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)

© 2012 Pearson Education, Inc.

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.

© 2012 Pearson Education, Inc.

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

© 2012 Pearson Education, Inc.

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.

© 2012 Pearson Education, Inc.

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

© 2012 Pearson Education, Inc.

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

2

1

3

4