Abstract
Sperm migration towards an oocyte in the female reproductive tract is an important step for successful fertilization. Although several sperm-chemotactic factors have been identified in mammals, it is unclear whether these chemoattractants contribute to sperm migration towards an oocyte that is the final destination for sperm. Furthermore, chemoattractants for bovine sperm are still undiscovered even though the follicular fluid attracts sperm in cattle. Here, we demonstrated that a single bovine cumulus-oocyte complex (COC) had the ability to attract sperm, suggesting that the COC secreted sperm chemoattractants. We identified stromal cell-derived factor 1 (SDF1), which was expressed in COCs, and its receptor CXCR4 in sperm, as a candidate. Our results showed that bovine sperm preferentially migrated to the area with a high SDF1 concentration and occasionally showed turn movements by asymmetric flagellar bends during the migration. We also demonstrated that increasing the intracellular Ca2+ concentration via Ca2+ channels was related to SDF1-induced sperm chemotaxis. Finally, a CXCR4 inhibitor significantly suppressed the in vitro bovine sperm migration towards a COC. Taken together, we propose that SDF1 is a chemotactic factor for bovine sperm to regulate their migration towards an oocyte via the CXCR4 receptor.
Highlights
Sperm migration towards an oocyte is the first crucial step for successful fertilization and subsequent conception
An in vitro sperm migration assay towards a cumulus-oocyte complex (COC) using 12-well dishes was performed to determine whether the COC secretes a chemoattractant for bovine sperm
The results showed that the number of migrated sperm in COC-containing wells was significantly higher than that of migrated sperm in the empty wells (Fig 1D, top), which was in agreement with our hypothesis
Summary
Sperm migration towards an oocyte is the first crucial step for successful fertilization and subsequent conception. After ejaculation or artificial insemination, mammalian sperm migrate to the ampulla of an oviduct, which is the fertilization site, and very few of them succeed in entering the oviduct [1]. This limited number of sperm still has to swim a long distance and overcome several obstacles to reach an oocyte in the ampulla of the oviduct. This fact and the very small dimension of the cumulus-oocyte complex (COC) relative to that of the oviduct indicate the need for sperm guidance mechanisms [2]. Sperm chemotaxis is a prevalent phenomenon from marine invertebrates with external fertilization to mammals with internal fertilization
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