Localization of fibronectin on the surface of human spermatozoa and relation to the sperm-egg interaction

Objectives The mechanism of human sperm-oolemma adhesion and penetration as well as localization of fibronectin on the sperm head and its relation to fertilization were investigated. Design Sperm-oolemma interaction was examined with an in vitro assay of the human spermzona-free hamster egg interaction. Localization of fibronectin on the surface of human spermatozoa was observed by the back scattered electron imaging mode of the scanning electron microscopy (SEM). Results It was confirmed by observations under SEM that the anterior tip of the sperm head is the first to come into contact with the egg plasma membrane but that the equatorial segment of the sperm head is the first to be trapped by microvilli of the plasma membrane and that the postacrosomal region is first incorporated into ooplasm. Localization of fibronectin on the equatorial segment of the human sperm head was detected by SEM. Antifibronectin antibodies inhibited human sperm-oolemma adhesion significantly. Conclusions Important involvement of fibronectin in the gamete interaction was made clear by the fact that fibronectin is localized in the region where a spermatozoon is fused first with the egg plasma membrane during fertilization and that the sperm adhering to the egg is inhibited by antifibronectin antibodies.