Sperm Swim Together To Help Each Other Reach The Egg

11:00 minutes

an illustration of an egg on the right, with many sperm swimming towards it from the left side
Credit: Shutterstock

New research is complicating our understanding of how, exactly, sperm are able to reach eggs. The predominant theory is that sperm compete against each other, with the strongest swimmer fertilizing the egg.

But a new study, using cow sperm, suggests that sperm might actually swim together, forming clusters to help each other swim upstream to reach the egg.

Researchers created a device that has some of the features of a female reproductive tract, which they tested using a polymer substance that mimics cervical mucus. The intensity of the flow of this mucus-like fluid influenced how well the sperm clustered together. The faster the flow, the more likely the sperm were to band together to swim upstream.

Ira talks with Dr. Chih-Kuan Tung, associate professor of physics at North Carolina Agricultural and Technical State University about his research on sperm motility, and how it could improve infertility testing in the future.

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Segment Guests

Chih-Kuan Tung

Dr. Chih-Kuan Tung is an associate professor of Physics at North Carolina A&T State University in Greensboro, North Carolina.

Segment Transcript

IRA FLATOW: This is Science Friday. I’m Ira Flatow. Later in the hour, we’re experts at sending rovers to Mars, but did you know we also have rovers deep down in the ocean? Yes. And how a 17-year-old inventor from Ukraine is trying to use drones to clear landmines.

But first, we’re continuing our look at the science behind reproductive health. This week, we’re focusing on a different piece of the reproductive process– sperm. On average, one teaspoon of semen contains about 200 to 500 million sperm, and how does all that sperm winnow down to a single winner? Think back to your high school biology textbook. Or maybe you talked about it in health class, specifically that chapter about human reproduction, the one everyone in class was just a little embarrassed to read.

Well, the story may have gone something like this. Those millions of sperm race against one another. The strongest swimmer gets to the egg first, beating out all the competing sperm. However, new research in cows suggests that sperm may actually swim together, forming clusters to help each other swim upstream to reach the egg.

Joining me now to share his fascinating physics of the swimming sperm is my guest, Dr. Chih-Kuan Tung, associate professor of physics at North Carolina A&T State University, based in Greensboro, North Carolina. Dr. Tung, welcome to Science Friday.

CHIH-KUAN TUNG: It is my pleasure.

IRA FLATOW: All right, let’s talk about this. Let’s start with the basics. Your previous research showed that sperm will swim together in groups, right? How did you originally discover this?

CHIH-KUAN TUNG: So the thinking was that, when we want to analyze sperm motility, we really want to look at how sperm swim in an environment that better resemble the environment sperm will encounter naturally in the female reproductive tract. So we started to use some microfluidic devices to mimic several features that will present in the female reproductive system. Sperm swim in mucus. They don’t swim in those watery that medium that we prepare.

So we started to add polymer into– some longterm molecules into the solution to create this mechanical property we call viscoelasticity. The word means that the fluid is both viscous, means they flow slowly. And elastic, at the same time, elastic means that in some short time scale, that fluid has a shape that it wants to come back. Regular fluid, say, water, the shape is the container you put that in. But once we did that, we put the polymer to increase viscoelasticity of the fluid, we started to see sperm swim very close to each other, mostly in parallel and forming those groups.

IRA FLATOW: Would it be fair to say that what you did was trying to imitate the fluid that sperm swim in and, by getting a better analog to that fluid, you discovered that they swim in groups?

CHIH-KUAN TUNG: Yes, that’s exactly what we did. So we also did some measurement to compare the polymer solution we used with the cervical mucus, and biological sample, they are not identical every time. But there are kind of ballpark similarities.

IRA FLATOW: So the fluid that the sperm are swimming through during actual conception is cervical mucus, correct?

CHIH-KUAN TUNG: A part of it, yes, through cervix, and then there is some different mucus in uterus and different fluids in oviduct.

IRA FLATOW: And you took a sample of cervical fluid from a cow and then tried to get as close as you could by making a polymer, a long chain of molecules.

CHIH-KUAN TUNG: Yes, yes, in some way, yes.

IRA FLATOW: And your latest research looks at how bovine sperm swim in cluster in different conditions. What did you find?

CHIH-KUAN TUNG: So there is actually a natural flow, generally speaking, in the female reproductive system, so this is quite relevant, physiologically. So typically, when sperm swim close to a solid surface, they will naturally form circular trajectories, and without anything else, they will just do that. And in the no-flow situation, we found that they don’t do those tight circle. But they are either a very large circle, or basically, they will be– the trajectory will become linearized.

So they are more directional. They don’t just circle somewhere. And we started to increase the flow rate, and there is some range of flow that the sperm will start to align against the flow. So there is this flow range that, when the sperm orient against the flow, we saw that the closer sperms are actually better align against the flow than the individual swimming sperm. And then finally, we keep increase the flow rate too. It’s higher. And there we can see typically 20% to 30% reduction of sperm being removed once they are in clusters.

