Researchers Find Previously Unknown Structure in Human Spermatozoon Tails

By using cryo-electron tomography, an international team of scientists has identified a completely new nanostructure — named the Tail Axoneme Intra-Lumenal Spiral (TAILS) — inside human spermatozoon tails.

The human spermatozoon tail is a highly complex machine that consists of around a thousand different types of building blocks. Image credit: University of Gothenburg.

The human spermatozoon tail is a highly complex machine that consists of around a thousand different types of building blocks. Image credit: University of Gothenburg.

A highly effective tail is needed in order for a sperm to be able to swim, and for an egg to be fertilized.

The tail is a highly complex machine that consists of around a thousand different types of building blocks.

The most important of these are called tubulins, which form long tubes (microtubules). The tubes are found inside the sperm tail.

Thousands of motorproteins — molecules that can move — are affixed to these tubes. By being fixed to one microtubule and ‘walking on’ the adjacent microtubule, the motorproteins in the sperm tail pull and the tail bends, enabling the sperm to swim.

“It’s actually quite incredible that it can work,” said Dr. Johanna Höög, a researcher in the Department of Chemistry and Molecular Biology at the University of Gothenburg in Sweden.

“The movement of thousands of motorproteins has to be coordinated in the minutest of detail in order for the sperm to be able to swim.”

The team’s research was started in order to see what human sperm tails look like in 3D. This would then provide clues about how sperms work, in the same way that a sketch of an engine helps to explain how it operates.

“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,” Dr. Höög said.

What the TAILS spiral is doing there, what it consists of and whether it is important in order for sperms to swim are questions that the scientists will now focus on answering.

“We speculate that TAILS may stabilize microtubules, enable rapid swimming or play a role in controlling the swimming direction of spermatozoa,” the authors said.

“We believe that the TAILS 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,” added Davide Zabeo, a Ph.D. student at the University of Gothenburg.

A paper reporting the discovery is published in the journal Scientific Reports.

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Davide Zabeo et al. 2018. A lumenal interrupted helix in human sperm tail microtubules. Scientific Reports 8, article number: 2727; doi: 10.1038/s41598-018-21165-8

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