Nature of Embryonic Induction

8/6/2019 Nature of Embryonic Induction

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NATURE OF EMBRYONICINDUCTION

Mrs. OFELIA SOLANO SALUDAR

Department of Natural SciencesUniversity of St. La Salle


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In anamniotes, instructions for establishing the body axes areset down during oogenesis, or fertilization.

In species with cytoplasmic determinants, unevenly distributedcytoplasmic determinants in the egg cell help establish thebody axes. These determinants set up differences in

blastomeres resulting from cleavage.

Establishing Cellular Asymmetries:Axes of the Basic Body Plan

My mothermade me do it!Egg of the tunicate Styela partita.


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Fertilization rearranges cytoplasmic determinants:Animal pole cytosol determines ectoderm.-catenin presence determines endoderm (like sea

urchins); macho-1 in yellow crescent determines musclecells. Maternally regulated aspects of vertebratedevelopment involve BMPs, Wnts, small GTPases,cytoskeletal components, and cell cycle regulators.

http://www.ncbi.nlm.nih.gov/books/NBK53189/
http://www.ncbi.nlm.nih.gov/books/NBK53189/http://www.ncbi.nlm.nih.gov/books/NBK53189/


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In amniotes, local environmental differences play themajor role in establishing initial differences betweencells and the body axes.

Cells are totipotent at cleavage, but as embryonic

development proceeds, potency of cells becomes morelimited. Cells begin to influence each others fates byinduction.

Based on their famous experiment, Hans Spemann andHilde Mangold concluded that the blastopores dorsal lip

is an organizer of the embryo.The Spemann organizer initiates inductions that result in

formation of the notochord, neural tube, and otherorgans.

Cell Fate Determination and PatternCell Fate Determination and PatternFormation by Inductive SignalsFormation by Inductive Signals


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Embryonic Induction:Embryonic Induction: signal from one group of cellsinfluences the development of an adjacent group of cells.Inductive signals play a major role inpattern formation, or

development of spatial organization.The molecular cues that control pattern formation specify

positional information,which tells a cell where it is withrespect to the body axes.

Requirements:

oInducing Tissue or InduceroInductive Signal - MorphogenoResponding TissueoCompetence

Output: Expression of target gene

which determines how the celland itsdescendents respond to future

molecular signals.


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Cellular responses to induction


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Embryonic Axis Formation of Drosophila and C. elegans

Initial studies on these 2 model systems (a LOOONG

story!) provided the basis for vertebrate embryogenesis.Summary of events :

vMaternal gene products (mRNAs and proteins) aredistributed unequally in the egg and provide initial

positional information.vMaternal gene products initiate a complex cascade ofzygotic gene activity, leading to the production of a suiteof transcription factors and intercellular signalingmolecules that provide more detailed positional

information during later stages.vmiRNAs and siRNA biogenesis contributes to mRNA

turnover during developmental transitions anddifferentiation (whew!)


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DevelopmentalDevelopmental

events need to beevents need to becoordinated both incoordinated both in

space and time.space and time.


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How is this

accomplished?

During developmentgroups of inducing

cells called organizingcenters secrete

graded growth factorsignals. The

concentration gradientof a morphogen caninduce multiple cell

differentiationchoices.


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A morphogen gradient can be generated by a source ofgrowth factor (such as BMP) or by a localized source ofinhibitor (such as Chordin). Both mechanisms are used.


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Organizer cellssecreting morphogen

Decreasingmorphogen

concentrationgradient

Distance from secretion site

Organ A Organ B Organ C

Embryo

Concentra

tion

ofmorphogen Organizers andOrganizers and

the morphogenthe morphogengradientgradient


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Develops intonotochordAnimal pole

Vegetal pole

Developsinto muscle

Secretion ofmorphogen

activin

Developsinto epidermis

MorphogensproMorphogensprovidevide

concentration-concentration-dependentdependentpositionalpositional

information.information.


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Target cells interpretTarget cells interpretsignal differentlysignal differently

depending ondepending onconcentrationconcentration. The

induced cellsproduce a factor that

has to reach asufficiently high

concentration for

muscle differentiation tooccur.


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Human Sonic hedgehog- secreted by notochord toinduce brain and spinal cord development.

High concentration-neural tube; low concentration-motor neurons

In limb- asymmetrical pattern of digits in zone ofpolarizing activity


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Morphogensand theMorphogensand thesignificance of timing.significance of timing.

An unchanging signalacting on otherwisesimilar cells at different

ages can evoke differentresponses. Spatial

patterns can be producedin this way by allowing anunchanging signal to act

at different times ondifferent members of an

array of initially similarcells.


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An independent timing mechanism controls thetime of expression of mesoderm-specific genes.

