03 February 2023
Uncovering sperm cell strategy for synchronized swimming
Published online 19 January 2023
Insights into the collaborative interactions between sperm cells could inform the design of more efficient motors for microscale robots.
Tiny microrobots could prove powerful for performing precision tasks in biomedical and industrial applications, but the design of such cellular-scale automata poses considerable challenges. By studying how sperm cells collaborate to improve the odds of fertilization, researchers, led by Islam Khalil of the University of Twente in the Netherlands and Veronika Magdanz of the University of Waterloo in Canada, are uncovering mechanisms of locomotion that could be replicated to power future microrobot units.
Spermatozoa travel under their own power using long tails known as flagella, which oscillate rhythmically to propel the cells forward. Although fertilization is inherently a competitive, zero-sum game, spermatozoa from some species can assemble into larger bundles that collectively gain an edge in terms of travel speed. This behaviour intrigued the authors, who work in the field of biologically-inspired robotics. “Sperm cells have a fascinating design and excellent capability to adapt to various environments,” says Khalil.
Scientists have struggled to dissect this bundling behaviour, which involves the complex interplay between multiple sperm cells and diverse environmental factors. Khalil, Magdanz and collaborators, including a researcher at The German University in Cairo, Egypt, were able to tackle this analysis by combining mathematical modeling with an experimental setup that allowed them to image the process by which individual bovine spermatozoa interact and assemble into collaborating pairs.
They identified a three-step process in which two sperm cells initially interact via their heads, then rotate relative to each other until they are aligned, before finally moving forward together with enhanced swimming efficiency. “Our study shows that the beating pattern of the cells is synchronized upon their interaction,” explains Khalil. This allows the cells to collectively move faster together while also consuming less energy.
These results could help illuminate a still-enigmatic aspect of reproductive biology. “It is very important for us to understand how sperm interact with nearby sperm through the fluid between them,” says Chih Kuan Tung, who was not involved in the study and investigates spermatozoa locomotion at North Carolina A&T State University, USA. He praises the team's modeling approach and hopes to see further experimental investigation of this process in the future.
For Khalil and Magdanz, the next step will be to leverage their findings to turbo-charge the movement of future microrobots with multiple flagella. “Our group is working on a new method… to artificially glue sperm cells into artificial bundles,” says Magdanz, adding that they will be coupling these to metallic nanoparticles that enable magnetic steering of these multi-spermatozoa assemblies.
Zhang, K. et al. Locomotion of bovine spermatozoa during the transition from individual cells to bundles. PNAS https://dx.doi.org/10.1073/pnas.2211911120 (2023).