Human Development Chapter 8

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  1. Early Development and Axis formation in Amphibians
    Amphibians are the organisms of choice for experimental embryology

    Early events following fertilization in Amphibians are similar to Sea urchins

    • 1.Restoration of diploid genome
    • 2.Egg activation – nucleic acid and protein synthesis
  2. Amphibian Egg
    Mesolecithal – disposition of yolk moderately towards the vegetal pole

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    Cleavage is holoblastic (complete) and radially symmetrical
  3. Cleavage of Amphibian egg
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    1st division – meridional – begins at the animal pole and slowly extends down into vegetal pole (about 1 mm/min at animal pole; 0.02 mm/min at vegetal pole)

    2nd division – also meridional, but perpendicular to first one, starts before the completion of the first cleavage through vegetal pole

    3rd division – equatorial (displaced towards animal pole) – 8 blastomeres (4 micromeres at animal pole) and 4 macromeres at vegetal pole

    • 4th, 5th, 6th ... result in animal region with numerous small cells and vegetal pole with few large cells
    • 14-64 cell stage – Morula (latin for malberry)128 cell stage – Blastula (several layers thick)
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    • Blastocoel
    •   – permits cell migration during gastrulation
    •   - prevents premature interaction of cells

    EP-Cadherins - Cellular adhesion – to keep dividing blastomeres together – by EP-Cadherin (mRNA for EP-Cadherin is in the egg cytoplasm)

    Depletion of Cadherin mRNA à obliteration of blastocoel
  4. Fate Map in Xenopus
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    Fate of cells of blastula depends on location along A-V axis

    Cells on the surface of animal hemisphere – Ectoderm (skin and nerves)

    Cells of the surface of vegetal hemisphere – endoderm (gut and associated organs)

    Inner cells around equator - mesoderm
  5. Cell fate - Role of Transcription factors
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    Fate of cells is imposed by the expression of:

    • -VegT – TATA box transcription factor (also involved in mid-blastula transition)
    •   Depletion of VegT --> loss of endoderm  

    • -Vg1 – paracrine factor (belongs to TGF-b family)
    •   mRNAs for these proteins are located in the cortex of the Vegetal pole –   apportioned to vegetal cells during cleavage
  6. Cellular Movements during Gastrulation
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    Gastrulation is initiated on the future dorsal side of embryo just below the equator in the region of gray crescent

    Begins by invagination of bottle cells in the marginal zone through dorsal lip of blastopore
  7. Fate of cells
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    First batch (bottle cells) --> leading edge of archenteron --> precursors of pharyngeal cells of foregut

    Second batch (prochordial plate) --> precursors of head mesoderm

    Third batch --> chorda-mesoderm cells --> notochord
  8. Cellular Movements during Mid-Gastrulation
    Mid-Gastrula – replacement of blastocoel with archenteron.

    Cells migrate from lateral and ventral lips of blastopore into embryo.

    Cells of animal hemisphere migrate down toward vegetal region --> displacement of blastocoel towards vegetal region

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  9. Cellular Movements during Gastrulation
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    • Late Gastrula – Displacement of blastocoel
    •   Epiboly of ectoderm --> surrounds the embryo.
    •   Internalization of endoderm  Mesodermal cells are positioned between   ectoderm and endoderm
  10. Blastopore
    • Blastopore formed on the opposite side to the point of sperm entry --> cortical cytoplasm rotates --> forming gray crescent
    • Side of sperm entry – Ventral; Gray crescent – Dorsal side

    Blastopore grows laterally and ventrally enclosing the endoderm (yolk plug)
  11. Movement of cells
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    A – formation of involuting marginal zone (IMZ) [Chordamesoderm and Head mesoderm]

    B – Vegetal rotation pushes bottle cells into the blastocoel

    C and D – further movements push bottle cells and mesoderm into the embryo towards animal pole

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    E – deep marginal cells flatten à superficial cells form wall of archenteron

    F – Radial intercalation in non-involuting marginal zone (NIMZ) à making thin bands of flattened cells

    Thinning of several layers cause extension towards blastopore lip pulling IMZ over the lip
  12. Forces driving convergent extension
    Positive regulators:

