1. Bilaminar ,Trilminar germ disc and neurulation
Dr A.Prasanna

2. The objective of the lecture is to discuss:
Formation of the bilaminar and trilaminar embryo disc
Establishment of the uteroplacental circulation
Enlargement of the syncytiotrophoblast
Formation of germ layers.

3. The embryoblast also differentiates into two layers.
A layer adjacent to blastocyst cavity which is cuboidal forms the hypoblast.
Another layer adjacent to amniotic cavity which is columnar forms the epiblast.
They together form a disc. Amniotic cavity is formed within the epiblast.
The uterine wall now secretes glycogen and is vascular.

4. In the meantime a new population of cells appears between the cytotrophoblast
And the outer surface of the exocoelomic cavity.
These cells form the extraembryonic mesoderm between the trophoblast externally and the amnion and the exocoelomic membrane internally.
A cavity is formed in this and enlarges to from a new space extraembryonic coelom or chorionic cavity.

5. This space surrounds the primitive yolk sac and the amniotic cavity except at the connecting stalk where the germ disc is connected to trophoblast.
The extraembryonic mesoderm lining the cytotrophoblast is termed as the extraembryonic somatopleuric mesoderm
The lining covering the yolk sac as the extraembryonic splanchnopleuric mesoderm.
The uterus changes towards increase in nutrition are called decidua reaction.

6. The blastocyst is more deeply embedded now.
Vacuoles appear in the syncytium and fuse to form large lacunae and called as the lacunar stage.
At the abembryonic pole cells originating from hypoblast form the exocoelomic (Heuser’s) membrane lining the inner surface of the cytotrophoblast
And together with the hypoblast forms the exocoelomic or the primitive yolk sac.

7. Because of additional cells produced by hypoblast
which go on to cover inner side of the exocoelomic membrane
give rise to a new cavity the secondary yolk sac or definitive yolk sac.
This is smaller than the primitive yolk sac.

8. The extraembryonic coelom expands and forms the chorionic cavity.
The extraembryonic mesoderm lining the inner side of the cytotrophoblast is called as chorionic plate.
The formation of the blood vessels in the connecting stalk forms the umbilical cord.

9. Third week of development: Trilaminar germ disc
Gastrulation
It includes the formation of embryonic mesoderm and endoderm.
It is the main event occurring in the 3rd week of gestation.
Three germ layers are established by gastrulation.

10. Third week of development: Trilaminar germ disc
It begins with the formation of the primitive streak on the surface of the epiblast.
It appears as a narrow groove.
The cephalic end of it is the primitive node and is surrounding a small pit the primitive pit.

11. The cells of the epiblast migrate towards the primitive streak.
After reaching the streak they get detached from the epiblast and slip beneath the epiblast.
The process is known as invagination.
The invaginated cells displace the hypoblast.

12. The displaced layer forms the endoderm layer.
The cells lying in between the epiblast and newly formed endoderm layer forms the mesoderm.
The mesoderm forming cells migrate beyond the disc and establish contact with the extraembryonic mesoderm covering yolk sac and amnion.

13. Cephalically the migrating cells pass on each side of the prechordal plate.
The prechordal plate forms between the tip of the notochord and the oropharyngeal membrane.
The prechordal plate is formed by the cells which have migrated in the beginning.

14. The prechordal plate is responsible for the induction of forebrain development.
The oropharyngeal membrane consists of tightly adherent ectoderm and endoderm cells and will make the future opening of the oral cavity.

15. Formation of notochord
Prenotochordal cells are the initial cells which invaginate the primitive node and move cranially to prechordal plate in the midline.
In the beginning they are attached to hypoblast and double layered called the notochordal plate.

16. Formation of notochord
Prenotochordal cells are the initial cells which invaginate the primitive node and move cranially to prechordal plate in the midline.
In the beginning they are attached to hypoblast and double layered called the notochordal plate.

17. Formation of notochord
As the hypoblast is replaced by the endoderm the notochord plate detaches from endoderm and proliferates.
They form a cord of cells called the definitive notochord.

18. Formation of notochord
This underlies the neural tube and serves as a basis of axial skeleton.
The notochord extends between the prechordal plate and the primitive pit.
Near the pit it is temporarily connected to amniotic and yolk sac cavities.

19. Formation of notochord
The cloacal membrane is formed at the end of the embryonic disc.
It consists of tightly adherent ectoderm and endodermal cells.
Yolk sac forms a diverticulum and it extends into the connecting stalk.
The diverticulum is called the allantois. This appears on 16th day.

