#sperm-egg interaction

Regeneration -ll

Fertilization cone formation involves:

After sperm-egg plasma membrane fusion, egg cortex reorganizes beneath fusion site into funnel-like protrusion called fertilization cone that engulfs sperm head and midpiece drawing nucleus inward. This structure depends on rapid polymerization of cortical actin microfilaments orchestrated by small GTPases RhoA, Rac, and Arp2/3 complex nucleating branched filaments. Actin mesh pushes membrane around sperm and provides traction. Myosin II later contracts cone, tubulin microvilli elongate slightly, and dynein remains sperm flagellar motor. Inhibition of actin polymerization with cytochalasin D prevents cone formation leaving sperm attached externally, proving actin assembly essential for incorporation of male pronucleus.

Ref: Gilbert, Developmental Biology, 12th ed., Chapter 7: Fertilization cone formation via actin polymerization in egg cortex.

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Sea urchin -l

Fertilization cone formation involves:

Immediately after sperm-egg plasma membrane fusion, egg cortex locally remodels beneath bound spermatozoon to form conspicuous fertilization cone. Process begins with transient depolymerization of cortical actin followed by rapid GTPase-dependent actin polymerization involving Arp2/3 complex and formins, pushing plasma membrane outward into blunt protrusion engulfing sperm head and midpiece while drawing sperm nucleus into ooplasm. Myosin II contractility and microtubule-based transport assist later incorporation and pronuclear migration but initial protrusion itself driven by localized actin polymerization triggered by calcium and small GTPases. Cytochalasin B inhibition blocks cone formation preventing sperm internalization, demonstrating dependence on actin dynamics.

Ref: Gilbert, Developmental Biology, Chapter 7: Fertilization cone - actin-based engulfment of sperm.

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Fertilization in mammals-lll

Fertilin protein primarily facilitates:

Fertilin heterodimer ADAM1/ADAM2 resides on sperm surface after epididymal processing, relocating to equatorial segment ready for fusion. Its disintegrin loop interacts with integrin alpha6beta1 on egg microvilli, stabilizing adhesion immediately after zona penetration and prior to Izumo1-Juno binding. This interaction lowers energy barrier for lipid bilayer merging, promoting fusion pore expansion and cytoplasmic continuity. While acrosin and hyaluronidase mediate zona penetration and acrosome formation occurs earlier, fertilin specifically mediates membrane apposition step. Knockout studies demonstrate reduced fertilization rates due to fusion failure despite intact motility and zona binding, underscoring role in gamete membrane unification.

Ref: Biggers et al., Biology of Reproduction 1997: Fertilins ADAM family mediating gamete membrane adhesion and fusion.

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Fertilization in mammals-iv

Glycosylation of ZP proteins affects:

Zona pellucida comprises sulfated glycoproteins ZP1-ZP4 bearing extensive N- and O-linked glycans that regulate mechanical stiffness and ligand function. Glycan moieties present terminal sialic acid and specific oligosaccharide epitopes recognized by sperm plasma membrane receptors such as galactosyltransferase, influencing species-restricted adhesion and triggering acrosome reaction. Altered glycosylation modifies zona solubility, protease resistance and sperm binding avidity without affecting sperm motility or implantation directly. Deglycosylation experiments reduce but may not abolish binding, underscoring glycans as key modulators of primary sperm-zona recognition and polyspermy block after fertilization.

Ref: Avella et al., J Cell Biol 2014: Zona pellucida glycosylation modulating sperm recognition and species-specific binding.

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