    • 1.Differential cell cohesion
    • - Expression of adhesion proteins like paraxial proto-cadherin and axial proto-cadherin in paraxial mesoderm and notochord facilitate convergent extension

    • 2.Calcium flux
    •   - dramatic wave of Ca++ surge in dorsal tissues undergoing convergent extension
    •   – required for microfilament contraction

    Negative regulators:

    Otx2 gene - involved in the formation of head region

      - negatively regulates convergent extension thus prevents the cells in the head region from further involution
  13. Cleavage and Developmental Determinants
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    • Autonomous specification (Independent or Mosaic development)
    • Heterogeneous distribution of cytoplasm and cytoplasmic determinants (Eg. Tunicates)

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    Conditional specification (Dependent or Regulative development): Egg cytoplasm is homogeneous, commitment of cells is determined by interactions with neighboring cells by induction (Eg. Amphibians)
  14. Nuclear Equivalence
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    • A. fertilized newt egg was constricted by a ligature – which restricted the nucleus to one half of the embryo
    • B. At 16-cell stage one nucleus was allowed to escape to the other side and constriction was re-tightened
    • C. After 140 days, each side developed into normal embryo
  15. Axis formation – Role of Gray Crescent
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    However, when constricted at different planes the results were different.

    • A. Longitudinal plane: dividing gray crescent into equal halves --> normal development into two tadpoles
    • B. Perpendicular plane: only one receiving gray crescent--> only this half develops into normal tadpole and the other develops into “Bauchstuck larva (no dorsal side)
  16. Stage of Gastrulation determines the fate of cells
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    Transplantation experiments with presumptive neural ectoderm at presumptive epidermis

    A. When transplantation was done in Early Gastrula - presumptive neural tissues develops into epidermis

    B. When transplantation was done in Late Gastrula – presumptive neural tissue forms another neural plate (not the epidermis)

    • Thus:
    • Early Gastrula – dependent on location
    • Late Gastrula – independent of location
  17. Dorsal lip of Blastopore
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    • Fate of cells at the dorsal lip of blastopore are determined very early compared to other cells of blastula.

    Primary Induction (Spemann and Hilde Mangold, 1924 - Spemann got Nobel prize in 1935): Transplantation of dorsal lip tissue from an early gastrula into another early gastrula

    Donor tissue invaginates,  - Second archenteron,  - Secondary embryonic axis,  - Somites  - Neural tubes etc --> two conjoined larvae.

    Cells of dorsal lip of blastopore are “organizer” –  as they change the fate of host cells and organize the  axis of the host cells by primary induction
  18. Induction by Vegetal pole cells
    When marginal cells are removed --> animal pole cells induced to from mesodermal tissue (notochord, muscle etc)

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    • Inducing Factor: Xbra (Xenopus Brachyury) activates genes that produce mesoderm-specific proteins – expressed in all vegetal cells
  19. How Organizer gets its Properties?
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    • b-Catenin is preferentially accumulated in the dorsal side 
    • - region containing 
    •      - organizer 
    •      - Nieuwkoop center
  20. Accumulation of b-Catenin
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    b-Catenin is synthesized throughout the embryo, but degraded by glycogen synthase kinase 3 (GSK-3)

    Whereas, Disheveled protein (Dsh) inhibits GSK-3

    In unfertilized egg, Vesicles containing Dsh are localized in Vegetal pole

    During fertilization – Dsh protein vesicles translocate dorsally during cytoplasmic rotation --> released from the vesicles --> distributed at future dorsal side of the embryo
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    Dsh blocks the action of GSK-3 --> prevents b-Catenin degradation.

    Thus b-catenin is selectively accumulated in the nuclei of blastomers on the dorsal side (Organizer and Nieuwkoop center)
  22. Functions of the Organizer
    • Organizer has the ability to:
    • 1.Initiate gastrulation

    2.Dorsalize the ectoderm into neural ectoderm

    3.Induce the neural plate to become neural tube

    4. Become dorsal mesoderm

    5. Dorsalize surrounding mesoderm into lateral mesoderm
Card Set:
Human Development Chapter 8
2014-02-27 02:01:40
Human Development Chap

Early Development and Axis formation in Amphibians
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