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

2. Review Preembryonic period: before 2 weeks Embryonic period: 3-8 weeks
Fetal period: after 9 weeks

3. Review
Zygote-Blastocyst (4-5th day)

4. End of the Week of Twos Blastocyst 2 major cell masses Trophoblasts
Inner cell mass
Cytotrophoblasts
Syncytiotrophoblasts
establish Uteroplacental Circulation
Embryonic Cavities
Amniotic cavity
Yolk sac
Bilaminar Germ Disc
Epiblast
Hypoblast
Blastocyst

5. “Periods” of Gestation
Preembryonic - First 2 weeks (i.e. prior to formation of Endoderm, Mesoderm & Ectoderm)
Gestation called:
Zygote, morula, blastocyst, bilaminar disc
Embryonic - 3 weeks to 8 weeks
Embryo
Morphogenesis
Organogenesis
Fetal - 9 weeks to 9 months
Fetus
Growth and maturation
Term Gestation
38 weeks from conception
40 weeks from last menstrual period

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

7. Trilaminar disc Migrating epiblasts will form:
Endoderm: migrated cells displace hypoblast cells
Mesoderm: cells btw the endo and ecto
Ectoderm: renamed epiblast
(Note - epiblasts=>all layers)
Primitive streak degenerates at the end of 4th week.

8. Distribution of intra-embryonic mesoderm
-mesoderm spread laterally and cranially
- Two region without mesoderm
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

9. Review/Background First Week zygote-morula-blastocyst Second Week
Bilaminar disc (Implantation)
Third- 8th week
Primitive streak
Induce migration of epiblast

10. Expansion and Elongation
Migration of Epiblasts to form
Endoderm
Mesoderm
Ectoderm
The disk expands cranially and laterally
Notochord:
-provide midline orientation for germ disc

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

12. Ectoderm Overlying notochord Thickens – Neural Plate
Edges curve upward- Neural Folds
Fuse at future neck region
Zippers shut
Neural Tube (future brain and spinal cord)
Anterior/Posterior Neuropores

13. Ectoderm Anterior Neuropore: close around 25th days
Posterior Neuropore:
close around 27th days

14. Ectoderm Failure of Neurotube closure abnormal CNS Anencephaly – Ant
Meningomyelocele – post

15. Ectoderm – Neural Crest
As neural tube closes
-Neural crest (paried cell cords)
Detach and migrate
Melanocytes – skin
Peripheral nervous system

16. Differentiation of the Ectoderm :
Neural tube--CNS
Neural crest cells-- melanocyte, PNS
Sensory epi. of ear, nose, eye
epidermis
In general terms it may be stated that the ectoderm gives rise to those organs and structures that maintain contact with the outside world.

17. Mesoderm Loose – condenses day 17 3 columns Paraxial mesoderm
Intermediate mesoderm
kidneys & Gonads
Lateral Plate mesoderm
body wall, limbs

18. Paraxial Mesoderm Cranial to caudal segmentation
form blocks of tissue along axis
Somite:20th days, 3 pairs/ per day,42-44 pairs by the end of 5th weeks
( evaluate the age of embryo of early stage)
-Sclerotome: will develop into vertebra
-Myotome: musculature of the region
-Dermatome: dermis of the skin

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

20. Lateral Plate Mesoderm
Folding of the embryo form intra-embryonic cavity: →body cavity
1.Outer layer: parietal mesoderm
→muscle, CT of the parietal layer of the body wall
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

21. Embryonic Folding Craniocaudal folding--CNS Lateral folding—somites
Endoderm-lined cavity is incorporated into the embryo properprimitive gut
Oropharyngeal membrane
Cloacal membrane to fold ventrally
Flat discpear, rounded body form

22. Endoderm From the roof of Yolk sac Form endodermally lined tube
Primitive gut
Besides the GI system
(ONLY EPITHELIUM)
-Foregut: thymus,thyroid,lung,liver,
pancreas, etc.
-Midgut: none
-Hindgut: bladder,urethra
Surrounded by visceral lateral plate mesoderm
( muscle, C.T. from mesoderm)

23. 3-8th week

24. OBJECTIVES 1.Bilaminar and trilaminar germ discs
2.Understand 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.

25. Exercise ( D ) ( C ) 1.The eight-day blastocyst:
A.is surrounded by zona pellucide
B.has about 6 to 8 cells
C.lies under uterine epithelium
D.is partially implanted
E.has a primitive streak
2.Ectopic implantations occur most commonly in the
A.Ovary
B.abdominal cavit
C.Oviduct
D.Cervix
E.mesentery
( D )
( C )

26. 3. Trilaminar germ disc is composed of _______,________ of _______,all of them from ________ ( epiblast or hypoblast)
ectoderm
mesoderm
endoderm
epiblast

27. 4. what’s the origin of following structure?
Neural tube ( ), connective tissue ( ), bone ( )
Epithelium of GI system ( ), submucosa of GI system ( )
Kidney and genital system ( ), melanocyte ( ), nerve system( )
A. ectoderm B. Mesoderm C.Endoderm A B B C B B A A