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Embryonic Development & Cell Differentiation. A Genetic Program for Embryonic Development. During embryonic development, a fertilized egg gives rise to many different cell types Cell types are organized into tissues, organs, organ systems, and the whole organism

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slide2

A Genetic Program for Embryonic Development

  • During embryonic development, a fertilized egg gives rise to many different cell types
  • Cell types are organized into tissues, organs, organ systems, and the whole organism
  • Gene expression controls this development
  • The transformation from zygote to adult results from cell division, cell differentiation, and morphogenesis
slide3

A Genetic Program for Embryonic Development

  • Cell differentiation is the process by which cells become specialized in structure and function
  • The physical processes that give an organism its shape constitute morphogenesis
  • Differential gene expression results from genes being regulated differently in each cell type
  • Materials in the egg can set up gene regulation that is carried out as cells divide
slide4

A Genetic Program for Embryonic Development

  • Determination commits a cell to its final fate
  • Determination precedes differentiation
  • Cell differentiation is marked by the production of tissue-specific proteins
slide5

Cytoplasmic Determinants and Inductive Signals

  • An egg’s cytoplasm contains RNA, proteins, and other substances that are distributed unevenly in the unfertilized egg
  • Cytoplasmic determinants are maternal substances in the egg that influence early development
  • As the zygote divides by mitosis, cells contain different cytoplasmic determinants, which lead to different gene expression
slide6

Unfertilized egg cell

Sperm

Nucleus

Two different

cytoplasmic

determinants

(a) Cytoplasmic determinants in the egg

slide7

Unfertilized egg cell

Sperm

Nucleus

Fertilization

Two different

cytoplasmic

determinants

Zygote

(a) Cytoplasmic determinants in the egg

slide8

Unfertilized egg cell

Sperm

Nucleus

Fertilization

Two different

cytoplasmic

determinants

Zygote

Mitotic

cell division

Two-celled

embryo

(a) Cytoplasmic determinants in the egg

slide9

NUCLEUS

Early embryo

(32 cells)

Signal

transduction

pathway

Signal

receptor

Signal

molecule

(inducer)

(b) Induction by nearby cells

slide10

Regulation of Gene Expression During Cellular Differentiation

  • Myoblastsproduce muscle-specific proteins and form skeletal muscle cells
  • MyoD is one of several “master regulatory genes” that produce proteins that commit the cell to becoming skeletal muscle
  • The MyoD protein is a transcription factorthat binds to enhancers of various target genes
slide11

Nucleus

Master regulatory gene myoD

Other muscle-specific genes

DNA

Embryonic

precursor cell

OFF

OFF

slide12

Nucleus

Master regulatory gene myoD

Other muscle-specific genes

DNA

Embryonic

precursor cell

OFF

OFF

OFF

mRNA

MyoD protein

(transcription

factor)

Myoblast

(determined)

slide13

Nucleus

Master regulatory gene myoD

Other muscle-specific genes

DNA

Embryonic

precursor cell

OFF

OFF

OFF

mRNA

MyoD protein

(transcription

factor)

Myoblast

(determined)

mRNA

mRNA

MyoD

Part of a muscle fiber

(fully differentiated cell)

slide14

Nucleus

Master regulatory gene myoD

Other muscle-specific genes

DNA

Embryonic

precursor cell

OFF

OFF

OFF

mRNA

MyoD protein

(transcription

factor)

Myoblast

(determined)

mRNA

mRNA

mRNA

mRNA

Myosin, other

muscle proteins,

and cell cycle–

blocking proteins

MyoD

Another

transcription

factor

Part of a muscle fiber

(fully differentiated cell)

slide15

Pattern Formation: Setting Up the Body Plan

  • Pattern formationis the development of a spatial organization of tissues and organs
  • In animals, pattern formation begins with the establishment of the major axes
  • Positional information, the molecular cues that control pattern formation, tells a cell its location relative to the body axes and to neighboring cells
slide16

Pattern Formation: Setting Up the Body Plan

  • Pattern formation has been extensively studied in the fruit fly Drosophila melanogaster
  • Combining anatomical, genetic, and biochemical approaches, researchers have discovered developmental principles common to many other species, including humans
slide17

