For more than 300 years, most scientists have suggested that sperm “swim” through the liquid by twisting its tail like an eel to propel itself forward. But according to a new report in Science Advances, this is really just an optical illusion, the result of seeing creatures from above using a 2D microscope. New observations with a 3D microscope have revealed that human sperm actually roll when they swim, like an otter, essentially turning themselves forward.
“Understanding how sperm tail works will essentially help develop future diagnostic tools to identify unhealthy sperm,” Co-author of the report, Hermes Gadelha from the University of Bristol said.
3D simulation of how to wave the sperm’s tail, they seem to move symmetrically from side to side.
The first honor to observe the sperm directly belonged to Antonie van Leeuwenhoek, a 17th-century Dutch diver who was particularly interested in science. He built microscopes and devised innovative production methods to improve microscope visibility. Only a few of his microscopes remain to this day, but they are capable of magnifying small objects up to 275 times. And historians believe some of his tools can achieve magnification as much as 500 times.
Van Leeuwenhoek has studied many substances under his microscope, including fluids like lake water, blood, milk, foam and tears, even plaque on his own teeth. He was shocked to discover small living things under his microscope, which he called “animalcules”. This is also the person who is considered the father of microbiology.
After observing the liquids, Van Leeuwenhoek turned his attention to semen samples, although he initially did not want to do so, fearing that it would not be taken seriously. But in 1677, he was “displeased”. Using a sample of his own sperm, he found in the sample many “animalcules”. He described these little creatures in a letter to the Royal Society saying: “Prison head, long tail, pushing yourself forward by waving its tail” with a movement like an eel swimming in water. “
First descriptions of images of rabbit sperm (1-4) and dogs (5-8)
He continued to observe sperm in samples of semen collected from other animals, such as rabbits and dogs. But Van Leeuwenhoek’s seminal observations turned out to have fallen into an optical trap. Gadelha and colleagues recorded sperm swimming freely with a high-speed camera, capable of shooting more than 55,000 frames per second. Eventually, it led to the reproduction of 3D images of the motions of the sperm tail. They observed that sperm tail twisted only on one side, which often causes sperm to swim in a circle. Therefore, sperm has adapted to regulate, by twisting themselves when swimming.
“Our findings show that sperm have developed a swimming technique to compensate for their physical inferiority and thereby cleverly solve a microscopic mathematical puzzle: to create symmetry from asymmetry “, Gadelha said. “However, the otter-like rotation of a human sperm is complex: the sperm’s head rotates at the same time as the tail of the sperm revolves around the swim direction. This is known in physics as when the Earth’s orbit. and Mars orbits the sun. “
The symmetry was created from asymmetric body architecture.
And according to the scientists, this makes a lot of sense, based on what we know about fluid dynamics related to the movement of microorganisms. Such creatures living in this environment have a low Reynold. This is an index that expresses the relative magnitude between the effect of inertia and the internal friction (viscosity) on the flow. This index is based on the viscosity, length and speed variables. It was named after 19th-century physicist Ostern Reynold.
Back in 1977, physicist Edward Purcell made some calculations to show how different sized animals would swim at different Reynold indices. For example, this number would be very high for a whale, so that it can go a good distance with a fan of its tail. However, according to Purcell’s calculations, bacteria swim with low Reynold numbers, so they can hardly overcome any distance just by pushing the body away by wagging its tail. It was like a human trying to swim in a thick liquid like honey and moving his arm at a speed as slow as a clockwise motion. Therefore, eels and sperm (or bacteria) will adopt very different swimming strategies because of the need for survival, as they are dealing with different Reynold indices.
Simulating sperm swimming, viewed from the front.
Going back to research, fertility clinics today still rely on a 2D perspective when examining sperm motility, so this new study provides a better understanding of how sperm motions move. overlap, which can lead to better diagnostic tools.
“This discovery will revolutionize our understanding of sperm motility and its impact on natural fertilization.”, co-author Alberto Darszon from the University of Nacional Autonoma de Mexico, a pioneer of 3D microscope techniques. “There is very little information about the complex environment within the female reproductive system and how sperm swim into fertilization. And new tools will help reveal the amazing ability of sperm.”