Model suggests that mammalian sperm cells have two modes of swimming

A new mathematical model predicts that mammalian sperm cells have two different swimming modes. This prediction opens new questions about possible links between the motor activity of sperm cells and their transitions to hyperactivation phases that may play an important role in fertilization. The finding is part of a larger effort to use mathematical and fluid dynamics to describe how mammalian sperm move

The research is led by a team of engineers at the University of California San Diego, and the work appears in the journal Physical assessment fluids and a preprint is currently available at the arXiv server.

Mammalian sperm cells propel themselves by moving their flagella back and forth, thanks to chemically powered motors that drive waves along their flagella, which are thread-like appendages.

The researchers’ new model of a swimming sperm cell captures the interactions between motor kinetics and changes in the shape (deformations) of the flagella, as well as the movements of the sperm head. The model also takes into account the complex fluid mechanics surrounding the sperm cell as it moves.

This new model predicts that the swimming speed of a mammalian sperm cell does not simply increase as the activity of its chemical motors increases. Instead, as the motor activity of a swimming sperm cell increases, this motor activity exceeds a threshold level, at which point a second, distinct swimming mode emerges. It is this second mode that may be associated with sperm hyperactivation.

In swimming mode one, the head of the mammalian sperm cell swings back and forth more than in swimming mode two. In swimming mode two, the wave-like beating of the flagellum is stronger than in swimming mode one.

“While we cannot say with certainty that this new model predicts the phenomenon of sperm hyperactivation that often occurs just before fertilization, it is certainly an interesting possibility. I hope that further research will clarify whether the motility transition we see in our model seeing is indeed related to fertilization.” to sperm hyperactivation,” says UC San Diego professor David Saintillan, the corresponding author of the new paper and a fluid mechanics researcher in the Department of Mechanical and Aerospace Engineering at the UC San Diego Jacobs School of Engineering.

“There are so many opportunities for engineers and mathematicians to contribute to our understanding of biology. For example, more and more of the models we are working on in fluid dynamics are emerging as important tools for understanding the dynamics of biological systems such as locomotion. In some cases, models allow us to test mechanisms or hypotheses that you can’t easily address experimentally. In these types of situations, models can be extremely useful,” Saintillan said.

The study of the mechanisms involved in sperm locomotion in mammals is an example of a problem where models have played a key role alongside experiments, Saintillan noted. “You can’t control the activity of the motors in living sperm cells with the turn of a knob, but with a model like ours you can speed up or slow down the motor activity of sperm and see how locomotion changes.”