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We recently posted a testis dissection video (which you can check out here), where we explained the anatomy and physiology of the testis, and of the male reproductive system in general. As you may already know, a primary function of the testes is to produce sperm via spermatogenesis, and fertilization happens when the sperm cell meets an egg cell in the fallopian tubes. (you can learn more about the fertilization process in our ovary dissection video here). Scientists for centuries have thought that upon ejaculation, sperm reaches the egg in the fallopian tubes by “swimming” through the female reproductive tract, using its tail to wiggle itself forwards. However, recent research using 3D microscopy and high-speed video technology has shattered these long-held assumptions about sperm motility, providing unforeseen insight into reproduction, the most essential and fundamental of all biological functions.
Antonie van Leeuwenhoek, the creator of the first compound microscope, was a Dutch scientist widely regarded as the father of microbiology. Leeuwenhoek was the first person to study the movement of sperm cells–in 1677, Leeuwenhoek used his newly developed 2D compound microscope to observe a semen sample up close.
Under the microscope, Leeuwenhoek observed tiny, wriggling “animalcules”–which later became known as individual sperm cells–and also noted that each “animalcule” had a rounded head, and a tail that moved side-by-side to project the sperm cell through fluid. Leeuwenhoek described the swaying, symmetrical manner in which the sperm cells seemed to move, as “that of a snake or an eel,” and until very recently, this 343-year-old observation was basically everything scientists knew about sperm movement.
Allan Pacey, a professor of andrology at the University of Sheffield, stated that "Many scientists have postulated that there is likely to be a very important 3D element to how the sperm tail moves, but to date we have not had the technology to reliably make such measurements," and Scott Pitnick, an evolutionary biologist at Syracuse University, stated that “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.” As explained by Dr. Pacey and Dr. Pitnick, many in the scientific community were beginning to think that there had to be more to sperm movement than the 2D swaying side to side, yet they lacked the technology to confirm their conjecture. However, all this changed in 2020, following an international study by the Universidad Nacional Autonoma de Mexico and the University of Bristol.
Dr. Gadêlha, a researcher at the Universidad Nacional Autonoma de Mexico, explained that the groundbreaking study on sperm motility used a microscopy technique that allowed for 3D imaging, and a high-speed camera able to capture 55,000 frames per second to record human sperm cells swimming on a microscope slide. Although the study was initially meant to be a “blue sky research (meaning basic, curiosity-driven research), the results were far from basic.
3D videos of sperm cells swimming revealed that the snake-like motion of the sperm cells described by Leeuwenhoek was actually an optical illusion, and that rather than moving side-to-side, sperm cells’ tails actually only move in one direction. Although a one-sided stroke of the tail on its own would result in the sperm pointlessly swimming in circles, researchers also discovered that the sperm compensates for the one-sided stroke of its tail by having the head rotate independently in a corkscrew-like motion in the direction opposite the tail’s stroke–combined, the tail and head’s respective one-sided motions enable the whole sperm cell to move forward in a straight line. Gadêlha described the newly discovered motion of the sperm by stating that the “sperm uses asymmetry to create symmetry,” and that the motion of the sperm is closer to drilling into the fluid, rather than swimming.
Interestingly, further research shows that human sperm cells are far from the only sperm cells that move using concurrent asymmetrical strokes–rat sperm, mouse sperm, and the flagella of many species of green algae also employ similar locomotive mechanisms. Scientists hypothesize that this may suggest an universal model of organizational structures across species. Fittingly, when taking into consideration the environment of the female reproductive tract–which is the “playing field” for sperm cells–the drilling method of movement employed by sperm cells would seem to provide an evolutionary advantage. The female reproductive tract is a very complex environment with narrow channels, sticky fluids, muscular contractions, and undulating cell walls, thus an aggressive drilling motion of movement would help the sperm navigate the “obstacle course” better than a smooth swimming motion of movement.
In addition to the scientific marvel that comes with redefining a key component of the reproductive system, a deeper insight into sperm motility has real life applications. Approximately 50 million couples globally are affected by infertility, with recent studies showing that the number is likely much higher. Male biological factors–most notably sperm motility issues–account for ~50% of all infertility cases, and are solely responsible for ~20-30% of all infertility cases. Recent research on sperm motility caused infertility had all worked under the now falsely proven assumption that sperm cells move by moving their tails side-to-side, and fertility clinics used 2D microscopes when observing sperm movement for diagnosis/treatment. Fortunately, the groundbreaking discovery scientists made regarding sperm movement and the newfound prospects of using 3D microscopy in diagnosis/treatment of male infertility issues could help open new pathways for addressing sperm motility issues that underlie many cases of infertility.
Sources:
https://www.smithsonianmag.com/science-nature/researchers-discover-how-human-sperm-really-swim-180975453/
https://www.livescience.com/sperm-swim-like-corkscrews.html
Image Credits:
https://www.sciencephoto.com/media/478928/view/historical-drawing-of-spermatozoa
https://www.sciencemag.org/news/2017/07/protein-tangles-behind-alzheimer-s-disease-may-be-critical-human-reproduction
https://www.researchgate.net/figure/Discovery-of-spermatozoa-by-Antonie-van-Leeuwenhoek-a-Scheme-of-Antonie-van_fig2_324310148
https://www.sciencemag.org/news/2020/07/watch-3d-video-reveal-how-sperm-really-swim
https://www.smithsonianmag.com/science-nature/researchers-discover-how-human-sperm-really-swim-180975453/
https://www.researchgate.net/figure/Configuration-of-the-3D-multi-sperm-image-acquisition-system-1-Piezoelectric-device_fig2_5301813