Early Embryology Kristine Krafts, M.D.

1. Early EmbryologyKristine Krafts, M.D.

2. Embryology Lecture Objectives • In general, what happens in the first and second phases of the embryonic period? • What happens during week 1 and 2? • Describe the main events in gastrulation. • How does the neural tube form, and what happens to it?

3. Embryology Lecture Objectives • What is the fate of the endoderm, mesoderm and ectoderm? • What are the pharyngeal arches? What structures arise from each arch, groove, and cleft? • What structures do neural crest cells form? major stuff forms from pouches?

4. Embryology Lecture Outline • General overview of prenatal development • Embryonic period phase 1 • Formation of bilaminar disk • Formation of trilaminar disk (gastrulation) • Embryonic period phase 2 • Formation of neural tube • Differentiation of mesoderm • Folding of embryo • Formation of pharyngeal arches

5. Embryology Lecture Outline General overview of prenatal development

6. Prenatal Development Embryo Fetus Fertilization Phase 1 Cellular proliferation and migration Phase 2 Differentiation of internal & external structures Phase 3 Growth and maturation 0 1 2 3 4 5 6 7 8 40 Delivery

7. This YouTube video is awesome at explaining early embryonic development: http://www.youtube.com/watch?v=rN3lep6roRI

8. Embryology Lecture Outline General overview of prenatal development Embryonic period phase 1

9. Prenatal Development Embryo Fetus Fertilization Phase 1 Cellular proliferation and migration Phase 2 Differentiation of internal & external structures Phase 3 Growth and maturation 0 1 2 3 4 5 6 7 8 40 Delivery

10. Embryology Lecture Outline • General overview of prenatal development • Embryonic period phase 1 • Formation of bilaminar disk

11. Week 1: Differentiation of Morula into Blastocyst Morula Blastocyst

12. Week 2: Formation of Bilaminar Germ Disk epiblast hypoblast

13. Embryology Lecture Outline • General overview of prenatal development • Embryonic period phase 1 • Formation of bilaminar disk • Formation of trilaminar disk (gastrulation) "It is not birth, marriage, or death, but gastrulation which is truly the most important time in your life.” - Lewis Wolpert (1986)

14. Gastrulation: formation of primitive streak primitive streak primitive node epiblast

15. Gastrulation: movement and differentiation of epiblast cells Bilaminar germ disk Primitive streak Epiblast Hypoblast Endoderm Ectoderm Mesoderm Endoderm Epiblast cells give rise to all three germ cell layers!(the hypoblast does NOT turn into endoderm)

16. Gastrulation: formation of notochord The notochord is super important because it tells the three layers what to do next.

17. Embryology Lecture Outline • General overview of prenatal development • Embryonic period phase 1 • Formation of bilaminar disk • Formation of trilaminar disk (gastrulation) • Embryonic period phase 2

18. Prenatal Development Embryo Fetus Fertilization Phase 1 Cellular proliferation and migration Phase 2 Differentiation of internal & external structures Phase 3 Growth and maturation 0 1 2 3 4 5 6 7 8 40 Delivery

19. Embryology Lecture Outline • General overview of prenatal development • Embryonic period phase 1 • Formation of bilaminar disk • Formation of trilaminar disk (gastrulation) • Embryonic period phase 2 • Formation of neural tube

20. Formation of Neural Tube

21. Formation of the Neural Tube

22. Embryology Lecture Outline • General overview of prenatal development • Embryonic period phase 1 • Formation of bilaminar disk • Formation of trilaminar disk (gastrulation) • Embryonic period phase 2 • Formation of neural tube • Differentiation of mesoderm

23. Differentiation of mesoderm Paraxial mesoderm forms bones and muscles of most of the body (except head), pharyngeal arches, and connective tissue Intermediate mesoderm forms urogenital system Lateral plate mesoderm forms hematopoietic system, heart, pharyngeal arches, and connective tissue

24. Central nervous system Cranial nervesBones and connective tissue of head Pharyngeal arches Neuroectoderm Urogenital system Neural crest Intermediate plate mesoderm Know this! Lateral plate mesoderm Surface ectoderm Paraxial mesoderm Epidermis Heart Hematopoietic system Pharyngeal arches Connective tissue Bones of most of the body (everything except the head)Muscles of the body and head Pharyngeal arches Connective tissue Endoderm Lining of GI tract

25. Clinical Correlation: Treacher Collins Syndrome Neural crest cells don’t migrate properly to the facial region. Structures derived from 1st and 2nd pharyngeal arches don’t develop properly.

