1. BILAMINAR & TRILAMINAR
EMBRYONIC DISCS
Dr. Ahmed Fathalla Ibrahim

2. SECOND WEEK Completion of implantation of the blastocyst
Production of a bilaminar embryonic disc
Formation of extraembryonic structures: amniotic cavity & amnion, yolk sac, chorionic sac & connecting stalk
Appearance of primary chorionic villi
Appearance of the prechordal plate

3. THIRD WEEK Appearance of the primitive streak
Development of the notochord
Differentiation of the 3 germ layers (formation of a trilaminar germ disc)
Beginning of formation of neural tube
Completion of formation of chorionic villi

4. BILAMINAR DISC
A flat, almost circular bilaminar plate of cells composed of 2 layers:
Epiblast a layer of high columnar cells
Hypoblast: a layer of small cuboidal cells

7. AMNIOTIC CAVITY & AMNION
Amniotic cavity: a space appears dorsal to the epiblast (primordium of amniotic cavity)
Amniogenic cells (amnioblasts) separates from epiblast & forms the amnion which encloses the amniotic cavity
N.B.:
The amnion forms the roof of amniotic cavity
The epiblast forms the floor of amniotic cavity & is continuous peripherally with amnion

8. YOLK SAC
The hypoblast becomes continuous peripherally with the exocoelomic membrane to enclose the exocoelomic cavity
PRIMARY YOLK SAC: has:
A roof: formed by hypoblast
A floor: formed by exocoelomic membrane
A cavity: formed by exocoelomic cavity

9. YOLK SAC SECONDARY YOLK SAC: Smaller than primary yolk sac
Formed by migration of endodermal cells from hypoblast to enclose a portion of primary yolk sac
The rest of primary yolk sac is pinched off

10. YOLK SAC SECONDARY YOLK SAC: has: A roof: formed by hypoblast
A floor: formed by endodermal cells migrating from hypoblast
A cavity: formed by a portion of the cavity of primary yolk sac

11. EXTRAEMBRYONIC MESODERM
Formed by:
Endodermal cells of yolk sac
later on by primitive streak
Surrounds amnion & yolk sac

12. CHORION
It is formed of:
Trophoblast
Extraembryonic mesoderm

13. EXTRAEMBRYONIC COELOM
Isolated spaces filled with fluid appear within the extraembryonic mesoderm
The spaces fuse to form the coelom
The coelom divides the meosderm into 2 layers:
Extraembryonic somatic mesoderm: between trophoblast & coelom
Extraembryonic splanchnic mesoderm: between coelom & yolk sac

14. CONNECTING STALK
It is a part of extraembryonic mesoderm that does not contain coelom
It attaches the amnion & yolk sac to the chorion
It indicates the future site of umbilical cord

15. CHORIONIC SAC
It contains embryo + amniotic sac + yolk sac suspended by the connectiong stalk
It is enclosed by the chorion

16. PRIMARY CHORIONIC VILLI

17. SECONDARY CHORIONIC VILLI

18. TERTIARY CHORIONIC VILLI

19. CHORIONIC VILLI
DEVELOPMENT: During implantation, the trophoblast thickens at region of contact with endometrium & differentiates into:
Outer syncytiotrophoblast: a layer containing multiple nuclei in a common cytoplasm with no cell boundaries (secretes HCG)
Inner cytotrophoblast: a layer of mononucleated cells

20. STAGES OF CHORIONIC VILLI
PRIMARY: At the end of 2nd week, finger-like processes formed of outer syncytiotrophoblast & inner cytotrophoblast appear
SECONDARY: Early in 3rd week, extraembryonic mesoderm extends inside the villi
TERTIARY: During 3rd week, arterioles, venules & capillaries develop in the mesenchyme of villi & join umbilical vessels
By the end of 3rd week, embryonic blood begins to flow slowly through capillaries in chorionic villi

21. PRECHORDAL PLATE

22. PRECHORDAL PLATE
In a localized area in the median plane, the cuboidal cells of the hypoblast become columnar & form a thickening: prechordal plate
The prechordal plate indicates the future cranial region & future site of mouth
It is an important organizer of the head region

23. PRIMITIVE STREAK

24. PRIMITIVE STREAK & NODE

25. BILAMINAR & TRILAMINAR DISCS

27. PRIMITIVE STREAK
It appears as a linear opacity in the epiblast of the caudal part of the embryo, in the median plane
It results from proliferation of epiblast
Primitive node: the enlarged cranial end of primitive streak
Primitive Pit: a depression in primitive node
Primitive groove: a narrow groove in primitive streak that is continuous with the primitive pit

28. PRIMITIVE STREAK IMPORTANCE: It forms embryonic mesoderm
It replaces cells of hypoblast from the roof of yolk sac & forms embryonic endoderm
The primitive node forms the notochordal process
It shares in the formation of extraembryonic mesoderm