Embryonic Development

Timing and coordination of gene activation

Genetic Basis of Development

Agenda• Turn in your take-home quiz (Place on music stand)• Timing and coordination of development

• Cytoplasmic Determinants and the maternal effect• Induction through cell signaling• Homeotic Genes and segment determination • Apoptosis

• Science Skills Practice (Homework if we don’t get to it)

Figure 21.8

“Copy Cat”

Cell Division - Morphogenesis –Differentiation

Figure 21.4a, b

Animal development. Formation of three germ layers, body cavity, gut, and nervous system.

Cells actively migrate during development.

Certain cells in each developed tissue remain as partially differentiated stem cells to replace cells that are old or damaged.

Plant development. Morphogenesis involves cell division and selective cell expansion. Cells cannot move.

The apical meristems located in the roots and shoots remain undifferentiated throughout the plants life for growth.

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 cells Zygote

(fertilized egg)

(a)

(b)

• Evidence that all cells in a developed organism have all the genetic material

Figure 21.6

EXPERIMENT Researchers enucleated frog egg cells by exposing them to ultraviolet light, which destroyed the nucleus. Nuclei from cells of embryos up to the tadpole stage were transplanted into the enucleated egg cells.

Frog embryo Frog egg cell Frog tadpole

Less differentiated cell

Donor nucleusTransplanted

Enucleatedegg cell

Fully differentiated(intestinal) cell

Donor nucleustransplanted

Most developinto tadpoles

<2% developinto tadpoles

Why are the fully differentiated cells less successful in developing into tadpoles?

Timing and coordination of development

• Cells specialize by activating master control genes

• What is the function of master control genes?

DNAOFF OFF

OFFmRNA

mRNA mRNA mRNA mRNA

Anothertranscriptionfactor

MyoDMuscle cell(fully differentiated)

MyoD protein(transcriptionfactor)

Myoblast (determined)

Embryonicprecursor cell

Myosin, othermuscle proteins,and cell-cycleblocking proteins

Other muscle-specific genesMaster control gene myoDNucleus

1

2

How do cells know which master control genes to activate during differentiation and morphogenesis?

• Cytoplasmic Determinants and the maternal effect• Induction by cell signaling

Induction: Cell signaling changes gene expression

Figure 21.11b

Signaltransductionpathway

Signalreceptor

Signalmolecule(inducer)

Induction by nearby cells. The cells at the bottom of the early embryo depicted here are releasing chemicals that signal nearby cells to change their gene expression.

NUCLEUS

Early embryo(32 cells)• Cell signals made by cells early on in

in differentiation• Affect transcription factors in the

nucleus of cells near by• Change gene expression of target cell

Pattern Formation in Drosophila

Translation of bicoid mRNAFertilization

Nurse cells Egg cell

bicoid mRNA

Developing egg cell

Bicoid mRNA in mature unfertilized egg

100 µm

Bicoid protein inearly embryo

Anterior end

(b) Gradients of bicoid mRNA and Bicoid protein in normal egg and early embryo.

1

2

3

Figure 21.14b

•Bicoid mRNA is placed in the egg cell by nurse cells (maternal effect)

•There is a gradient of Bicoid mRNA and Protein

•What experimental evidence suggests that high Bicoid protein concentration causes Anterior (head) segments develop?

C. elegans- a model of induction• What experimental evidence showed that cell to cell signaling or

cell to cell contact between adjacent cells was essential for correct differentiation and morphogenesis?

4

Anterior

EMBRYO

Posterior

ReceptorSignalprotein

Signal

Anteriordaughtercell of 3

Posteriordaughtercell of 3

Will go on toform muscle and gonads

Will go on toform adultintestine

12

43

3

Induction of the intestinal precursor cell at the four-cell stage.(a)

Figure 21.11a

Unfertilized egg cell

Molecules of a a cytoplasmicdeterminant Fertilization

Zygote(fertilized egg)

Mitotic cell division

Two-celledembryo

Nucleus

Cytoplasmic Determinants• mRNA, protein or other signaling

molecules in the cytoplasm of the unfertilized egg

• Unevenly distributed

• Mitosis creates cells with different sets of cytoplasmic determinants

• How might these cytoplasmic determinants regulate gene expression?

Molecules of another cyto-plasmic deter-minant

Homeotic Genes• Homeotic genes are regulatory genes that

determine where certain anatomical structures, such as appendages, will develop in an organism during morphogenesis.

• These seem to be the master genes of development

Normal Mutant with legs growing out of head

What are the functional (protein) products of Homeotic genes that enable them to determine cell fate? What part of the DNA would they interact with?

Four general phases for body formation

1. Organize body along major axes2. Organize into smaller regions (organs,

legs)3. Cells organize to produce body parts4. Cells themselves change

morphologies and become differentiated

Programmed Cell Death (Apoptosis)• Cell signaling is involved in programmed cell death• Is essential for normal development.

2 µmFigure 21.17

Apoptosis is essential for morphogenesis of hands and feet

Figure 21.19

Interdigital tissue

1 mm

Homeotic Genes and Evolution • What is the evidence that homeotic

genes are evolutionarily conserved?

• What does this figure mean?

Adultfruit fly

Fruit fly embryo(10 hours)

Flychromosome

Mouse chromosomes

Mouse embryo(12 days)

Adult mouse

Figure 21.23

A. Drosophila's eight Hox genes in a single cluster and 39 HOX genes in humans.B. Expression patterns of Hox and HOX genes along the anterior-posterior axis in invertebrates and vertebrates.

Hox genes in the Animal Kingdom: increasing numbers and types of Hox genes (animal homeotic genes), increased body plan complexity

Hox genes determine the number and types of vertebrae in animals

• Hoxc-6 determines that in the chicken the 7 vertebrae will develop into ribs• Snake: Hoxc-6 is expanded dramatically toward the head and toward the rear so all

these vertebrae develop ribs.

How does homeotic gene regulation help organisms evolve different body plans?

Start homework!

• Science Skills Practice: how do we know enhancers regulate gene expression

• Science Skills practice: Hox genes and segment development