BIMA82 - HT2018 Early development in vertebrates I: Amphibians (Anamniotes)
←
→
Page content transcription
If your browser does not render page correctly, please read the page content below
Animals are made of repeating units Lobopodian 500 mio år Salamander 150 mio år Dinosaur 150 mio år 5 Trilobite 500 mio år
Diversity arises from variations in the numbers and kinds of repeating units All vertebrate limbs are variations on a common design in which the number, size and shape of elements differ 6 Human hand
Evolution as variation in number and kind Samuel Williston’s Law (1914): In the course of evolution the parts in an organism tend toward a reduction in number and the fewer parts showing a great specialization in function. 7
How is form encoded in the genome? Modularity, symmetry and polarity are universal features of animal design • bilateral symmetry • anterior-posterior axis • proximal-distal axis • dorsal-ventral axis 8
Early asymmetry in the amphibian egg Are factors required for development asymmetrically localized in the fertilized egg? Hans Spemann and Hilde Mangold Nobel prize 1935 9
Homeotic Selector Genes Wild type Wild type Ultrabithorax Antennapedia Antennapedia 11
Drosophila Homeotic (Hox) Genes • Specify identity of body region • Located in homeotic complexes: Antennapedia complex, Bithorax complex • Complexes are conserved throughout the animal kingdom 12 Adapted from: Carroll et al., From DNA to Diversity , Blackwell Science, 2001 and S.F. Gilbert, Sinauer
Drosophila Homeotic and Vertebrate Hox Genes Control Anterior-Posterior Identity 13
Pax6: A Master Control Gene for Eye Development Come back to polydactyly, cancer and hedgehog 14
The genetic toolkit So what’s in the toolkit? Transcription Factors 15
Cyclopia, Polydactyly, Cancer and Hedgehog Drosophila hedgehog ZPA & AER Polydactyly Cyclopia Basal Cell Carcinoma in Sheep 16
The toolkit paradox • Humans and mice share nearly identical sets of 25000 genes, and humans and chimps are 99% identical at the DNA level? • How can the same set of tool kit genes sculpt the different anatomies of arthropods and vertebrates? • At what point in development is a cells fate sealed? 17
Fate Maps The fate map reveals that at some point in development cells know where they are in the embryo and to what tissue or structure they belong. How do cells learn their position or identity? 18
The Coordinate system Define identities of modules Defining the poles The third axis is defined Subdivide into smaller regions Form new worlds at specific coordinates Initially similar modules are distinguished according to Within the worlds polarity is their position 19 Refine into finer modules established
The Making of a Fly Subdividing the Drosophila embryo Wing primordia Leg primordia marked by Distalless Subdivision of the D-V axis 20
The Making of a Vertebrate k-l: chordin and Frzb marking the early axes in the frog m-n: subdivision of the mouse brain by hox genes o-p: toolkit genes marking the segmented organization of the somites q: toolkit genes marking the position where the limbs will form 21
The Making of a Vertebrate w: BMP4 marking tissue between the digits that will die x: Patched receptor marking feather buds in the chicken u: Gdf5 marking the future position of joints in the digits x: Scleraxis gene marking future position of tendons of the limbs 22
Genetic Switches The positioning of toolkit genes determines the fate of cells and builds tissues. But where are the operating instructions for the toolkit? How do toolkit genes know in what order to act or where to act in the embryo? What is the mechanism that positions toolkit genes? Genetic switches are the key link between toolkit genes that build animal complexity and diversity. 23
Regulatory Sequences: Genetic “Switches” One gene can have multiple “switches”: enhancers The physical integrity of switches is very important for normal development! 24
How do Genetic Switches Work? A is an activator B and C are repressors 25
Changing Switches and Evolution Solving the toolkit paradox A, B, U: enhancer binding proteins (U= ubx) Gene 1: required for forewing development Gene 2: required for fore- and hindwing Gene 3: required for hindwing development 26
Summary § Animals are built of repeating units and have a modular design. § Diversity is created by evolutionary change of individual modules. § There is a universal gene toolkit used to build all animals. § Expression of toolkit genes foreshadows the development of tissues and organs § Evolutionary change is created by modulation of gene regulatory switches. § Every animal form is the product of two processes: development from an egg and evolution from its ancestors. 27
Axis formation in vertebrates In vertebrates differences in axis formation are mainly due to differences in the mode of reproduction 28
The Xenopus life cycle 29
The Xenopus body plan 30
The Xenopus fate map • Regulative development: The individual cell’s potential is greater than it’s normal embryonic fate. 31 • Cell fate is determined by the interaction with neighboring cells: called induction!
