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The First Three Weeks of Human Embryogenesis

The First Three Weeks of Human Embryogenesis. Department of Histology, Cytology and Embryology Kharkiv State Medical University. Week 1-3:. Early Stages : 1. Fertilization 2. Cleavage 3. Gastrulation 4. Formation of the embryo body Late stages: Histogenesis, Organogenesis. Week 1.

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The First Three Weeks of Human Embryogenesis

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  1. The First Three Weeks of Human Embryogenesis Department of Histology, Cytology and Embryology Kharkiv State Medical University

  2. Week 1-3: Early Stages: • 1. Fertilization • 2. Cleavage • 3. Gastrulation • 4. Formation of the embryo body • Late stages: Histogenesis, Organogenesis

  3. Week 1 • 1.Fertilization – is the fusion of the sperm and ovum = Zygote formation

  4. Fertilization. Cleavage Cleavage 2 cells stage 3-5 cells stage Morula Blastula . uterus . uterine tube Implantation Day 6 - 8 Fertilization (positive rheotaxis, gyno-androgamones, capacitation, acrosomal reaction) = zygote

  5. Week 1 • 2.Cleavage – is the division of the zygote in the uterine tube = Blastula formation (2 cell stage – 4 cell stage …16-32 cell stage (morula) – blastocyst) • During cleavage the germ is moved to the uterus

  6. Inner Cell Mass (embryoblast) Trophoblast Blastocoele At the end of the cleavage outer cells (trophoblast) involve nutritive fluid, which forms cavity, moving inner cells to the periphery (embryoblast). Blastula is formed.

  7. Week 2. 3. Gastrulation Gastrulation by the delamination and migration leads to formation of three germ layers:ectoderm, endoderm and mesoderm, and axial organs: notochord, neural tube, somites

  8. Early Gastrulation.At the beginning of gastrulation germ sinks into the uterine wall – implantation. Gastrulationbegins from delamination – division of embryoblast into two germ layers -ectoderm and endoderm, forming embryonic disc and two sacs – ectoblast and endoblast

  9. Trophoblast Ectoblast Ectoderm Endoderm Endoblast Delamination Embryonic disc:

  10. Migration - extraembryonic mesoderm formationFrom embryonic disc cells begin to migrate which form extraembryonic mesoderm, surrounding upper and lower sacs,and underlying trophoblastAs a result appear amnion, yolk sac and chorion(extraembryonic organs)

  11. Week 2 Extraembryonic Mesoderm Chorion Trophoblast Ectoderm Amnion Endoderm Yolk Sac

  12. Late gastrulation – migration continues within embryonic disk and leads to formation of the embryonic mesoderm (third germ layer) andaxial organs (neural tube, notochord and somites - 43, 44 pairs)

  13. Transverse section. Moving ectodermal and endodermal cells form group of cells = primitive streak Amniotic Cavity Embryonic mesoderm Ectoderm Primitive streak Yolk Sac Endoderm

  14. Notochord is formed from invaginated ectoderm and primitive streak

  15. Notochord formation (longitudinal section)

  16. Development of the Neural Tubeby the invagination of ectoderm

  17. Development of the Neural Tube Neural groove

  18. Development of the Neural Tube

  19. Development of the Neural Tube

  20. Development of the Neural Tube Surface Ectoderm Neural Crest Neural Tube

  21. Neural tube formation

  22. Neural tube formation

  23. Somites locate between ectoderm and endoderm, arise from primitive streak and notochord and • consist of mesoderm

  24. Axial organs –neural tube, notochord, somites (mesoderm) Neural tube Somite Notochord

  25. Formation of the Body AxisLeft – Right Sidedness FGF8 Nodal Lefty PITX2

  26. Left – Right sidedness • Situs inversus • 20% (Kartagener syndrome) • Dysfunctional cilia • Respiratory problems • Male fertility problems

  27. 4. Formation of the embryo body (20 days) Body flexion, head and tail folds formation. Amnion accumulates fluid and increases, yolk sac decreases. Formation of a gut, allantois

  28. Body flexion formation

  29. Body flexion formation(Gut formation. The upper part of yolk sac forms tube inside the forming body = gut - primitive digestive tube gut

  30. Body flexion formation (Gut. Allantois – is extention of hindgut) Allantois FUTURE PLACENTA

  31. Body formation hindgut foregut midgut Cloacal Plate Oral Plate Allantois Heart • Yolk Sac

  32. Body formation Chorionic plate participates in the formation of placenta Amniotic cavity Uterine Wall Chorionic Cavity Yolk Sac and allantois participate in the formation of ambilical cord

  33. What should we study by heart ?

  34. EXTRAEMBRYONIC ORGANS chorion • Allantois • FUTURE PLACENTA Amnion Yolk sac

  35. Extraembryonic organs (supportive, nutritive) • Amnion – protective bag of water • Yolk sac – gut, germs of gametes first blood vessels, cells • Allantois – urinary bladder • Chorion – protection, hormones, placenta • Placenta – main nutritive, protective, hormonal

  36. Differentiation of GERM LAYERS: 1. Differentiation of Ectoderm A. Surface Ectoderm B. Neural Tube 2. Differentiation of Endoderm A. G.I. Tract B. Respiratory Tree C. Endocrine glands 3. Differentiation of Mesoderm A. Somites-dermatome, myotome, sclerotome B. Intermediate- nephrotome C. Lateral-splanchnotome D. Mesenchyme

  37. Surface Ectodermdifferentiates to skin, its derivatives, oral, rectal epithelium, corneal epithelium, tooth enamel amnion ectoderm proctodeum stomatodeum

  38. Ectoderm forms neuroectoderm –neural tube(neurons and neuroglia of the brain and the retina) and neural crests(nerve ganglia, neuroglia, adrenal medulla, melanocytes, APUD-system).---15-20 weeks Surface Ectoderm Neural Tube Neural Crest

  39. Endodermdifferentiates toepithelium of the stomach, intestine, liver, pancreas, respiratory, endocrine systems -- 3-4 weeks - gut endoderm gut

  40. Endoderm(gut, allantois) foregut hindgut midgut Cloacal Plate Heart Oral Plate Stomatodeum Allantois • Yolk Sac

  41. Mesoderm -formation of the first blood vessels in the wall of yolk sac and allantois blood vessels

  42. Mesoderm Ectoderm Amniotic Cavity (Somite) Intermediate mesoderm (nephrotome) Lateral plate mesoderm (somatopleuric, splanchnopleuric mesoderm) Yolk Sac Notochord Endoderm

  43. dermatome - dermis of skin myotome - muscles, sclerotome - skeleton, except skull Somite Mesoderm urogenital system including kidneys, gonads, ducts, and accessory glands Intermediate Mesoderm. Nephrotome Lateral Mesoderm - serous membranes of pleura, pericardium and peritoneum Mesenchyme (loose part) – connective tissue of viscera and limbs, blood and lymph cells, cardiovascular and lymphatic systems

  44. Chorionic plate participates in the formation of placenta Amniotic cavity Uterine Wall Chorionic Cavity Yolk Sac and allantois participate in the formation of ambilical cord

  45. Late embryonic stages • Histogenesis • Organogenesis

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