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

2. Review

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

5. 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

6. 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

7. 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

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

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

10. 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

11. Ectoderm - Neuralation
Overlying notochord Thickens – Neural Plate
Edges - Neural Folds
Fuse at future neck region
Zippers shut
Neural Tube
Ant/Post Neuropores

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

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

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

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

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

17. 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

18. Intermediate Mesoderm
Kidney and genital system “Urogenital tissue”

19. 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

20. 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

21. 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

22. 3-8th week

23. 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.