1. Gastrulation It is not birth, marriage or death, but gastrulation, which is truly the most important event in your life Lewis Wolpert

2. 17 hours in the life of a zebrafish embryo

3. Blastula Stages 1- cell2-cell 4-cell 8-cell 32-cell 512-cell 1000-cell Transcription begins sphere dome 30% epiboly 4 2/3 hrs

4. Gastrula Stages 50% germ ring shield 75%90% bud 10 hr

5. Somitogenesis 3-somite 6-somite18-somite 26-somite 22 hr

6. Zebrafish development

7. Early development ?

8. Three germ layers Side view top view Skin & nervous system Muscle & cardiovascular Gut and liver

9. Gastrulation simplified

10. Involution Mesoderm & endoderm inside Ectoderm outside

11. Dorsal-Ventral Side view top view dorsal ventral

12. Involuting cell movements during gastrulation

13. Convergence-Extension convergence extension

14. Convergence-extension

15. Cell fates Dorsal view Cross section dorsal

16. Convergence-extension

19. Summary Formation of germ layers Convergence- extension Formation of tissue boundaries Cleavage

20. Two major Questions How are the germ layers established? What determines polarity?

21. Signaling from the YSL

22. Nodal mutant wild-type sqt;cyc -/- No endoderm No head & trunk mesoderm

23. Germ layer patterning YSL signal Sqt and Cyc Endoderm Head and Trunk Mesoderm Tail mesoderm

24. Dorsal-ventral patterning shield

25. Bozozok

26. The Organizer

27. Bmp promotes ventral fates Organizer signals are BMP binding factors: Noggin, Chordin Bmp signals promote ventral fates (skin, ventral mesoderm) and oppose dorsal fates (neural, dorsal mesoderm)

28. Mutants in the dorsal-ventral pathway Snailhouse (snh) = bmp7 mutant Chordino (din) = chordin mutant

29. Bmp regulates cell movement High Bmp Low Bmp

30. Summary

31. Xenopus egg

33. Cleavage of Xenopus embryo Midblastula transition (MBT) occurs at 12th cleavage = start of zygotic transcription Animal pole view

34. Gastrulation in Xenopus vegetal pole view

35. Fate map

36. Cross sectional view

37. Gastrulation Gastrulation begins dorsally and spreads ventrally

38. Gastrulation in Xenopus

40. Two major Questions How are the germ layers established? What determines polarity?

41. Nieuwkoop’s experiment 1967

43. Nodals Xnrs induce mesoderm (and endoderm)

44. Why are the Xnrs expressed vegetally?

45. VegT VegT is a transcription factor

46. VegT

47. VegT activates the transcription of the Xnr genes

48. VegT

49. Formation of the dorsal side

51.  -catenin is a transcriptional activator

52. Creation of the early gastrula

56. The role of the Organizer

57. Organizer patterns the mesoderm and converts epidermis to neural

58. What are the Organizer signals? Inhibitors of Bmp signaling: –Noggin, Chordin Inhibitors of Wnt signaling: –Dickkopf, Frzb

59. Function of Bmp BMP promotes ventral fates O Some organizer signals (Chordin, Noggin) inhibit BMP signals and promote dorsal fates