Even sperm gotta stick together.
A new study has shown that bull sperm swims more efficiently in groups, possibly offering insight into human fertility. This behavior can be seen in simulated reproductive tracts, such as those of cattle or humans. Groups of bovine sperm from cooperative cows will outperform meandering lonersChih-kuan Tung, physicist, and his colleagues race to fertilize a female egg cells. Frontiers in Cellular and Developmental Biology.
The benefits of clustering don’t come down to flat-out speed. “They are not faster,” says Tung, of North Carolina Agricultural and Technical State University in Greensboro. “In terms of speed, they are comparable or slower” than sperm traveling alone. As in the race of individual hares against herds or tortoises in sperm, the winners don’t always have to be the fastest, but those who are able to stay on target.
On their own, sperm tend to follow curved paths — which is a problem, because the shortest distance between two points is a straight line. When sperm are in large groups, however, they follow straighter paths. It’s behavior that a couple of the same researchers noted in a previous study where They tracked sperm swimming in stationary fluids (SN: 3/17/16). This might give sperm groups an advantage, but it only works if they’re moving in the right directions. Other benefits of sperm clustering weren’t clear until the researchers developed an experimental setup that introduced flowing fluid into their experiments.
Animals like humans and cattle have sperm that swim against a current of mucus. This pushes the sperm towards the ovum. It’s difficult to study what benefits clustering might confer while swimming upstream inside living beings. Tung and co-workers created an analog in the lab. A narrow channel measuring 4 centimeters long was filled with a thick fluid. It mimics natural mucus flow rates and is shallow.
Regardless of whether sperm are swimming upstream by themselves or in groups, they naturally swim upstream. The experiment showed that clusters of sperm performed better at heading upstream to the mucus stream than individual sperm. Although some sperm moved faster than others, their ability to point upstream was slower and hindered the progress of sperm loners.
Clusters were also able to maintain their position despite rapid-flowing mucus. Individual sperm was washed away after researchers turned up the flow. Clusters of sperm were less likely to be swept downstream.
Tung states that even though the sperm studied in this study were bovine, the benefits of clustering should be applicable to human sperm. Both species have similar sizes. Swimmers often compete to fertilize one ovum. Contrary to other animals, where semen is placed directly in the uterus of pigs and other animals, both human and bovine embryos start in the vagina. They then travel through the cervical region to reach the uterus.
Studying sperm in fluids that closely resemble the flowing mucus in reproductive tracts could reveal problems that don’t turn up in conventional observations of sperm swimming in stationary fluids, Tung says. “One hope is that this sort of knowledge can help us do better diagnoses” to provide clues to understand Fertility in humans (SN: 3/31/03).
The lab might soon allow sperm to be subjected to realistic conditions. Help for people having difficulty conceivingChristopher Barratt, a fertility researcher at the University of Dundee in Scotland, said that the answer is yes. He was not associated with the study.SN: 6/9/21).
“How a sperm cell responds to its surroundings and how that may change its behavior is a very important subject,” Barratt says. “This type of technology could be used, or adapted, to select better quality sperm,” for people in need of fertility assistance. “That would be a very big deal.”
