Maths solves sperm movement mystery
UK scientists may have unravelled one of the long-standing mysteries of human fertility: why sperm cells tend to behave like tiny, surface-seeking missiles.
Researchers have known for more than 40 years that sperm cells in a drop of fluid tend to congregate at a specific distance from the surfaces where the fluid meets solid objects like microscope slides. Exactly why has long been a puzzle.
Now UK mathematicians Dr David Smith and Professor John Blake at the University of Birmingham think they have figured out what is going on. Their results, published on the pre-press website arXiv.org and in the journal The Mathematical Scientist , are based on a mathematical model of a swimming sperm cell.
The researchers say that various factors work together to accumulate sperm near surfaces. First is a weak fluid dynamic force that pulls the cells towards the surface.
Acting against that, however, is the fact that the tails of sperm cells tend to end up closer to the surface than the head. This causes the sperm to swim away from the surface.
The final factor is the thrashing motion of the sperm's tail, or flagellum, which gives it a pitching motion as it swims, causing it to swim first away from and then towards a surface.
When the researchers simulated the effect of these forces over long periods of time, they found that a sperm's swimming trajectory would always tend towards a specific distance from the surface.
"The surface does not simply attract the cell; it causes alternate pitching towards and away from the surface that steers the cell to this finite distance," they say.
One in six Australian couples suffer infertility and male factors are thought to be the cause of half that problem. Fertility expert Professor Rob McLachlan, director of Andrology Australia that one of those factors is the sperm's motility—or the way it moves.
"We understand that problems with motility are a major factor, and often go along with poor sperm shape and low sperm number," he says.
McLachlan says, while the new findings might not have an immediate impact on how infertility is treated, a better understanding of how sperm move is likely to be of benefit in the longer term.
"Understanding the process of normal sperm motility will help us unravel motility problems in infertile men and impact on clinical practice in the future," he says. "Such basic science often pays real dividends."