It turns out that sperm swim against the current better when they swim together.
Despite the popular notion that the fastest and strongest male reproductive cell is the one that wins the fertilization race, research has shown that sperm often team up to navigate the female reproductive tract in a variety of mammalian species. A new study published in the journal Frontiers in cell and developmental biology provides some compelling reasons for a newly identified clustering behavior.
Previous research by the team, led by scientists from North Carolina A&T State University and Cornell University, first discovered that sperm naturally contract without sticking together when floating in viscoelastic fluid. This is the type of fluid encountered by sperm traveling through the cervix and uterus to the fallopian tube where the egg is fertilized. The term viscoelasticity refers to both thickness and elasticity – think melted cheese.
However, teams of unbound sperm do not outrun the individual swimmers as is the case in other examples of group behavior. For example, the wood mouse’s sperm head has a hook that physically attaches it to other sperm, connecting hundreds to thousands in a type of sperm train that’s faster than single sperm.
Swim against the current
The researchers wanted to experience the potential biological benefits of this seemingly odd behavior on a scale and in an environment that is not easy to study — specifically, streams of viscoelastic fluid flowing through narrow channels in the female reproductive tract. In a series of experiments using bovine sperm (a good model for the human variant) and a microfluidic device to mimic the physical parameters of the female tract, they observed how sperm clustered in viscoelastic fluid responded to different flow scenarios.
They found three potential biological benefits of sperm clustering based on the strength of the current the sperm must travel against. First, in the absence of flow, clustered sperm seem to change direction less frequently and swim in a straighter line. Against a light to medium current, clustered sperm are better aligned, like a school of fish going upstream. Finally, at high physiological flow velocities, there seems to be a certainty of not being swept away by the strong current.
“In general, I would say that identifying motility benefits that are not speed increases is not common and therefore significant. In a way, we are opening up new avenues for studying sperm performance,” noted Dr. Chih-kuan Tung, co-author and associate professor of physics at North Carolina A&T State University.
Fertility needs physics
A physicist by training, Tung said he’s particularly fascinated by the protective dynamics when the current is at its strongest. “This may resemble the peloton formation seen in bicycling, although the flow mechanics for sperm are drastically different from those of bikers. We would certainly want to know more about that.”
Watching sperm swim is not just a scientific sport. A better understanding of the physics of how sperm navigate the complicated female reproductive tract to fertilize the egg may have implications for infertility treatment and beyond.
“Longer term, our understanding may allow better selection of sperm to be used for interventions, such as in vitro Fertilization or other assisted reproductive technologies,” Tung said. “This may be necessary [these methods] typically skip some or all of the selection mechanisms present in the female tract and lead to less favorable outcomes.”
Relation: Phuyal S, Suarez SS, Tung CK. Biological advantages of sperm swimming collectively in a viscoelastic fluid. Front Cell Dev Biol. 2022;10. doi: 10.3389/fcell.2022.961623
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