1. BIO624: Developmental Genetics GASTRULATION PART I Suk-Won Jin, Ph.D.

2. “It is not birth, marriage or death, but gastrulation which is truly the most important time in your life.” Lewis Wolpert, 1986

3. Overview of next three lectures 1.1 st Class: Introduction to gastrulation 1.Basic concept 2.Germ layer formation 2.2 nd Class: Interaction of signals 1.Nodal and its relatives 2.FGF and its relatives 3.Other minor players 3.3 rd Class: Formation of body axis 1.Body axis formation 2.Breaking the asymmetry

4. Overview of next three lectures 1.1 st Class: Introduction to gastrulation 1.Basic concept 2.Germ layer formation 2.2 nd Class: Interaction of signals 1.Nodal and its relatives 2.FGF and its relatives 3.Other minor players 3.3 rd Class: Formation of body axis 1.Body axis formation 2.Breaking the asymmetry

5. Overview of today’s lecture 1.Basic concepts and cell movement 2.Gastrulation in different species 3.Mesoderm induction

6. Overview of today’s lecture 1.Basic concepts and cell movement 2.Gastrulation in different species 3.Mesoderm induction

7. Basic concepts of gastrulation

8. What is gastrulation? Gastrulation is the event to create different cell types

9. What is the goal of gastrulation?

10. Historical perspectives 1.Morphological features first described by Rusconi and Dutrochet, and again by Ernst von Baer. 2.The actual name of gastrulation was first used by Ernst Haeckel in 1872. 3.The germ layer concept and the names of ectoderm, mesoderm, and endoderm were named by Sir Ray Lankester.

11. Fate map of three germ layers

12. How to generate three layers?

13. Making inner layers Invagination Involution ingression

14. Covering wider areas epiboly intercalation Convegent extension

15. Cell movement in gastrulation 1.Invagination: Inward movement of a sheet of cells 2.Ingression: Individual cells migrate from the epithelial sheet as mesenchymal cells 3.Involution: A sheet of cells roll over to generate additional layer of cells inside 4.Epiboly: a sheet of cells spread and become thinner 5.Intercalation: Two adjacent sheets of cells become one sheet 6.Convergent extension: specialized intercalation that occurs in certain direction

16. 1.Ectoderm, e.g. epidermis, neural plate, neural crest and cartilage 2.Mesoderm, e.g. notochord, muscle, kidney, blood, heart, blood, and vasculature 3.Endoderm, e.g. pharynx, liver, stomach, and pancreas The three germ layers

17. Vertebrate embryonic lineages

18. 1.Ectoderm, e.g. epidermis, neural plate, neural crest and cartilage 2.Mesoderm, e.g. notochord, muscle, kidney, blood, heart, blood, and vasculature 3.Endoderm, e.g. pharynx, liver, stomach, and pancreas The three germ layers

19. Overview of today’s lecture 1.Basic concepts and cell movement 2.Gastrulation in different species 3.Mesoderm induction

20. Specification of germ layers: Evolutionary perspectives Deuterostome Protostome blastopore Anterior Posterior Anterior Posterior

21. Specification of germ layers in Drosophila, a protostome

22. Specification of germ layers in Sea urchin

23. Specification of germ layers in fish and amphibians

24. Formation of blastopore in amphibians

25. Coordinated movement of ectoderm and mesendoderm

26. Specification of germ layers in birds and mammals

27. Specification of germ layers: ingression at the node

28. Specification of germ layers in mammalian embryos

29. Specification of germ layers

30. Comparison on the Specification of germ layers in vertebrates

31. Overview of today’s lecture 1.Basic concepts and cell movement 2.Gastrulation in different species 3.Mesoderm induction

32. How mesendoderm is specified? Pieter Nieuwkoop’s experiments and mesodermal induction

33. Mesendoderm C. elegans zebrafish

34. 1.Induction is a process by which one group of cells signals to a second group of cells altering their ultimate developmental state. 2.Competency is an Ability of a group of cells to respond to an inducing signal. competence is directly related to the age of a tissue during development. Induction and Competency

35. 1.A signal generated by the inducing cell. 2.A receptive system that directly or indirectly controls gene expression in the responding cell, i.e. competence. Two components of inductive system

36. Nieukoop’s induction experiments I All layers Ectoderm endoderm all layers

37. Conclusions from the experiment 1.Mesoderm is induced when endoderm and ectoderm are juxtaposed. 2.Signals arise from endoderm and received by ectoderm; i.e. the ectoderm is induced to form mesoderm.

