Sperm is an incredibly good swimmer for its size. It needs quite a bit of strength to break through the cervical mucus barrier. But where does that strength come from? Scientists from the universities of York and Oxford say that the secret is the outer-layer which coats the tails of human sperm. This reinforcing coating provides the strength to make the powerful rhythmic strokes.
The reinforcing outer-layer of the tails of human sperm is what gives them the strength. Image credit: Bobjgalindo via Wikimedia (CC BY-SA 4.0)
The journey through the female reproductive tract is one of the toughest in nature. 55 million sperm embark on this endeavour, but only 15 reach the end to fertilize the egg. It is not easy, because cervical mucus is one hundred times thicker than water. Tiny little sperm need a lot of power to finish this race on top, but what provides that power for these rhythmical strokes? Scientists say that the secret may be in the flagella – sperm tail, which is incredibly complex, despite being just the length of the breadth of a hair.
Scientists created a virtual model of a human sperm in order to compare it with sperm of sea urchins, fertilize their eggs outside of the body. Even though humans and sea urchins are obviously completely different animals, the bendy inner core of the sperm is rather similar. However, mammals evolved a reinforcing outer layer, which provides extra strength and stability, which is required to overcome the barrier of cervical mucus. In fact, in their models scientists tested the ability of sea urchin-like sperm to swim through a liquid as thick as human cervical mucus and found that they simply couldn’t do it. Meanwhile human sperm was quite useless in water, but in thick mucus began demonstrating powerful rhythmical movement. That solves the mystery of strength and power, but even bigger puzzles remain unsolved.
Sperm has no central nervous system that would allow it to make decisions and to strategize its way through the female reproductive tract. How it propels itself being so simple is quite a mystery. Dr Hermes Gadêlha, one of the researchers in the study, said: “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”. The best guess is that sperms move automatically, maybe relying on their number or maybe even utter coincidence.
It is pretty much a miracle that you’re alive. 15 sperms out of 55 million go through the thick of cervical mucus. Others get lost and never reach their goal. Many families are unable to conceive a child and maybe this research will help understanding at least some of these cases.
Source: University of York