Short Wave - Is The Sperm Race A Fairy Tale?

Episode Date: March 5, 2021

A lot of us were taught that conception happens with a survivor-style sperm race — the fastest and strongest sperm fight to make it to the egg first. In this Back To School episode, we revisit this misleading narrative and learn just how active the egg and reproductive tract are in this process. You can find Ariela @arielazebede, Lisa @CampoEngelstein, and Kristin @kristin_hook on Twitter. Email us at [e-mail] Note - The introduction of this episode has been updated to reflect anthropologist Emily Martin's crucial role in first making this issue widely known.REFERENCES:The Egg and the Sperm: How Science Has Constructed a Romance Based on Stereotypical Male-Female Roles, Emily Martin (1991)Revisiting "The fertilization fairytale:" an analysis of gendered language used to describe fertilization in science textbooks from middle school to medical school, Lisa Campo-Engelstein & Nadia Johnson (2014)Misconceptions about Conception and Other Fallacies: Historical Bias in Reproductive Biology, Virginia Hayssen (2020)See pcm.adswizz.com for information about our collection and use of personal data for sponsorship and to manage your podcast sponsorship preferences.NPR Privacy Policy

Hey everybody, Emily Kwong here. So this is a back-to-school episode where we revisit something you may have learned in school. And this time, we are focusing on conception, using all of the medical terminology you might expect in an episode about how babies get made. So just a heads up for you and any young listeners out there. All right. Enjoy. Well, hello, Ariel Zabidi. Hello, hello. What do you got for us today? Well, Emily Kwong, today I think we need to have the talk. Ugh, Aurella, I got the talk in elementary school. I think I'm all set. Well, this is a different kind of talk. It's a talk to correct for the one you got the first time. Oh, okay. Tell me a little bit about what you learned way back when about how conception works.

Well, they showed us this video that described conception as a kind of obstacle course, where the sperm are these little tadpole-looking things, and when they enter the vagina, they're in this hostile environment. and they have to fight their way through all these obstacles and make it to the egg, and the sperm that reaches the egg wins. It's kind of how it was told. Yeah, that's pretty standard. It's similar to what I was taught, too. And I spoke to Lisa Campo Engelstein.

She's a reproductive bioethicist. And Lisa built off the work of anthropologist Emily Martin, who pointed out that we use really gendered language to describe this biology. It's like a fertilization fairy tale. So the sperm is this shining night who's there to save the egg who is a damsel in distress. And the sperm has all the agency. The sperm is on a mission. The sperm is fighting off other sperm to be the one to conquer the egg.

Whereas the egg is just sort of passively floating around waiting for the night and doesn't do anything itself. That is exactly what they told us. Yeah. And Lisa examined tons of textbooks at all levels, from middle school to middle school to medical school for this kind of bias. And she found some pretty wild stuff. For example, sperm had this little hat-like structure called the acrosome. Textbooks described it as a motorcycle helmet. I mean, they could have called it, you know, a bicycle helmet. They could have called it a horseback riding helmet, a ski helmet. They couldn't have called it, you know, any type of helmet, but it was a motorcycle

helmet, right? And that conjures up images of masculinity, right? Like this tough guy in leathers. Well, clearly, once again, the patriarchy finds a way. But in this case, isn't this story exactly what happens biologically, how it all goes down? Actually, not at all. Oh, no. It's a lie. It's a lie. All right.

I am ready to go back to school. I want this post-talk. Talk. Ariela, let's do it. All right, Emily, well, buckle up. Today on the show, we go back to school to read. visit the sperm race narrative and look at the ways that the egg and the reproductive tract play an active role in this process. I'm Ariel Zabidi. And I'm Emily Kwong. You are listening

to Shortwave, the Daily Science podcast from NPR. All right, classes in session. We're going back to school. Shortwave school. The best kind of school. Yes, to learn about conception. Yeah, and just to be clear, today we're talking about this process as it plays out internally. But a lot of folks conceived through assisted reproductive technologies like IVF. Yeah, which are very cool. Okay, just to recap, when I was taught conception in school, it was basically described as a survivor-style sperm race, but Aurella, you are telling me that this is a lie. Yes? Yes. There are a few really big problems with this narrative. When sperm first arrive in the vagina, they can't really race. I talk to Ginny Hyson. She's a biology professor at Smith College. They don't have enough energy to make it to the

site of conception. They don't have enough directional ability. But isn't that what the cute little tails are for? Like don't the sperm use them to swim? Yeah, the tails do give sperm some swimming ability, but that's not a complete picture. The sperm are getting there faster than they could all on their own. And we've seen in rats and other mammals that even dead sperm can reach the fallopian tubes. So it seems like sperm don't rely that much on their own mobility. So how are they getting there? The reproductive tract is bringing them along. Oh, that is amazing.