38. Nieukoop’s induction experiments II Ectoderm endoderm all layers all layers

39. Conclusions from the experiment 1.Mesoderm is induced when endoderm and ectoderm are juxtaposed. 2.Signals arise from endoderm and received by ectoderm; i.e. the ectoderm is induced to form mesoderm. 3.The signal from the endoderm is secreted. 4.Signal only acts over a short distance. 5.Ectoderm is only competent for this signal for a short period during development.

40. 1.How is the “Signal” propagated? 2.How far is the signal propagated? Remaining questions

41. How is the signal propagated? Diffusion Relay

42. Methods to Identify the Mesoderm Inducing Signal 1.Cut and dump: Place animal cap tissue in the presence of potential mesodermal inducer-score for expression of mesodermal markers and cellular movements. 2.Functional screen: Inject embryos with potential gene or cDNA library, cut animal caps score as above. 3.Expression screen: Differential screen between ectoderm and endoderm. Alternatively Pick cDNA from tissue specific library and perform in situs.

43. Methods to Identify the Mesoderm Inducing Signal 1.Cut and dump: Place animal cap tissue in the presence of potential mesodermal inducer-score for expression of mesodermal markers and cellular movements. 2.Functional screen: Inject embryos with potential gene or cDNA library, cut animal caps score as above. 3.Expression screen: Differential screen between ectoderm and endoderm. Alternatively Pick cDNA from tissue specific library and perform in situs.

44. Animal cap assay

45. Methods to Identify the Mesoderm Inducing Signal 1.Cut and dump: Place animal cap tissue in the presence of potential mesodermal inducer-score for expression of mesodermal markers and cellular movements. 2.Functional screen: Inject embryos with potential gene or cDNA library, cut animal caps score as above. 3.Expression screen: Differential screen between ectoderm and endoderm. Alternatively Pick cDNA from tissue specific library and perform in situs.

46. What is Brachyury? 1.It is expressed throughout the mesoderm from its formation. 2.Immediate early response factor to mesoderm inducing signals in a dose dependent manner 3.It is required for the proper mesoderm formation 4.Over-expression of Brachyury is sufficient to convert ectoderm to mesoderm 5.Evolutionarily conserved.

47. Brachyury Expression in development Mouse Xenopus

48. 1.Family of transcription factors that has T- box domain. 2.Function as ‘Classical Transcription Factors’ 1.Localize to the nucleus 2.Bind DNA in a sequence specific fashion 3.Regulate transcription of neighboring genes 3.Multiple family members are expressed in specific domains 4.Mutated in development and disease Brachyury is a T-Box protein

49. Methods to Identify the Mesoderm Inducing Signal 1.Cut and dump: Place animal cap tissue in the presence of potential mesodermal inducer-score for expression of mesodermal markers and cellular movements. 2.Functional screen: Inject embryos with potential gene or cDNA library, cut animal caps score as above. 3.Expression screen: Differential screen between ectoderm and endoderm. Alternatively Pick cDNA from tissue specific library and perform in situs.

50. Subtractive Hybridization e.g. Veg Identify endoderm specific signal

51. Criteria for Mesodermal Inducer Based on Nieuwkoop’s Studies 1. Gene or protein must be expressed and active in the correct time and place. 2. Protein must be secreted and act at a distance. 3. Protein must be necessary and sufficient to induce mesoderm in ectoderm in a protein synthesis independent fashion.