Doing more with less: the flagellar end piece enhances the propulsive effectiveness of human spermatozoa

TL;DR

This study demonstrates that an inactive distal end of the human sperm flagellum significantly improves propulsion and efficiency, with optimal inactive lengths varying by species and conditions.

Contribution

It introduces a biophysical model showing the advantageous role of an inactive flagellar end in sperm motility, a feature previously overlooked.

Findings

Inactive end piece increases propulsion efficiency by up to 430%.

Optimal inactive length ranges from 2% to 18% of total flagellum length.

The benefit varies with wavenumber and viscous-elastic ratio.

Abstract

Spermatozoa self-propel by propagating bending waves along a predominantly active elastic flagellum. The organized structure of the "9 + 2" axoneme is lost in the most-distal few microns of the flagellum, and therefore this region is unlikely to have the ability to generate active bending; as such it has been largely neglected in biophysical studies. Through elastohydrodynamic modeling of human-like sperm we show that an inactive distal region confers significant advantages, both in propulsive thrust and swimming efficiency, when compared with a fully active flagellum of the same total length. The beneficial effect of the inactive end piece on these statistics can be as small as a few percent but can be above 430%. The optimal inactive length, between 2-18% of the total length, depends on both wavenumber and viscous-elastic ratio, and therefore is likely to vary in different species. Potential implications in evolutionary biology and clinical assessment are discussed.