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#sea urchin

41 public questions tagged with this topic.

Specification Determination-l

Which experiment demonstrated regulative development in sea urchins?

Hans Driesch in 1891 performed landmark sea urchin blastomere isolation demonstrating regulative potential. By mechanically shaking apart two- or four-cell embryos, he found each isolated blastomere developed into complete, proportionate pluteus larva smaller than normal but correctly patterned. This unexpected outcome directly contradicted Roux mosaic results, proving fate not irrevocably allocated through cytoplasmic division. Remaining cells compensated via intercellular signaling respecification, establishing paradigm of conditional development and potency exceeding fate. Driesch experiment shifted embryology from deterministic preformationist determinant views toward modern concept of embryos as self-organizing morphogenetic fields governed by inductive interactions and positional information.

Ref: Gilbert, Developmental Biology, 12th ed., Chapter 3: Driesch Experiment and Regulative Development.

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Specification Determination-ll

If the blastomeres of a 4-cell stage sea urchin embryo are isolated, each forms a complete larva. This demonstrates:

Formation of a complete pluteus larva from a single isolated blastomere of a four-cell stage sea urchin embryo demonstrates classical conditional, regulative development. Fate is not prefixed by inherited determinants; instead, blastomeres continuously interpret positional information through Delta-Notch, Wnt/beta-catenin and Nodal signaling, adjusting lineage allocation to replace missing cells. Such compensation via community effect ensures robustness and whole-embryo regeneration from partial embryos. By contrast, mosaic embryos produce only partial structures upon blastomere isolation. Sea urchin results, pioneered by Hans Driesch, founded the concept of potency exceeding fate.

Ref: Gilbert, Developmental Biology, 12th ed., Chapter 3: Driesch Experiment and Conditional Specification.

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Regeneration -ll

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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Regeneration -ll

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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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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Regeneration -ll

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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Regeneration -ll

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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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

Species specificity during fertilization in sea urchins primarily depends on:

Fertilization requires species-specific molecular lock-and-key recognition to prevent cross-species hybrids in mixed spawning assemblages. In sea urchins this specificity resides in interaction between sperm protein bindin on acrosomal process and its complementary glycoprotein receptor EBR1 embedded in vitelline envelope. Carbohydrate moieties and protein loops vary rapidly between species, creating selective adhesion. Mixing experiments show bindin from Strongylocentrotus binds strongly only to conspecific EBR1; antibodies to EBR1 block fertilization species-specifically. Flagellar proteins, actin filaments, or ATPases provide motility and exocytosis machinery but lack discriminatory binding capacity, making bindin-receptor coevolution primary determinant of reproductive isolation.

Ref: Gilbert, Developmental Biology, 12th ed., Chapter 7: Species specificity - bindin and EBR1 receptor co-evolution.

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Regeneration -ll

In sea urchins, the sperm acrosomal reaction begins when sperm contacts:

The acrosome reaction is triggered before sperm contacts vitelline envelope or egg plasma membrane, ensuring bindin exposure at correct time. Initial stimulus comes from fucose-sulfate-rich glycoconjugates and low molecular weight peptides within egg jelly surrounding egg. Jelly components induce calcium influx via voltage-gated channels and activation of Na+/H+ antiport raising intracellular pH, causing outer acrosomal membrane fusion with plasma membrane releasing acrosomal enzymes and polymerizing actin to project acrosomal process. Vitelline envelope interaction follows jelly-induced priming; egg membrane fusion occurs afterwards, and nuclear contact never directly triggers reaction.

Ref: Gilbert, Developmental Biology, 12th ed., Chapter 7: Acrosome reaction triggered by egg jelly polysaccharides.

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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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