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V. Organizing Power and Axis Formation
A. Background Information
B. Invertebrates1. Sea Urchins
2. Snails
3. Tunicates
4. C. Elegans
5. Drosophila melanogaster
C. Vertebrates1. The Frog
2. Zebrafish
3. The Chick Embryo
4. Mammals
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• Part of these processes is the determination of axes in the organism
– The first few cleavages may produce little or no directionality to the embryo
– It starts at varying stages in various animals and can result from different mechanisms
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Remember our primary axes....
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Figure 5.8 Fate map and cell lineage of the sea urchin Strongylocentrotus purpuratus
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Step 1: Specification of Micromeres
disheveled expression blocks B-catenin degradation
egg
Two Big Changes: Specified to become skeletogenic mesenchyme Specified to become “Organizer” for other cells
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-catenin’s job
NML
ALL
Allendoand meso
NONEAllecto
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Step 2: “Organizing Power”
• Secrete Wnt-8 into autocrine loop• Wnt-8Blimp-1B-cateninWnt-8
• Paracrine “early signal” induces macromeres and vegetal cells to differentiate to vegetal endoderm– Unknown signal as of yet
• Delta-Notch juxtacrine signal induces non-skeletogenic mesenchyme
• Wnt-8 makes a come-back to induce invagination
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Axis Determination
• Anterior-Posterior: Cytoplasmic determinants in the egg cytosol, such as disheveled and B-catenin
• Left-Right: Nodal expression (TGF-B family member)
• Dorsal-Ventral: unclear
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Spiral cleavage in molluscs
The spirally cleaving mollusks havea strong autonomous specificationfrom cytoplasmic determinants in egg.
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Step 1: Polar lobe formation
The polar lobeis a cytoplasmoutpouching from the egg prior to cleavage
It isolates criticaldeterminants intoonly one of thefirst cell pair.
TF’s associatedwith the lobeturn CD into“The Organizer”
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Figure 5.27 Association of decapentaplegic (dpp) mRNA with specific centrosomes of Ilyanassa
Decapentaplegic is TGF-B family member used toinduce specific cell fates secreted by the Organizer
The Organizer induces mesodermal and endodermalfates in cells that would otherwise remain ectodermal
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MAP kinase activity activated by D-quadrant snail blastomeres
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Figure 5.30 MAP kinase activity activated by D-quadrant snail blastomeres (Part 2)
Normal MAPK Blocked
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Axis Determination
• Anterior-Posterior: Cytoplasmic determinants in the lobe
• Left-Right: Nodal expression (TGF-B family member)
• Dorsal-Ventral: Cytoplasmic determinants in the lobe
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Bilateral, Holoblastic Cleavage of the Tunicate
The 8-cell embryo isalready autonomouslyspecified for cell fates
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Figure 5.35 Cytoplasmic rearrangement in the fertilized egg of Styela partita
Fertilization rearranges cytoplasmic determinants
1. Animal pole cytosol determines ectoderm2. B-catenin presence determines endoderm (like urchins)3. Macho-1 in yellow crescent determines muscle cells
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Figure 5.38 Antibody staining of -catenin protein shows its involvement with endoderm formation
Wherever B-catenin shows up, endoderm is formed
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Figure 5.37 Autonomous specification by a morphogenetic factor
Where Macho-1 shows up tail muscle will form
Zinc-finger TF for muscle actin, myosin, TBX-6Also TF for Snail TF which blocks notochord induction
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Conditional Specification also plays a role
Integrates with the autonomous specification patterns
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Axis Formation accomplished prior to cleavage!
Fertilization rearranges cytoplasmic determinants
determines dorsal-ventral
determinesanterior-posterior
Left-right: unclearbut nodal shows it later
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Rotational, Holoblastic Cleavage in the nematode Caenorhabditis elegans
hermaphrodite
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Figure 5.42 The nematode Caenorhabditis elegans (Part 2)
Both autonomous and conditional specification at work early on.
