It depends on the kind of lashing of a flagellum how close a sperm remains to the curve of a wall.
Millions set out on a quest and only one finds what they are looking for: we are talking about sperm and the miracle of fertilisation. But how do the 60 micrometre-sized future-makers find their way through narrow, winding channels such as the fallopian tube? Jülich biophysicists explain.
Sperm are long-distance swimmers. To reach their destination, the egg cell, they have to travel about 15 centimetres. That is 2,500 times their own length. For a human, that would amount to about 4.5 kilometres. It has been known for quite some time what drives sperm forward – whip lashes of their tail, the flagellum. But how do the DNA carriers swim through tiny channels? Dr. Jens Elgeti and his colleagues from the Institute of Complex Systems and Institute for Advanced Simulation (ICS-2/IAS-2) have simulated this with the help of the supercomputer JURECA.
Picture above: It depends on the kind of lashing of a flagellum how close a sperm remains to the curve of a wall.
“During the forward movement, a sperm pushes itself – always headfirst – at a slight angle against the wall of the microchannel so that it remains close to the wall. This way, it even passes slight twists effortlessly,” says Jens Elgeti. However, contact with the wall is lost from a certain radius of curvature onwards. The researchers found out that the kind of lashing of a sperm’s flagellum determines when the body contact with the wall will break off. The lashing is not always the same: the flagellum can wiggle in two or three spatial directions. This has consequences for the swimming route: “The sperm with a three-dimensional lashing of the flagellum remain along the wall even when the curvature is considerably stronger – in contrast to those that only strike in one plane,” explains the researcher.
So sperm move through winding ducts in different ways depending on the lashing of their flagellum. That can be put to good account. In the laboratory, for example, sperm can be sorted out for artificial insemination. “Studies by other researchers have already shown that the genetic make-up of sperm is related to their swimming style,” says Elgeti. “If more details are known, a kind of obstacle course may be used to separate inferior sperm from high-quality sperm, thus improving the likelihood of successful artificial insemination.” The fidings could also help with contraception: If it is clear which lashing technique brings the sperm to the ovum, drugs could interfere with the wagging, thus slowing down the DNA carriers on their way.
But first, the biophysicist Elgeti and his colleagues hope to gain a better understanding of the sperm’s swimming technique : “We are currently investigating how the interplay of active forces and flagellum elasticity influences the lashing,” says Elgeti – among other things by observing sperm under the high-speed microscope.
Photo: Forschungszentrum Jülich/Ralf-Uwe Limbach, Graphic: Forschungszentrum Jülich/SeitenPlan, Videos: Sebastian Rode et al., New J. Phys. (2019), DOI: 10.1088/1367-2630/aaf544 (CC BY 3.0)