Axis Formation and Gastrulation II

1. Axis Formation and Gastrulation II

2. The Xenopus Oocyte is Asymmetric “Animal” VegT Vg1 “Vegetal” Maternal Determinants VegT: Transcription factor -Promotes “vegetal” identity (endoderm) -Activates mesoderm inducers (nodals/TGF-ß’s) Vg1: Another nodal/ TGF-ß’s

3. Wnt11 VegT Wnt11 VegT Maternal Determinants VegT: Transcription Factor Promotes “vegetal” identity (endoderm) Wnt11: Signaling Ligand Promotes Dorsal identity Sperm Entry Point Determines D/V Axis in Xenopus “Animal” An Cortical Rotation D V “Vegetal” Veg

4. Specifying the Germ Layers Mesoderm Inducers are TGF-ß Family Members (Vg1, Nodals (Xnr’s), Activin) -higher in dorsal mesoderm Wnt11

5. Specifying The Spemann-Mangold Organizer V D Wnt11 Nodals (TGF-ß’s)

6. Transplant Dorsal Blastopore Lip Donor: Pigmented (newt) V D Host: Unpigmented (newt) V D Spemann and Mangold, 1924

7. V D Form Second Body Axis V D Spemann and Mangold, 1924

8. Also two sites of gastrulation Normal Body Axis V D Second Body Axis V D Second Body Axis: Dorsal Mesoderm (Notocord)--Donor Cells Neural Plate--Host Cells Other mesoderm--mix of Donor and Host Donor tissue “organized” host tissue to take on new cell identities Blastopore lip = an Organizer Spemann and Mangold, 1924

9. Specifying The Speman-Mangold Organizer V D V D wnt11 nodals The Organizer: -Patterns the mesoderm along the D/V axis -Determines the site of gastrulation -Allows for induction of the nervous system (neuroectoderm)

10. Fate Map of the Xenopus Blastula Xenopus An V D Veg

11. TGF-ß (BMP, Dpp) Frog Fly Xolloid/Tolloid D Chordin, Sog xolloid sog V Note: BMP4 is different type of TGF-ß than nodals -nodals form early gradient that is high in dorsal regions -BMPs form later gradient that is high in ventral regions Hypothesis: Neural Induction The dorsal lip secretes a signaling molecule that patterns the mesoderm and induces neural plate specification Search for molecules that can mimic organizer activity Find secreted INHIBITORS of TGF-ß ligands (Chordin, Noggin, Follistatin) Neural is DEFAULT state and TGF-ß signaling is required to INDUCE ectoderm

12. Fish and Frog Embryos Appear Very Different Yolk Cell

13. Fish and Frogs Do Things Similarly -The oocyte has an Animal-Vegetal Axis -The Wnt pathway initiates the D/V Axis (unclear how--doesn’t seem to be sperm) -BMPs and Wnts pattern the mesoderm V D V D BMPs V D Organizer (Shield) V D nodals V D Wnts -Nodal (and FGF) signaling specifies the mesoderm (and endoderm) with a dorsal bias -Dorsal mesoderm makes the organizer (shield)

14. Fate Mapping (Lineage Tracing) to Investigate Cell Identity and Developmental Potential Xenopus An V D Veg Zebrafish

15. It is not birth, marriage or death, but gastrulation, which is truly the most important time in your life.               - Lewis Wolpert (1986) Gastrulation

16. Moving cells around Spreading tissues out Making tissues longer Convergence/extension Cell Movements Relevant for Gastrulation Getting cells inside

17. Xenopus Gastrulation Initiates at the Organizer (aka Dorsal Blastopore Lip)

18. Xenopus Gastrulation

19. Zebrafish Fate Map Schier and Talbot, 2005

20. Zebrafish Gastrulation

21. Early Zebrafish Cleavages

22. Zebrafish Epiboly Transforms the Embryo Into a Hemisphere Solnica-Krezel, 2006

23. Cells Involute and Ingress to Form the Dosal Shield Cells Migrate Anteriorly After Involution Solnica-Krezel, 2002

24. Convergence/Extension Also Contributes to A/P Axis Formation Extension of cells along the A/P axis Convergence of lateral cells toward the midline of the embryo Convergence/Extension Requires the PCP Pathway Solnica-Krezel, 2006

