Do you know how sperm swim?
Do you also think that sperm swims with their tails swinging from side to side?
In fact, conventional wisdom has misled us for years.
Some studies have found that the sperm head is actually constantly rotating, and the tail rolls around the swimming axis, rather than simply swinging from side to side.
What's going on here?
The "sperm hypothesis" of scientists has deceived for more than 300 years
In the 17th century, Dutch scientist Leeuwenhoek invented the microscope, which opened the door to the first observation of sperm in humans.
In 1677, while observing his semen, Hook was amazed to find a myriad of tiny "creatures" with large heads and tails, wriggling in the fluid, writhing like snakes and winding like eels.
At the time, he lamented that these sperm were like small snakes or eels in the water, swimming by wagging their tails.
Hooke was curious about these "creatures", but did not know their function, so he tried to trace their form with a microscope, but due to technical limitations, he had to draw them intuitively by hand.
In his sketches, the sperm resemble fish, swung from side to side with their tails swimming in the fluid.
100 years later, another scientist, Hart Sock, referred to Hooke's research and proposed a link between sperm and human reproduction, arguing that sperm dwells small people who grow up after entering a woman's body.
This "sperm hypothesis" explains the function of sperm, but there is also a clear loophole because the egg was not found at that time.
In the 19th century, scientists finally discovered the egg and observed the union of sperm and egg.
Until then, the scientific community had been using Hooke's theory about the movement of sperm, which believed that sperm moved forward by swinging their tails from side to side, squirming like snakes or eels.
This "snaketail hypothesis" does not raise questions, after all, similar patterns of movement can be observed under the microscope.
Advances in microscopy have allowed scientists to see sperm more clearly, but the way they move hasn't changed for more than 150 years.
In the scientific community, the "snaketail hypothesis" has been settled, and no one doubts that Hooke's hand-drawn drawings accurately reflect the truth about the movement of sperm.
It wasn't until 2020 that Mexican scientists used new technology to re-study the movement of sperm, and it turned out that the "snake tail hypothesis" since Hook was actually wrong!
It turns out that the sperm does not swing its tail from side to side, but burrows forward in a spiral fashion.
The "snake tail hypothesis" has deceived the scientific community for more than 300 years!
This is how the sperm propelled it
With advances in microscopy and high-speed videography, researchers have found that the way human sperm moves is very different from what has long been recognized.
It turns out that instead of simply swinging their tails from side to side to propulsion, they rotate forward in a drill-like way through the liquid.
In 2020, a team of researchers from the National Autonomous University of Mexico used a 3D microscope and a high-speed camera at 55,000 frames per second to find that sperm advances more like a miniature drill — it rotates in a spiral while the head rotates slowly.
This discovery shattered the common human impression of how sperm moves.
What makes people wonder why sperm chooses this seemingly strange way of moving?
The researchers point out that this is precisely the result of long-term natural selection.
Unlike side-to-side swings, spiral rotation can adapt to the changing environment in the uterus, such as fluids of different viscosities, helping sperm save energy to overcome "obstacles" and finally achieve the goal of fertilization.
In fact, the sperm's tail and head movements are controlled by two separate mechanisms, with the tail providing the power of spiral propulsion while the head turns at a different rhythm, which helps to counteract the imbalance caused by the unilateral swing of the tail.
What provides energy to sperm is the large number of mitochondria in its body, like a mini battery.
In order to make the most of the limited volume, these mitochondria are arranged in a spiral pattern, and sperm need to store enough "fuel", because the journey from departure to completion of fertilization is 15-18 centimeters, and there are "traps" along the way, and more than 99% of sperm cannot complete the final task.
In other words, only an efficient way of movement can accomplish the task with extremely limited energy, and the rotation of the drilling fluid can minimize resistance, which is the professional "skill" of the highly evolved sperm.
In addition, although it has long been thought that the faster the sperm competition, the better, but in fact, speed does not determine the winner, and what really determines the egg is the screening of the eggs after the arrival of many sperm.
So, the existence of each of us does not mean that we have won first place in the great sperm competition in the past.
For more than 300 years, we have understandably misunderstood how sperm is propelled, after all, sperm are so small, only 50 microns long, that their high-speed rotation is difficult to observe with the naked eye.
Now, high-tech has uncovered the mystery of this mini life, giving us a glimpse of the incredible survival of sperm, and the discovery of sperm has brought mankind one step closer to the mystery of life.
Mathematical model of sperm freestyle
About 55 million sperm are produced for each male ejaculation, but very few reach the egg, and only a few of the hundreds of millions of sperm end up bonding to the egg.
Sperm needs to survive in the female reproductive system before it reaches the egg, and a large number of sperm will be eliminated on the way from ejaculation to the uterus.
Sperm have to adapt to the acidic environment of the vagina, and the cervical mucus also blocks them, and after entering the uterus, white blood cells will kill some sperm, and the female body also has a natural defense against sperm.
Even if it successfully enters the fallopian tube, the sperm will inevitably hit by mistake, because the woman only releases one mature egg a month, and the egg does not necessarily appear in the same fallopian tube, so a large number of sperm run in the tube without encountering the egg.
Brazilian mathematician Gadelha once built a mathematical model in order to accurately describe the peristalsis of sperm.
He found that the "connecting spring" of the sperm's tail transmits mechanical information as it swims, and different parts of the body bend in different ways, and the tail even takes on the opposite direction of bending from the head.
When the sperm head turns, the end of the tail creates an opposite bend, which is also known as the "reverse tendency", but it is also this peristalsis of the tail that provides the sperm with the power to help it move forward in the cervical mucus.
In addition to being the "helmsman" of itself, sperm also faces "screening" of eggs, and some studies have shown that eggs may have a tendency to attract or reject sperm from certain genes.
The metabolism of vitamins involved in the reproductive process may form important signaling molecules on the surface of sperm and egg cells, thus changing the degree of attraction of both.
Drawing on mathematical logic, physics, engineering, and other knowledge, Gadelha devised a mathematical formula to describe sperm movement, and he pointed out that after the sperm head calculates the direction of movement, the tail is a "soft machine" that executes it.
The use of mathematical models to depict sperm structure and peristalsis not only helps to understand the reproductive process, but also provides inspiration for the design of bionic soft robots.
After all, sperm is completely calculated by the body to achieve movement, without the involvement of the brain, which is exactly what soft robots are pursuing.
However, it remains to be verified whether Gadelha's model can fully describe sperm motility, and with advances in microscopy, scientists can observe sperm structure more clearly at the microscopic and nanoscale, and discover some new mechanisms that affect motility.
It has also been found that there may be differences in sperm morphological parameters in different populations and individuals, and more data needs to be collected to test whether Gadelha's model is universal.
At present, many scientists are working to develop new sperm motility testing technologies, which can measure the motility parameters of individual sperm and assess their quality, which can help treat male infertility.
This precise test can provide more valuable information than traditional methods of looking at the overall motility of sperm, and perhaps in the future, the technology may also be able to detect sperm DNA damage and screen healthy sperm for eugenics.
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