1. 人胚胎早期发生II Trilaminar Germ Disc (Third Week of Development) 周俊 浙江大学医学院

2. OBJECTIVES 1.Bilaminar and trilaminar germ discs.
2.Describe the formation and derivatives of ectoderm, mesoderm and endoderm.
3.The components of fetal membrane and their functions.
4.Structure and function of placenta.
5.Composition of Placenta barrier.

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

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

5. Gastrulation Migrating epiblasts will form:
-Endoderm（内胚层）: migrated cells displace
hypoblast cells
-Mesoderm（中胚层）: cells btw the endo and ecto
-Ectoderm（外胚层）
(Note - epiblasts=>all layers)
Mesoderm cranial and lateral extension
Fused membranes
Oral: oropharyngeal membrane（口咽膜）
Cloacal: cloacal membrane（泄殖腔膜）

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

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

8. Differentiation of trilaminar germ disc
3rd –8th weeks
embryonic period（胚期）: 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

9. Ectoderm （外胚层）- Neuralation
Overlying notochord Thickens – Neural Plate
（神经板）
Edges - Neural Folds
（神经褶）
Fuse at future neck region
Zippers shut
Neural Tube（神经管）
Ant/Post Neuropores
（前/后神经孔）

10. 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

11. Ectoderm – Neuralation
Failure of Neurotube closure
Ant
Anencephaly （无脑儿）
post
Meningocoele（脊膜膨出）

12. Ectoderm – Neural Crest
As neural tube closes
-Neural crest （神经嵴）
Detach and migrate
PNS
Melanocytes – skin
肾上腺髓质

13. Differentiation of the Ectoderm :
神经管：CNS，神经垂体
神经嵴：PNS，melanocyte，肾上腺髓质
表面外胚层：表皮及其衍生物，感觉上皮，腺垂体
In general terms it may be stated that the ectoderm gives rise to those organs and structures that maintain contact with the outside world.

14. Mesoderm Loose – condenses day 17 3 columns Paraxial mesoderm （轴旁中胚层）
Intermediate mesoderm
（间介中胚层）
Lateral Plate mesoderm
（侧中胚层）

15. Paraxial Mesoderm Segmentation Blocks of tissue along axis
Somite（体节）:20th days, 3 pairs/ per day,42-44 pairs by the end of 5th weeks
-Sclerotome（生骨节）: vertebra
-Myotome（生肌节）:skeletal muscle
-Dermatome（生皮节）:dermis

16. 生皮节
生肌节
生骨节

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

18. Lateral Plate Mesoderm
Intra-embryonic coelom: →body cavity
1.Outer layer: parietal mesoderm, （体壁中胚层）surrounding the intraembryonic cavity
2.Inner layer: visceral mesoderm （脏壁中胚层）→muscle, CT of the gut
3.Cavity: pericardial, pleural and
peritoneal cavities
Mesenchyme（间充质）: →cardiovascular and lymph system

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

20. Endoderm From the Yolk sac Forms Primitive gut （原始消化管）
Besides the GI system
-Foregut: thymus,thyroid,lung,liver,
pancreas, etc.
-Midgut: none
-Hindgut: bladder,urethra
Surrounded by visceral lateral plate mesoderm

21. The end of 8th week

22. Anne Geddes