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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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IP3 in fertilization is generated by:

Phosphoinositide signaling during egg activation begins with hydrolysis of phosphatidylinositol 4,5-bisphosphate PIP2 residing in inner leaflet of plasma membrane. Enzyme catalyzing cleavage is phospholipase C activated via sperm-derived PLCzeta or Src kinase-mediated phosphorylation of PLCgamma after gamete fusion. Cleavage yields membrane-retained diacylglycerol activating protein kinase C and soluble inositol trisphosphate IP3 diffusing to endoplasmic reticulum triggering calcium release. Protein kinase C is downstream target not producer of IP3, dynein ATPase powers axonemal motility, Na+/H+ exchanger regulates pH independent of phosphoinositide turnover, making PLC sole generator of IP3 surge at fertilization.

Ref: NCBI Bookshelf, Cell Signaling, Chapter 9: Phospholipase C cleavage of PIP2 to generate IP3 and DAG.

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Which of these is true for bindin protein?

Bindin represents classic example of rapidly evolving reproductive protein under positive selection driving speciation. It is insoluble acrosomal protein retained on acrosomal process after exocytosis, contacting vitelline envelope receptor EBR1. Sequence analysis reveals lectin domains mediating species-specific adhesion plus amphipathic helical regions destabilizing membranes promoting fusion, qualifying as fusogenic. It is not jelly component dissolving in seawater, not cortical granule enzyme released by egg, nor constituent of hardened fertilization envelope that consists of crosslinked vitelline glycoproteins. Its localization strictly on sperm ensures sperm side contributes adhesive and fusogenic activity essential for membrane merger and reproductive isolation.

Ref: Gilbert, Developmental Biology, 12th ed., Chapter 7: Bindin as sperm fusogenic and adhesive protein.

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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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Egg jelly in sea urchins primarily functions in:

Egg jelly is thick layer of sulfated fucan polysaccharides and glycoproteins surrounding vitelline envelope, deposited during oocyte growth and containing embedded chemotactic peptides resact and speract. Its primary adaptive functions are attracting sperm from distance via diffusion gradients ensuring fertilization in dilute seawater and inducing acrosome reaction through fucose-sulfate ligands that raise sperm calcium and pH. Jelly also contributes to species-specific agglutination of sperm. Polyspermy block is achieved by fertilization envelope elevation and hardening after cortical reaction, nutrient supply comes from yolk platelets, and embryonic genome activation depends on calcium and pH changes inside egg not jelly components.

Ref: Gilbert, Developmental Biology, 12th ed., Chapter 7: Egg jelly functions - chemotaxis and acrosome induction.

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Which enzyme cleaves bindin receptors during slow block?

Permanent block to polyspermy requires irreversible removal of sperm attachment sites. Cortical granule serine protease, trypsin-like enzyme activated at neutral pH upon exocytosis, cleaves extracellular domain of Egg Bindin Receptor EBR1 within vitelline envelope, releasing peptide fragments and destroying lectin-binding interface. This ensures even if fertilization envelope incompletely hardens, supernumerary sperm cannot remain bound. Acrosomal protease digests egg jelly to allow sperm penetration, ovoperoxidase crosslinks envelope proteins for hardening, phospholipase C generates IP3 and DAG for signaling but does not cleave receptors. Specific serine protease inhibitors prevent receptor loss while envelope still elevates.

Ref: Gilbert, Developmental Biology, 12th ed., Chapter 7: Cortical granule serine protease clipping bindin receptor EBR1.

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What molecule initiates calcium release in egg cytoplasm after fertilization?

Egg cytoplasmic calcium rise is triggered by diffusible second messenger inositol 1,4,5-trisphosphate IP3 acting on endoplasmic reticulum calcium stores. IP3 binds ligand-gated IP3 receptors, tetrameric channels releasing sequestered calcium producing self-propagating wave due to calcium-induced calcium release. Upstream sperm factor activates Src-family kinase that phosphorylates and recruits phospholipase C gamma to membrane where it cleaves PIP2 into DAG and IP3. Cyclic AMP predominates in sperm chemotaxis signaling, diacylglycerol activates protein kinase C regulating Na+/H+ exchange, ATP fuels processes but does not gate calcium release directly; IP3 is immediate chemical trigger.

Ref: Molecular Biology of the Cell, Chapter 15: IP3-mediated calcium release via phospholipase C during egg activation.

