1. Gastrulation, Neurulation and Folding
Dr Rania Gabr

2. OBJECTIVES By the end of this lecture , the student should be able to:
Define Gastrulation
Describe the formation of the primitive streak
Describe the formation of the intraembryonic mesoderm and the Trilaminar disc
Explain the formation , function and fate of the notochord
Define Neurulation
Describe the formation of the neural plate, groove, fold , crest and canal
Understand the process of folding its timing and results

3. The significant event of third week is Gastrulation
The Third Week
The significant event of third week is Gastrulation
Gastrulation: is the process of formation of the 3 germ layers (ectoderm, mesoderm & endoderm).

4. 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

5. 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 the primitive node
A groove, primitive groove, appears in the primitive streak, which continues with a small depression, primitive pit, in the primitive node.

6. A circular thickening appears in the hypoblast near the cranial end, in the midline, to form the prechordal plate ( oropharyngeal membrane), 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

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

8. 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

9. Trilaminar Embryonic Disc

10. 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

11. 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

12. 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

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

14. Formation of Notochord
Mesenchymal cells migrate cranially from the primitive pit towards 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.

15. 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.

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

17. Notochordal plate folds to form the notochord.

18. 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).

19. 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

20. Ectodermal Derivatives The Neurulation
It is the process by which the neural tube is formed.
The stages of neurulation include the formation of:
Neural plate
Neural groove
Neural folds & their fusion
Neural crest cells
Neural tube
Begins during early part of the 4th week ( days)
Ends by the end of 4th week (27 days)
Is induced by the notochord

21. The Neurulation
Under the inducing effect of the developing notochord, the overlying ectodermal cells thickens to form the neural plate

22. The neural plate first appears:
Cranial to the primitive node and
Dorsal to the developing notochord & the mesoderm adjacent to it

23. As the notochord forms & elongates:
The embryonic disc elongates and becomes club-shaped
The neural plate broadens and extends cranially as far as the buccopharyngeal membrane, and later on grows beyond it

24. Neural fold
On 18th day: the neural plate invaginates to form neural groove & neural folds

25. Some neuroectodermal cells along the crest of the neural fold differentiate as the neural crest cells.
Neural crest cells
Neural fold

26. By the end of 3rd week, the neural folds move to the midline and fuse to form the neural tube
The fusion begins in the future cervical region and then extends both in cranial and caudal direction

27. The neural tube separates from the surface ectoderm, lies in the midline, dorsal to the notochord

28. Neural tube is open at both ends, communicating freely with the amniotic cavity.
The cranial opening, the rostral neuropore closes at about 25th day & the caudal neuropore closes at about the 27th day

29. The cranial ⅓ of the neural tube represent the future brain
The caudal ⅔ represents the future spinal cord

30. Folding Of Embryo
Folding means conversion of the flat trilaminar embryonic disc into a cylindrical embryo.
 Time:
Folding of the embryo begins by the end of the 3rd week. It is completed by the 4th week.

31. Folding of the embryo is due to rapid growth of the embryo specially the nervous system.
The head folds first then the tail . At the same time, side to side folding occurs.