P1 will develop autonomously
Stem cell divisions are meridional
Founder cell divisions are equatorial
If cells are separated:
AB requires input from P lineage
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• Autonomous specification in P1– SKN-1, PAL-1 and PIE-1 TFs from egg– as P1 divides these determine daughter fates– P lineage becomes “Organizer”
• Conditional specification in AB– P2 secretes Wnt family member MOM-1 to
induce endodermal specification in AB lineage– P2 use Delta-Notch signals to induce
ectodermal fates in AB lineage
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Axis Determination in C. elegans
Anterior-Posterior axisis determined by egg shape
Which end is posterioris determined by sperm(the closest end is back)
Sperm CYK-4activates egg rho,actin rearrangementcauses assymetricfirst cleavage division
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AB division leads to both dorsal ventral and left-right axes
Assymetrical division of AB-MS forces AB dorsal and MS ventral
Delta-notch recognition between daughters of AB and MS gives left-right
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Cytoskeletal rearrangement also pushes P-granules into the germ line
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The cells of the blastula have specified fates in Xenopus.....
Gastrulation changes all of that, .....afterwards all cell fates are determined!
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Development of “Organizing Power” at the dorsal blastopore lip
The bottle cellsget the ball rollingbut the real poweris conferred on thefirst cells throughthe blastopore.
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The dorsal mesoderm keeps the power to determine other cell’s fates throughout gastrulation: “Spemann’s Organizer”
This ability to determinecell fates is called...
“Primary Embryonic Induction”
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The dorsal lip cells first have to become competent to be “Organizer”
The area of Dshaccumulation isseen as a graycrescent in someamphibian embryos
Cortical rotation shiftsdisheveled, GBP, Wnt-11to dorsal side of embryo
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β-catenin starts out everywhere in the embryo but only survives GSK3 in the dorsal portion due to Dsh, GBP and Wnt-11
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The dorsal vegetal cells of the Nieuwkoop Center turn on “Organizer”
Wnt and Vg-1 (TGF-B family)induce pre-dorsal lip mesoderm
FGF needed for all mesoderm
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Figure 7.22 Summary of events hypothesized to bring about induction of the organizer in the dorsal mesoderm
Vg-1
Nodal
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Figure 7.23 Vegetal induction of mesoderm (Part 2)
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So, what can the “Organizer” do?
• Initiate gastrulation
• Become the notochord and other dorsal mesoderm
• Dorsalize ventral mesoderm into paraxial mesoderm, somites, etc.
• Dorsalize the ectoderm into the neural plate and neural tube
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Figure 7.26 Localization of chordin mRNA
Dorsal blastopore lip Blastopore Dorsal mesoderm
The “Organizer” is induced prior to gastrulation
Continues to organize events throughout its own differentiation
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Interestingly, the primary mechanism is by means of inhibition....
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Presumably, the Wnt, FGF and RA signals arise from endoderm and ectoderm
Without the “Organizer”you get mainly skin and gut
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Figure 7.31 Cerberus mRNA injected into a single D4 blastomere of a 32-cell Xenopus embryo induces head structures as well as a duplicated heart and liver
Don’t underestimate the power of the “Organizer”!
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Axis Formation
• Dorsal-Ventral: sperm penetration and cortical rotation
• Anterior-Posterior: migration direction of the dorsal mesoderm
• Left-Right: nodal expression exclusively on left side of the lateral plate mesoderm
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Nodal expression causes Pitx2 expression
Nodal and Pitx2 on left Injected on both sides
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Relationships between the frog and chick “Organizers”
• The hypoblast = dorsal vegetal cells
• Koller’s sickle = pre-dorsal lip mesoderm
• Hensen’s node = dorsal blastopore lip and dorsal mesoderm
• Primitive streak = blastopore
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Formation of Hensen’s node from Koller’s sickle
Wnt and FGF from the hypoblastinduce Koller’s sickle epiblast
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Figure 8.10 Induction of a new embryo by transplantation of Hensen’s node (Part 1)
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Possible contribution of inhibition of BMP signaling
Appears to be similar to the frog....
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In the chick, the hypoblast plays a large role much like the frog endoderm
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Figure 8.8 Specification of the chick anterior-posterior axis by gravity
Anterior-Posterior axis parallels the rotation inside the shell
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Rostral-Caudal (Anterior-Posterior) axis extension in chick embryos
The combination ofpositional specification,complex signaling andTF (Hox, etc.) expressionis thought to cause axis.
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Left-right asymmetry in the chick embryo
This is farther along Nodal and Pitx2 again are implicated