New structure discovered in human sperm

Scientists have identified a never-before-seen structure inside human sperm tails, an advance that may help us understand why some sperms are better swimmers than others.

A highly effective tail is needed in order for a sperm to be able to swim, and for a baby to be conceived. Researchers, including those from the University of Gothenburg in Sweden, identified the completely new nanostructure inside sperm tails, using cryo-electron tomography. The method produces 3D images of cellular structures.

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 Johanna Hoog from the University of Gothenburg, who led the study published in the journal Scientific Reports. "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," said Johanna. The researchers looked at the first 3D images of the very end section of a sperm tail. They spotted something 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.

"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 spermâs energy to be fully focused on swimming quickly towards the egg," said Davide Zabeo, the lead PhD student behind the discovery.