#cortical reaction

Regeneration -ll

The fertilization envelope is formed by:

Fertilization envelope does not arise de novo but represents transformed vitelline envelope. Before fertilization vitelline envelope tightly apposes egg plasma membrane and bears sperm receptors. Upon calcium-triggered cortical granule exocytosis, glycosaminoglycans released become highly hydrated osmotically drawing seawater into perivitelline space lifting envelope. Serine protease removes bound sperm and cleaves EBR1, hyalin creates inner supporting layer, ovoperoxidase catalyzes dityrosine crosslinks hardening envelope against mechanical penetration. This elevated modified structure is fertilization envelope, mechanically strong and biochemically non-adhesive, protecting embryo. Hyalin, glycosaminoglycans, or acrosomal proteins alone cannot form envelope without vitelline precursor.

Ref: Gilbert, Developmental Biology, 12th ed., Chapter 7: Vitelline to fertilization envelope conversion via cortical granule components.

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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 -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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Sea urchin -lll

Cortical reaction in sea urchins involves:

Cortical reaction represents slow block to polyspermy activated by fertilization calcium wave propagating across egg. Calcium binds synaptotagmin on cortical granules docked beneath plasma membrane triggering SNARE-mediated fusion and exocytosis. Contents including proteases, sulfated glycosaminoglycans, hyalin, ovoperoxidase, and transglutaminase are released into perivitelline space. Enzymes cleave vitelline envelope attachments, separate it from plasma membrane, osmotically expand space, and modify it into hardened fertilization envelope. This elevation completes in about minute after fast electrical block based on depolarization, providing permanent mechanical barrier preventing additional sperm entry and protecting embryo from environmental damage.

Ref: Gilbert, Developmental Biology, 12th ed., Chapter 7: Cortical reaction cortical granule release fertilization envelope.

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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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