Fate Map of a Frog Blastula 32
Early asymmetry in the amphibian egg Are factors required for development asymmetrically localized in the fertilized egg? Hans Spemann and Hilde Mangold Nobel prize 1935 33
Hans Spemann and Hilde Mangold: Primary embryonic induction 34
The dorsal blastopore lip is the Spemann organizer The Spemann organizer: • initiates gastrulation • induces dorsal mesoderm (notochord) and neural tube formation • organizes tissues into an embryo with anterior-posterior polarity 35
How does the organizer work? - How is the organizer itself specified - How do cells in the dorsal blastopore lip become different from other cells? - What factors are secreted by the organizer? - How is anterior-posterior polarity established? 36
The Spemann organizer is induced by the Nieuwkoop center 37
Transplantation experiments localize the Nieuwkoop center 38
The Nieuwkoop center is induced by cortical rotation of the egg cytoplasm + - + - 39
Experimental evidence: Cortical rotation untreated UV irradiated 40
Dishevelled stabilizes β-catenin on the dorsal side 41
Various factors can rescue axis formation in UV- irradiated embryos Injection of blastocysts at the 2-cell stage with dominant-inactive GSK3. 42
β-Catenin, VegT and Vg-1 induce the Nieuwkoop center Oocyte β-Catenin VegT, Vg-1 Vg-1 (TGF-β family) is localized during oogenesis 43
The Spemann organizer is induced by the Nieuwkoop center Nieuwkoop center Nodal-related molecules (Xnr, TGF-β family) 44
Mechanism inducing the Spemann organizer VegT in endoderm 45
Model for mesoderm induction and organizer formation by β-catenin and TGFβ family molecules 46
Transplantation 47
Functions of the Spemann organizer 1. Differentiation of dorsal mesoderm (prechordal plate, axial mesoderm) 2. Dorsalization of surrounding mesoderm into paraxial mesoderm (somites) 3. Dorsalization of the ectoderm inducing neural tube formation 4. Initiation of gastrulation movements 48
The homeodomain transcription factor Goosecoid is expressed in the organizer - activates the migration properties of dorsal blastopore lip cells. - autonomously determines the fate of dorsal mesoderm cells. - enables goosecoid-expressing cells to recruit neighboring cells into the dorsal axis. goosecoid encodes a transcription factor and must therefore activate diffusible factors for its non-cell autonomous effects! 49
The Spemann organizer secretes growth factor antagonists Noggin, Chordin, Follistatin BMP-4 (induces ventral structures) Frzb-1, Dickkopf-1, Cerberus Wnts (prevent head formation) Lefty, Activin TGF-β BMP-4 Chordin 50
Signals ① From vegetal cells to induce mesoderm in marginal cells of the animal hemisphere (Vg1, low Xnr)! ② In dorsal vegetal cells Vg1 and β-catenin induce high concentrations of Xnr (Nieuwkoop center). ③ From Nieuwkoop center cells (high Xnr) to marginal cells ( β- catenin) to dorsalize the mesoderm (Spemann organizer)! ④ From Spemann organizer to ventrally adjacent marginal cells to induce mediolateral mesoderm (noggin, chordin)! ⑤ From ventral mesodermal cells to oppose the dorsalizing signal emanating from the Spemann organizer (BMP4 gradient)! 51
Gastrulation in Amphibians 52
Gastrulation in the Frog Embryo 53
Migration of the germlayers in Xenopus 54
Cell intercalation brachyury Epiboly Convergent extension 55
Summary: Gastrulation in Xenopus • Combination of involution, convergent extension and epiboly. • Starts below the equator in the marginal zone on the dorsal side. • Endodermal cells invaginate to form the blastopore. Cells change their shape by apical constriction to form bottle cells. • Mesoderm starts to involute across the dorsal blastopore lip migrating towards the animal pole. • Region of invagination (and the blastopore) widens laterally and ventrally and more ventral cells involute. • At the same time the ectoderm expands towards the vegetal pole by epiboly (convergent extension + division). • Finally the blastopore contracts and closes. The germlayers have been internalized. 56
Determination of the germlayers • Closely linked to axis formation w Animal vegetal axis formed by maternal factors. Animal pole: ectoderm, vegetal pole: endoderm. w Mesoderm formation by “embryonic induction”: ability of cells to determine the embryonic fate of other neighboring cells by cell communication. 57
Temporal specificity of induction 58 Transplantation of the dorsal blastopore lip at different times during gastrulation
Regional specificity of induction 59 Transplantation of tissue from different regions of the neural plate
Signals from the mesoderm regionalize the anterior-posterior axis 60
Signals from the mesoderm regionalize the anterior-posterior axis 61
Wnt inhibitors regionalize the anterior-posterior axis Frzb chordin 62 Developmental Biology, 9e, Figure 7.32
Double gradient model of patterning the Xenopus body plan β-catenin 63
Sequence of events • Four stages of specification in Xenopus: ① Dorsal ventral axis is determined by the point of sperm entry. Radial symmetry is broken by cortical rotation. Opposite to the sperm entry will be dorsal. ② Vegetal cells (Nieuwkoop center) induce cells above them to become the Spemann organizer (mesoderm). ③ Organizer converts neighboring mesoderm into dorsal mesoderm. Invagination through the blastopore establishes the anterior-posterior axis (anterior structures invaginate first). ④ Regional specificities are induced in the neural ectoderm. Fertilization Cortical Rotation Late blastula Tailbud stage 64
Literature • Gilbert/Barresi, Developmental Biology, chapter 11 (11th edition), chapter 8 (10th edition). • Planar Cell Polarity, Vladar EK. & JD. Axelrod, Cold Spring Harbor Perspectives Biol. 2009 • Carroll S., Endless Forms Most Beautiful, Quercus, 2011 65
You can also read