A team of researchers using electronic cryotomography, which provides a microscopic image in 3D, has recently identified a nano-structure unknown to date inside the tail of spermatozoa.
Spermatozoa are essential for our reproduction, so it would be easy to assume that we have a detailed knowledge of their appearance. Yet, an international team of researchers has indeed identified a completely new nano-structure inside the sperm tails, thanks to the use of cryoelectronic tomography. The method, for which Joachim Frank, Jacques Dubochet and Richard Henderson received a Nobel Prize in Chemistry in 2017, allows experts to produce 3D images of cellular structures.
Thanks to this revolutionary method, small individual structures can be observed inside a cell. For this study, the researchers looked at the tail of a spermatozoon, essential to allow the little swimmer to fertilize an egg, or at least to try his luck. The tail of a sperm is a very complex machinery made up of about a thousand different types of building blocks. The most important of these are called tubulins, which form long tubes.
Thousands of motor proteins — molecules that can move — are affixed to these tubes. By being attached to a microtubule and evolving on the adjacent microtubule, the motor proteins can generate the movement of the tail of the spermatozoon, allowing the spermatozoa to swim. The movement of thousands of motor proteins must therefore be coordinated in great detail so that spermatozoa can swim.
To try to better understand the mechanisms at work, the researchers used electronic cryotomography. “When we looked at the first 3D images of the last part of a sperm tail, we noticed something we had never seen before inside the microtubules: a kind spiral structure that extends from the tip of the sperm and measures about one-tenth of the length of the tail, “says Johanna Höög, of the Department of Chemistry and Molecular Biology of the University of Gothenburg, Sweden.
The discovery leads researchers to question the role of this spiral structure. “We think that this spiral structure acts a little like a plug inside the microtubules, preventing them from lengthening and shrinking, which allows the energy of spermatozoa to be entirely devolved to swimming fast towards the egg,” suggests the researcher.
New work will be needed to get to the bottom of it. The fact is that understanding all the mechanisms involved could lead to new treatments to increase the mobility of poor sperm, a major cause of infertility in men. It could also open the door to a new method of male contraception.