Mammalian sperm-egg fusion: the development of rat oolemma fusibility during oogenesis involves the appearance of binding sites for sperm protein "DE".

Rat epididymal protein DE mediates gamete fusion through complementary sites localized on the egg surface. To investigate whether these egg components are involved in the development of rat oolemma fusibility, both the presence of DE-binding components and the ability of the oolemma to fuse with sperm during oogenesis were examined. Localization of DE-complementary sites by indirect immunofluorescence revealed the absence of fluorescent labeling on growing oocytes with a diameter < 50 microns, and the presence of a uniform staining over the entire surface of germinal vesicle oocytes with a diameter > 50 microns. This localization of oolemma components changed progressively to a patchy distribution during maturation. Whereas sperm incorporation was observed only in maturing oocytes, the development of the Hoechst transfer technique to evaluate membrane fusion revealed that germinal vesicle oocytes with a diameter > 50 microns were already competent to fuse with sperm. The involvement of the DE-complementary sites in the oolemma fusibility of these oocytes was confirmed by the fact that the presence of DE during gamete coincubation significantly (p < 0.001) reduced the percentage of oocytes with fused sperm. Together, these observations indicate that the acquisition of fusibility by the rat oolemma occurs during the growth period and involves the appearance of DE-binding components on the oocyte surface. This study provides novel information on the molecular mechanism by which the mammalian egg plasma membrane becomes competent to fuse with sperm during oogenesis.