Bull Sperm Get by With a Little Help From Their Friends | Science


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A bull runs in front of a cow. If they mate, the bull’s sperm will likely pool as they swim through the female’s reproductive organs.
Xurxo Lobato/Getty Images

Scientists began looking at semen under a microscope nearly 350 years ago, and since then their sperm sightings have raised as many questions as answers. Back then, they couldn’t figure out exactly what the fidgety little things were or what they were doing, let alone the different ways in the animal kingdom that sperm fulfill their reproductive role.

Part of the problem comes from looking in the wrong place. Sperm doesn’t do much under a microscope; They once thrive in the female reproductive tract – a very difficult place to see what happens when sperm plumes spring into action. The situation has created some persistent misconceptions, such as the idea that reproduction is always an “every sperm for itself” sprint.

Despite the often competitive aspects of animal reproduction, scientists now know that some groups of sperm from the same ejaculate actually gather to work together in some sort of social cooperation. Researchers have recently documented the sperm of mice, mollusks and possums pooling, although they don’t always know why this happens.

A study published today in Frontiers in cell and developmental biology has revealed a reason at least among the cops; Swimming together helps sperm move through the sticky fluids found as they migrate through the female reproductive tract. Using a microfluidic machine to simulate the mucus-like conditions inside a cow, the researchers learned that sperm pooling has benefits that help them efficiently navigate the female tract and swim upstream – better than individual sperm. The study and others trying to recreate the environments in which sperm swim may help improve sperm analysis, which could be used to improve human fertility techniques.

Sperm science has a long and turbulent history. The field was started by Anton van Leeuwenhoek, inventor of the compound microscope, who observed spermatozoa in his own semen and published a paper on his findings in 1678 – but only after fearing that “these observations would repel scholars or could be outraged”. After van Leeuwenhoek threw the spotlight on sperm, many sometimes hilarious theories attempted to explain what exactly sperm were and how conception happened. His contemporary Nicolass Hartsoeker claimed to have seen sperm a few years before van Leeuwenhoek’s publication, but like others later dismissed them as a type of seminal parasite. For nearly two centuries, one school of thought insisted that each sperm contained a very small, preformed human.

Sperm are individual cells with a unique mission. They pass on a man’s genetics to the next generation. Unlike other cells, they are not designed to be part of the body but are produced to be ejaculated and live in a foreign environment. “Our main field is the female reproductive system, but it’s an incredibly difficult place to visualize and experiment with,” says Scott Pitnick, a Syracuse University biologist and sperm specialist who is not affiliated with the study. “It’s probably easier to study ice fish in Antarctica.”

Chih-kuan Tung, a physicist at North Carolina Agricultural and Technical State University, and colleagues addressed this problem by recreating key aspects of the female reproductive system so sperm could be easily observed. Tung notes that researchers at a typical fertility clinic or bull semen trading service simply place sperm in an aqueous laboratory solution, place it between two panes of glass, and watch them float under a microscope. While the method reveals obvious problems, like sperm that can’t swim, it can’t provide much real-world information.

“We really should look at a swimming environment that’s closer to what sperm would encounter in a female reproductive system,” notes Tung. To that end, his team from North Carolina A&T and Cornell University began studying how bull sperm — a decent surrogate for our own among mammals — moves in a sticky environment resembling the conditions found in the cervix, uterus, and fallopian tubes of cattle.

The group knew from previous studies with the liquid that bull sperm formed clumps, but also that these clumps could not swim faster than individuals, so this apparent advantage was not the reason the sperm stuck together. Looking for another benefit, the team designed a new experiment that added currents, or currents, as sperm encounter them in life. They uncovered three different ways sperm benefited from clustering, depending on the flow of fluid in the environment.

When there was no flow, the clusters traveled to their destination in a much more direct way than individual sperm could. “It’s an advantage for them because they want to go somewhere,” Tung explains.

With moderate current, clustered sperm could align better to swim directly against the current. That’s the direction they want to go in the female tract because fluid would generally flow outward.

As flow currents increased to the highest levels in the reproductive tract, clustering allowed sperm to stand strong and resist the flow, so they were much less likely to be swept away downstream than individual sperm.

Taken together, the results show that sperm travel through the reproductive tract fluids is aided by social cooperation. They can detect and maintain correct direction more efficiently, or even use slipstream techniques favored by packs of cyclists and race cars in strong currents.

bull sperm

Bull semen that swims in clusters (marked in yellow ovals) has advantages when moving through fluids such as those found in cows’ reproductive organs.

S Phuyal, SS Suarez, CK Tung

By trying to mimic the environment of the female tract, from fluid flows to three-dimensional shapes, studies like this one can improve semen analysis and help design more effective infertility treatments for people — as well as better contraception for those hoping to conceive avoid .

“People are going to more realistic types of environmental setups to study sperm function, and that’s been totally absent in the history of sperm research,” says Pitnick. In his own work with fruit flies, he uses fluorescent protein markers to visualize sperm heads so he can observe how they interact and compete in the female reproductive tract.

In species other than bulls, research has uncovered some instances of social cooperation where sperm move collectively in very interesting ways. Each wood mouse sperm has a hook on its head that the sperm use to connect in trains of hundreds to thousands that swim faster than individuals. In some molluscs, an oversized sperm serves as a sort of mobile penis, a bus that transports and deposits other fertilizing sperm on its way through the reproductive tract. In the opossum, sperm have evolved to get where they want to go by swimming in pairs, connected by asymmetric heads, only separating when they are close to an opportunity to fertilize the egg. However, scientists still don’t know all the reasons why these sperm cells work together. By teasing out some clear advantages among bull semen, this study moves the ball forward.

“To me, this study shows that it is likely that even in species that have not evolved physical attachment mechanisms for this cooperation, there are still advantages for sperm to cooperate during their migration through the female tract,” says Pitnick. “And they demonstrate biophysically, in terms of flow dynamics, how it actually works.”

Such work is also key to understanding the evolutionary biology of sperm, how they function in amazing and unique ways within the female reproductive tract – which Pitnick calls one of the great unexplored frontiers in all of biology. “We need to understand this environment,” he says, “to understand what sperm are doing in it.”



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