1. 2nd WEEK OF DEVELOPMENT Formation of Bilaminar Disc
Dr Rania Gabr

2. OBJECTIVES By the end of this lecture the student should be able to:
Describe the formation of the Bilaminar disc
Describe the formation of the yolk sac and amniotic cavity
Explanation of formation of Extraembryonic mesoderm and its types

3. End of the First week of Development
11/19/2018
BLASTOCYST
Inner cell mass or embryoblast- EMBRYO
Outer cell mass or trophoblast

4. Inner cell mass develops into bilaminar embryonic disc
Hypoblast – Yolk sac forms here
Epiblast – amniotic cavity forms here
Yolk sac – formed by an extension of hypoblast
Digestive tube forms from yolk sac
Tissues around yolk sac
Gives rise to earliest blood cells and blood vessels
Amniotic sac – formed by an extension of epiblast
Outer membrane – forms the amnion
Inner membrane – forms the amniotic sac cavity
Filled with amniotic fluid

5. 2- (bilaminar germ disc)
The inner cell mass is differentiated into: a- hypoblast = endoderm (small cuboidal cells facing the blastocyst cavity and b- epiblast= ectoderm (tall columnar cells facing the amniotic cavity).

6. Trophoblast  Bilaminar Embryonic Disc

7. Formation of Amniotic Cavity
As implantation of the blastocyst progresses, changes appear in the inner cell mass (embryoblast)
A cavity, amniotic cavity appears separating embryoblast from the trophoblast, which soon becomes lined by amnioblasts derived from inner cell mass
The cavity gradually increases in size and is filled with amniotic fluid

8. Formation of Primitive Yolk Sac
The blastocyst cavity becomes lined with exocelomic membrane and is called exocelomic cavity
The hypoblastic cells soon replace the exocelomic membrane and the cavity is then named as the primitive (primary) yolk sac

9. 3- FORMATION OF THE EXTRA-EMBRYONIC MESODERM AND EXTRAEMBRYONIC COELOM
- Cells appears between the inner surface of the cytotrophoblasts and the outer surface of the yolk sac cavity
a. These cells are derived from the yolk sac cells and form the extra-embryonic (primary) mesoderm.
b. They fill the space between the trophoblasts, the amnion and yolk sac cavities.

11. Large spaces develop in the extraembryonic mesoderm and coalesce to form extraembryonic coelom.
These spaces rapidly fuse to form a large fluid filled, C-shaped cavity, the extraembyonic celome surrounding the amniotic cavity and the yolk sac

12. Formation of Connecting Stalk
The region where no cavity has appeared, forms the connecting stalk, that connects the amniotic cavity, yolk sac and the embryonic disc to the outer wall
The site of the connecting stalk determines the caudal pole of the embryonic disc

13. With the formation of extraembryonic ceolum:
The extraembryonic mesoderm splits into two layers:
an outer extraembryonic parietal (somatic) mesoderm
an inner extraembryonic visceral (splanchnic) mesoderm

14. Clinical application:
The syncytiotrophoblast secretes (human chorionic gonadotrophin hormone) HCG.
It stimulates the production of progesterone which in turn is important in sustaining the placenta. By the end of the 2nd week
the amount of this hormone HCG will be sufficient to be detected in the maternal blood and urine. This is the basis of pregnancy test.

15. EXTRAEMBRYONIC MESODERM
11/19/2018
Cells are derived from epiblast and hypoblast
2 layers:
Somatopleuri c mesoderm- lining the amnion
Splanchnople uric mesoderm- lining the yolk sac
Free template from

16. The primary yolk sac decreases in size and becomes the secondary (definitive) yolk sac
Wall of the yolk sac, amnion & chorion are formed

17. The second week of development is the week of two`s, because of the following:
The trophoblast differentiates into 2 layers, cytotrophoblast & syncytiotrophoblast
The inner cell mass differentiates into 2 layers, epiblast & hypoblast.
The primary mesoderm splits into somatopleuric primary mesoderm & splanchnopleuric primary mesoderm.
Starting of formation of the amniotic and yolk sac cavities.

18. 11/19/2018

19. Thank you