20. Establishment of the body axis
It includes anteroposterior, dorsoventral and right and left body axis.
It occurs before and during the gastrulation.
Fate map established during gastrulation
Fate map of regions is established by cell migration and ingression from epiblast.
The cells that enter in through the cranial region of the node become prechordal plate and notochord.

21. Those that migrate at the lateral edge near the cranial end become the paraxial mesoderm.
Those migrating from the middle of the streak become the intermediate mesoderm.
Those migrating from the caudal part of the streak form the lateral plate mesoderm.
And those migrating from the caudal most part of the streak form the extraembryonic mesoderm.
Extraembryonic mesoderm also develops from primitive yolk sac.

22. Development of Mesoderm

23. Growth of the embryonic disc
The embryonic disc in the beginning is flat and round
Becomes elongated with a broad cephalic and a narrow caudal end.
The region of the primitive streak does not change.
Cells continue to migrate from the primitive streak up to fourth week .
The primitive streak disappears afterwards.
The formation of the germ layers and the differentiation is from cranial to caudal regions.

24. Further development of the trophoblast
In the beginning of the 3rd week the primary villi appear.
They are made of cytotrophoblast core covered by syncytial layer.
The mesodermal cells penetrate the core and the newly formed villi are called the secondary villi.
Blood vessels and blood cells are formed by the mesodermal cells in the core and establish the capillary system.
The villous is now known as the tertiary villus or definitive villus.

25. The capillaries of the definitive villi establish contact with capillaries of the chorionic plate and the connecting stalk.
The vessels of the placenta establish contact with the vessels of the embryo.
Heart begins beat in the fourth week of development.

26. The villi that extend from the chorionic plate to the decidua basalis or plate are called anchoring villi.
The chorionic cavity becomes larger and the embryo is attached to trophoblast by the connecting stalk .
The stalk later develops into umbilical cord forms connection between the placenta and embryo.

27. Third to eighth weeks: the embryonic period
It is the period of organogenesis.
In this period the three germ layers give rise to specific tissues and organs of the body.
By the end of the embryonic period main organ systems are established and human form is recognizable.

28. Derivatives of ectoderm
The appearance of the notochord
And the prechordal plate
Induces the overlying ectoderm to thicken and form the neural plate.

29. Neurulation
It is the process by which the neural plate forms the neural tube.
By the end of the third week the lateral edges of the neural plate elevate to form the neural folds
And the depressed region in between as the neural groove.
The neural folds fuse in the midline.
The fusion begins in the cervical region proceeds both cephalically and caudally.

30. The neural tube formed communicates with amniotic cavity through the anterior and posterior cranial neuropores.
The anterior neuropore closes on 25th day and the posterior neuropore closes on 28th day with which the neurulation is complete.

31. The central nervous system is represented as narrow spinal cord and a broad cephalic portion with brain vesicles.
Before the closure of the neural tube two bilateral thickenings are formed and they are the otic and lens placodes.

32. Neural crest cells
As the neural folds elevate to fuse and form the neural tube
The cells at the lateral borders disassociate and these cells are called the neural crest cells.
They undergo mesenchymal transition, migrate and enter into underlying mesoderm.
Mesenchyme means loosely organized embryonic connective tissue of all germ layers.

33. Neural crest cells
Neural crest cells from the trunk region enter the dermis of the skin dorsally and form melanocytes.
The cells which migrated ventrally in the anterior part of each somite form
sensory ganglia
sympathetic ganglia,
Schwann cells
Adrenal medulla.

34. Some cells also migrate before the closure of the neural tube
Neural crest cells
Some cells also migrate before the closure of the neural tube
These cells contribute to craniofacial skeleton
Cranial ganglia
Glial cells
Melanocytes.

35. The neural crest cell derivatives are:
Connective tissue
Bones of face and skull
Dermis of face and neck
Smooth muscle of blood vessels of face
Cranial nerve ganglia
Cells of thyroid gland
Conotruncal septum of heart
Odontoblasts

36. The neural crest cell derivatives are:
Spinal root ganglia
Sympathetic ganglia and chain
Parasympathetic ganglia of GIT
Adrenal medulla
Schwann cell
Glial cells
Meninges of forebrain
Melanocytes

37. The neural crest cell derivatives are:
In general the ectodermal cells give rise to structures which keep us in contact with the outside world.
They are the nervous system, the sensory organs of ear, nose and the eye and the skin including the hair and nails.
In addition it gives rise to the mammary glands, the pituitary gland and the enamel of the teeth.

38. At the end of the lecture, students should be able to:Outline
Formation of bilaminar disc
Map the events of gastrulation
Formation of trilaminar disc
Describe the appearance of primitive streak
Trace the sequence of events in notochord formation leading to neuralation
Enumerate the derivatives of neural crest cells.