IRA FLATOW: So there’s an advantage you found to sperm clustering together when there’s more mucus-like liquid flowing.

CHIH-KUAN TUNG: In all flow rate, there is some different kinds of advantage that we found.

IRA FLATOW: Why is it an advantage, then? What is there about the fluid, then, the way they’re swimming, that when they cluster, they do a better job of getting where they want to go?

CHIH-KUAN TUNG: The mechanical question how exactly this happens is still remain to be studied that we cannot answer for sure, but it does look like there is some kind of helping each other in this process.

IRA FLATOW: Do all sperm swim in groups or some swimming solo? Do you see an intrepid spermatocytes there trying to push past the group and get to the egg first?

CHIH-KUAN TUNG: There are different kinds of collective behaviors that have been reported across different species. So the one we use is with bull sperm. A lot of different kind of mice, there have been reports of different kinds of– say, the head will attach to each other, or the head can hook up to a tail or some Guinea pig that can form different structures to swim together. So there are a lot of reports regarding how sperm actually cooperate with each other to reach the goal of fertilization.

IRA FLATOW: Interesting. I understand that you originally were using your physics expertise in cancer research. How did you end up studying the physics of sperm?

CHIH-KUAN TUNG: It was potentially accidental. I was at Cornell University at the time, and the initial project got me there was a project for cancer cells to build a device to see how the flow within the tissue influence the cancer cell migration. And that was a skill I acquired earlier as a PhD student, and then the sperm project came up. They wanted someone to build a device, so that’s basically how it got into this.

IRA FLATOW: So you became the expert at building the device to mimic the female reproductive tract.

CHIH-KUAN TUNG: Initially, it was just building some devices, and I listened to people, what you want me to build?

IRA FLATOW: [LAUGHS] And so they brought you in.

CHIH-KUAN TUNG: Yeah, yeah, yeah. That’s how it happened.

IRA FLATOW: And how long have you been doing this?

CHIH-KUAN TUNG: Our first paper related to sperm was published in 2014, I believe, so a little while now.

IRA FLATOW: Yeah, so you really have a niche expertise in this, right?

CHIH-KUAN TUNG: Yeah, yeah, yeah.

IRA FLATOW: There aren’t many people who know how to do this like you do. Has this research changed how you think about how sperm swim?

CHIH-KUAN TUNG: A lot. I learned a lot through this process. I never thought about it. I guess the common picture that sperm just compete with each other and then one win is so deeply rooted. But since I started, I realized the whole process is so complicated that I never knew anything about.

So for example, whatever we talk about here is probably more relevant in the lower part of the female reproductive tract, say, cervix or uterus, because, in order for them to cluster, you need certain– you need a higher number of counts. So we are probably not talking about something closer to the fertilization site, which is in the oviduct.


CHIH-KUAN TUNG: And yeah, and the whole process is just so much more complicated than we learned in, say, high school.

IRA FLATOW: Yes. Do you have physicians and people who are interested in fertility of patients, who study fertility, have you found them asking you about your research?

CHIH-KUAN TUNG: Yeah, we talk to clinicians at conferences and those, and I actually heard from one person who told me that, when they look at the sperm samples after the intercourse from the female body, that they actually saw sperm swim like next to each other. But because it wasn’t something they were interested in, so it was never explicitly reported. There were some interesting conversations along the line, yes, certainly.

IRA FLATOW: Well, I would think that maybe you could have a fertility test, perhaps, about sperm.

CHIH-KUAN TUNG: Yeah, that’s the goal down the line. So at the moment, we are generating some knowledge of how those behaviors help the goal of reaching the egg. So the first thing is that we would like to use this knowledge to develop a better diagnostic tool for, say, male infertility because, right now, the semen analysis has not been very helpful in quite a bit of cases, those situations that is called unexplained.

Once we can get there, potentially, we can also talk about– maybe we can do some sperm selection for infertility treatment. But that’s further down the road and require a lot more other expertise that I do not currently have myself.

IRA FLATOW: Well, I’m sure you’ll find that expertise from teach it yourself like this, or you’ll find from someplace else, doctor. Thank you for taking time to be with us today.

CHIH-KUAN TUNG: Thank you for having me.

IRA FLATOW: Dr. Chih-Kuan Tung, associate professor of physics at North Carolina A&T State University based in Greensboro.


Meet the Producers and Host

About Shoshannah Buxbaum

Shoshannah Buxbaum is a producer for Science Friday. She’s particularly drawn to stories about health, psychology, and the environment. She’s a proud New Jersey native and will happily share her opinions on why the state is deserving of a little more love.

About Ira Flatow

Ira Flatow is the host and executive producer of Science FridayHis green thumb has revived many an office plant at death’s door.

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