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Third to Eighth Week: The Embryonic Period

Third to Eighth Week: The Embryonic Period. Jun Zhou （周俊） School of Medicine, Zhejiang University 20130108. Review. End of the Week of Twos. 2 major cell masses Trophoblasts Inner cell mass Trophoblasts Cytotrophoblasts Syncytiotrophoblasts Uteroplacental Cir.

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Third to Eighth Week: The Embryonic Period

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  1. Third to Eighth Week: The Embryonic Period Jun Zhou（周俊） School of Medicine, Zhejiang University 20130108

  2. Review

  3. End of the Week of Twos • 2 major cell masses • Trophoblasts • Inner cell mass • Trophoblasts • Cytotrophoblasts • Syncytiotrophoblasts • Uteroplacental Cir. • Embryonic Cavities • Amniotic cavity • Yolk sac • Germ Disc • Epiblasts • Hypoblasts

  4. Formation of trilaminar disc • Early of three weeks • Primitive streak: caudal end,a midline indentation on the epiblast surface • Primitive node/knot • Primitive pit • Proliferation & Invagination of epiblasts • Intra-embryonic mesoderm

  5. Gastrulation • Migrating epiblasts will form: • Endoderm: migrated cells displace hypoblast cells • Mesoderm: cells btw the endo and ecto • Ectoderm • (Note - epiblasts=>all layers) • Mesoderm cranial and lateral • extension

  6. Distribution of intra-embryonic mesoderm -spread laterally and cranially -Fused membranes • Oral: oropharyngeal membrane • Cloacal: cloacal membrane • Formation of notochord • Invaginate in through primitive pit/knot • a solid rod of mesoderm • Basis of the axial skeleton in the midline

  7. Review/Background • First Week • Second Week • Third- 8th week • Gastrulation • Primitive pit • Primitive node • Primitive streak • Notochord

  8. Expansion and Elongation • Migration of Epiblasts to form • Endoderm • Mesoderm • Ectoderm • Expansion of the disk • Notochord: • -midline rod of mesoderm • -orientation the germ disc

  9. Differentiation of trilaminar germ disc 3rd –8th weeks embryonic period: the major body form and organ structure take Bilaminar disc: flat and roundelongated with a broad cephalic and a narrow caudal end Trilaminar disc:a pear-shaped appearance

  10. Ectoderm - Neuralation • Overlying notochordThickens – Neural Plate • Edges - Neural Folds • Fuse at future neck region • Zippers shut • Neural Tube • Ant/Post Neuropores

  11. Ectoderm - Neuralation • Overlying notochordThickens – Neural Plate • Edges - Neural Folds • Fuse at neck region • Zippers shut • Neural Tube • Ant Neuropore: 25th days • Post Neuropore: 27th days

  12. Ectoderm – Neuralation • Failure of Neurotube closure • Anencephaly – Ant • Meningocele – post

  13. Ectoderm – Neural Crest • As neural tube closes • -Neural crest • Detach and migrate • Melanocytes – skin • Dorsal root ganglia

  14. Differentiation of the Ectoderm : • the central nervous system (CNS) • Neural crest cells (melanocyte, PNS) In generalterms it may be stated that the ectoderm gives rise to those organs and structures that maintain contact with the outside world.

  15. Mesoderm • Loose – condenses day 17 • 3 columns • Paraxial mesoderm • Intermediate mesoderm • Lateral Plate mesoderm

  16. Paraxial Mesoderm • Segmentation • Blocks of tissue along axis • Somite:20th days, 3 pairs/ per day,42-44 pairs by the end of 5th weeks • -Sclerotome: vertebra • -Myotome:muscle of the back • -Dermatome:dorsal dermis of the overlying skin

  17. Intermediate Mesoderm • Kidney and genital system “Urogenital tissue”

  18. Lateral Plate Mesoderm • Intra-embryonic coelom: →body cavity • 1.Outer layer: parietal mesoderm • →muscle, CT of the body wall • →parietal layer of cavities • 2.Inner layer: visceral mesoderm →muscle, CT of the gut • →Visceral layer of cavities • 3.Cavity: peritoneal, pleural and pericardial cavities mesenchyme: →CT, cardiovascular and lymph system

  19. Endoderm • From the Yolk sac • Forms Primitive gut • Besides the GI system • -Foregut: thymus,thyroid,lung,liver, • pancreas, etc. • -Midgut: none • -Hindgut: bladder,urethra • Surrounded by visceral lateral plate mesoderm

  20. Embryonic Folding • Craniocaudal folding--CNS • Lateral folding—somites • Endoderm-lined cavity is incorporated into the embryo properprimitive gut • Yolk sacvitelline duct • Oropharyngeal membrane • Cloacal membrane • Flat discpear, round

  21. 3-8th week

  22. OBJECTIVES 1.Bilaminar and trilaminar germ discs 2.Undertand the origin of the three embryonic “germ” layers that make up the trilaminar disk. 3.Describe the formation and derivatives of ectoderm, mesoderm and endoderm.

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