Swimming in groups aids sperm retention in the reproductive tract.
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| On their way to fertilize a female egg cell, bull sperm that clusters together in groups of two or more swims straighter and more directly than individual sperm. S. PHUYAL, S.S. SUAREZ, AND C.-K. TUNG/CELL AND DEVELOPMENTAL BIOLOGY 2022 |
A new study finds that bull sperm swims more effectively in clusters, potentially providing insight into human fertility. The behavior increases the chances that groups of cooperative bovine sperm will outpace meandering loners as they race to fertilize a female egg cell in simulated reproductive tracts of animals like cattle and humans, physicist Chih-Kuan Tung and colleagues report September 22 in Frontiers in Cell and Developmental Biology.
The advantages of clustering are not solely related to speed. Tung of North Carolina Agricultural and Technical State University in Greensboro says, "They are not faster." "They are comparable or slower in terms of speed" than sperm traveling alone. The winners in the sperm equivalent of herds of tortoises racing individual hares are not necessarily the fastest, but rather the ones who can stay on target.
Sperm tends to follow curved paths on their own, which is a problem because the shortest distance between two points is a straight line. When sperm congregates in groups of two or more, it takes a more direct path. The same researchers observed this behavior in a previous study in which they tracked sperm swimming in stationary fluids (SN: 3/17/16). Although this could give sperm clusters an advantage, it would only be beneficial if they were heading in the right direction. Other advantages of sperm clustering were not apparent until the researchers devised an experimental setup that included flowing fluid.
Sperm tends to follow curved paths on their own, which is a problem because the shortest distance between two points is a straight line. When sperm congregates in groups of two or more, it takes a more direct path. The same researchers observed this behavior in a previous study in which they tracked sperm swimming in stationary fluids (SN: 3/17/16). Although this could give sperm clusters an advantage, it would only be beneficial if they were heading in the right direction. Other advantages of sperm clustering were not apparent until the researchers devised an experimental setup that included flowing fluid.
Sperm travels to the ovum in mammals such as humans and cattle by swimming against a current of mucus that flows through the cervix and away from the uterus. It's difficult to study the benefits of clustering while swimming upstream within living beings. Tung and colleagues devised an analog in their lab: a shallow, narrow 4-centimeter-long channel filled with a thick fluid that mimics natural mucus and flows at rates that the researchers could control.
Sperm naturally swims upstream, whether alone or in groups. Clusters of sperm in the experiment, on the other hand, did a better job of moving upstream into the mucus flow, whereas individual sperm were more likely to go in other directions. Despite the faster travels of some individual sperm, the ability of sperm loners to point upstream hampered their progress when compared to slower moving clusters.
Clusters also remained steady in the face of rapidly flowing mucus. Many individual sperm were washed away when the researchers increased the flow in their apparatus. Sperm clusters had a much lower chance of being swept downstream.
While the sperm in the study was bovine, Tung believes the benefits of clustering should also apply to human sperm. Both species' sperm have similar dimensions. Typically, the swimmers compete to fertilize a single ovum. In addition, unlike pigs or other animals, where sperm is deposited directly in the uterus, both human and bovine sperm begin in the vagina and travel through the cervix to reach the uterus.
Tung believes that studying sperm in fluids that closely resemble the flowing mucus in reproductive tracts could reveal problems that aren't apparent in traditional observations of sperm swimming in stationary fluids. "One hope is that this type of knowledge can help us do better diagnoses" in order to provide clues to understanding human infertility (SN: 3/31/03).
According to fertility researcher Christopher Barratt of the University of Dundee in Scotland, who was not involved in the study, subjecting sperm to realistic settings in the lab may soon offer practical help for people who are having difficulty conceiving (SN: 6/9/21).
"How a sperm cell responds to its surroundings and how that affects its behavior is a very important subject," says Barratt. "This type of technology could be used or adapted to select better quality sperm" for people seeking fertility treatment. "That would be a huge deal."
CITATIONS
C.-K. Tung, S. Phuyal, and S.S. Suarez. The biological advantages of sperm swimming collectively in a viscoelastic fluid. Frontiers in Cell and Developmental Biology, vol. 10, 961623, September 22, 2022. fcell.2022.961623. doi:10.3389/fcell.2022.961623.
Fluid viscoelasticity promotes sperm collective swimming, according to C.-K. Tung et al. Scientific Reports, Vol. 7, No. 3152, June 9, 2017. doi: 10.1038/s41598-017-03341-4.