One Phantasmic Way Sperm Have Fooled Microscopes Since 1678

The sperm tail moves very rapidly in 3D, not from side-to-side in 2D, as it was believed.

Source: Image credited to polymaths-lab[dot]com

New state-of-the-art 3D microscope technology combined with advanced mathematical calculations reveal that human sperm propel themselves forward by spinning like corkscrews in an "otter-like" fashion. This research suggests that, contrary to popular belief, sperm don't swim by moving their tails bilaterally from side-to-side like eels.

This week, Hermes Gadêlha of the University of Bristol in the UK along with Gabriel Corkidi and Alberto Darszon of the Universidad Nacional Autónoma de México and other co-authors, unveiled groundbreaking new research that reconstructs the actual movement of the sperm tail in 3D. The findings (Gadêlha et al., 2020) were published on July 31 in the journal Science Advances.

The researchers found that the human sperm tail is surprisingly wonky; it only wiggles on one side. As a way to adapt to this lopsided stroke—which would cause every sperm to swim around in futile circles without some type of compensation—it appears that sperm use a corkscrew method of spinning "like an otter" that propels them forward.

Anyone who's struggled with fertility issues knows that the ability of a male partner's sperm to "swim well" is something that gets tested and ranked using the "sperm motility index." I know from first-hand experience that being poked, prodded, and subjected to a "Sperm Quality Analyzer" can trigger a wide range of emotions and irrational feelings of being "less than" if you don't get high scores.

One of the most deflating—yet kind of hilarious—experiences I had while struggling to conceive a baby via in vitro fertilization (IVF) was having a motility expert on the Upper East Side of Manhattan tell me in a nonchalant and kind of cliché, jaded New Yorker way that my sperm were just "meh" swimmers.

My defensive, knee-jerk reaction to his blasé insights about my sperm: "What do you mean they're 'meh' swimmers?!?" Like a character from a Seinfeld episode, the fertility specialist proceeded to say: "When I look under my microscope, I can immediately tell if a guy has really fast swimming, long-tailed sperm that charge ahead in a straight line; or if his short-tailed sperm swim slowly and tend to go around in circles; or if he has middle-of-the-pack sperm (like you) that have a medium-length tail and swim kind of straight but not particularly fast."

I didn't know it at the time, but what this so-called "sperm motility expert" was seeing under his microscope was first observed in the fall of 1677 by a Dutch medical student from Leiden named Johan Ham. While watching a live human semen sample closely under his microscope, Ham noticed countless 'animalcules' that appeared to be swimming around in the semen. (Meyer, 1938)

In late 1677, Ham brought this discovery to the attention of his mentor, Antonie van Leeuwenhoek, who confirmed what looked like "semen animals" (i.e., spermatozoa) with a "tail, which, when swimming, lashes with a snakelike movement, like eels in water." It wasn't until the winter of 1678 that Leeuwenhoek confirmed this phenomenon by looking at the semen of other animals under a microscope; he published these findings the following year.

For the past 342 years or so, scientists who've only had access to a two-dimensional view of semen "swimming" under a 2D microscope (much like the one used by Leeuwenhoek and Ham in the 1600s) thought that sperm's motility and its ability to propel forward was caused by the side-to-side movement of an "eel-like" tail. As you can see in the illustration (below), it turns out that this swimming technique is an optical illusion.

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Sperm tail moves like a precessing spinning top that cancels out the one-sided swimming stroke in an ingenious corkscrew motion: Symmetry is achieved through asymmetry, enabling human sperm to swim forwards.

Source: Image credited to polymaths-lab[dot]com

"Human sperm figured out if they roll as they swim, much like playful otters corkscrewing through water, their one-sided stoke would average itself out, and they would swim forwards," Gadêlha, who specializes in the mathematics of fertility and is head of the Polymaths Laboratory at Bristol's Department of Engineering Mathematics, said in a news release.

"The sperms' rapid and highly synchronized spinning causes an illusion when seen from above with 2D microscopes—the tail appears to have a symmetric side-to-side movement, 'like eels in water,' as described by Leeuwenhoek in the 17th century," Gadêlha noted. "However, our discovery shows sperm have developed a swimming technique to compensate for their lop-sidedness and, in doing so, have ingeniously solved a mathematical puzzle at a microscopic scale: by creating symmetry out of asymmetry."

As you can see in the YouTube video (above) of 3D sperm motility in action, the researchers used a high-speed camera capable of recording over 55,000 frames per second and a "microscope stage with a piezoelectric device to move the sample up and down at an incredibly high rate."

"With over half of infertility caused by male factors, understanding the human sperm tail is fundamental to developing future diagnostic tools to identify unhealthy sperm," Gadêlha said.

Anecdotally, I'm living proof that even with mediocre sperm motility, fertility is still a possibility. After numerous unsuccessful IVF pregnancy attempts, we were able to conceive and had a healthy baby. Hopefully, for other couples who face sperm-related fertility issues in the future, the latest (2020) findings by Gadêlha et al. will be of help. The researchers hope that their 3D sperm-imaging technology will be widely available in clinical centers someday.

"This discovery will revolutionize our understanding of sperm motility and its impact on natural fertilization. So little is known about the intricate environment inside the female reproductive tract and how sperm swimming impinge on fertilization. These new tools open our eyes to the amazing capabilities sperm have," Darszon concluded.