Scientists first began looking at semen under a microscope about 350 years ago. At the time, they were unable to understand exactly what the undulating little things were or what they did, let alone the different ways throughout the animal kingdom that sperm play a reproductive role.
Part of the problem comes from looking in the wrong place. Sperm do not do much under the microscope. They breed in the female reproductive tract, which is a very difficult place to see what happens when the sperm population kicks in. This situation creates persistent misconceptions, such as the idea that reproduction is always a splint “every sperm for itself”.
Despite the often competitive aspects of animal reproduction, scientists now believe that several groups of sperm from the same semen actually come together and work together in a sort of social cooperation. Researchers recently documented mouse, mollusk and opossum sperm joining forces, but it’s not always known why that happens.
Research published today The forefront of cell developmental biology We made it clear why, at least among the bulls. Swimming along helps sperm move through the sticky fluid found as it travels through the female reproductive tract. simulated and found that sperm clustering has the advantage of helping them move efficiently through the female duct and swim upstream against the current. This study, and others attempting to recreate the environment in which sperm swim, may help improve sperm analysis, which could be used to enhance human fertilization techniques.
Sperm science has a long and colorful history. The field was started by Anton van He Leeuwenhoek, inventor of the compound microscope. He observed sperm in his semen and published a paper on his findings in 1678. Once van Leeuwenhoek brought sperm to the limelight, many, sometimes comical, theories attempted to explain exactly what sperm are and how conception occurred. His contemporary Nicholas Hart Sawker claimed to have seen spermatozoa several years before Van Leeuwenhoek’s publication, but, like others afterward, dismissed them as a type of seminal parasite. argued that each sperm contains a very small, preformed human being.
A sperm is a single cell with a unique mission. They pass the male gene to the next generation. Unlike other cells, they weren’t meant to become part of the body, but were created to be ejaculated and live in a foreign environment. Scott, a biologist and sperm expert at Syracuse University “Our main field site is the female reproductive system, which is very difficult to visualize and experiment with,” says Pitnick, who was not involved in the study. “It’s probably easier to study icefish in Antarctica.”
Chih-kuan Tung, a physicist at North Carolina Agricultural Technology State University, and his colleagues tackled this problem by recreating key aspects of the female reproductive system so that sperm can be easily observed. . At a typical fertility clinic or bull semen service, Tung said, researchers simply put the sperm in a laboratory water solution, sandwiched it between two pieces of glass, and watched it swim under a microscope. See. While this method reveals obvious problems such as non-swimming sperm, it fails to provide real-world information.
“We really need to look at a swimming environment that approximates what sperm encounter in the female reproductive system,” Tung said. To that end, his team at North Carolina A&T and Cornell University found that bull sperm (our own suitable stand-in in mammals) had a sticky consistency similar to the condition of a cow’s cervix, uterus, and fallopian tubes. I started researching how it behaved in the environment.
The group knew from previous work that spermatozoa of bulls form clusters, but those clusters cannot swim faster than individuals, so their obvious advantage is why sperm stick together. There wasn’t. Seeking another advantage, the team designed a new experiment that added a flow and flow like sperm encounters in life. They revealed three different ways that sperm benefit from clustering, depending on fluid flow in the environment.
In the absence of flow, the clusters advanced toward their targets in a much more direct path than individual sperm could. I will explain.
At moderate flow levels, the clustered sperm were able to align themselves to swim against the flow.
When current levels of flow were raised to the highest levels found within the reproductive tract, clustering enabled sperm to stand strong and withstand the flow, and was shed far less frequently downstream than individual sperm. rice field.
Taken together, the results indicate that the process of sperm passage through the reproductive tract fluid is facilitated by social cooperation. They are able to identify and maintain proper orientation more efficiently. You can even use the drafting technique favored by cyclists and race car flocks in strong currents.
By attempting to mimic the environment of the female tract, from fluid flow to three-dimensional shapes, such research could help improve semen analysis and create more effective fertility treatments for humans. there is. .
“People are trying to have a more realistic kind of setting to explore sperm function, which has been completely absent in the history of sperm research,” says Pitnick. used fluorescent protein tags to visualize sperm heads, allowing us to observe how sperm interact and compete in the female reproductive tract.
In species other than bulls, research has revealed several examples of social cooperation in which sperm move collectively in very interesting ways. , whereby the sperm are linked in hundreds to thousands of rows and swim faster than an individual. This is the bus that transports and unloads other fertilizing sperm on its route through the reproductive tract. In opossums, sperm have evolved to reach their destination by swimming in pairs connected by asymmetric heads, separating only when the opportunity to fertilize an egg approaches. But scientists don’t know all the reasons why these sperm cells work together.
“To me, this study shows that even in species that have not evolved physical binding mechanisms for this cooperation, there are still benefits to sperm cooperating as they move through the female duct. “And they show how this actually works from a biophysics, flow dynamics perspective.”
Such studies are also key to understanding the evolutionary biology of sperm and how they came to function in a strikingly unique way within the female reproductive tract. This is what Pitnick calls his one of the great unexplored frontiers in all biology. “You have to understand the environment,” he says.