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#egg activation

17 public questions tagged with this topic.

Regeneration -ll

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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Regeneration -ll

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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Regeneration -ll

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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Regeneration -ll

The cortical granule reaction requires an increase in intracellular:

Cortical granule exocytosis is strictly calcium-dependent exocytosis analogous to neurotransmitter release. Fertilization activates phospholipase C producing IP3 that triggers calcium release from endoplasmic reticulum via IP3 receptors. Cytoplasmic free calcium increases from nanomolar resting to micromolar levels propagating as wave across cortex. Calcium binds synaptotagmin-like calcium sensors on granule membranes promoting assembly of SNARE complexes driving membrane fusion and content discharge. Elevation of sodium, potassium, or magnesium accompanies fertilization potential but cannot substitute. Chelation of calcium by intracellular BAPTA injection completely abolishes cortical reaction demonstrating absolute requirement for intracellular calcium surge.

Ref: NCBI Bookshelf, Egg Activation: IP3-mediated calcium wave and cortical granule exocytosis in sea urchin.

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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

IP3 in fertilization is generated by:

Fertilization induces phosphoinositide signaling cascade generating crucial second messenger IP3. Phospholipase C isoforms, including sperm-contributed PLCζ and egg PLCγ activated by Src family tyrosine kinases downstream of sperm-receptor interaction, hydrolyze phosphatidylinositol 4,5-bisphosphate abundant in inner leaflet of plasma membrane. Cleavage yields membrane-retained diacylglycerol that activates protein kinase C and soluble inositol 1,4,5-trisphosphate that diffuses to endoplasmic reticulum. IP3 generation absolutely requires phospholipase C enzymatic catalysis; without PLC activity calcium wave fails. Protein kinase C, dynein ATPase and Na+/H+ exchanger act downstream modulating pH, cytoskeleton and motility but are not synthesizing IP3 upstream.

Ref: Carroll et al., Dev Biol 1997, Phospholipase C and IP3 generation at fertilization; Alberts Chapter 15.

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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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Sea urchin -l

Sea urchin eggs activate upon fertilization primarily due to:

Fertilization triggers awakening termed egg activation converting quiescent oocyte into metabolically active totipotent zygote preparing for mitosis. Central event is massive rise of intracellular free Ca2+ from endoplasmic reticulum after IP3 production by phospholipase C. Propagating calcium wave stimulates cortical granule exocytosis, cytoplasmic alkalinization via Na+/H+ antiporter, increased oxygen consumption, enhanced protein synthesis from maternal mRNAs, and cell cycle resumption through cyclin activation. Without Ca2+ elevation, shown by BAPTA chelation, none of these events occur, proving rise in intracellular Ca2+ drives activation and initiates embryogenesis.

Ref: Stricker, Dev Biol 1999, Egg activation Ca2+; Gilbert Chapter 7: Calcium wave as activation signal.

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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 cortical granule reaction requires an increase in intracellular:

Cortical granule exocytosis is triggered by calcium wave sweeping from sperm entry point across cortex. Phospholipase C zeta introduced by sperm hydrolyzes PIP2 generating IP3, which binds receptors on endoplasmic reticulum releasing stored Ca2+ into cytosol. Rising Ca2+ binds synaptotagmin and triggers SNARE-mediated fusion of cortical granules with plasma membrane. Calcium ionophore A23187 can artificially activate eggs mimicking envelope elevation. Sodium, potassium and magnesium fluctuations follow depolarization but essential fusogenic trigger is intracellular Ca2+ rise linking fertilization to polyspermy block and metabolic activation.

Ref: Whitaker, Physiol Rev 2006, Calcium wave in egg activation; Gilbert Chapter 7: Ca2+ dependent cortical reaction.

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Sea urchin -l

Slow block to polyspermy involves:

Slow block provides permanent long-term mechanical barrier after fast electrical block wanes within minute window. Rising intracellular Ca2+ wave triggered by phospholipase C-mediated IP3 production induces SNARE-dependent exocytosis of cortical granules arranged beneath cortex. Their contents include serine proteases that cleave bindin receptors, mucopolysaccharides that swell and draw water elevating envelope, and ovoperoxidase that crosslinks tyrosine residues hardening envelope into fertilization membrane. Assembly of elevated hardened fertilization envelope physically renders sperm unable to bind or penetrate. Ca2+ influx from seawater unnecessary; internal endoplasmic reticulum release via IP3 receptors alone drives reaction and envelope transformation.

Ref: Wong et al., 2007, Cortical reaction; Gilbert, Developmental Biology, Chapter 7: Slow block and fertilization envelope assembly.

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Drosophila -l

Activation of Drosophila eggs occurs:

Unlike vertebrates where sperm induces activation, Drosophila oocytes activate during passage through female reproductive tract independent of fertilization. As mature stage-14 oocytes exit ovary into lateral oviduct, mechanical compression and osmotic swelling trigger calcium influx through Trpm channels, initiating a propagating calcium wave throughout ooplasm. This wave drives meiosis resumption from metaphase I, vitelline membrane crosslinking, mRNA polyadenylation via PNG kinase, and translation of stored transcripts. Fertilization occurs later in uterus after activation processes started, demonstrating fertilization-independent activation and explaining capacity for parthenogenesis observed in some insects.

Ref: Gilbert, Developmental Biology, 12th ed., Chapter 7: Drosophila egg activation independent of fertilization.

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