Developmental Biology

AP Bio18:421:6

47:2 (part), 3

Wild-type mouse embryo 9.5 days post coitum

Our focus:

• Timing and Coordination• Gene Expression• Interactions, Cell Signaling

Insights into development have been obtained froms studying

• Slime Molds• Nematode worm C. elegans• Fruit Flies• Zebrafish• Frog Embryos• Chick Embryos• Mice

Fig. 18-14

(a) Fertilized eggs of a frog (b) Newly hatched tadpole

Zebrafish are often used for embryological research.

Their embryos are transparent.

A program of differential gene expression leads to the different cell

types in a multicellular organism

• During embryonic development, a fertilized egg gives rise to many different cell types

• Cell types are organized successively into tissues, organs, organ systems, and the whole organism

• Gene expression orchestrates the developmental programs of animals

A Genetic Program for Embryonic Development

• The transformation from zygote to adult results from cell division, cell differentiation, and morphogenesis

• 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

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

Fig. 18-15a

(a) Cytoplasmic determinants in the egg

Two differentcytoplasmicdeterminants

Unfertilized egg cell

Sperm

Fertilization

Zygote

Mitoticcell division

Two-celledembryo

Nucleus

• The other important source of developmental information is the environment around the cell, especially signals from nearby embryonic cells

• In the process called induction, signal molecules from embryonic cells cause transcriptional changes in nearby target cells

• Thus, interactions between cells induce differentiation of specialized cell types

• Work with the nematode C. elegans has shown that induction requires the transcriptional regulation of genes in a particular sequence.

Fig. 18-15b

(b) Induction by nearby cells

Signalmolecule(inducer)

Signaltransductionpathway

Early embryo(32 cells)

NUCLEUS

Signalreceptor

Sequential Regulation of Gene Expression During Cellular

Differentiation

• Determination commits a cell to its final fate: it is the progressive restriction of developmental potential as the embryo develops

• Determination precedes differentiation• Cell differentiation is marked by the expression of

tissue-specific proteins

For example

• Myoblasts produce 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 factor that binds to enhancers of various target genes

Fig. 18-16-1

Embryonicprecursor cell

Nucleus

OFF

DNA

Master regulatory gene myoD Other muscle-specific genes

OFF

Fig. 18-16-2

Embryonicprecursor cell

Nucleus

OFF

DNA

Master regulatory gene myoD Other muscle-specific genes

OFF

OFFmRNA

MyoD protein(transcriptionfactor)

Myoblast(determined)

Fig. 18-16-3

Embryonicprecursor cell

Nucleus

OFF

DNA

Master regulatory gene myoD Other muscle-specific genes

OFF

OFFmRNA

MyoD protein(transcriptionfactor)

Myoblast(determined)

mRNA mRNA mRNA mRNA

Myosin, othermuscle proteins,and cell cycle–blocking proteinsPart of a muscle fiber

(fully differentiated cell)

MyoD Anothertranscriptionfactor

Why doesn’t myoD change any type of embryonic cell?

• Probably a combination of regulatory genes are necessary for differentiation is required.

Pattern Formation: Setting Up the Body Plan

• Pattern formation is 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 (cytoplasmic determinants and inductive signals) control pattern formation, and tell a cell its location relative to the body axes and to neighboring cells

• 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

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

Fig. 18-17a

ThoraxHead Abdomen

0.5 mm

Dorsal

Ventral

RightPosterior

LeftAnteriorBODY

AXES

(a) Adult

Fig. 18-17bFollicle cell

Nucleus

Eggcell

Nurse cell

Egg celldeveloping withinovarian follicle

Unfertilized egg

Fertilized egg

Depletednurse cells

Eggshell

FertilizationLaying of egg

Bodysegments

Embryonicdevelopment

Hatching

0.1 mm

Segmentedembryo

Larval stage

(b) Development from egg to larva

1

2

3

4

5

Genetic Analysis of Early Development: Scientific Inquiry

• Edward B. Lewis, Christiane Nüsslein-Volhard, and Eric Wieschaus won a Nobel 1995 Prize for decoding pattern formation in Drosophila

• Homeotic genes control pattern formation in the late embryo, larva, and adult.

