1. Axis Formation in Amphibians
How does a radially symmetrical egg develop the axes of bilateral symmetry?
dorsal
posterior
anterior
ventral

2. SP
Gray crescent

3. Cortical rotation L R

5. 2-cell
Nuclei stained Nuclei not stained
dorsal ventral
β-catenin (TF) is localized in the nuclei of dorsal cells due to cortical rotation.
gastrula

7. Canonical Wnt Pathway

8. Dsh binds to and blocks GSK3. GSK3 degrades β-catenin.

9. β-catenin activates a transcription factor cascade that establishes the inducing power of the dorsal lip.

10. Inject goosecoid mRNA
Inject dominant inactive GSK3 in both blastomeres at 2-cell stage. GSK3 suppresses dorsal specification of ventral half.

11. Unequal radial holoblastic cleavage

12. Fate Map of Xenopus laevis
Where is the
mesoderm?

13. Fate mapping
Walter Vogt, 1929

14. Gastrulation movements
Formation of endodermal bottle cells
Convergent extension
Crawling of lead cells of sheet

16. Epiboly
Phayngeal endoderm moves to blastocoele ceiling and becomes leading (crawling) edge of forming archenteron.

17. Fibronectin stained yellow
Early gastrula Late gastrula
Injection with integrin binding fibronectin peptide (competitive inhibition)

19. Epiboly of ectoderm

21. Epiboly: cell division and convergent extension

22. The gray crescent is necessary for axial development
baustück

23. The dorsal lip of the blastopore (the organizer) derives from the gray crescent and it has the power to ORGANIZE a second embryo. Thus is primary induction. Spemann and Mangold, 1924

24. Pigmented markers showed the NT developed from host ectoderm

25. Depletion of EP-cadherin mRNA
Why a blastocoele?
1. Keep animal cap away from inducing endoderm
2. Provide space into which involuting endoderm and mesoderm can move.

26. Where does the organizer come from
Where does the organizer come from? The induction of the mesoderm by endoderm

27. Pieter Nieuwkoop: Nieuwkoop center

28. Mesodermal induction: Nieuwkoop Center acts in blastula

29. Maternal mRNA for Veg1 (white) tethered to the vegetal cortex

30. (Xenopus nodal related) TGF-β paracrine factor TGF-β paracrine factor
Maternal mRNA for TF

32. The Organizer Initiates gastrulation (bottle cells)
Autonomously differentiates into the notochord
Dorsalizes the lateral mesoderm to form somites.
Dorsalizes the overlying ectoderm to form a neural tube. The search for the inducer.

33. Is neural induction permissive, rather than instructive?

35. BMP induces ventral mesoderm and epidermis
BMP induces ventral mesoderm and epidermis. Neural ectoderm and dorsal mesoderm and endoderm require that BMP be blocked! This is what the inducers do! Does this explain Holtfreter’s surprising results?
Nodal
Review figure A

36. Find it: Chordin mRNA expression
Chordin is a paracrine factor that blocks BMP-2 and BMP-4 from binding its receptor

37. Morpholino against chordin (and noggin and follistatin) Control
Sox2 expression
(neural marker)
DevBio9e-Fig jpg
Morpholino against BMP-2, 4, and 7)

38. Noggin mRNA can “rescue” UV-irradiated embryos (belly pieces)

40. Regional specification
Saxén and Toivonen (film)

41. Saxén and Toivonin. Neuralizing (head) and mesodermalizing (trunk and tail) gradients
Please watch Saxen interview