Okay, how is the reproductive tract doing that? So I talked to Kristen Hook. She's an evolutionary biologist. And she told me it's doing this in tons of ways by changing the thickness of the reproductive tract fluid. Just like if we were swimming in a swimming pool with water versus a swimming pool full of honey, you know, you're going to move differently in these different fluids. Or with contractions. Similar to contractions in your stomach, you know, after you've had a big meal or whatnot,

to move your food through your intestines. So it's like the sperm are on one of those moving sidewalks. Yeah, they're being transported along, eventually reaching the fallopian tubes. Okay, and what happens after that? So the sperm start to move their tails more intensely, which makes those pretty useless movements we talk. talked about earlier, more powerful. Research suggests that fluids in the reproductive tract kind of give the sperm more energy. Think of it like taking a bath and coffee. One, that sounds

dreamy. Two, the idea that the reproductive tract literally gives the sperm their strength is giving me strength right now. That is fantastic. I know. And Emily, the official name for this process is hyperactivation. That's from the books. Oh, that's really. That's really. That's really. And wait, there's even more. The reproductive tract also has to prepare the sperm for when it eventually meets the egg. Right now, the sperm is a little overdress for the occasion. It's got a layer of stuff on that prevents it from binding to the egg. And molecules in the reproductive tract helps strip off this layer so that the sperm is ready to bind. Ooh, la la, naked sperm, okay. And Emily, remember, the sperm don't have eyes. They have no idea.

where the heck they're going. So the egg provides them with a GPS. It releases these super attractive chemicals that show the sperm where to go. Oh, so it's like leaving breadcrumbs for them to follow. Yeah, and you have to realize that fallopian tubes aren't this straightforward path. It's really complex and windy in there. There are tons of little crevices. So without those crumbs, the sperm probably wouldn't know where to go. We were taught to think of it as a racetrack, right? But Kristen says we know better now. If you want to go with a racetrack idea, at least recognize that it's a dynamic racetrack. So it's not like the German Audubon.

It's more like, you know, more like a rainbow road where you have twists and turns and places to fall off. And there are checkpoints that you get asked for your license, registration, your proof of insurance. I'm sorry, proof of insurance? What does that mean? Honestly, that's not too far off from reality. And this brings me to maybe the coolest. part of all of this. Remember that hostile environment you described earlier? Yeah, but, you know, I was brainwashed back then in health class. And I regret saying that because it sounds like the

reproductive tract is actually far more helpful than hostile. Here you, totally. But it is true that there are tons of obstacles along the way that seem to be counterproductive. Like, at one point, these big immune cells surround the sperm and literally eat them. No, that's terrifying. Yeah, you don't want to be the sperm in that face-off. So it makes sense that you and me and teachers everywhere described this as a hostile environment. But now we're starting to realize that these obstacles actually have a purpose. It works to separate sperm that are dysfunctional from those that are functional. It works to separate debris that enters into the reproductive track with coitus.

And it separates the wheat from the chaff, shall we say? and then it takes what it needs or wants to the site of conception with selection processes happening all the way along. There's some evidence that the reproductive tract is holding back sperm with DNA damage or other defects and getting rid of potentially dangerous pathogens that can enter during sex. So the reproductive track has evolved all these ways to select the best sperm from the pack. Seems like it. And once some sperm makes the cut and reaches the egg, what happens there? Well, before we even get to that step, we have to talk about how the egg travels to the site of conception too.

It doesn't just stay in one place waiting for the sperm to arrive. Granted, the sperm definitely have to travel over a longer distance. But the ovaries release the egg and it gets picked up by these finger-like ends of the fallopian tubes, and then it moves some way down the fallopian tubes. So it doesn't just stay in one place, the reproduct. productive tract is moving the egg and the sperm towards each other, like bringing them closer together? Yeah, and it's commonly accepted that they meet somewhere along the fallopian tube. And once they're in the same place, we usually hear this story that sperm burrow their way into the egg by sheer force, right?

Yeah, I'm guessing that's a lie, too. Yeah, it's this really complicated, cooperative binding process with lots of chemicals and receptors involved. And the egg is surrounded by these special cells. and we've seen in other mammals that these cells can basically attract and then trap the sperm. Are you telling me that the egg is a sperm catcher? Yes, ma'am. Yes, ma'am. That is a radically different narrative than what we were spoon fed in school. Right?

Yeah, and Emily, I should mention, you and I both know, science is always ongoing. There's a lot about this process we're just discovering. But a lot of the research we talked about today isn't new. Ginny says that we've known that it's unlikely the sperm race to the site of conception for at least 70 years. But it's still really dominated our popular understanding. Yeah, and clearly our textbooks. Right. And Emily, today we focused on the contributions from the egg side that are often ignored.

But the reality of conception is that both egg and sperm are working together. And really, it's a cooperative process. It's really not one is active and the other's passive. There are mutual interactions that are going in between there. I mean, Ariel, that makes total sense from an evolutionary standpoint, that that's exactly how it would go down. Yeah, but that's not the story we choose to tell. And Lisa, the bioethicist we heard from earlier,

reminds us to think about who is telling the story. Historically, most of this research was done by men. And so it's not surprising then that they are seeing the sperm in this active role as the hero. Whereas when we get people of different backgrounds who are scientists, we hear a different story. Science is done by people, Emily. And every person brings their own biases and perspectives to everything they do. So science, even though we do our best, can never be truly objective. It's always told through a certain lens.

Yeah, and this is why we go back to school, right? Exactly. We get to question and re-examine these fundamental science concepts that are really easy to take for granted. Well, Aurella, thank you for introducing a lot of depth to this idea of going back to school. I've really learned a lot from listening to you, and I thank you so, so much. Thank you, Emily. This episode was produced by Britt Hansen, edited by Viet Le, and fact-checked by Rasha. The audio engineer for this episode was Ted Mebain.

Special thanks to Emily Martin for first making this issue widely known. I'm Emily Kwong. And I'm Ariel Zabidi. Thanks for listening to Shortwave from NPR.