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Neural induction
Model organism: Xenopus
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Late blastula
neurula
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P31. During gastrulation, tissue interaction between the involuting dorsal cells (prospective pharyngeal endoderm and dorsal mesoderm, collectively referred to as mesendoderm) and the overlying ectoderm define the region of the ectoderm that will form the nervous system and establish principal axes, and direct cells within this region towards a neural fate. This process is known as neural induction.
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Two topics in this chapter are neural induction and axial formation
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discovery of neural induction
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Hensen’s node
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Organizer releases inductive signals.
All ectoderm are competent.
Polarity has been determined.
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Search for the inducers
Animal cap experiment, artificial inducers, 60 years pass
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Dissociated cells neurons (neural default)
Dominant negative experiments
Activins, Vg-1
BMPs
Activins, inhibins
Homodimer, hetrodimers
Promiscuous receptors
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BMP2, BMP4, BMP7
Noggin, chordin, follistatin induce primitive neural tissue
BMP inactivation triggers early neural markers, other factors are required to elicit differentiated markers
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WNTs as another class of neural inhibitors in animal cap explants.
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Conserved between Drosophila and vertebrate
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Polarity and establishment of the neuraxis
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Early ingressing cells become prechodal plate and induce forebrain.
Late ingressing cells become notochord and induce midbrain, hindbrain, and spinal cord.
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Where do signals come from?
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Holtfreter, urodele
Xenopus, primitive induction OK and showed AP axis
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Caveats of planar experiments
1. Can’t rule out vertical signals (cryptic gastrulation)
2. Deeper cells before gastrulation release cerberus
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The two-signal model
1. General inducers: noggin, chordin, follistatin, induce anterior-like neural tissue, NCAM, Otx2, BF1.
2. Posterior factors
3. Anterior factors, Wnt vs. Wnt antagonist
Source of inducers
1. Mesendoderm, anterior-to-posterior ingression
posterior-to-anterior translocation
2. The organizer
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Posteriorizing signals (transforming signals)
• Retinoic acid (constitutive and dominant-negative receptors)
• FGF (avian beads)• Wnt• TGF members• Animal cap assay, beads
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Posteriorizing signals (transforming signals)
• Retinoic acid (constitutive and dominant-negative receptors)
• FGF (avian beads)• Wnt• TGF members• Animal cap assay, beads
Forebrain marker: BF1, Otx2Midbrain marker: Otx2, Engrail
Hindbrain marker: Hox, Krox20
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Specific pathways for head induction
Cerberus
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Dickkopf
• In presumptive prechordal plate• Antagonist of Wnt
Activin and nodal-related factors induce posteriorization in zebrafish
Antivin converts posterior CNS into anterior
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Competence of the ectoderm
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Neural induction in the amniote embryo
In mouse, the anteriormost region of the neural axis is induced by signals from presumptive extraembryonic (visceral) endoderm.
Two effects of Otx2: induction, maintenance
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Mediolateral extent of the neural plate
• BMP conc gradient decides fate of epidermis, neural crest,
neural plate. Experiments from zebrafish.
epidermis
neural crest
neural plate
BMP conc
E NPNC
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epidermis
neural crest
neural plate
BMP conc
E NPNC
epidermis
neural crest
BMP conc
E NPNC
neural plate
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ectoderm ectodermNeural plateNC NC
Normal
BMP2-/-
Chordin-/-
Chordin-/-,somitabun