the wall for a second.
Within about forty minutes of fertilisation, Juno vanishes from the surface of the egg. The egg sheds the receptor. Which is very likely how the egg stops any further sperm from fusing, a problem called polyspermy that would be fatal to the embryo.[2]
The egg does not have a door that gets broken down. It holds the handle. And once it has what it needs, it removes the handle entirely.
Then, in 2020, the choosing
Which brings me to the study you may have seen, usually under a headline claiming the egg picks its favourite. The real thing is more careful than the headline and more interesting.
John Fitzpatrick, then at Stockholm University, working with a team at the University of Manchester and Saint Mary's Hospital, took follicular fluid from women in fertility treatment. That is the fluid surrounding the egg in the follicle, and it is known to release chemoattractants: chemical signals that draw sperm toward them.
Then they built a choice test at the scale of a cell. Sperm from a given man, offered follicular fluid from two different women at once. Every combination, crossed.
Follicular fluid from different women consistently and differentially attracted sperm from specific men.[3]
Not better sperm being attracted more. Specific combinations. One woman's follicular fluid pulled harder on this man's sperm; another woman's fluid pulled harder on a different man's. The chemistry is not ranking the men. It is expressing a preference between them, and the preference belongs to the fluid.
Run it below and change the man.
The Choice Test
Two samples of follicular fluid, from two different women. One man's sperm released between them. Switch the man and watch what changes. Stylised from the published design, not real measurements.
And now the part that unsettles people, which the researchers state plainly. This gamete-level preference did not reinforce the couples' own choices.[3]
The people had already chosen each other. Their cells were running a separate assessment, and reaching separate conclusions.
Nobody knows yet what the fluid is reading for. Immune-gene compatibility is the leading suspect, on the grounds that pairing with a partner whose immune genes differ from yours tends to produce offspring able to recognise a wider range of pathogens. It is a suspect rather than an answer, and I want to be clear that the mechanism is open.
What is not open is that something is being read.
So why did we get taught the other thing
To answer that I want to hand over to an anthropologist, because in 1991 she did the work that explains the picture.
Emily Martin read the biology textbooks. Not one or two. The standard ones, the medical and premedical texts that trained the doctors who trained the doctors.[4]
She catalogued the language, and the pattern is not subtle once it is laid out.
Sperm production is a marvel, described with real enthusiasm, millions a day, an astonishing feat of engineering. Egg production is a depletion, described in terms of loss and waste and degeneration. Menstruation is debris and failure, the shedding of a lining that has failed at its purpose. The same monthly process in the male, the continuous production and loss of vastly more cells, is a triumph.[4]
Sperm are streamlined, propelled, strong, active, on a mission. They penetrate and they burrow. The egg is transported, swept, drifts. It is large and passive. It waits.[4]
Martin's argument is that this is not neutral description with some unfortunate word choices. It is a story about a man and a woman, told in the vocabulary of cell biology, and taught as anatomy. Her paper has been cited over eight hundred times, and it holds up.
The most damning part is what happened when the mechanics were corrected. Once researchers established that the egg's coat captures and holds the sperm, the language did not become neutral. The egg was recast as an aggressor, a trapper, a spider. It went from damsel to femme fatale without ever being allowed to be a cell doing chemistry.[4]
Try the toggle. Left is roughly how the textbooks put it. Right is the same event with the story taken out.
Rewrite the Textbook
Same biology, two vocabularies. The one on the left is the one most of us were taught.
What this does not mean
Now the part where I argue with my own headline, because you have earned that.
The egg is not deciding anything in the way you decide things. There is no preference in there, no wanting, no judgement of character. This is receptor chemistry and diffusion gradients, and the word choice matters because replacing a heroic sperm with a discerning egg is still a story with a protagonist. I would rather have the mechanism.
The 2020 work was done with follicular fluid in a controlled assay, not with eggs inside a body. It shows that the fluid attracts sperm differentially by combination. It does not show what happens across a whole tract, or what any of it means for a pregnancy.[3]
It cannot tell you whether you and a partner are compatible. There is no test here, and anyone selling you one is ahead of the science by a considerable distance.
And the Juno work was done in mice, with the interaction confirmed to be conserved in humans.[2] That is a strong position, and it is not the same as having watched it happen in a human egg.
What I am claiming is narrower and I think more useful. The passive egg was never a finding. It was an assumption that survived because nobody had a reason to check it, and because the picture it produced was satisfying.
Six things you were told
Have a go at these before you read the answers.
True or False
Six statements about fertilisation. Not all of them are wrong.
Why it matters what we call it
You could read all of this and conclude it is a quarrel about adjectives.
I do not think it is, for one specific reason. The mechanics of tethering were published in 1988. The receptor on the egg was found in 2014. The chemoattraction work is from 2020. The lag between the sperm-side questions and the egg-side questions is decades long, and it is not because the egg side was harder.
It is because of what people thought was there to find. If you already know the egg is passive, its surface chemistry is not an interesting research question. You do not fund the study. Nobody looks, and then the absence of findings looks like confirmation.
That is the same thing I wrote about with the post-menopausal ovary, and with the transition, and with women in clinical trials. A story arrives before the evidence, sets what counts as a question worth asking, and then quietly gets called common knowledge.
The story here is unusually easy to see because it is so obviously a story. A hero. A quest. A prize that does not move.
The actual event is stranger and better. A cell releasing a chemical signal that pulls harder on some sperm than others, for reasons we cannot yet read. A coat that catches what arrives and will not let go. A receptor that permits the fusion and then disappears from the surface, so that nothing else can follow.
Not waiting. Never waiting. Doing chemistry we are only now bothering to describe.
For the women reading
You were taught, at about thirteen, a story in which your half of this was the part that lay still. It was not true, and it was not a small thing to be told.
Take the useful version with you: your biology is not the backdrop to somebody else's achievement. It was doing the selecting, the holding, and the closing, in every account we have now bothered to measure.
And take the question, because it travels further than this topic. When you are told something about your own body in language that sounds like a fairy tale, ask who checked, and when, and what they expected to find before they started.
References
- Baltz, J. M., Katz, D. F., & Cone, R. A. (1988). Mechanics of sperm-egg interaction at the zona pellucida. Biophysical Journal, 54(4), 643–654. https://doi.org/10.1016/S0006-3495(88)83000-5
- Bianchi, E., Doe, B., Goulding, D., & Wright, G. J. (2014). Juno is the egg Izumo receptor and is essential for mammalian fertilization. Nature, 508(7497), 483–487. https://doi.org/10.1038/nature13203
- Fitzpatrick, J. L., Willis, C., Devigili, A., Young, A., Carroll, M., Hunter, H. R., & Brison, D. R. (2020). Chemical signals from eggs facilitate cryptic female choice in humans. Proceedings of the Royal Society B, 287(1928), 20200805. https://doi.org/10.1098/rspb.2020.0805
- Martin, E. (1991). The egg and the sperm: How science has constructed a romance based on stereotypical male-female roles. Signs: Journal of Women in Culture and Society, 16(3), 485–501. https://doi.org/10.1086/494680