These Researchers Put Sperm Through a Kind of ‘Hunger Games’

Karen Hopkin: This is Scientific American’s Science, Quickly. I’m Karen Hopkin.

Swimming against a current can be tough. But imagine having to do it in a fluid with the consistency of corn syrup. That’s more or less the challenge faced by mammalian sperm as they race to reach an egg.

Reza Nosrati: That’s such a hard race for sperm. It’s like a very tough marathon.

Hopkin: Reza Nosrati is a mechanical engineer at Monash University in Melbourne, Australia. He says that sperm are not deterred by tide or averted by viscosity.

In fact, Reza and his team have found that having to navigate the physiological slalom of the female reproductive tract actually helps sperm swim with optimal efficiency and may guide them to their desired destination. The results appear in the journal Cell Reports Physical Science.

Nosrati: When you look at the female reproductive tract, I think it’s been known for such a long time that it’s a very complicated environment.

Hopkin: As sperm swim upstream, they make their way through secretions that vary in thickness and flow rate. One of the first hurdles comes at the cervix, the gateway to the uterus.

Nosrati: Right at the port of entry to the cervix, there is a very strong flow because of mucous secretions by the cells within the cervix.

Hopkin: That current helps to flush out any harmful, disease-causing bacteria, which are not known to be strong swimmers.

Nosrati: It’s kind of like a barrier and a filter to keep the environment essentially favorable for fertilization.

Hopkin: And once sperm get to the fallopian tube, where fertilization typically takes place, they face not only fluids that are gooier but a course whose undulating curves lead to blind alleys…

Nosrati: …and to the increasing level of geometrical confinement that the sperm need to leverage to find their way forward.

Hopkin: But rather than hampering the swimmers’ progress, maybe these changes serve as cues—or clues—that give the cleverest cells a leg (or in this case a flagellum) up.

Nosrati: Would that be a mechanism that enables sperm of better quality, or smarter cells, to adjust their swimming behavior in order to gain some competitive advantage over millions of other sperm cells that are swimming in the reproductive tract?

Hopkin: To find out, Reza and the gang put bull sperm to the test.

Nosrati: We use bull sperm because it’s—in terms of its morphology and shape and beating behavior is quite similar to human sperm.

Hopkin: But they didn’t want to look at sperm by the handful.

Nosrati: We wanted to study these behaviors at the single-cell level—looking at the same cell while we’re replacing the flow, while we’re changing the viscosity.

Hopkin: And they wanted to focus on the flagellum because it’s that whiplike tail that propels a sperm forward.

Nosrati: Now, how that flagellum beating and how the movement of that structure changes in response to changing flow conditions was completely ignored.

Hopkin: So the researchers built a sperm-sized testing arena. Or in engineer speak…

Farin Yazdan Parast: We designed a microfluidic channel.

Hopkin: Farin Yazdan Parast is a research fellow in mechanical engineering at Monash University. She says each microfluidic channel… 

Yazdan Parast:: …had three different inlets for different viscosities so we can expose sperm to different viscosities, and also we have different flow rates to also mimic the fluid condition in the female reproductive tract.

Hopkin: To make sure the sperm didn’t swim out of her microscopic field of view, Farin tethered their heads to the chamber floor, which was also key to keeping an eye on things once the researchers cranked open the faucet.

Nosrati: The flow could push the cells out of our observation window.

Hopkin: But with the sperm heads stuck to the surface, the researchers were free to fiddle with the fluid dynamics….

Nosrati: And then I’m looking at the cell while it can’t go anywhere. Then we could see in real time how that cell respond[s] to that change in the environment.

Hopkin: So after all this setup, what did they find?

Yazdan Parast: What we observed was that the viscosity had a more dominant role than the shear rate in influencing the sperm flagellar waveform and the overall sperm behavior.

Nosrati: And I think it was a little bit surprising. Like, when we started the study, I was expecting to see a more dominant effect from shear rate than viscosity. But we actually realized it’s the other way around: the viscosity plays a more crucial role.

Hopkin: When the sperm are swimming through fluid that’s more syrupy …

Nosrati: … they reduce their flagellar beat amplitude; then they beat with a smaller wave. Those cells who can demonstrate this type of behavior actually swim in a way that is more energy-efficient.

Hopkin: That behavior could give them a biological boost.

Nosrati: Maybe there’s another cell, which is trying much harder. But they’re not doing it the right way, and they can’t propel forward.

Hopkin: So a sperm that’s able to change gears and breeze effortlessly along…

Nosrati: That will be the sperm that can reach the site of fertilization quicker and get to the egg.

Hopkin: Reza says the findings could advance fertility treatments in which a clinician chooses which sperm to present to an egg.

Nosrati: Maybe when it comes to treatment strategies, we need to move towards media that are more viscous and more closely mimics those properties of in vivo fluids because that can potentially lead to selecting better cells and a strategy which is better informed by that natural swimming behavior.

Hopkin: In other words, make the sperm work for it.

Nosrati: I think if you have a sample, which has cells, relatively motile cells, then making the race a little bit tougher increases your chance of getting the best cells.

Hopkin: Whether or not a viscosity challenge could lead to better outcomes for treatments such as in vitro fertilization, or IVF…

Nosrati: We need to do animal studies to make sure what we are essentially hypothesizing makes sense.

Hopkin: While they wait for those results, Reza and Farin will continue to talk about their work—or try to.

Yazdan Parast: Um … it’s a bit weird to start a conversation about sperm and [these] kinds of things.

Nosrati: I think it’s starting—talking about it is always difficult….

Yazdan Parast: Yeah.

Nosrati: But when you start, especially at parties or stuff like that, people will follow up. And they remember it [laughs].

Hopkin: Even without the party, I suspect I may have tethered bull sperm stuck in my head for some time to come.

Science, Quickly is produced by Jeffery DelViscio, Tulika Bose, Kelso Harper and Carin Leong. Subscribe wherever you get your podcasts, and visit for updated and in-depth science news.

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For Scientific American’s Science, Quickly, I’m Karen Hopkin.