Strictures of the female reproductive tract impose fierce competition to select for highly motile sperm

TL;DR

This study uses microfluidic devices to mimic the female reproductive tract, revealing that strictures act as a selective barrier favoring highly motile sperm and influencing their accumulation and movement patterns.

Contribution

It introduces a microfluidic model of reproductive tract strictures and uncovers their role in selecting for sperm with higher motility during fertilization.

Findings

Strictures prevent low-motility sperm from passing.

High-motility sperm accumulate and form hierarchical clusters.

Sperm with lower velocities spread further apart before the stricture.

Abstract

Investigating sperm locomotion in the presence of an external fluid flow and geometries simulating the female reproductive tract can lead to a better understanding of sperm motion during the fertilization process. In this study, using a microfluidic device featuring a stricture that simulates the biophysical properties of narrow junctions inside the female reproductive tract, we observed the gate-like role the stricture plays to prevent sperm featuring motility below a certain threshold from advancing towards the fertilization site. At the same time, all sperm slower than the threshold motility accumulate before the stricture and swim in a butterfly-shaped path between the channel walls which maintains the chance of penetrating the stricture and thus advancing towards the egg. Interestingly, the accumulation of sperm before the stricture occurs in a hierarchical manner so that sperm with higher velocities remain closer to each other and as the sperm velocity drops, they spread further apart.