In 1677 Anton van Leeuwenhoek, Dutch scientist and inventor of the first compound microscope, finally gave into peer pressure from his colleagues and used the tool to examine his own semen. The wriggling “animalcules” that he described would come to be known as individual sperm cells, or spermatozoa. Each had a rounded head and, van Leeuwenhoek thought, a tail that moved side to side to project it through fluid. Until now, pretty much everything scientists know about human sperm movement has been based on van Leeuwenhoek’s primitive observations. But a paper published today in Science Advances has upended roughly 350 years’ worth of assumptions about reproduction, the most essential of biological functions.
“There's just complete misinformation in almost the entire history of understanding sperm functional biology, and it needs to be corrected, but it's a real challenge,” says Scott Pitnick, an evolutionary biologist who studies sperm biology at Syracuse University and who was not involved in the study. “And this is one of the first studies that has really risen to that challenge and cracked sort of a complex problem.”
Using 3-D microscopy and advanced mathematical analyses, an international team of researchers from the University of Bristol in England and the Universidad Nacional Autonoma de Mexico discovered that the human sperm tails’ snakelike movement is an optical illusion. Rather than moving side-to-side, sperm tails actually turn in only one direction. Without other adjustments, a one-sided stroke would result in sperm swimming in circles and never reaching their destination, the female egg. To compensate, the scientists found, the body or head of the sperm rotates independently in a corkscrew-like motion in the opposite direction, enabling the whole cell to move forward in a straight line.
“We were not expecting to find what we found,” says Hermes Gadêlha, head of the Polymaths Lab at University of Bristol and lead author on the study. “The aim of the project was ‘blue sky’ [or broad] research, to understand how sperm moves in three dimensions. And the result has completely changed the belief system that we have.”
The limitations of van Leeuwenhoek’s description of sperm motility were no fault of his own; he was using the most advanced technology available at the time. “To see the true movement, you would have to swim with the cell, and the way you do this, is almost like if you could get a GoPro camera and attach it to the head of the sperm, and look at the tail,” says Gadêlha.
To get an accurate picture of how a sperm cell moves, Gadêlha and his team vertically suspended sperm in a solution. They set the sperm solution in a stabilized 3-D microscope to scan for motion as a high-speed camera recorded more than 55,000 frames per second at many angles. They also attached a piezoelectric device—which measures changes in pressure, acceleration, and force by converting these properties to electrical charges—to the 3-D microscope. That device gathered information about sperm movement at the level of submicron resolution, smaller than one-millionth of a meter. By running the combined data gathered from all the machines through advanced mathematical transformations, the scientists were able to find movement averages and “see” the true directionality of the tails.
Each sperm cell moved like a spinning top, rotating around its own axis, and also around a middle axis. “What nature is telling us is that there is more than one way to achieve symmetry,” says Gadêlha. “Sperm use asymmetry to create symmetry.”
Human spermatozoa are not the only microorganisms to function this way—mouse and rat sperm and the flagella of Chlamydomonas, a type of green algae, also have asymmetric movements and an underlying asymmetric shape. This, says Gadêlha, may be indicative of universality in organizational structures across species.
Whether or not a sperm’s movement is the most efficient way to swim is hard to quantify. “We like to think that nature is optimizing things but we always have to remember that there are many competing aspects. A sperm cell is not just made to swim and find the egg, it has to find chemical cues, react to different viscosities, activate,” says Gadêlha. “At every stage you need a new super power that enables you to do these things.”
To understand the evolution of structural mechanisms within an organism, Pitnick says, it’s about understanding the familiar biological concept of form fitting function; the shape of something is designed for the job it is meant to perform. To truly understand sperm, it must be observed in its intended, selective environment—the female reproductive tract, which scientists also need to study more. “The female is a complex three-dimensional environment.,” says Pitnick. “And we don't know very much about it, and in part that's just been a historical, obscene male bias in doing biology.”
Doctors think that this new discovery showing how sperm move can help treat infertility, a condition that affects roughly 50 million couples globally. Male biological factors are solely responsible for an average of 20 to 30 percent of cases of infertility, and contribute to about 50 percent over all. Still, these statistics are biased based on countries where data from IVF and other fertility treatments are common, so sperm factors could be even more significant than recorded. “[Male infertility] is really quite common, perhaps more common than the general public realizes,” says Cori Tanrikut, a reproductive urologist with Shady Grove Fertility Center in Maryland. “And right now, if you want to think about this study, currently, we really have limited means of improving or optimizing sperm motility.”
The more accurately scientists can understand the fundamental molecular biology of sperm motility, the better doctors may be able to address motility issues associated with infertility, says Tanrikut. She hopes that knowledge gained from future work in the field will help her offer patients less aggressive fertility treatment options, or even improve their chances of conceiving without assistance.
Implications of Gadêlha and his team’s discovery could also go far beyond the scope of what this study demonstrates about sperm. The cell as an organism makes unconscious computations and corrections, adjusting torque and movement patterns depending on the conditions around it. Understanding these mechanisms could inform soft robotics research and materials science. One of Gadêlha’s students, for example, is looking at how the body’s slight, undetectable oscillations could be useful in developing foot and ankle prosthetics.