Researchers have figured out what gives human sperm the power to swim and break through the thick cervical mucus and allow fertilization to happen.

A team from the University of York and the University of Oxford created virtual models to compare the sperm of humans and sperm of various animals. They found that a reinforcing outer layer of the tail of the human sperm is responsible for giving it the strength to swim through the cervical mucus, which is a hundred times thicker than water.

The study can aid fertility clinics to develop a better sperm-selection method that mimics the natural processes that take place inside the body.

The findings were published in the Journal of the Royal Society Interface.

Powerful Swimmers

The researchers created virtual models of human sperm which fertilizes the human inside the body (internal fertilization). They compared it to the sperm from sea urchins which fertilizes the egg outside the body (external fertilization).

They found that while both have a bendy inner core, the human sperm might have evolved a reinforcing outer layer that gives it the exact amount of power to break through the thick cervical mucus.

The team added and removed the sperm tail called flagella of the models from different species to confirm its function.

"Using virtual sperm we were able to see how mammalian sperm is specially adapted to swim through thicker fluids," stated Hermes Gadêlha of the University of York, co-author of the study. "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."

The researchers also tested the virtual sperm of sea urchin to swim through a liquid that is as viscous as the cervical mucus. The creatures reproduce by releasing their sperms and eggs into the water where the fertilization happen. When released into a viscous liquid, the virtual sea urchin sperm buckled, unabled to propel itself forward.

Meanwhile, the human sperm, when placed in a low-viscosity liquid like water. It thrived and swim in a steady rhythm in thicker liquids.

Picking The Strongest Swimmers

The researchers believe that the study would be helpful to couples who are affected by fertility issues. Right now, they said that IVF clinics do not use high-viscous liquids to find the best sperm. The study could lead to a better selection method that will be as effective as the thick cervical mucus.

"Cervical mucus forms part of the process in the female body of ensuring only the best swimmers make it to the egg," Gadêlha explained.