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#Spemann organizer

13 public questions tagged with this topic.

amphibian Development-l

Organizer cells in amphibians are derived from:

Spemann-Mangold organizer responsible for secondary axis induction derives from dorsal marginal zone. After cortical rotation, dorsal vegetal cells with nuclear beta-catenin and Nodal signaling induce overlying dorsal marginal zone to become organizer expressing goosecoid, chordin, noggin. At gastrulation, this region forms dorsal lip of blastopore containing bottle cells and involuting chordamesoderm forming notochord. Fate maps show dorsal lip contributes to prechordal plate and notochord, while ventral lip forms ventral mesoderm. Transplantation of dorsal lip to ventral side induces secondary axis with neural tissue, whereas ventral lip lacks organizing ability, confirming dorsal origin and inductive power.

Ref: Gilbert, Developmental Biology, 12th ed., Chapter 9: Spemann organizer derived from dorsal lip of blastopore.

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amphibian -ll

Transplanting dorsal lip tissue into a ventral region causes:

Spemann and Mangold transplanted dorsal lip of blastopore containing prospective chordamesoderm, which normally involutes as notochord, into ventral marginal zone of host gastrula opposite side. Donor tissue retained organizer program expressing Chordin and Noggin, antagonizing ventral BMP4 and eliciting host ventral ectoderm to form secondary neural plate marked Sox2. Host also contributed somites around graft forming second axis. Resulting conjoined twins with secondary dorsal axis bearing notochord and neural tube demonstrated induction not self-differentiation alone. No ventralization occurs; secondary embryo formation proves organizer instructive capacity and neural induction capability.

Ref: Gilbert, Developmental Biology, 12th ed., Chapter 9: Transplantation of dorsal lip causing secondary embryo formation.

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amphibian -ll

The primary embryonic induction process involves:

Primary embryonic induction defined by Spemann and Mangold as organizer dorsal mesoderm inducing overlying ectoderm to become neural plate rather than epidermis, mediated by BMP antagonists Chordin, Noggin. This mesoderm-ectoderm interaction is archetypal induction: dorsal mesoderm secreting inhibitors instructs competent ectoderm whose fate changes upon contact requiring time for transcription. Secondary inductions like lens or limb involve similar principles. Although endoderm and blastocoel form earlier and neural crest arises later via Wnt and BMP border signals, fundamental conceptual breakthrough concerned mesoderm signaling to ectoderm generating central nervous system and body pattern.

Ref: Gilbert, Developmental Biology, 12th ed., Chapter 10: Primary embryonic induction - mesoderm-ectoderm interactions in neural induction.

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amphibian -ll

Dorsal mesoderm during organizer formation specifically induces:

Dorsal mesoderm from Spemann organizer involutes as chordamesoderm: anterior prechordal plate induces forebrain, while posterior notochordal and paraxial dorsal mesoderm secretes Wnt antagonists Frzb plus FGF and retinoic acid gradients that posteriorize overlying neural ectoderm, converting default anterior neural fate to hindbrain rhombomeres expressing Krox20 and spinal cord trunk expressing HoxB9. Experiments grafting dorsal mesoderm beneath animal caps induced Krox20-positive hindbrain and HoxB9 trunk markers. Ventral mesoderm and epidermis lack this caudalizing ability, confirming dorsal mesoderm induces hindbrain and trunk, essential for AP neural patterning.

Ref: Wolpert, Principles of Development, 5th ed., Chapter 6: Dorsal mesoderm patterning hindbrain and trunk neural tissue.

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amphibian -ll

Organizer formation in Xenopus requires:

Organizer formation requires intersection of dorsal Wnt-beta-catenin pathway providing dorsal competence and vegetal VegT-Nodal pathway providing mesodermal competence. Dorsal signal stabilizes beta-catenin activating Siamois, while VegT and Vg1 induce mesodermal genes such as Xbra and Xnrs. Only cells at dorsal marginal zone receiving both Siamois and VegT-dependent Nodal achieve high Goosecoid and Chordin expression marking functional organizer. Single dorsal signal produces endoderm only, mesoderm signal alone ventral mesoderm. Intersection ensures single organizer at dorsovegetal marginal boundary rather than entire embryo, essential for normal body axis patterning.

Ref: Wolpert, Principles of Development, 5th ed., Chapter 5: Organizer requires dorsal and mesodermal signals intersection.

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amphibian -ll

Dorsal signaling in Xenopus involves:

Dorsal specification integrates three interacting components: maternally stored Wnt11 mRNA polyadenylated after fertilization secretes dorsalizing ligand activating Frizzled receptors; GBP protein bound to kinesin transported dorsally binding and inhibited GSK-3; Dishevelled scaffold recruited to membrane inhibiting Axin destruction complex. Together they stabilize beta-catenin dorsally activating Siamois and Twin transcription in Nieuwkoop center which activates Nodal. BMP antagonists Noggin and Chordin are downstream organizer products, not dorsal inducers. VegT cooperates for mesoderm induction later. Triad GBP, Dsh and Wnt11 constitutes core dorsal signaling module for axis initiation.

Ref: Gilbert, Developmental Biology, 12th ed., Chapter 9: Dorsal signaling involves GBP, Dishevelled and Wnt11 collaboration.