The Life Cycle of Drosophila

  • In Drosophila, cytoplasmic determinants in the unfertilized egg determine the axes before fertilization
  • After fertilization, the embryo develops into a segmented larva with three larval stages
slide18

Follicle cell

Egg cell

developing within

ovarian follicle

1

Nucleus

Egg

cell

Nurse cell

2

(a) Development from egg to larva

slide19

Follicle cell

Egg cell

developing within

ovarian follicle

1

Nucleus

Egg

cell

Nurse cell

Egg

shell

Unfertilized egg

2

Depleted

nurse cells

4

(a) Development from egg to larva

slide20

Follicle cell

Egg cell

developing within

ovarian follicle

1

Nucleus

Egg

cell

Nurse cell

Egg

shell

Unfertilized egg

2

Depleted

nurse cells

Fertilization

Laying of egg

Fertilized egg

3

(a) Development from egg to larva

slide21

Follicle cell

Egg cell

developing within

ovarian follicle

1

Nucleus

Egg

cell

Nurse cell

Egg

shell

Unfertilized egg

2

Depleted

nurse cells

Fertilization

Laying of egg

Fertilized egg

3

Embryonic

development

Segmented

embryo

4

0.1 mm

Body

segments

(a) Development from egg to larva

slide22

Follicle cell

Egg cell

developing within

ovarian follicle

1

Nucleus

Egg

cell

Nurse cell

Egg

shell

Unfertilized egg

2

Depleted

nurse cells

Fertilization

Laying of egg

Fertilized egg

3

Embryonic

development

Segmented

embryo

4

0.1 mm

Body

segments

Hatching

Larval stage

5

5

(a) Development from egg to larva

slide23

Thorax

Head

Abdomen

0.5 mm

Dorsal

Right

Body

Axes

Posterior

Anterior

Left

Ventral

(b) Adult

slide24

Axis Establishment

  • Maternal effect genes encode for cytoplasmic determinants that initially establish the axes of the body of Drosophila
  • These maternal effect genes are also called egg-polarity genesbecause they control orientation of the egg and consequently the fly
slide25

Axis Establishment

Bicoid: A Morphogen Determining Head Structures

  • One maternal effect gene, the bicoid gene, affects the front half of the body
  • An embryo whose mother has a mutant bicoid gene lacks the front half of its body and has duplicate posterior structures
slide26

Tail

Head

A8

T1

T2

A7

T3

A6

A1

A5

A2

A3

A4

Wild-type larva

A8

A8

A7

A7

A6

slide27

Tail

Head

A8

T1

T2

A7

T3

A6

A1

A5

A2

A3

A4

Wild-type larva

Tail

Tail

A8

A8

A7

A7

A6

Mutant larva (bicoid)

slide28

Axis Establishment

  • This phenotype suggests that the product of the mother’s bicoid gene is concentrated at the future anterior end
  • This hypothesis is an example of the gradient hypothesis, in which gradients of substances called morphogens establish an embryo’s axes and other features
slide29

100 µm

Bicoid mRNA in mature

unfertilized egg

Axis Establishment

slide30

Fertilization,

translation

of bicoid

mRNA

Anterior end

100 µm

Bicoid mRNA in mature

unfertilized egg

Bicoid protein in early

embryo

Axis Establishment

slide31

Nurse cells

Egg

bicoid mRNA

Developing egg

Axis Establishment

slide32

Nurse cells

Egg

bicoid mRNA

Bicoid mRNA in mature

unfertilized egg

Developing egg

Axis Establishment

slide33

Nurse cells

Egg

bicoid mRNA

Bicoid mRNA in mature

unfertilized egg

Developing egg

Bicoid protein

in early embryo

Axis Establishment

slide34

Axis Establishment

  • The bicoid research is important for three reasons:
    • – It identified a specific protein required for some early steps in pattern formation
    • – It increased understanding of the mother’s role in embryo development
    • – It demonstrated a key developmental principle that a gradient of molecules can determine polarity and position in the embryo