principles of embryonic development © 2012 pearson education, inc
TRANSCRIPT
PRINCIPLES OF EMBRYONIC DEVELOPMENT
© 2012 Pearson Education, Inc.
Developmental Stages in an Amphibian
Where do tissues/organ derive during embryonic development?What regulates cell differentiation and organ formation?How can new body plans arise?
"It is not birth, marriage, or death, but gastrulation, which is truly the most important time in your life.”
Lewis Wolpert (1986)
Most animals proceed through these stages during development:
1. Zygote2. Early cleavage stages
• Blastula• Establish polarity and
body axes3. Gastrulation
• Establish germ layers4. Body plan (segmentation)
• In vertebrates, this involves neurulation
5. Morphogenesis (organogenesis)
LE 21-4
Animal development
Zygote(fertilized egg)
Eight cells Blastula(cross section)
Gastrula(cross section)
Adult animal(sea star)
Cellmovement
Gut
Cell division
Morphogenesis
Observable cell differentiation
Seedleaves
Shootapicalmeristem
Rootapicalmeristem
PlantEmbryoinside seed
Two cellsZygote(fertilized egg)
Plant development
Fertilization results in a zygote and triggers embryonic development
Embryonic development begins with fertilization,
– the union of sperm and egg, forming diploid zygote
Sperm contributes 1/2 genetic material (23 chromosomes in humans)
Egg contributes:
– Cytoplasm and all organelles
– Nourishment
– mRNAs for first proteins synthesized
– Regulatory factors that guide early development (cytoplasmic determinants)
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Cleavage produces a multicellular embryo and begins to organize body axes
Cleavage is a rapid series of cell divisions that produces
– more cells,
– smaller cells, and
– a fluid-filled embryo called a blastula.
© 2012 Pearson Education, Inc.
Video: Sea Urchin Embryonic Development
LE 47-7
Fertilized egg Four-cell stage Morula Blastula
Starfish development, unfertilized egg.2 blastomeres.
Starfish development,
nonmotile blastula.
4 blastomeres.
16 blastomeres. 32 blastomeres.
morula
Cleavage produces a multicellular embryo and begins to organize body axes
Cleavage is a rapid series of cell divisions that produces
– more cells,
– smaller cells, and
– a fluid-filled embryo called a blastula.
© 2012 Pearson Education, Inc.
Video: Sea Urchin Embryonic Development
Sperm
Molecules of acytoplasmicdeterminant Fertilization
Nucleus
Molecules of anothercytoplasmic determinant
Unfertilized egg cell
Zygote(fertilized egg)
Mitotic cell division
Two-celledembryo
Cleavage Defines Body Axes
•Cytoplasmic determinants become asymmetrically distributed.
•Most of these cytoplasmic determinants are maternal factors (mRNA or proteins) inherited from egg cytoplasm
•Maternal factors generally code for or are transcription factors that regulate gene expression
These cells now have different fates.
Bicoid Protein Provides Positional Information
Bicoid is an example of cytoplasmic determinant (maternal
factor)
Animation: Development of Head-Tail Axis in Fruit Flies
LE 21-14a
Head
Tail
Tail
Tail
Wild-type larva
Mutant larva (bicoid)
Drosophila larvae with wild-type and bicoid mutant phenotypes
Gastrulation produces 3 primary tissue (germ) layers in embryo
During gastrulation
– cells migrate to new locations,
– a rudimentary digestive cavity forms, and
– the basic body plan of three layers is established with
– ectoderm outside—becomes skin and nervous systems,
– endoderm inside—becomes digestive tract,
– mesoderm in the middle—becomes muscle and bone.
© 2012 Pearson Education, Inc.
Most animals proceed through these stages during development:
1. Zygote2. Early cleavage stages
• Blastula• Establish polarity and
body axes3. Gastrulation
• Establish germ layers4. Body plan (segmentation)
• In vertebrates, this involves neurulation
5. Morphogenesis (organogenesis)
Figure 27.11_s3
Blastula(end of cleavage)
Animal pole
Blastocoel
Vegetal pole
Gastrulation(cell migration)
Formation of asimple digestivecavity
Blastopore
Blastocoelshrinking
Gastrula(end of gastrulation)
Simpledigestivecavity
EndodermMesoderm
Ectoderm
Starfish development, gastrula during invagination. Starfish development, mid-gastrula. LM X75.
Starfish, late bipinnaria. Starfish, young adult.
Table 27.11
27.12 Organs start to form after gastrulation
Organs develop from the three embryonic layers.
Neurulation is formation of nervous system
– notochord forms from mesoderm
– later replaced by the vertebral column in most chordates.
– The neural tube develops above the notochord from ectoderm
– Will become brain and spinal cord.
The nervous system is the first organ system to develop
Once the nervous system is in place, basic body pattern and dorsal/ventral axis established.
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Video: Frog Embryo Development
Neural folds
Endoderm
Mesoderm
Ectoderm
Notochord
Neuralfold
Neuralplate
Neural fold
Neural plate
Neural tube
Outer layerof ectoderm
Neural tube
Notochord
Coelom
Digestivecavity
Somite
Somites Tail bud
Eye
Somites are derived from mesoderm and form vertebrae
By end of neurulation, body
has been segmented and basic body plan
established
Body Pattern Formtion
Pattern formation,
– Shaping of animals major body structures in proper positions within each body segment
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Fruit fly embryo (10 hours)
Adult fruit fly
Homeotic Genes Control Pattern Formation
Homeotic genes (Hox genes)
– Code for protein transcription factors and regulate timing and expression of body pattern genes
– contain common nucleotide/amino acid sequence (homeoboxes), that allows Hox protein to bind DNA
– occur in diverse groups such as
– prokaryotes,
– yeast,
– plants, and
– animals.
– Homeotic genes reveal the shared evolutionary history of life.
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Hox Genes in Drosophila Determine Segment Identity
Hox Genes Control Body Segmentation
Figure 27.14BFly chromosome Mouse chromosomes
Fruit fly embryo (10 hours) Mouse embryo (12 days)
Adult mouseAdult fruit fly
Multiple processes guide development
Induction
– Signals from neighboring cells direct gene expression and development
Cell migrations around developing embryo
Apoptosis (cell death)
Regulated gene expression
– Gene expression cascades
– Homeotic Genes
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Figure 27.13A
Cell signaling and cascades of gene expression direct animal development
1. Head-to-tail, top-to-bottom, and side-to-side axes are determined by proteins and mRNA in the egg from mother.
2. Cleavage results in asymmetrical distribution of these proteins within embryo - regulating expression of genes specific to the cell’s fate.
3. Leads to a cascade of gene expression - one protein acts as transcription factor for another, inducing pattern formation (anatomy of body parts and development of segments)
Example: hox genes
© 2012 Pearson Education, Inc.
Follicle cells
“Head”mRNA
Embryo
Adult fly
Expression of homeotic genesand cascades of gene expression
Body segments
Cascades ofgene expression
Gene expressionGrowth of egg cellLocalization of “head” mRNA
Egg cell
Egg cell
Egg cell and folliclecells signalingeach other
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1
3
4