Even the sperm must stay together.
Bull sperm swim more effectively when in groups, a new study shows, which could offer insights into fertility in humans. In the simulated reproductive tracts of animals such as cattle and humans, the behavior increases the chances that Cooperative bovine sperm packs will outcompete meandering loners as they race to fertilize a female ovum, physicist Chih-kuan Tung and colleagues report September 22 in Frontiers in cell and developmental biology.
The benefits of clustering are not reduced at full speed. “They’re not faster,” says Tung, of North Carolina Agricultural and Technical State University in Greensboro. “In terms of speed, they are comparable to or slower” than sperm traveling alone. Like the sperm equivalent of herds of tortoises competing with individual hares, the winners are not necessarily the fastest, but rather those who can 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. But when sperm come together in groups of two or more, they swim along straighter routes. It’s a behavior that a couple of the same researchers observed in a previous study where tracked sperm swimming in stationary fluids (Serial number: 03/17/16). While that might give sperm pools an advantage, it would only help if they’re headed in the right direction. Other benefits of sperm pooling remained unclear until the researchers developed an experimental setup that introduced fluid fluid into their experiments.
In creatures like humans and cattle, the sperm reach the egg by swimming against a current of mucus that flows through the cervix and away from the uterus. It is difficult to study what benefits clustering might confer while swimming upstream within living things. So Tung and his colleagues created an analog in their lab: a narrow, shallow channel 4 centimeters long filled with a thick fluid that mimics natural mucus and flows at a rate the researchers could control.
Whether alone or in groups, sperm naturally tend to swim upstream. However, groups of sperm in the experiment did a better job of heading upstream toward the mucus flow, while individual sperm were more likely to head in other directions. Despite the faster travels of some individual sperm, a lower ability to point upstream hampered the progress of solitary sperm compared to slower-moving groups.
The clusters also stayed the course in the face of rapidly flowing mucus. When the researchers increased the flow in their apparatus, many individual sperm were eliminated. The sperm clusters were much less likely to be swept downstream.
While the sperm in the study were bovine, the advantages of pooling should also apply to human sperm, says Tung. The spermatozoa of both species have similar dimensions. Swimmers often compete to fertilize a single egg. And unlike pigs or other animals where semen is deposited directly into the uterus, both human and bovine sperm start in the vagina and travel through the cervix to reach the uterus.
Studying sperm in fluids that closely resemble the mucus that flows in reproductive tracts could reveal problems that don’t show up in conventional observations of sperm swimming in stationary fluids, says Tung. “One hope is that this kind of knowledge can help us make better diagnoses” to provide clues to understanding infertility in humans (Serial number: 03/31/03).
Subjecting sperm to realistic environments in the laboratory may soon offer practical help for people who have trouble conceivingsays fertility researcher Christopher Barratt of the University of Dundee in Scotland, who was not involved in the study (Serial number: 9/6/21).
“How a sperm responds to its environment and how that can change its behavior is a very important topic,” says Barratt. “This type of technology could be used, or adapted, to select for better quality sperm” for people who need fertility assistance. “That would be a big problem.”