Sperm evolved reinforced tails for swimming strength and stability

A new study led by the University of York has uncovered a mechanism that helps sperm achieve their mission of fertilizing an egg. The researchers discovered an outer layer that reinforces the tails of human sperm, which gives them the strength they need to break through the protective barrier of the cervix.

Only about 15 out of 55 million sperm manage to break through the cervical mucus, which is 100 times thicker than water. Sperm tails are incredibly complex for their tiny size. The researchers used a virtual sperm model to compare the tails of sperm from humans to other mammals with the sperm of sea urchins.

“Using virtual sperm we were able to see how mammalian sperm is specially adapted to swim through thicker fluids. We don’t know which adaptation came first – the stronger sperm or the cervical mucus, or whether they co-evolved – but nothing in nature is by chance and precisely what is required for species to reproduce has been added due to evolutionary pressure over millions of years,” said study co-author Dr. Hermes Gadêlha.

The tails of sea urchin and human sperm were found to share the same bendy inner core, but the tails of sperm in mammals appear to have evolved a reinforcing outer layer to give them the strength and stability that is required to survive the cervical mucus barrier.

The researchers tested the ability of virtual sea urchin sperm to swim through liquid as viscous as cervical mucus and found that their tails could not endure the pressure and they were unable to keep moving. Human sperm, however, struggled in the low-viscosity liquid and thrived in the thicker liquids. The findings could lead to better sperm-selection methods in IVF clinics.

“We still don’t fully understand how, but a sperm’s ability to swim could be associated with genetic integrity. Cervical mucus forms part of the process in the female body of ensuring only the best swimmers make it to the egg,” said Dr. Gadêlha.

“During the sperm selection process, IVF clinics don’t currently use a highly viscous liquid to test for the best sperm as until now it was not entirely clear whether this is important. Our study suggests that more clinical tests and research are needed to explore the impact of this element of the natural environment when selecting sperm for IVF treatments.”

Sperm have no central nervous system to make decisions about how or when to move, and what regulates this movement remains a mystery.

“We know that, just like in our arms and legs, sperm have tiny muscles which allow their tails to bend – but nobody knows how this is orchestrated inside the tail, at the nanometric scale,” explained Dr. Gadêlha. “Sperm are an archetype of self-organization – movement seems to be happening automatically, perhaps because of a complex combination of many mechanisms at play.”

The study is published in the journal Royal Society Interface.

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By Chrissy Sexton, Earth.com Staff Writer