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In 1963, the zoologist Lord Rothschild found that sperm cells in a drop of bull semen tended to distribute themselves in a specific non random way. For some reason, they were much more likely to be near the surface of the drop than near its centre.

Since then many biologists have noted the fact that sperm cells of all kinds tend to congregate near a surface rather than on it or far away from it. In fact, they tend navigate at a specific distance from a surface such as the cover slip or slide under a microscope, a behaviour that has turned out to be very useful for biologists studying these cells.

Just why sperm cells should behave in this way has long puzzled biologists. Now David Smith and John Blake at the University of Birmingham in the UK throw some light on the matter by creating a fluid dynamics model of a swimming sperm cell and the forces that operate on this scale.

Their conclusion is that the accumulation near surfaces is an effect purely of fluid dynamics rather than any kind of surface-seeking behaviour of the sperm themselves. However, the effect is subtle and emerges only in lengthy simulations of sperm behaviour in fluids

Smith and Blake say there are several effects at work. First, there is a very weak fluid dynamic force that pulls cells towards a surface.

However, for various reasons, a sperm cell tends to take up a specific attitude when it comes close to a surface: the tail tends to sit closer to the surface than the head. This inclination causes the sperm to swim away from the surface.

Added to this is a sperm’s natural pitching motion as it swims that causes it to swim first away from and then towards a surface.

Smith and Blake simulated the effect of these forces and iterated them. They found that after 8000 cycles, 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 fifinite distance,” they say.

This convergence to a specific distance away from a surface takes time. But Smith and Blake says the size and shape of the head of the cell significantly speed this up.

So that’s a subtle effect that goes a long way to clearing up this long standing mystery.

However, it raises a number of other questions, such as why sperm come to have this strange property in the first case. Is it an artifact observed only in the highly artificial circumstances that occur under a cover slips and microscopes? Or is it biologically significant, in which case how did it evolve and why?

And finally, can this better understanding of sperm behaviour be put to good use in fertility treatments? Some 1 in 6 couples in the western world have trouble conceiving and of these about half are thought to be the result of problems relating to sperm.

A better understanding of sperm motility will surely one day help and could also be important in other areas too such as domestic animal breeding and wildlife conservation.

Ref: arxiv.org/abs/1007.2153: Surface Accumulation Of Spermatazoa: A Fluid Dynamic Phenomenon

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