Reorganization of the Flagellum Scaffolding Induces a Sperm Standstill During Fertilization

Abstract

Mammalian sperm delve into the female reproductive tract to fertilize the female gamete. Within the midpiece of the sperm flagellum lies a cortical network of actin that is arranged as a double helix sheltering the mitochondrial sheath. This work demonstrates that the actin network of the midpiece undergoes structural changes that result in motility cessation. This structural modification is accompanied by a decrease in diameter of the midpiece and is driven by intracellular calcium changes that occur concomitant with a reorganization of the actin helicoidal cortex. Although midpiece contraction may occur in a subset of cells that undergo acrosomal exocytosis, live-cell imaging during in vitro fertilization showed that the midpiece contraction is required for motility cessation during the fusion process. These findings provide the first evidence of the F-actin network's role in regulating sperm motility, adapting its function to meet specific cellular requirements during fertilization.

Morphometric analysis of the sperm midpiece during capacitation

“…Although the quanti cation of sperm characteristics is important for understanding the physiology of these cells, few studies have analyzed the morphological changes in agella during capacitation. When live cells were imaged during in vitro fertilization, a decrease in the diameter of the midpiece was observed over time after acrosomal exocytosis (32). This observation is consistent with our results in xed cells.…”

“…2d and 3f). Changes in the cytoskeleton are known to occur during the acrosome reaction (39), but little information is available on possible changes in the agellar cytoskeleton during capacitation (32,40,41). The cytoskeleton of the agellum consists mainly of the axoneme, which is composed of 9 doublets and a central pair of microtubules (42).…”