Teams of sperms swim more smoothly against the current

Coexistence of spermatozoa swimming individually and grouped. Cluster sperm are labeled with yellow ovals. Scale bar: 50 µm. Credit: S Phuyal, SS Suarez, CK Tung

It turns out that the sperm goes against the flow better when they swim together.

Despite the popular idea that the fastest and fit male reproductive cell is the one that wins the fertilization contest, research has shown that sperm often team up to navigate the female reproductive tract in a wide range of mammalian species. A new study published in the journal Frontiers in cellular and developmental biology offers some compelling reasons behind a newly identified clustering behavior.

Previous research by the team, led by scientists from North Carolina A&T State University and Cornell University, found for the first time that sperm naturally bond without attaching to each other when swimming in viscoelastic fluid. This is the type of fluid encountered by sperm as it migrates through the cervix and uterus to the oviduct where the egg is fertilized. The term viscoelasticity refers to both thickness and elasticity.

However, unattached sperm teams do not outperform solo swimmers, as they do in other examples of group behavior. For example, the wooden rat’s sperm head has a hook that physically attaches it to other sperm, connecting hundreds by thousands into a kind of sperm train that is faster than lone sperm.

Going against the tide

The researchers wanted to learn the possible biological benefits of this seemingly strange behavior on a scale and in an environment that is not easy to study, particularly the currents of viscoelastic fluid that flow through narrow channels in the female reproductive tract. In a series of experiments using bovine sperm (a good model for the human variety) and a microfluidic device to mimic the physical parameters of the female tract, they observed how sperm clustered in viscoelastic fluid reacted to different flow scenarios.

They discovered three potential biological benefits for sperm grouping, based on the strength of the current against which the sperm must travel. First, in the absence of flow, cluster sperms appear to change direction less frequently and swim in a straight line. Against a mild to intermediate flow, the clustered sperm are better aligned, like a school of fish heading upstream. Finally, with high physiological flow rates, there appears to be security in numbers against being carried away by the strong flow.

“In general, I would say that identifying motility benefits that are not speed improvement is not usual and therefore significant. We somehow open new avenues for examining sperm performance,” noted Dr. Chih-kuan Tung, co-author and associate professor of physics at North Carolina A&T State University.

Fertility needs physics

As a trained physicist, Tung said he is particularly intrigued by the protective dynamics at play when the flow is heavier. “This could resemble group formation on a bicycle, even if the fluid mechanics for sperm are drastically different from that of motorcyclists. We would definitely like to know more.”

Watching sperm swim isn’t just a scientific sport. A better understanding of the physics of how sperm navigates through the complicated female reproductive tract to fertilize the egg may have implications for infertility treatments and beyond.

“In the longer term, our understanding could provide better selection of sperm used for interventions such as IVF or other assisted reproductive technologies,” said Tung. “This may be necessary like [these methods] generally some or all of the selection mechanisms present in the female trait are skipped and give less favorable results “.

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More information:
Biological benefits of the collective swimming of spermatozoa in a viscoelastic fluid, Frontiers in cellular and developmental biology (2022). DOI: 10.3389 / fcell.2022.961623

Citation: Teams of sperm swim more smoothly against the tide (2022, September 22) recovered on September 22, 2022 from

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