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Actin polymerization in sperm cells occurs primarily during:

Actin polymerization in sperm is temporally restricted to acrosomal reaction. G-actin monomers stored bound to profilin in sperm head rapidly assemble into F-actin bundle forming slender acrosomal process that extends bindin to reach vitelline envelope. Calcium influx and pH rise activate actin nucleation via formin and depolymerization of capping proteins, generating force to protrude process through egg jelly remnants. Cytochalasin B or latrunculin blocking polymerization prevents process formation and fertilization fails. Cortical reaction involves secretory exocytosis in egg, fertilization cone actin polymerization occurs in egg cortex during sperm incorporation, and slow block relies on envelope hardening rather than actin assembly.

Ref: Gilbert, Developmental Biology, 12th ed., Chapter 7: Actin polymerization driving acrosomal process extension in sea urchin sperm.

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During cortical granule reaction, glycosaminoglycans:

Cortical granules discharge heterogeneous mixture; glycosaminoglycans such as chondroitin sulfate proteoglycans are major osmotically active component. Upon release into narrow perivitelline space they rapidly hydrate and swell, generating high colloid osmotic pressure drawing water between vitelline envelope and plasma membrane. Resulting influx expands space beneath envelope lifting it outward to create perivitelline space and nascent fertilization envelope. Hardening occurs later via ovoperoxidase-mediated dityrosine crosslinking, while receptor removal depends on protease cleavage. Glycosaminoglycans do not harden envelope, digest receptors, or induce acrosomal exocytosis; their physical swelling provides elevating force essential for block.

Ref: Gilbert, Developmental Biology, 12th ed., Chapter 7: Glycosaminoglycans and osmotic elevation of fertilization envelope.

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Sperm motility in sea urchins is initiated by:

Sea urchin sperm are kept quiescent in gonads where pH is acidic and carbon dioxide high, maintaining intracellular pH around 7.2. Upon spawning into seawater pH 8.0, sodium-hydrogen exchangers extrude protons driven by sodium gradient, raising internal pH to 7.6. Alkalization activates dynein ATPase, increases mitochondrial oxygen consumption, enhances flagellar beat frequency, and sensitizes guanylate cyclase to chemotactic peptides. Acidifying agents inhibit motility, while artificial alkalization with ammonium chloride activates motility without egg factors. Thus increased internal pH, not decreased pH or mere external pH change without internal effect, initiates forward motility and chemosensory competence.

Ref: Gilbert, Developmental Biology, 12th ed., Chapter 7: Sperm activation via intracellular pH increase and Na+/H+ exchange.

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Bindin receptors in sea urchins are located on the:

Species-specific sperm binding occurs on vitelline envelope where Egg Bindin Receptor glycoprotein complex is concentrated. Biochemical isolation shows EBR1 is 350 kDa transmembrane protein with large extracellular domain containing sulfated oligosaccharides recognized by bindin lectin domain. Immunofluorescence localizes EBR1 to vitelline envelope before fertilization, not soluble jelly, internal cortical granules, or deep plasma membrane domains. After cortical granule serine protease cleaves EBR1 extracellular domain, binding sites are lost, contributing to permanent block. Egg plasma membrane contains fusogenic lipids and integrins mediating final membrane merger after initial vitelline anchoring, but primary bindin receptor resides in envelope.

Ref: Gilbert, Developmental Biology, 12th ed., Chapter 7: Localization of bindin receptor EBR1 on vitelline envelope.

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Sea urchin eggs activate upon fertilization primarily due to:

Egg activation involves resumption of meiosis, elevation of metabolism, and preparation for embryonic cleavage. Central event is single propagated calcium wave sweeping from sperm entry point throughout cytoplasm, raising cytosolic calcium over tenfold. Calcium activates calmodulin-dependent kinase II leading to cyclin B degradation and release from M-phase arrest, stimulates NAD kinase increasing NADPH, triggers cortical granule fusion, and opens Na+/H+ exchangers raising pH to promote protein synthesis. ATP increase, pH rise, and sperm nuclear incorporation are downstream consequences of calcium signaling. Without calcium transient, eggs remain arrested even if sperm fuses, proving calcium rise as primary activator.

Ref: Gilbert, Developmental Biology, 12th ed., Chapter 7: Calcium wave as primary activator of sea urchin egg metabolism.

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