Muscle-specific gene expression is at mid-gastrula stage


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Modes of Inductive Interaction :Modes of Inductive Interaction :Instructive interactionInstructive interaction

INDUCTION AND INTERCELLULAR SIGNALING


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Permissive interaction-Permissive interaction- responding tissue contains all thepotentials that are to be expressed, and needs only an

environment that allows the expression of such traits. For

instance, fibronectin and laminin in the substrate ofresponding tissues that need them to develop do not alterthe type of cell to be produced, but only enable what has

been determined to be expressed.


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4

Anterior

EMBRYO

Posterior

Receptor

Signal

protein

Signal

Anteriordaughtercell of 3

Posteriordaughtercell of 3

Will go on toform muscleand gonads

Will go on toform adultintestine

1

2

43

3

Examples of induction:the dorsal lip of amphibian embryose lens

b


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Development of the Vertebrate Eye Lens by InductionDevelopment of the Vertebrate Eye Lens by Induction


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Embryonic induction and Pax6 expression:Embryonic induction and Pax6 expression:linking Genetics and Embryologylinking Genetics and Embryology

Signal from optic cup induces placode to developPax6 initiates and orchestrates program of eyedevelopment in optic cup and lens placode

Heterozygous humans- aniridia; homozygotes-completeabsence of eye


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DNAOFF OFF

OFFmRNA

mRNA mRNA mRNA mRNA

Anothertranscriptionfactor

MyoD

Muscle cell(fully differentiated)

MyoD protein(transcriptionfactor)

Myoblast(determined)

Embryonicprecursor cell

Myosin, othermuscle proteins,and cell-cycleblocking proteins

Other muscle-specific genesMaster control gene myoDNucleus

Determination. Signals from othercells lead to activation of a masterregulatory gene called myoD, andthe cell makes MyoD protein, atranscription factor. The cell, nowcalled a myoblast, is irreversibly

committed to becoming a skeletalmuscle cell.

1

Differentiation. MyoD protein stimulatesthe myoD gene further, and activatesgenes encoding other muscle-specifictranscription factors, which in turnactivate genes for muscle proteins. MyoDalso turns on genes that block the cellcycle, thus stopping cell division. Thenondividing myoblasts fuse to becomemature multinucleate muscle cells, alsocalled muscle fibers.

2

Determination and differentiation of muscle cellsDetermination and differentiation of muscle cells


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(a) Organizer regions

Apicalectodermal

ridge (AER)

Limb budsPosterior

Anterior

Limb bud

AER

ZPA

50 m

Formation of theFormation of the

Vertebrate LimbVertebrate Limb

The wings andlegs of chicks,

like all vertebratelimbs, begin asbumps of tissuecalled limb buds


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Apical Ectodermal Ridge (AER) secretes FGFZone of Polarizing Activity (ZPA) or Polarizing Region

region near proximal posterior margin of early limb bud that

secretes Sonic Hedgehog (Shh). Tissue transplantationexperiments support the hypothesis that the ZPA produces amorphogen which produces dose-dependent effects. thatconveys positional information indicating posterior.

Progress Zone Zone of mesodermal cells that underlie theAER.


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Three axes of limb development:1.Proximal-distal axis regulated primarily by

Fibroblast Growth Factors (FGF) produced byApical Ectodermal Ridge (AER).

2.Anterior-posterior axis regulated primarily bySonic Hedgehog (SHH) produced by Zone ofPolarizing Activity (ZPA).

3.Dorsal-ventral axis regulated primarily byWNT-7a produced by Dorsal Ectoderm.

Nested expression of Hox genes provides fine-tuned positional information and precise

control of limb patterning.


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Hostlimbbud

Posterior

2

34

AnteriorNewZPA

EXPERIMENT

RESULTS

ZPA

Donorlimbbud

2

34

What role doesthe zone of

polarizing activity(ZPA) play in

limb patternformation invertebrates?


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Experiment: Remove AER of limb buds.Result: Proximaldistal development endsprematurely, only proximal elements form.

Conclusion: The AER is necessary to promotenormal proximaldistal limb elongation.

Proximal Distal Limb Development: The Role ofProximal Distal Limb Development: The Role ofthe AER and FGF in Chick Embryosthe AER and FGF in Chick Embryos


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Experiment: Remove AER but replace it with a beadsoaked in FGF.Result: Normal (or nearly normal) proximal distal

development.Conclusion:

oFGF is the signal molecule that promotesproximal-distal limb elongation.

oFGF binds to FGF receptors in mesodermal cells

of progress zone, by activating a tyrosine kinasepathway.


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Experiment: Remove AER from wing bud, replace withAER from leg bud.

Result: Normal wing development.

Experiment: Remove progress zone mesoderm from awing bud and replace it with leg bud mesoderm.

Result: Distal limb develops as leg, not wing!Conclusions:

AER and FGF are necessary for growth but notlimb identity.

Limb identity is specified by underlyingmesodermal cells of the progress zone.


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Role of the ZPA and SHH in Chick EmbryosRole of the ZPA and SHH in Chick Embryos

Experiment: Transplantposterior limb bud (includingZPA) from one chick toanterior limb bud of a secondchick embryo.

