Even the sperm gotta coagulate.
Bull sperm swim more efficiently when in clusters, a new study shows, offering potential for fertility in humans. In simulated reproductive tracts of animals such as cattle and humans, the behavior increases the chances that groups of bovine sperm will jump out cooperatively, like the race to fertilize a female egg cell, scientist Chih-kuan Tung and colleagues report on September 22. Frontiers in Cell and Developmental Biology.
The benefits do not come down to flat speed. “They’re not faster,” said Tung, of North Carolina State Agricultural and Technical University in Greensboro. “To what extent are the speeds comparable or slower” than sperm traveling alone? Like the sperm of a herd of tortoises, like the individual running hares, the winners are not necessarily the fastest, but those who can stay on target.
Their sperm tend to follow curved paths, which is a problem because the shortest distance between two points is a straight line. But when the sperm are gathered in groups of two or more, they swim in straight paths. Its behavior is noted by two of the same researchers in a previous study where they investigated sperm swimming in stationary fluids (SN: 3/17/16). Although that would give the advantage of sperm clusters, it only helps if it happens to be going in the right direction. Other good sperm clusters weren’t clear until the researchers developed an experimental kit that introduced the fluid into their experiments.
In creatures such as humans and cattle, the sperm tend to swim to the egg against the flow of mucus that flows through the cervix and leaves the uterus. It is difficult to study how many benefits the riverbanks can bring to the animals swimming in them. So Tung and his colleagues created an analogy in their lab: a shallow, narrow, 4-centimeter-long channel filled with a thick fluid that mimics natural mucus flows and at rates the researchers could control.
Whether alone or in groups, sperm naturally tend to swim in the river. However, the clusters of sperm in the experiment did a better job of streaming into the mucus, while individual sperm were more likely to be blocked in other parts. Despite the faster travel of some individual sperm, the weaker ability to target downstream impedes the progress of sperm loners compared to slower moving clusters.
A cluster of mucus also flows rapidly in the face. When the researchers turned on the flow of their media, many individual sperm were washed away. A sperm cluster is much less likely to drag downstream.
While the bovine sperm were in the study, the benefits of the vaccine should also apply to human sperm, Tung says. The sperm of both species have similar dimensions. Swimmers typically strive to fertilize one egg. And unlike pigs or other animals in which semen is deposited directly into the uterus, both human and bovine sperm begin in the vagina and pass through the cervix to reach the uterus.
Studying sperm in fluids that flow with mucus in reproductive tracts closely resembles problems that don’t occur in observations of sperm floating in stationary fluids, he says. “The only hope is that this kind of knowledge can help us to make better diagnoses”, to understand the causes of infertility in people (SN: 3/31/03).
Subjecting sperm to real-world settings in the lab could soon provide practical help for people struggling to conceive, says fertility researcher Christopher Barratt of the University of Dundee in Scotland, who was not affiliated with the study.SN: 6/9/21).
“How a sperm cell responds to its surroundings and how it changes its behavior is a very important subject,” says Barratt. “This type of technology can be used, or adapted, to select better sperm quality” for men in need of fertility assistance. “That would be a very big deal.”
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