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#egg membrane

3 public questions tagged with this topic.

Sea urchin -lll

Egg membrane depolarizes within:

Upon sperm-egg fusion in sea urchin, sodium channels open causing rapid influx of sodium ions into egg cytoplasm raising positive charge. Membrane potential shifts from resting about minus seventy millivolts to positive twenty millivolts within one to three seconds, constituting fast electrical block to polyspermy. Depolarized membrane is refractory to additional sperm fusions because fusion machinery requires negative potential for electrostatic interaction. This transient block lasts about minute until cortical granule-derived fertilization envelope elevates permanently. Later cortical reaction provides slow block, while ATP synthesis and meiosis completion occur after electrical block, preventing lethal polyspermy during spawning.

Ref: Gilbert, Developmental Biology, 12th ed., Chapter 7: Membrane depolarization within 1-3 seconds blocks polyspermy.

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Fertilization in mammals-l

Structure guiding sperm entry into eggs:

Zona pellucida is transparent glycoprotein envelope surrounding mammalian oocyte, ovulated complex and early embryo until blastocyst hatching. Synthesized by oocyte and follicle cells, it comprises ZP1-4 filaments forming porous elastic matrix that supports cumulus retention, mediates species-restricted sperm binding, triggers and completes acrosome reaction, and after cortical granule release hardens preventing polyspermy. By confining blastomeres it also prevents premature adhesion and extrauterine implantation. Hence zona functions as multifunctional gatekeeper guiding sperm entry, protecting early development, ensuring implantation timing, embryo transport coordination and monospermy enforcement.

Ref: Gilbert, Developmental Biology, 12th ed., Chapter 7: Zona pellucida functions in sperm guidance and polyspermy block.

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Fertilization in mammals-ll

Izumo protein on sperm binds to egg membrane protein:

Izumo1 is sperm-specific immunoglobulin superfamily transmembrane protein that becomes exposed on equatorial segment after acrosome reaction. Its cognate receptor on egg plasma membrane is Juno, also known as folate receptor 4, enriched on microvillar oolemma. Structural studies show Izumo1 helical bundle binds Juno with nanomolar affinity, providing essential adhesion preceding fusion. CD9 organizes membrane microdomains facilitating clustering. After fertilization Juno is rapidly shed in extracellular vesicles, preventing additional sperm attachment. Izumo-Juno interaction is conserved, indispensable for mammalian fertilization success, monospermy enforcement and initiation of egg activation cascade.

Ref: Bianchi et al., Nature 2014, Izumo1-Juno interaction: essential sperm-egg adhesion pair in mammalian fertilization.

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