Fig. 18-18

Antenna

MutantWild type

Eye

Leg

A mutation in regulatory genes, called homeotic genes, caused this.

Homeotic Genes

• One example are the Hox and ParaHox genes which are important for segmentation,

another example is the MADS-box-containing genes in the ABC model of flower development.

Chap 21:6

The Homeobox

• Homeotic genes contain a 180 nucleotide sequence called a homeobox found in regulatory genes .

• This homeobox has been found in inverts and verts as well as plants.

• The homeobox DNA sequence evolved very early in the history of life and has been conserved virtually unchanged for millions of years.

• Differences arise due to different gene expressions.

ABC model of flower development

Molecular basis of differentiation: • The A, B, and C genes are transcription

factors. Different transcription factors are needed together to turn on a developmental gene program--such as A and B needed to initiate the program for petals. What turns on the different transcription factors in different cells?

• Induction and inhibition by one cell signaling to a neighboring cell.

Fate Mapping

• Fate maps are general territorial diagrams of embryonic development

• Classic studies using frogs indicated that cell lineage in germ layers is traceable to blastula cells

*Chap 47 (3)

http://education-portal.com/academy/lesson/how-fate-mapping-is-used-to-track-cell-development.html

Fig. 47-21

Epidermis

(b) Cell lineage analysis in a tunicate(a) Fate map of a frog embryo

Epidermis

Blastula Neural tube stage(transverse section)

Centralnervoussystem

Notochord

Mesoderm

Endoderm

64-cell embryos

Larvae

Blastomeresinjected with dye

Fig. 47-21a

Epidermis

(a) Fate map of a frog embryo

Epidermis

Blastula Neural tube stage(transverse section)

Centralnervoussystem

Notochord

Mesoderm

Endoderm

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 genes because they control orientation of the egg and consequently the fly

• 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 at both ends

Bicoid: A Morphogen Determining Head Structures

Fig. 18-19a

T1 T2T3

A1 A2 A3 A4 A5 A6A7

A8

A8A7 A6 A7

Tail

TailTail

Head

Wild-type larva

Mutant larva (bicoid)

EXPERIMENT

A8

Fig. 18-19b

Fertilization,translationof bicoidmRNA Bicoid protein in early

embryo

Anterior endBicoid mRNA in matureunfertilized egg

100 µm

RESULTS

Fig. 18-19c

bicoid mRNA

Nurse cells

Egg

Developing egg Bicoid mRNA in matureunfertilized egg

Bicoid proteinin early embryo

CONCLUSION

Bicoid mRNA, Bicoid Protein (red)

• 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 (amounts) of substances called morphogens establish an embryo’s axes and other features

• 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

Cancer results from genetic changes that affect cell cycle control

• The gene regulation systems that go wrong during cancer are the very same systems involved in embryonic development

Types of Genes Associated with Cancer

• Cancer can be caused by mutations to genes that regulate cell growth and division

• Tumor viruses can cause cancer in animals including humans

• Oncogenes are cancer-causing genes• Proto-oncogenes are the corresponding normal

cellular genes that are responsible for normal cell growth and division

• Conversion of a proto-oncogene to an oncogene can lead to abnormal stimulation of the cell cycle

Fig. 18-20

Normal growth-stimulatingprotein in excess

Newpromoter

DNA

Proto-oncogene

Gene amplification:Translocation ortransposition:

Normal growth-stimulatingprotein in excess

Normal growth-stimulatingprotein in excess

Hyperactive ordegradation-resistant protein

Point mutation:

Oncogene Oncogene

within a control element within the gene

• Proto-oncogenes can be converted to oncogenes by– Movement of DNA within the genome: if it

ends up near an active promoter, transcription may increase

– Amplification of a proto-oncogene: increases the number of copies of the gene

– Point mutations in the proto-oncogene or its control elements: causes an increase in gene expression

Tumor-Suppressor Genes• Tumor-suppressor genes help prevent

uncontrolled cell growth

• Mutations that decrease protein products of tumor-suppressor genes may contribute to cancer onset