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amphibian -ll

The Nieuwkoop center primarily induces:

Nieuwkoop center comprises dorsovegetal endodermal blastomeres enriched with nuclear beta-catenin which activates transcription of Siamois, Twin and Nodal-related genes Xnr1,2,5,6 through TCF binding. These secreted Nodals act vertically on overlying dorsal marginal mesoderm to induce organizer genes Goosecoid, Chordin and Noggin, effectively creating mesoderm from ectodermal marginal zone. Vegetal cells themselves remain endoderm expressing Sox17 and Mix. Without Nieuwkoop signals, no dorsal mesoderm forms and embryo ventralized with excess blood. Ventral vegetal cells lacking beta-catenin induce ventral mesoderm via BMPs. Thus Nieuwkoop center primarily induces dorsal mesoderm organizer precursors.

Ref: Gilbert, Developmental Biology, 12th ed., Chapter 9: Nieuwkoop center induction of mesoderm and Spemann organizer.

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amphibian -ll

The gray crescent in amphibians forms:

Upon fertilization, sperm entry triggers calcium wave and formation of microtubule array from sperm aster at vegetal cortex. Dynein-dependent cortical rotation shifts vegetal cortex 30 degrees relative to inner cytoplasm, transporting dorsal determinants like Dishevelled, GBP and Wnt11 mRNA toward opposite side of sperm entry. This opposite side shows depigmentation as melanosomes move, forming gray crescent rich in beta-catenin stabilizing factors. Future dorsal organizer arises there. Sperm entry thus defines ventral pole, gray crescent dorsal pole marking organizer position for gastrulation patterning.

Ref: Gilbert, Developmental Biology, 12th ed., Chapter 9: Gray crescent formation opposite point of sperm entry via cortical rotation.

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chick development-l

The equivalent of the amphibian Spemann organizer in chick is:

Spemann organizer induces neural tissue and dorsalizes mesoderm through BMP antagonists Chordin, Noggin, Follistatin and Shh. Equivalent capacity in chick resides at Hensen's node at anterior primitive streak, not along entire streak, hypoblast or yolk plug. Grafting node laterally induces ectopic notochord and neural plate expressing Sox2, secreting identical BMP antagonists and Shh to block BMP4. Molecular signature includes Goosecoid, FoxA2, Chordin identical to amphibian organizer. Primitive streak forms mesoderm broadly but organizer function localized to node, confirming Hensen's node as avian Spemann organizer homolog across vertebrate evolution.

Ref: Gilbert, Developmental Biology, 12th ed., Chapter 9: Hensen's node as avian equivalent of Spemann organizer.

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Chick development-ll

The avian equivalent of amphibian Spemann organizer is:

Avian functional equivalent of amphibian dorsal blastopore lip identified by Waddington via transplantation assays. Hensen's node located at anterior tip of primitive streak expresses organizer markers goosecoid, chordin, noggin, FoxA2 and Shh, homologous to Spemann organizer transcriptome. When transplanted to lateral epiblast it induces secondary embryonic axis with notochord and neural plate recruiting host cells. It also directs left-right asymmetry via Shh and secretes BMP antagonists neuralizing ectoderm. Hypoblast, primitive groove and streak itself lack full organizer inductive capability. This illustrates conserved developmental logic of morphogen gradients patterning embryonic axes through Wnt and BMP antagonism.

Ref: Waddington, Hensen's node as chick organizer, classic transplantation, Developmental Biology textbook Chapter 12.

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frog development

Spemann organizer activity involves secretion of:

Spemann organizer functions not by secreting inducing ligands but by secreting extracellular inhibitors neutralizing ventralizing signals. Embryological and biochemical work identified chordin, noggin and follistatin that bind BMP4 and prevent Smad1/5/8 activation, plus Frzb, Dickkopf1 and Cerberus blocking Wnt8. Combined inhibition creates dorsal territory free of BMP and Wnt allowing expression of chordin-goosecoid and neural genes. Injection of these antagonists ventrally reproduces organizer grafts. Thus organizer activity involves secretion of BMP antagonists establishing permissive environment for dorsal development and neural competence. This illustrates conserved developmental logic of morphogen gradients patterning embryonic axes through Wnt and BMP antagonism.

Ref: De Robertis and Kuroda, Dorsal-ventral patterning - BMP antagonists from Spemann organizer, Annu Rev Cell Dev Biol.

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frog development

Organizer-induced neural tissue formation involves inhibition of:

Ectodermal competence model proposes both neural and epidermal fates possible. BMP4 secreted ventrolaterally binds BMPR inducing Smad1/5/8 phosphorylation, activating epidermal genes like keratin and Xvent while repressing neural genes Sox2, Zic1 and FoxD5. Organizer secretes three BMP antagonists chordin, noggin and follistatin that bind BMPs extracellularly preventing signaling. BMP inhibition dorsalizes ectoderm allowing neural transcription factors to dominate, neural plate forms. FGF and Wnt modulation assist but central switch for neural induction is BMP inhibition demonstrated by neuralization after dominant-negative BMP receptor expression. This illustrates conserved developmental logic of morphogen gradients patterning embryonic axes through Wnt and BMP antagonism.

Ref: NCBI Bookshelf, Developmental Biology Gilbert: Neural induction - BMP inhibition by organizer signals.

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