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V. Organizing Power and Axis Formation

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1 V. Organizing Power and Axis Formation
Background Information B. Invertebrates Sea Urchins Snails Tunicates C. Elegans Drosophila melanogaster C. Vertebrates The Frog Zebrafish The Chick Embryo Mammals

2 Part of these processes is the determination of axes in the organism
The first few cleavages may produce little or no directionality to the embryo It starts at varying stages in various animals and can result from different mechanisms

3 Remember our primary axes....
DevBio9e-Fig jpg

4 Figure 5.8 Fate map and cell lineage of the sea urchin Strongylocentrotus purpuratus
DevBio9e-Fig jpg

5 Step 1: Specification of Micromeres
Two Big Changes: Specified to become skeletogenic mesenchyme Specified to become “Organizer” for other cells egg DevBio9e-Fig R.jpg disheveled expression blocks B-catenin degradation

6 -catenin’s job NML All endo and meso ALL All ecto NONE
DevBio9e-Fig R.jpg All ecto NONE

7 Step 2: “Organizing Power”
Secrete Wnt-8 into autocrine loop Wnt-8Blimp-1B-cateninWnt-8 Paracrine “early signal” induces macromeres and vegetal cells to differentiate to vegetal endoderm Unknown signal as of yet Delta-Notch juxtacrine signal induces non-skeletogenic mesenchyme Wnt-8 makes a come-back to induce invagination

8 Axis Determination Anterior-Posterior: Cytoplasmic determinants in the egg cytosol, such as disheveled and B-catenin Left-Right: Nodal expression (TGF-B family member) Dorsal-Ventral: unclear

9 Spiral cleavage in molluscs
The spirally cleaving mollusks have a strong autonomous specification from cytoplasmic determinants in egg. DevBio9e-Fig jpg

10 Step 1: Polar lobe formation
The polar lobe is a cytoplasm outpouching from the egg prior to cleavage It isolates critical determinants into only one of the first cell pair. TF’s associated with the lobe turn CD into “The Organizer” DevBio9e-Fig jpg

11 Decapentaplegic is TGF-B family member used to
Figure Association of decapentaplegic (dpp) mRNA with specific centrosomes of Ilyanassa Decapentaplegic is TGF-B family member used to induce specific cell fates secreted by the Organizer DevBio9e-Fig jpg The Organizer induces mesodermal and endodermal fates in cells that would otherwise remain ectodermal

12 MAP kinase activity activated by D-quadrant snail blastomeres
DevBio9e-Fig R.jpg

13 Figure 5.30 MAP kinase activity activated by D-quadrant snail blastomeres (Part 2)
Normal MAPK Blocked DevBio9e-Fig R.jpg

14 Axis Determination Anterior-Posterior: Cytoplasmic determinants in the lobe Left-Right: Nodal expression (TGF-B family member) Dorsal-Ventral: Cytoplasmic determinants in the lobe

15 Bilateral, Holoblastic Cleavage of the Tunicate
The 8-cell embryo is already autonomously specified for cell fates DevBio9e-Fig jpg

16 Fertilization rearranges cytoplasmic determinants
Figure Cytoplasmic rearrangement in the fertilized egg of Styela partita Fertilization rearranges cytoplasmic determinants DevBio9e-Fig jpg 1. Animal pole cytosol determines ectoderm 2. B-catenin presence determines endoderm (like urchins) 3. Macho-1 in yellow crescent determines muscle cells

17 Wherever B-catenin shows up, endoderm is formed
Figure Antibody staining of -catenin protein shows its involvement with endoderm formation Wherever B-catenin shows up, endoderm is formed DevBio9e-Fig jpg

18 Figure 5.37 Autonomous specification by a morphogenetic factor
Where Macho-1 shows up tail muscle will form DevBio9e-Fig jpg Zinc-finger TF for muscle actin, myosin, TBX-6 Also TF for Snail TF which blocks notochord induction

19 Conditional Specification also plays a role
Integrates with the autonomous specification patterns DevBio9e-Fig jpg

20 Axis Formation accomplished prior to cleavage!
Fertilization rearranges cytoplasmic determinants determines dorsal-ventral DevBio9e-Fig jpg determines anterior-posterior Left-right: unclear but nodal shows it later

21 Rotational, Holoblastic Cleavage in the nematode Caenorhabditis elegans
DevBio9e-Fig R.jpg hermaphrodite

22 Figure 5.42 The nematode Caenorhabditis elegans (Part 2)
Both autonomous and conditional specification at work early on. If cells are separated: P1 will develop autonomously Stem cell divisions are meridional DevBio9e-Fig R.jpg Founder cell divisions are equatorial AB requires input from P lineage

23 Autonomous specification in P1
SKN-1, PAL-1 and PIE-1 TFs from egg as P1 divides these determine daughter fates P lineage becomes “Organizer” Conditional specification in AB P2 secretes Wnt family member MOM-1 to induce endodermal specification in AB lineage P2 use Delta-Notch signals to induce ectodermal fates in AB lineage