• Tumor-suppressor proteins

– Repair damaged DNA

– Control cell adhesion

– Inhibit the cell cycle in the cell-signaling pathway

Interference with Normal Cell-Signaling Pathways

• Mutations in the ras proto-oncogene and p53 tumor-suppressor gene are common in human cancers

• Mutations in the ras gene can lead to production of a hyperactive Ras protein and increased cell division

Fig. 18-21a

Receptor

Growthfactor

G protein GTP

Ras

GTP

Ras

Protein kinases(phosphorylationcascade)

Transcriptionfactor (activator)

DNA

HyperactiveRas protein(product ofoncogene)issuessignalson its own

MUTATION

NUCLEUS

Gene expression

Protein thatstimulatesthe cell cycle

(a) Cell cycle–stimulating pathway

11

3

4

5

2

Fig. 18-21b

MUTATIONProtein kinases

DNA

DNA damagein genome

Defective ormissingtranscriptionfactor, suchas p53, cannotactivatetranscription

Protein thatinhibitsthe cell cycle

Activeformof p53

UVlight

(b) Cell cycle–inhibiting pathway

2

3

1

Fig. 18-21c

(c) Effects of mutations

EFFECTS OF MUTATIONS

Cell cycle notinhibited

Protein absent

Increased celldivision

Proteinoverexpressed

Cell cycleoverstimulated

• Suppression of the cell cycle can be important in the case of damage to a cell’s DNA; p53 prevents a cell from passing on mutations due to DNA damage

• Mutations in the p53 gene prevent suppression of the cell cycle

The Multistep Model of Cancer Development

• Multiple mutations are generally needed for full-fledged cancer; thus the incidence increases with age

• At the DNA level, a cancerous cell is usually characterized by at least one active oncogene and the mutation of several tumor-suppressor genes

Hedgehog Signaling Pathway

http://www.youtube.com/watch?v=FCNJp6Y901M

• The Hedgehog Pathway is very important in development but after adult state is reached it is used in maintenance of stem cells.

Development in AnimalsChap 47 (part of 2, 3)

• Timing and coordination to produce stages• After fertilization, embryonic development

proceeds through cleavage, gastrulation, and organogenesis

• The sperm’s contact with the egg’s surface initiates metabolic reactions in the egg that trigger the onset of embryonic development

• Important events regulating development occur during fertilization and the three stages that build the animal’s body:

– Cleavage: cell division creates a hollow ball of cells called a blastula

– Gastrulation: cells are rearranged into a three-layered gastrula

– Organogenesis: the three layers interact and move to give rise to organs

Fig. 47-6

(a) Fertilized egg (b) Four-cell stage (c) Early blastula (d) Later blastula

Gastrulation• Gastrulation rearranges the cells of a blastula into a

three-layered embryo, called a gastrula, which has a primitive gut

• The three layers produced by gastrulation are called embryonic germ layers– The ectoderm forms the outer layer – The endoderm lines the digestive tract– The mesoderm partly fills the space between the

endoderm and ectoderm

These germ layers become:

• Ectoderm – skin and nervous system• Mesoderm – skeleton, muscles, circulatory,

lining of body cavity• Endoderm – lining of digestive and

respiratory tract, liver, many glands (pancreas, thymus, thyroid, parathyroid)

Fig. 47-14

ECTODERM MESODERM ENDODERM

Epidermis of skin and itsderivatives (including sweatglands, hair follicles)Epithelial lining of mouthand anusCornea and lens of eyeNervous systemSensory receptors inepidermisAdrenal medullaTooth enamelEpithelium of pineal andpituitary glands

NotochordSkeletal systemMuscular systemMuscular layer ofstomach and intestineExcretory systemCirculatory and lymphaticsystemsReproductive system(except germ cells)Dermis of skinLining of body cavityAdrenal cortex

Epithelial lining ofdigestive tractEpithelial lining ofrespiratory systemLining of urethra, urinarybladder, and reproductivesystemLiverPancreasThymusThyroid and parathyroidglands

Fig. 47-9-6

Future ectoderm

Key

Future endoderm

Digestive tube (endoderm)

