#sperm entry

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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amphibian Development-l

The region opposite sperm entry forms the:

Cortical rotation moves determinants opposite sperm entry, forming gray crescent with high nuclear beta-catenin. This region becomes Nieuwkoop center and later Spemann organizer, establishing dorsal side of embryo that will form notochord, somites, and neural tube. Sperm entry side maintains high BMP and ventralizing signals, becoming ventral side producing blood, lateral plate mesoderm, and epidermis. Manipulations that reverse rotation reverse dorsal-ventral polarity, demonstrating opposite side rule. Anterior-posterior axis forms later via Wnt and retinoic acid gradients, not directly from sperm entry point positioning alone during development.

Ref: Wolpert, Principles of Development, Chapter 6: Cortical rotation establishes dorsal side opposite sperm.

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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-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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