#polyspermy

Regeneration -ll

Slow block to polyspermy is stabilized by:

Slow block stabilization requires chemical hardening making fertilization envelope resistant to proteases and mechanical penetration. Ovoperoxidase released from cortical granules uses hydrogen peroxide generated by NADPH oxidase to catalyze dityrosine crosslinks between tyrosine residues of vitelline envelope glycoproteins, increasing tensile strength. Transglutaminase subsequently introduces covalent epsilon-gamma-glutamyl-lysine isopeptide bonds further reinforcing matrix. Together enzymatic crosslinking converts soft elevated envelope into tough protective shell around early embryo. Glycosaminoglycans provide osmotic lifting force, hyalin forms inner hyaline layer supporting blastomeres adhesion, sodium influx drives electrical block but not envelope stabilization.

Ref: Gilbert, Developmental Biology, 12th ed., Chapter 7: Fertilization envelope stabilization by ovoperoxidase and transglutaminase crosslinking.

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Regeneration -ll

The fast block to polyspermy lasts approximately:

Fast block is transient electrical depolarization preventing extra sperm fusion until slow block completes. Sea urchin resting potential -70 mV shifts to +20 mV within seconds after first sperm entry. This positive potential makes membrane refractory because sperm-egg fusion voltage-dependent and favored only at negative potentials. Depolarization lasts about a minute until cortical granule exocytosis elevates fertilization envelope. Voltage clamp experiments show positive-held eggs reject sperm, return to negative restores receptivity. One to two minutes ensures overlap with slow block; shorter seconds insufficient, longer many minutes would impair subsequent ionic homeostasis required for development.

Ref: Jaffe LA, Fast block to polyspermy in sea urchin eggs - membrane potential duration about one minute, CSIRO Reproduction.

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Regeneration -ll

Slow block to polyspermy involves:

Slow block provides long-lasting barrier after momentary electrical block decays. Fertilization triggers phospholipase C activation generating IP3 that opens endoplasmic reticulum calcium channels, creating traveling calcium wave across egg cortex. Elevated calcium drives SNARE-dependent exocytosis of cortical granules releasing proteases that sever bindin receptor EBR1 connections, glycosaminoglycans that osmotically lift envelope, and ovoperoxidase producing dityrosine crosslinks hardening envelope. Resulting fertilization envelope is physically elevated and biochemically unreceptive, preventing late sperm entry. Membrane potential change mediates fast block, seawater calcium influx insufficient, and acrosomal vesicle fusion belongs to sperm not egg.

Ref: Gilbert, Developmental Biology, 12th ed., Chapter 7: Slow block - cortical reaction and fertilization envelope formation.

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Regeneration -ll

Fast block to polyspermy is mainly due to:

Fast block to polyspermy in sea urchin eggs is achieved electrically rather than structurally. Within one to three seconds of sperm-egg fusion, sodium channels open producing rapid depolarization from resting -70 mV to positive +20 mV, known as fertilization potential. This voltage shift renders plasma membrane refractory to additional sperm fusions because sperm-egg fusion is voltage-dependent and favored only at negative potentials. Depolarization is transient lasting about a minute until slow block secures envelope elevation. Cortical granule discharge, protease activation, and bindin binding correspond to slow permanent block, not immediate electrical mechanism preventing supernumerary entries.

Ref: NCBI Bookshelf, Developmental Biology: Fast block to polyspermy - Jaffe's electrophysiological studies of membrane depolarization.

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Sea urchin -l

Slow block to polyspermy is stabilized by:

After initial elevation triggered by mucopolysaccharide swelling, vitelline envelope transformed into fertilization envelope requires chemical hardening to provide durable mechanical barrier against supersperm and environmental stress. Two cortical granule enzymes accomplish covalent stabilization: egg-specific ovoperoxidase catalyzes oxidative crosslinking forming dityrosine bridges between adjacent envelope glycoproteins using hydrogen peroxide, while transglutaminase catalyzes formation of ε-(γ-glutamyl)lysine isopeptide bonds. Together these enzymatic crosslinks convert soluble vitelline envelope into insoluble, tough, impermeable protective coat encasing embryo. Glycosaminoglycans drive swelling but not stabilization, hyalin builds hyaline layer for cell adhesion, Na+ mediates fast electrical block. Thus stabilization depends on peroxidase-transglutaminase system.

Ref: Foerder & Shapiro, PNAS 1977, Peroxidase hardening; Gilbert, Developmental Biology, Chapter 7: Envelope crosslinking.

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Sea urchin -l

Which enzyme cleaves bindin receptors during slow block?

