2. Bilaminar & Trilaminar Embryonic Disc
Dr. Zeenat Zaidi

3. Bilaminar Embryonic Disc
The Second Week

4. 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

5. Formation of Embryonic Disc
The inner cell mass becomes flattened forming a circular bilaminar plate, the embryonic disc, consisting of two layers:
Epiblast (ectoderm), thicker layer, consists of high columnar cells, and is related to the amniotic cavity
Hypoblast (endoderm), consists of small cuboidal cells, and lies adjacent to the blastocyst cavity

6. 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

7. At this stage the embryonic disc is:
A circular bilaminar disc, that lies between the amniotic cavity and the primitive yolk sac
The epiblast forms the floor of the amniotic cavity & the hypoblast lies in the roof of the primitive yolk sac

8. Formation of Extraembryonic Mesoderm
Endoderm of the yolk sac gives rise to a layer of loosely arranged connective tissue, extraembryonic mesoderm (EEM), which surrounds the amniotic cavity and the yolk sac.

9. Formation of Extraembryonic Celome
Isolated spaces appear in the EEM
These spaces rapidly fuse to form a large fluid filled, C-shaped cavity, the extraembyonic celome surrounding the amniotic cavity and the yolk sac

10. 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

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

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

13. Wall of the yolk sac: Two layers:
Amnion: Two layers:
Amnioblasts
Extraembronic splanchnic mesoderm
Wall of the yolk sac: Two layers:
Endoderm
Chorion: Three layers:
Extraembryonic somatic mesoderm
Cytotrophoblast
Syncytiotrophoblast

14. Extraembryonic celome is now called the
chorion
amnion
bilaminar disc
wall of the yolk sac
Connecting stalk
EEC
Extraembryonic celome is now called the
CHORIONIC CAVITY

15. Trilaminar Embryonic Disc
The Third Week
The significant event of third week is Gastrulation

16. Gastrulation
The process by which the bilaminar disc is converted into a trilaminar disc
It is the beginning of morphogenesis (formation of body form)
Consists of formation of the primitive streak, the three germ layers & the notochord
Embryo is referred to as a Gastrula

17. Primitive Streak
The primitive streak results from proliferation of the epiblastic cells in the median plane, in the caudal half of the epiblast, and lies along the cranio-caudal axis.
Its cranial end forms primitive node
A groove, primitive groove, appears in the primitive streak, which continues with a small depression, primitive pit, in the primitive node.

18. A circular thickening appears in the hypoblast near the cranial end, in the midline, to form the prechordal plate, that marks the future site of mouth
A circular thickening appears in the hypoblast caudal to primitive streak in the midline to form the cloacal membrane, the future site of the anus

19. By this stage of development, it is possible to identify the embryo’s:
craniocaudal axis
cranial and caudal ends
dorsal and ventral surfaces
right and left sides.
Connecting stalk

20. Formation of Intraembryonic Mesoderm
The epiblastic cells from the primitive streak (groove) proliferate to form mesenchymal tissue
The newly formed cells invaginate, migrate ventrally, laterally & cranially between the epiblast and hypoblast & organize to form the intraembryonic mesoderm

21. Formation of Intraembryonic Mesoderm cont’d
Intraembryonic mesoderm merges with the extra-embryonic mesoderm at the periphery of the embryonic disc
By the end of 3rd week, mesoderm lies between embryonic ectoderm and endoderm everywhere except in the region of prechordal plate and cloacal membrane, as the embryonic ectoderm & endoderm are fused at these regions

22. Formation of Intraembryonic Mesoderm cont’d
Some mesenchymal cells displace the hypoblasts forming the embryonic endoderm
Cells remaining in the epiblast form the embryonic ectoderm

23. Thus the EPIBLAST gives rise to all three germ layers, Ectoderm, Mesoderm, Endoderm in the embryo
Each of the three germ layers gives rise to specific tissues and organs

24. Fate of Primitive Streak
Actively forms mesoderm until the early part of 4th week
Then it starts regressing and becomes an insignificant structure in the sacrocooccygeal regions
Normally it degenerates and disappears by the end of 4th week
Remnants may persist and give rise to a large tumor called Sacrococcygeal Teratomas

25. Notochord
A rod of mesenchymal cells located cranially, in the midline, extending between the primitive node and the prechordal plate

26. Formation of Notochord
Mesenchymal cells migrate cranially from primitive pit toward the prechordal plate, and form a rod like notochordal process
The notochordal process becomes canalized forming a hollow tube, the notochordal canal, communicating with the primitive pit.

27. Formation of Notochord cont’d
The floor of the tube and the underlying endoderm break down, forming a notochordal plate
The notochordal plate becomes continuous with the endodermal layer.

28. Formation of Notochord cont’d
A temporary communication is established between the amniotic cavity and the yolk sac, termed the neurenteric canal.

29. Notochordal plate folds to form the notochord.

30. Functions of Notochord
Defines primordial axis of the embryo
Provides rigidity to the embryo
Serves as a basis for the development of the axial skeleton
Indicates the future site of the vertebral bodies/column
Regulates differentiation of surrounding structures including the overlying ectoderm (neural plate) and mesoderm (somites).

31. Fate of Notochord
Degenerates and disappears as the bodies of the vertebrae develop, but it persists as the nucleus pulposus of each intervertebral disc
Remnants of notochordal tissue give rise to tumors called Chordomas

32. Differentiation of the Intraembryonic Mesoderm
Induced by the notochord
Differentiates (in the region of notochord) into:
Paraxial mesoderm
Intermediate cell mass
Lateral plate mesoderm

33. thank you