26. Embryology Lecture Outline • General overview of prenatal development • Embryonic period phase 1 • Formation of bilaminar disk • Formation of trilaminar disk (gastrulation) • Embryonic period phase 2 • Formation of neural tube • Differentiation of mesoderm • Folding of embryo

27. Folding of the Embryo

28. Lateral Folding of the Embryo Lateral plate mesoderm splits in two. One part remains near the ectoderm. The other part follows the endoderm.

29. Lateral Folding of the Embryo Endodermal layer (lined by mesoderm) bends, the edges reaching towards each other, meeting in front to form the gut. Ectodermal layer (lined by mesoderm) grows forward, reaches around the gut, and zips up the front to form the anterior body wall.

30. Lateral Folding of the Embryo Back, with ectoderm overlying neural tube Gut (lined by endoderm, surrounded by mesoderm) Amnion (and amniotic cavity) comes along for the ride, eventually surrounding entire embryo. Anterior thoracic wall (mesoderm covered with ectoderm)

31. Head-Tail Folding of the Embryo

32. Head-Tail Folding of the Embryo Day 24 Before folding Stomatodeum Day 26 Day 28

33. What happens to the neural tube? It turns into the brain(forebrain, midbrain and hindbrain) and the spinal cord. • Hey, what’s that mesoderm doing there? It’s forming: • Somatomeres (which turn into muscles of the head and neck) • Somites (which turn into the bones and muscles of the back).

34. Embryo, day 23-26 Neural tube still open Neural tube closed Somites Neural tube still open

35. Embryo, day 26-30 Neural tube closed Somites Neural tube still a little bit open

36. Embryo, day 26-30

37. Embryology Lecture Outline • General overview of prenatal development • Embryonic period phase 1 • Formation of bilaminar disk • Formation of trilaminar disk (gastrulation) • Embryonic period phase 2 • Formation of neural tube • Differentiation of mesoderm • Folding of embryo • Formation of pharyngeal arches

38. Formation of Pharyngeal Arches

39. Pharyngeal Arch Anatomy Arches have a core that’s derived from lateral and paraxial mesoderm and neural crest cells. They are covered with ectoderm on the outside and lined by endoderm on the inside. Each arch has its own cartilage, artery, and nerve.

40. Formation of Pharyngeal Arches 25-day-old embryo 35-day-old embryo No arches yetBuccopharyngeal membrane intact Arches and pouches nicely formed Mouth now open to esophagus

41. 26-day-old embryo showing stomatodeum and first two arches

42. Embryo, day 26-30 Frontal prominence 1 2 3 Somites

43. Optic placode Nasal placode Stomatodeum Maxillary 1 Mandibular Heart 2 3 4 Somites First four arches in a 32-day-old embryo

44. Pharyngeal arches, grooves, pouches

45. Wait, are they branchial or pharyngeal arches? Fish have branchial (“gill”) arches, with slits in between for water flow. Humans don’t have gills. So our arches are called pharyngeal.

46. Groove/cleft Arch Pouch Pharyngeal arches, grooves/clefts and pouches

47. Know the stuff in red! Important! Meckel’s cartilage indicates where the mandible will develop – but it does not turn into the mandible!

48. What happens to the pouches and grooves? • 1st groove and pouch -> ear stuff • Rest of grooves disappear • 2nd pouch obliterated by tonsil • 3rd pouch -> inferior parathyroid, thymus • 4th and 5th pouches -> superior parathyroid, ultimobranchial body

49. Head and Neck Anomalies From Improper Groove Closure Pharyngeal sinuses and cysts Auricular sinuses and cysts

50. Prenatal Development Embryo Fetus Fertilization Phase 1 Cellular proliferation and migration Phase 2 Differentiation of internal & external structures Phase 3 Growth and maturation 0 1 2 3 4 5 6 7 8 40 Delivery We covered this in this lecture.