Mouth

Ectoderm

Mesenchyme(mesodermforms futureskeleton)

Anus (from blastopore)

Future mesoderm

Blastocoel

Archenteron

Blastopore

Blastopore

Mesenchymecells

Blastocoel

Blastocoel

Mesenchymecells

Archenteron

Vegetalplate

Vegetalpole

Animalpole

Filopodiapullingarchenterontip

50 µm

http://www.gastrulation.org/Movie9_3.mov

Gastrulation in the frog

• Early in vertebrate organogenesis, the notochord forms from mesoderm, and the neural plate (which will becomes the nervous system) forms from ectoderm

Fig. 47-12

Neural folds Tail bud

Neural tube

(b) Neural tube formation

Neuralfold

Neural plate

Neuralfold

Neural plate

Neural crestcells

Neural crestcells

Outer layerof ectoderm

Mesoderm

Notochord

Archenteron

Ectoderm

Endoderm

(a) Neural plate formation

(c) Somites

Neural tube

Coelom

Notochord

1 mm1 mm

SEM

Somite

Neural crestcells

Archenteron(digestivecavity)

SomitesEye

Fig. 47-21a

Epidermis

(a) Fate map of a frog embryo

Epidermis

Blastula Neural tube stage(transverse section)

Centralnervoussystem

Notochord

Mesoderm

Endoderm

In humans,

• At completion of cleavage, the blastocyst forms

• A group of cells called the inner cell mass develops into the embryo

• The trophoblast, the outer epithelium of the blastocyst, initiates implantation in the uterus.

• As implantation is completed, gastrulation begins

Fig. 47-16-1

Blastocoel

Trophoblast

Uterus

Endometrialepithelium(uterine lining)

Inner cell mass

will become embryo

Restriction of the Developmental Potential of Cells

• In many species that have cytoplasmic determinants, only the very early stages of the embryo are totipotent.

• That is, only the zygote can develop into all the cell types in the adult

• As embryonic development proceeds, potency of cells becomes more limited

Stem Cells of Animals• A stem cell is a relatively unspecialized cell that

can reproduce itself indefinitely and differentiate into specialized cells of one or more types

• Stem cells isolated from early embryos at the blastocyst stage are called embryonic stem cells; these are able to differentiate into all cell types

• The adult body also has stem cells, which replace nonreproducing specialized cells

Spemann Experiment

Gray crescent notbisected equally

• The gray crescent acts as an “organizer” by inducing cells to become certain parts of the embryo.

• A signaling protein perhaps?

Importance of Apoptosis in development

• Elimination of transitory organs and tissues. Examples include tadpole tails and gills.

• Tissue remodeling. Vertebrate limb bud development,

removal of interdigital skin. • Nutrients are reused!

When the grim and reaper genes work together, they help guide cells in flies through their death process, apoptosis—much like that spectre of 15th century folklore, the Grim Reaper.

Comparing plant and animal development

• Since plants have rigid cell walls, there is no morphogenetic movement of cells.

• plant development depends upon differential rates of cell division then directed enlargement of cells.

All postembryonic growth occur at meristems which give rise to all adult structures (shoots, roots, stems, leaves and flowers) and have the capacity to divide repeatedly and give rise to a number of tissues (like stem cells).

Two meristems are established in the embryo, one at the root tip and one at the tip of the shoot.

The developmental patterning of organs therefore continues throughout the life of the plant.

• Their fate is determined largely by their position but they do have signaling.

• Homeotic genes control organ identity (ABC model) but genes are called Mad-box genes instead of Hox genes

Similarities in development of plants and animals

• Both involve a cascade of transcription factors

• But differences in regulatory genes as stated in previous slide

Evo-DevoComparing developmental processes of different multicellular organisms

• Many groups of animals and plants, even distantly related ones, share similar molecular mechanisms for morphogenesis and pattern formation.

• These mechanisms can be thought of as “genetic toolkits”.

• Development produces morphology and much of morphological evolution occurs by modifications of existing development genes and pathways rather than the introduction of radically new developmental mechanisms.

Our common ancestor