Polyspermy blockage process in mammals and zebrafish maternal proteins involved in cortical granule lifecycle. (A) The mouse zona pellucida (ZP) is a glycoprotein matrix composed of several proteins, including ZP1, ZP2, and ZP3, which play key roles during fertilization. 1) Sperm binding and acrosomal reaction: During fertilization, the sperm recognizes and binds to ZP2, triggering the acrosomal reaction. This reaction releases hydrolytic enzymes that help the sperm penetrate the ZP. 2) ZP penetration: As the sperm progresses, it interacts with ZP2, which guides its passage through the extracellular matrix of the ZP. 3) Membrane fusion and nuclear entry: Fusion between the sperm and oocyte membranes enables the sperm pronucleus to enter the oocyte cytoplasm, starting a Ca2+ signaling cascade and where it merges with the oocyte pronucleus to form the zygote nucleus. 4) CGE and zona reaction: Following membrane fusion, CGs within the oocyte release their contents into the perivitelline space. This exocytosis induces a zona reaction that reorganizes the structure of the ZP and creates a physical and chemical barrier to block additional sperm from entering, ensuring monospermy. (B) CG biosynthesis begins in the early stages of oogenesis, where hypertrophied Golgi units synthesize the vesicles. These vesicles then undergo fusion and maturation, with Suf protein participating in CG maturation. Rab proteins aid in recruiting CGs to the oocyte cortex, assisted by an actin network. The CGs are subsequently anchored to the PM and, under the control of proteins such as HnRNP I, Ybx1, Dschs1 in zebrafish or SNAP23, Rab3a, and Rabphilin3A in mice, exocytose their contents into the PVS, resulting in chorion expansion (or zona pellucida in mammals). ISV, immature secretory vesicle; CGs, cortical granules; PM, plasma membrane; PVS, perivitelline space.
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