From back and forth to either side to simple rotation, many models have been proposed for the movement of spermatozoa. But it would seem that the reality is much more complex: according to a recent study, it is in fact a very elaborate movement whose study could help research, for example in the context of infertility.
In 1677, Anton Van Leeuwenhoek became the first known person to observe spermatozoa under a microscope. He noticed that under their glass slide, these small entities seem to move “Like an eel in water”, Propelling themselves with a symmetrical movement of their flagellum. For centuries, his observations have served as a benchmark, and it is only in the modern era that they have been clarified, in the light of our new knowledge in genetics, bioengineering and imaging. We now know that spermatozoa don’t just “whip” back and forth to move forward, but that they spin their flagellum like a small propeller. This is an optical illusion, linked to the angle of observation and the fact that the sperm is stuck between two glass slides. However, this movement remains difficult to study because of its extreme finesse and speed, and we are still far from knowing all the subtleties. But in a study published in Science Advances, an international team of researchers took matters into their own hands to study it with a precision unmatched until now.
And the result was a resounding success: they discovered that a simple “propeller” was not enough to describe the movement of the scourge, which is actually surprisingly complex. For starters, it is not symmetrical and regular as it seemed commonly accepted in the past! In fact, it is not only asymmetric but also anisotropic, that is to say that some components of this movement do not take place in a direction planned in advance; in short, it is therefore the sum of two already very complex movements. But in addition, the head of the sperm itself turns on itself, supposedly to balance the whole! A hell of a ballet, far from the simple back and forth conceptualized by Van Leeuwenhoek.
https://www.youtube.com/watch?v=LNYVSHx3jPk
Large technical resources
To achieve this level of precision, they developed a camera capable of taking pictures at the microscopic scale at a frequency of over 55,000 images per second. A necessary capture speed, because to create a reliable model, researchers need to be sure not to miss any movement, and therefore you need a camera capable of taking two images quickly enough that the subject cannot move in the meantime. . The researchers then immersed all these beautiful people in a fluid even less viscous than water and in very particular lighting conditions, to shoot portraits with great precision. They were then able to stitch together this gigantic amount of images to create different scans of individual sperm.
This is already a technical tour de force in itself, but at this point the research team still only had simple recordings of individual sperm, with no scientific value as such. To be able to use this data, they then had to cross-reference it to create a model of the general movement of a sperm. And as much to say that this step is not trivial. It is not for nothing that this study was directed by Hermes Gadêlha, a mathematician by training. We had to break down the movement in order to study every component from every angle, so that it could be reconstructed by computer using a complex mathematical approach. A real riot of know-how and technology for such a small object!
Potential applications in research
If Gadêlha and her team worked their brains at this point, it’s not just for the pleasure of collecting data that is very difficult to obtain and making models of it. Theinfertility is a problem which takes more and more place in our society. Thousands of men all over the world find themselves unable to bear children and this problem is often linked to a problem with the mobility (or more precisely, motility) of the sperm. It is therefore crucial to understand the way they move to understand why some do not do well. This could offer new perspectives on the mechanisms behind certain forms of infertility, but also on other questions of evolution and natural selection.
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