A new structure found in human sperm tails

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A team of researchers, using cryo-electron tomography (which can take microscopic 3D images), recently identified a previously unknown nano-structure in sperm tails. 

It goes without saying: sperm is essential to our reproduction. Thus, you could be forgiven for thinking that we know all there is to know about their appearance and structure. However. An international team of researchers have in fact identified a completely new nano-structure inside sperm tails, thanks to cryo-electron tomography. This method, for which Joachim Frank, Jacques Dubochet and Richard Henderson received the Noble Prize for chemistry in 2017, can produce 3D images of cellular structures.

Thanks to this revolutionary method, tiny individual structures can be observed even inside a cell. The researchers in this study examined sperm tails, which are essential for helping the little swimmers fertilise an egg, or at least to try. The sperm tail is a very complex mechanism made up of a thousand different types of ‘construction blocks’. The most important of them are called tubulins, which form long tubes.

Thousands of motor proteins -molecules which can move -are fixed to these tubes. Being fixed to a microtube and pressing on an adjacent microtube, the motor proteins allow for movement of the sperm tail. The tail thus turns, allowing the sperm to swim. The movement of thousands of motor proteins needs to be extremely precisely coordinated in order for the sperm to be able to swim.

In order to try and better understand the mechanisms at work, the researchers looked to cryo-electron tomography. And much to their surprise: “When we looked at the first 3D images of the very end section of a sperm tail, we spotted something we had never seen before inside the microtubules: spiral that stretched in from the tip of the sperm and was about a tenth of the length of the tail”, explains Johanna Höög from the Department of Chemistry and Molecular Biology in the University of Göteborg in Sweden.

The discovery led the researchers to investigate the role of this spiral structure. They came up with the following hypothesis: “We believe that this spiral may act as a cork inside the microtubules, preventing them from growing and shrinking as they would normally do, and instead allowing the sperm’s energy to be fully focused on swimming quickly towards the egg”, suggests the lead researcher.

Further works will be necessary to clear the matter up. It is always the case that understanding all the mechanisms at play could lead to developing new treatments aimed at increasing the mobility of sperm, which is a major factor in male infertility. Such knowledge could also pave the way for new methods of male contraception.

You can find all the details of this study in the journal Scientific Reports.