Permanent elimination of sperm binding capacity during slow block involves enzymatic destruction of specific recognition molecules. Cortical granules discharge large trypsin-like serine protease that cleaves peptide linkages anchoring EBR1 bindin receptor complex to vitelline envelope glycoprotein scaffold and degrades residual fertilizing sperm proteins adhering to envelope. This proteolytic destruction ensures no new bindin-receptor interactions can reform even before envelope hardening physically completes. Acrosomal protease facilitates sperm entry through jelly, ovoperoxidase catalyzes dityrosine crosslinks hardening envelope, phospholipase C generates IP3 for calcium release but does not degrade receptors. Thus enzyme responsible for cleaving bindin receptors is cortical granule serine protease.

Ref: NCBI Bookshelf, Developmental Biology, Chapter 7: Cortical granule protease cleaves bindin receptor.

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Sea urchin -l

The fast block to polyspermy lasts approximately:

Fast electrical block depolarizes plasma membrane within one to three seconds after sperm fusion, creating immediate short barrier preventing additional fusions. By holding membrane positive, further sperm cannot undergo voltage-sensitive merger. This transient depolarization provides critical window lasting roughly sixty to one hundred twenty seconds before permanent slow block assembles. During this calcium wave triggers cortical exocytosis and progressive elevation and hardening of fertilization envelope. Voltage clamp experiments showing artificially negative holding permits supernumerary fusions confirm mechanism. Duration of fast block generally quoted as one to two minutes covering overlapping transition between blocks.

Ref: Jaffe, Development 1976, Duration of fast block; Gilbert Chapter 7: Timescale of electrical and slow blocks.

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Sea urchin -l

The fertilization envelope is formed by:

After successful sperm fusion, cortical granule exocytosis rapidly releases paracrystalline protein fraction and enzymatic cocktail into perivitelline space between plasma membrane and vitelline layer. Mucopolysaccharide hydration induces osmotic water influx elevating vitelline envelope away from egg surface. Simultaneously proteases sever connections linking bindin receptors to envelope, while transglutaminase and ovoperoxidase catalyze covalent crosslinking of glycoproteins and newly deposited structural proteins hardening envelope into fertilization envelope, tough impermeable protective coat preventing further sperm entry and shielding embryo from mechanical stress. Hyalin, glycosaminoglycans and acrosomal proteins contribute but driving mechanism is vitelline envelope modification and crosslinking.

Ref: NCBI Bookshelf, Developmental Biology, Chapter 7: Fertilization envelope formation from vitelline envelope.

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Fertilization in mammals-ll

Zinc spark prevents polyspermy by:

Upon fertilization, oocytes release massive stores of chelatable zinc in discrete spark events concomitant with cortical granule exocytosis. Each spark liberates billions of zinc ions into perivitelline space, zinc concentration in zona increases threefold. Zinc binding alters supramolecular architecture of ZP filaments increasing density and rigidity observed by electron microscopy, phenomenon termed zona hardening. Additionally extracellular zinc transiently inhibits sperm motility and CatSper activity. Collectively these physiochemical changes erect rapid structural barrier blocking supernumerary sperm penetration, complementing ZP2 cleavage for durable polyspermy prevention and embryo protection during early development.

Ref: Que et al., Integrative Biology 2017, Zinc sparks induce ZP hardening and block to polyspermy in mouse eggs.

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Fertilization in mammals-ll

Slow block to polyspermy modifies zona protein:

Slow block to polyspermy involves biochemical modification of zona pellucida called zona reaction. Cortical granules release ovastacin, an astacin-family metalloendoprotease that specifically cleaves ZP2 near its N-terminus, destroying N-terminal sperm-binding domain. Cleaved ZP2 no longer supports sperm adhesion, zona loses elasticity and hardens, preventing secondary sperm from penetrating or binding. This irreversible proteolysis, combined with glycosidase release and zinc crosslinking, provides durable post-fertilization barrier ensuring monospermic fertilization essential for diploid development, viability, normal embryogenesis, preventing triploid abortive conceptus formation and maintaining genomic integrity.

Ref: Burkart et al., J Cell Sci: Ovastacin cleavage of ZP2 mediates slow block to polyspermy and zona hardening.

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Fertilization in mammals-iv

Polyspermy block by zinc involves:

Zinc spark released upon fertilization delivers concentrated zinc into perivitelline space and zona matrix. Zinc binding causes physicochemical alterations of ZP glycoproteins, facilitating ovastacin-mediated cleavage of ZP2 and enhancing disulfide crosslinking, reducing zona solubility and sperm binding sites. This zona hardening provides sustained block to polyspermy complementing fast electrical block. Zinc does not lyse sperm nor alter egg membrane potential directly, and does not regulate capacitation. Experimental chelation of zinc preserves zona softness allowing supernumerary sperm binding, while exogenous zinc induces hardening even without fertilization, demonstrating zinc as key regulator of zona-based polyspermy defense.

Ref: Duncan FE et al., PNAS 2016: Zinc-induced zona pellucida hardening as physicochemical block to polyspermy in mice.

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