#polyspermy prevention

Sea urchin -l

Slow block to polyspermy involves:

Slow block provides permanent long-term mechanical barrier after fast electrical block wanes within minute window. Rising intracellular Ca2+ wave triggered by phospholipase C-mediated IP3 production induces SNARE-dependent exocytosis of cortical granules arranged beneath cortex. Their contents include serine proteases that cleave bindin receptors, mucopolysaccharides that swell and draw water elevating envelope, and ovoperoxidase that crosslinks tyrosine residues hardening envelope into fertilization membrane. Assembly of elevated hardened fertilization envelope physically renders sperm unable to bind or penetrate. Ca2+ influx from seawater unnecessary; internal endoplasmic reticulum release via IP3 receptors alone drives reaction and envelope transformation.

Ref: Wong et al., 2007, Cortical reaction; Gilbert, Developmental Biology, Chapter 7: Slow block and fertilization envelope assembly.

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Sea urchin -l

Fast block to polyspermy is mainly due to:

Fast block to polyspermy prevents immediate entry of extra sperm within seconds of first fusion. Sperm-egg fusion depolarizes plasma membrane from about -70 mV to +20 mV via rapid Na+ channels, creating electrical barrier because voltage-sensitive sperm fusion cannot merge with depolarized membrane. This depolarization occurs within one second and persists roughly one minute until permanent slow block develops via fertilization envelope. Cortical granule exocytosis, protease release constitute slow block acting later. Fast electrical block thus provides initial transient defense ensuring monospermy before permanent barrier assembles.

Ref: Jaffe, Nature 1976, Fast electrical block to polyspermy; Gilbert Chapter 7: Membrane potential shift.

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Sea urchin -lll

Cortical reaction prevents polyspermy by:

Cortical reaction and fast electrical block together prevent polyspermy, but initial rapid depolarization provides immediate transient protection. Upon first sperm fusion, sodium influx depolarizes egg plasma membrane from negative to positive within seconds, making membrane refractory to additional sperm fusions that require negative potential. This electrical change precedes cortical granule exocytosis by about half minute. Although some textbooks attribute depolarization to cortical granule release, it represents initial phase of reaction sequence. Membrane depolarization thus serves as fast mechanism within cortical reaction leading to polyspermy prevention before permanent fertilization envelope elevation completes protection of embryo.

Ref: Gilbert, Developmental Biology, 12th ed., Chapter 7: Cortical reaction depolarization fast block polyspermy prevention.

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Fertilization in mammals-lll

Polyspermy prevention primarily relies on:

Prevention of polyspermy in mammals employs fast electrical depolarization and slow zona block. Upon gamete fusion, cortical zinc stores release massive burst called zinc spark into extracellular milieu. Zinc binds zona pellucida, increasing stiffness and inducing structural remodeling that blocks additional sperm binding, complementing ovastacin-mediated ZP2 cleavage. Calcium influx triggers exocytosis but zinc directly hardens matrix and reduces sperm motility in perivitelline space. Sodium influx mediates fast block in some non-mammalian species, potassium efflux not implicated. Thus zinc spark is recognized as essential chemical signal coupling egg activation to physicochemical hardening of zona ensuring monospermy and viability.

Ref: Fissore et al., PNAS 2016: Zinc spark contribution to zona hardening and block to polyspermy in mammalian eggs.

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Fertilization in mammals-iv

Polyspermy prevention involves:

Mammalian eggs prevent polyspermy through rapid membrane depolarization and slower zona hardening. A critical chemical event is fertilization-induced zinc spark, where cortical vesicle-like zinc stores expel billions of zinc ions within minutes of sperm-egg fusion. Extracellular zinc binds zona pellucida proteins, induces conformational changes and crosslinking involving ovastacin-mediated ZP2 cleavage, increasing matrix rigidity and preventing additional sperm penetration. This zinc-dependent hardening works alongside calcium wave-triggered cortical granule exocytosis. Sodium, potassium or calcium alone do not mediate this structural zona modification; zinc provides specific physicochemical block essential for monospermic fertilization.

Ref: Que et al., Integr Biol 2017 & Duncan et al., PNAS 2016: Zinc sparks induce zona hardening establishing slow block to polyspermy.

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