25. The Anterior-Posterior Axis is Also Coupled to Gastrulation The developmental potential and inducing properties of cells in the dorsal blastopore lip change with time: -Cells in the lip early become anterior mesoderm and induce anterior neural tissue -Latter cells become posterior and induce more posterior neural structures -A gradient of Wnt activity is high in posterior and low in anterior

26. Chick Embryos Look Different but Act Similarly The Chick Embryo Forms as a Flattened Disc of Cells On the Yolk The primitive streak/node is the organizer and expresses goosecoid and chordin The primitive streak elongates with the node/organizer at the leading edge The posterior marginal zone initiates primitive streak/organizer formation and expresses Vg1 and Wnt8

27. Hensen’s Node is the Avian Organizer Similar to Xenopus blastopore and Fish dorsal shield in terms of both patterning and gastrulation movements The node can induce nervous system development and a secondary axis when transplanted The node expresses BMP antagonists, like the Xenopus and Zebrafish organizers

28. Fish Frog Mouse Bird Blastopore Lip Henson’s Node Node Dorsal Shield All Vertebrate Embryos Have a Spemann-Mangold Organizer Organizer Bird

29. Chick Gastrulation Movements -In the Node and Primitive Streak, cells delaminate from the epiblast and ingress to form the endoderm and mesoderm -Epiblast cells continue to enter the streak from lateral regions -Once they have ingressed in the streak, newly formed endoderm and mesoderm move laterally again

30. Gastrulation and Patterning Follow the Same Logic as Fish and Frogs As with fish and frogs, the first cells gastrulating through the chick organizer (node) become anterior endoderm and prechordal plate mesoderm The next cells through will form notochord These first cells also induce the nervous system from the overlying ectoderm Cells gastrulating through more posterior primitive streak become other mesoderm and endoderm derivatives BUT, note that the organizer MOVES as the primitive streak first advances and then regresses More posterior regions of notocord are formed from cells migrating through node as it regresses

31. The A/P Axis is “laid down” During Primitive Steak Regression This is in contrast to the active extension of the A/P axis seen in frog and fish embryos (although some active mechanisms do further elongate the chick A/P axis) Consequently, posterior development is delayed relative to anterior development “Laying down” the notocord during regression of Henson’s node

32. Human Embryos Gastrulate Like Chick Embryos

33. Different Embryos, Common Themes

34. (After Green et al., 1992) Activin (TGF-ß): A morphogen for the mesoderm Animal cap assay (After Asashima, 1994)

35. Lineage Analysis and Fate Mapping Goal: Identify which cells in the early embryo give rise to particular cells in the later embryo or adult

36. Lineage Analysis and Fate Mapping Approaches: 1) Just watch closely

38. Lineage Analysis and Fate Mapping Approaches: Just watch closely Cell transplantation Use a Lineage Tracer -Inject into single cells or few cells -Activate in single cells or few cells

39. Fate Mapping by Single Cell Injection of Lineage Tracer e.g. Woo and Fraser, 1995

40. Photo-activatable GFP Absorbance spectrum before PA Absorbance spectrum after PA Patterson and Lippincott-Schwartz, 2002 Laser Activation of Lineage Tracer “Caged” fluorescein: e.g. Jopling and den Hertog, 2005

41. Lineage Analysis and Fate Mapping Approaches: Just watch closely Cell transplantation Use a Lineage Tracer -Inject into single cells or few cells -Activate in single cells or few cells 4) Genetic labeling -Random labeling -Labeling a specific lineage

42. Genetic Mosaics Genetic mosaics can be created by mitotic recombination induced by X-rays or a site-specific recombinase

43. Genetic Tracking of Specific Lineages Question: What do cells that express YFG at time X develop into? loxP Actin Promoter STOP GFP YFG CRE-ER Add tamoxofen at time X Cells expressing YFG at that time permanently labeled with GFP

44. Some things to worry about: -Can you identify and label your cells of interest? -Does your labeling technique interfere with normal development? -Is your lineage tracer stable over time? -Is your lineage tracer diluted by cell division?