2. Intraembryonic Mesoderm
Origin: The epiblastic cells from the primitive streak (groove)
The newly formed cells migrate ventrally, laterally & cranially between the epiblast and hypoblast
At the margins of the embryonic disc, the intraembryonic mesoderm merges with the extra-embryonic mesoderm

3. By the end of 3rd week, mesoderm lies between embryonic ectoderm and endoderm everywhere, EXCEPT in the region of:
Buccopharyngeal membrane (fused prechordal plate + ectoderm)
Cloacal membrane, as the embryonic ectoderm & endoderm are fused at these regions

4. Notochord Notochord is a rod of mesenchymal cells
located in the midline
extending cranially from the primitive node to the buccopharyngeal membrane

5. Formation of Notochord
Origin: Primitive node/pit
Like the primitive streak, the primitive pit cells proliferate and then migrate cranially in the midline, toward the buccopharyngeal membrane, and form a rod like notochordal process
The notochordal process becomes canalized forming a hollow tube, the notochordal canal, which communicates with the amniotic cavity at the primitive pit.

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

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

8. Notochordal plate folds to form the notochord, which gets separated from the underlying endoderm.

9. 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 and the mesoderm

10. Fate of Notochord
Degenerates and disappears as the bodies of the vertebrae develop
The part that lies between the vertebral bodies persists as the nucleus pulposus of each intervertebral disc
Remnants of notochordal tissue give rise to tumors called Chordomas

11. Differentiation of the Intraembryonic Mesoderm
Induced by the notochord
Differentiates into the:
Paraxial mesoderm
Intermediate cell mass
Lateral plate mesoderm

12. Ectodermal Derivatives
Dr. Zeenat Zaidi

13. 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 (22-23 days)
Ends by the end of 4th week (27 days)
Is induced by the notochord

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

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

16. 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

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

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

19. 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

20. Following fusion of the neural folds, the neural crest cells get separated and move laterally to form the sensory neurons of the spinal (dorsal root) ganglia

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

22. 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

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

24. Congenital Anomalies of the Nervous System
Disturbance of neurulation may result in severe abnormalities of the brain and the spinal cord
Most defects are the result of non-closure or defective closure of the neural tube:
In the brain region (e.g. anencephaly)
In the spinal cord regions (e.g. spina bifida)
High level of alpha-fetoprotein (AFP) in the amniotic fluid is a strong sign of neural tube defects

26. Ectoderm
Surface ectoderm
Neuroectoderm

27. Surface Ectoderm Derivatives
Epidermis of the skin
Hair
Nail
Sweat & Sebaceous glands
Mammary glands
Enamel of the teeth
Lens of eye
Internal ear
Anterior lobe of the pituitary gland

28. Neuroectoderm
Neural Tube
Neural Crest Cells

29. Neural Tube Derivatives
Central nervous system
Peripheral nervous system
Retina
Sensory epithelia of nose & ear
Pineal gland
Posterior lobe of the pituitary gland

30. Neural Crest Cells Derivatives
Sensory ganglia (cranial & spinal)
Autonomic ganglia
Meninges (Pia mater & Arachnoid mater) of the brain & spinal cord
Schwann cells
Satellite cells
Melanoblasts
Suprarenal medulla (chromaffin cells)
Several skeletal & muscular components in the head (derived from pharyngeal arches)

31. Thank U & Good Luck