Experiment: Implant a beadsoaked with SHH in anteriorregion of limb bud.Result: Mirror-image

duplication of posteriorstructures (digits) in a dose-dependent fashion!


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Dorsal-Ventral Limb Development:Dorsal-Ventral Limb Development:Role of Dorsal Ectoderm and WNT-7aRole of Dorsal Ectoderm and WNT-7a

AER

ZPA

Ventral

Dorsal

Dorsal ectoderm secretesWNT-7a, (wingless ofDrosophila).WNT-7aloss-of-function

mutants: no dorsallimb structures!

Expression of WNT-7aonventral ectoderm: noventral limb structures!


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Hox Gene Expression During Limb DevelopmentHox Gene Expression During Limb Development

Nested expression

along P-D and A-Paxis.Expression pattern

mirrors genearrangement on

chromosome.Also seen in

homeotic/hox genecontrol of developmentalong the A-P body

axis in Drosophila andother animals.


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Cell signaling is also involved in programmed cell death.Apoptosis is essential for normal morphogenesis of hands

and feet in humans and paws in other animals.

Interdigital tissue

1 mm

2 m


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Synpolydactyly phenotype with mutation in HOXD13 in heterozygous(left) and homozygous (hand and foot) individuals.


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Evolutionary conservationEvolutionary conservationof homeobox (Hox) genesof homeobox (Hox) genes

Found in virtually all

multicellular animals.Conserved DNA

sequence and genearrangement.

Duplications of genes orcomplexes common invertebrates.

Often functionally similar(e.g. mouse Hoxgene

works in fruit flies).


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Vertebrates have 4 Hoxcomplexes, withabout 10 genes each. They can be aligned in

13 groups of paralogues. They displaycolinearity:

Temporal colinearity: genes on one end of thecomplex are expressed first, those on the other

(posterior) end are turned on last.Spatial colinearity: the more anteriorly

expressed genes are in one end, the moreposterior ones at the other end of the gene

complex.Anterior Hox genes are activated sequentially byretinoic acid.


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Temporal and spatial colinearity: order of Hox genes inDNA follows the antero-posterior body axis.


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RARE

Hox complexes have a retinoic acid receptor responseelement (RARE) in the DNA before paralogue1. This DNAenhancer element controls expression of many genes in

the complex. In retinoic acid teratogenesis, Hox geneexpression borders move into more anterior regions.


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Pharyngeal arch1 does not

express any Hoxgene. It gives

rise to maxillaryand mandibular

structures.Retinoic acid can

cause cleftpalate and

micrognathia.


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Only a few signaling pathways pattern the embryo, butthere are hundreds of differentiated cell types in the

human body. WHY?

The same signals can trigger different types of celldifferentiation responses in cells of different

developmental history.

TGF/BMP Serine/Threonine kinase receptorsReceptor Tyrosine kinases such as FGF, EGF, IGF,

InsulinWnt

Sonic HedgehogNotchG protein-coupled receptorsNuclear hormone receptors


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Three kinds of signaling for three styles of pattern formation(A) Intracellular signals can organize cytoplasmic determinants in the egg,

which are inherited by different blastomeres when the egg divides. (B)Long-range diffusible signals from a signaling center can direct the global

pattern of cell specialization in the surrounding tissue. (C) Short-range, cell-cell contact interactions can create a fine-grained mosaic of cells in

different states; they often play a crucial part in deciding the final step ofdifferentiation in intricate tissues such as the retina and other sensory

epithelia.


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Contact-Dependent Notch Signaling and CellContact-Dependent Notch Signaling and CellFate in Neurogenic Ectoderm of DrosophilaFate in Neurogenic Ectoderm of Drosophila

In Drosophila, undifferentiated neurogenic ectodermalcells have two potential fates: Neuroblasts (about 25%

adopt this fate); epidermal cells (about 75% adopt thisfate).

Cells differentiate in a regular lattice-like pattern. How?

Lateral inhibition via Notch signaling!

UndifferentiatedNeurogenicEctoderm

NeuroblastsEpidermal Cells


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Initially equivalentneurogenic

ectodermal cells

Initially equivalentneurogenic

ectodermal cells Neuroblast

Epidermalcell

Stimulateneuronal

differentiation

Lateral Inhibition of Neural Cell Development via Contact-Lateral Inhibition of Neural Cell Development via Contact-Dependent Notch Signaling: The Molecular DetailsDependent Notch Signaling: The Molecular Details


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Regional specificity of inductions:Regional specificity of inductions:Epidermal signals (Sonic hedgehog and TGF 2)

cause dermis to form condensed mesenchyme

In turn the dermis responds by secreting factors thatcause the epidermis to form regionally specific skinstructures (e.g. forelimb/hindlimb feathers).


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GeneticGeneticspecificity ofspecificity ofinduction.induction.

Instructions sentby themesenchymal

tissue can crossspecies barriers.

Salamandersrespond to frog

signals, andchick tissueresponds tomammalianinducers.


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Nature of Embryonic Induction