Poster U16 - Sperm long-distance migration in complex environments

Introduction: To fertilize the oocyte, sperm must traverse the complex and fluctuating environment of the female reproductive tract (FRT), survive the intensive selective pressure imposed by the tract, undergo physiological transitions, reach the site of fertilization, and search for the oocyte. The long-distance, inefficient nature of this migration, which results in the success of only one sperm from billions, along with the poor statistical picture of sperm motility, impedes proposing a physical model to capture the essential features of this migration. Here, performing motility profiling for bull chamically stimulated sperm over long timeframes, as well as stochastic simulations, we propose a three-phase model for sperm migration in complex environments. This model, which is inspired by a previous model proposed for the long-distance navigation of birds, suggests a cascade of transitions in sperm motility as it migrates within the FRT and appropriately tunes its navigational strategies according to each functional region.

Materials and

Methods: Sperm Sample: Cryopreserved semen sourced from Bos taurus was diluted in egg yolk extender to maintain viability and stability during storage. Prior to experimentation, antibiotics including gentamicin, tylosin, lincomycin, and spectinomycin were added to prevent microbial contamination and ensure the integrity of the sperm sample.

TALP Medium Composition: TALP medium, essential for maintaining sperm viability and motility during experimentation, was carefully formulated with precise concentrations of salts, buffers, and nutrients. Components such as glucose, pyruvic acid, penicillin G, and bovine serum albumin were included to support sperm health and activity. To induce hyperactivation, caffeine was used as a chemically known agonist.

Microfluidic Device Fabrication: Microfluidic devices were custom-made using polydimethylsiloxane (PDMS) through a soft lithography process, ensuring precise control over channel dimensions and features.

Microscopy Techniques for Sperm Observation: Sperm motion within the microfluidic devices was observed using a Nikon Eclipse TE300 inverted phase-contrast microscope, enabling high-resolution imaging. Sperm behavior was recorded in real-time using an Andor Zyla 5.5s camera, allowing for detailed analysis of motility patterns and responses to experimental conditions.

Cell Tracking

Methods: Both manual and automatic tracking methods were employed to monitor sperm trajectories and behavior. Automatic tracking relied on image processing techniques, including image subtraction and centroid tracking algorithms, to accurately follow individual sperm cells over time. Specialized software tools like MATLAB were utilized for analyzing sperm movement, including measurements of midpiece bending and orientation relative to surfaces within the microfluidic device.

Results, Conclusions, and Discussions: Our experimental and numerical results reveal the existence of three phases of migration during sperm long-distance navigation in complex environments: 1) A long-distance phase where sperm exhibit ballistic motion with the capability of receiving directional cues from the tract, 2) an exploration phase where sperm exhibit persistent random walks (PRW), allowing them to explore complex geometries, and 3) an exploitation phase where sperm's localized chiral swimming is suitable for pinpointing the target. We also observe the existence of a mixed-phase consisting of both exploration and exploitation phases, indicating a smooth transition, or potentially a trade-off, between exploration and exploitation phases. These results provide a potential physical model to explain sperm migration in complex environments such as the FRT. Our results could also lay the foundation for the next generation of assisted reproductive technologies.

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