24 Axis Determination in C. elegans
Anterior-Posterior axis is determined by egg shape Which end is posterior is determined by sperm (the closest end is back) DevBio9e-Fig jpg Sperm CYK-4 activates egg rho, actin rearrangement causes assymetric first cleavage division

25 AB division leads to both dorsal ventral and left-right axes
Assymetrical division of AB-MS forces AB dorsal and MS ventral DevBio9e-Fig jpg Delta-notch recognition between daughters of AB and MS gives left-right

26 Cytoskeletal rearrangement also pushes P-granules into the germ line
DevBio9e-Fig jpg

27 The cells of the blastula have specified fates in Xenopus.....
DevBio9e-Fig jpg Gastrulation changes all of that, .....afterwards all cell fates are determined!

28 Development of “Organizing Power” at the dorsal blastopore lip
The bottle cells get the ball rolling but the real power is conferred on the first cells through the blastopore. DevBio9e-Fig R.jpg

29 This ability to determine cell fates is called...
The dorsal mesoderm keeps the power to determine other cell’s fates throughout gastrulation: “Spemann’s Organizer” This ability to determine cell fates is called... “Primary Embryonic Induction” DevBio9e-Fig R.jpg

30 The dorsal lip cells first have to become competent to be “Organizer”
Cortical rotation shifts disheveled, GBP, Wnt-11 to dorsal side of embryo DevBio9e-Fig R.jpg The area of Dsh accumulation is seen as a gray crescent in some amphibian embryos

31 β-catenin starts out everywhere in the embryo but only survives GSK3 in the dorsal portion due to Dsh, GBP and Wnt-11 DevBio9e-Fig R.jpg

32 The dorsal vegetal cells of the Nieuwkoop Center turn on “Organizer”
Wnt and Vg-1 (TGF-B family) induce pre-dorsal lip mesoderm DevBio9e-Fig R.jpg FGF needed for all mesoderm

33 Figure Summary of events hypothesized to bring about induction of the organizer in the dorsal mesoderm Nodal DevBio9e-Fig jpg Vg-1

34 Figure 7.23 Vegetal induction of mesoderm (Part 2)
DevBio9e-Fig R.jpg

35 So, what can the “Organizer” do?
Initiate gastrulation Become the notochord and other dorsal mesoderm Dorsalize ventral mesoderm into paraxial mesoderm, somites, etc. Dorsalize the ectoderm into the neural plate and neural tube

36 DevBio9e-Table jpg

37 Figure 7.26 Localization of chordin mRNA
The “Organizer” is induced prior to gastrulation Dorsal blastopore lip Blastopore Dorsal mesoderm DevBio9e-Fig jpg Continues to organize events throughout its own differentiation

38 Interestingly, the primary mechanism is by means of inhibition....
DevBio9e-Fig R.jpg

39 Without the “Organizer” you get mainly skin and gut
Presumably, the Wnt, FGF and RA signals arise from endoderm and ectoderm Without the “Organizer” you get mainly skin and gut DevBio9e-Fig jpg

40 Don’t underestimate the power of the “Organizer”!
Figure Cerberus mRNA injected into a single D4 blastomere of a 32-cell Xenopus embryo induces head structures as well as a duplicated heart and liver DevBio9e-Fig jpg Don’t underestimate the power of the “Organizer”!

41 Axis Formation Dorsal-Ventral: sperm penetration and cortical rotation Anterior-Posterior: migration direction of the dorsal mesoderm Left-Right: nodal expression exclusively on left side of the lateral plate mesoderm

42 Nodal expression causes Pitx2 expression
Nodal and Pitx2 on left Injected on both sides DevBio9e-Fig jpg

43 Relationships between the frog and chick “Organizers”
The hypoblast = dorsal vegetal cells Koller’s sickle = pre-dorsal lip mesoderm Hensen’s node = dorsal blastopore lip and dorsal mesoderm Primitive streak = blastopore

44 Formation of Hensen’s node from Koller’s sickle
DevBio9e-Fig jpg Wnt and FGF from the hypoblast induce Koller’s sickle epiblast

45 Figure 8.10 Induction of a new embryo by transplantation of Hensen’s node (Part 1)
DevBio9e-Fig R.jpg

46 Possible contribution of inhibition of BMP signaling
DevBio9e-Fig jpg Appears to be similar to the frog....

47 In the chick, the hypoblast plays a large role much like the frog endoderm
DevBio9e-Fig R.jpg

48 Anterior-Posterior axis parallels the rotation inside the shell
Figure 8.8 Specification of the chick anterior-posterior axis by gravity DevBio9e-Fig jpg Anterior-Posterior axis parallels the rotation inside the shell

49 Rostral-Caudal (Anterior-Posterior) axis extension in chick embryos
The combination of positional specification, complex signaling and TF (Hox, etc.) expression is thought to cause axis. DevBio9e-Fig jpg

50 Left-right asymmetry in the chick embryo
This is farther along Nodal and Pitx2 again are implicated DevBio9e-Fig jpg

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