In mammals, two integral membrane proteins, sperm IZUMO1 and egg CD9, regulate sperm-egg fusion, and their roles are critical but as yet unknown. In such situation, a recent study has shown that CD9-containing exosome-like vesicles, which are released from wild-type eggs, can induce the fusion between sperm and Cd9 -defi cient egg, even though Cd9 -defi cient eggs are highly refractory to the fusion with sperm. This result provides compelling evidence for the crucial involvement of CD9-containing, fusion-facilitating vesicles in sperm-egg fusion. On the other hand, similarities have been observed between the generation of retroviruses in host cells and the formation of small cellular vesicles, termed exosomes, in mammalian cells. The exosomes are thought to regulate intercellular communication through transfer of proteins and RNAs. These collective studies provide an insight into the molecular mechanisms of gamete fusion and other membrane fusion events.

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