1. Third week of Embryological development
EVENTS:
GASTRULATION
NEURULATION
FOLDING OF THE EMBRYO
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2. Events during 3rd wk appear to occur in threes
3 germ layers of derived from bilaminar embryonic disc
3 new structures appear
Primitive streak, notochord & allantois
3 layers appear in chorionic villi
Syncytiotrophoblast
Intermediate cytotrophoblast
Inner mesodermal layer
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3. Other events in third week
Formation of neural
-plate
-tube
-crest
Formation of somites
Formation of intraembryonic coelom
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4. GASTRULATION
The main event that occurs during the third week of development is the formation of the trilaminar embryo. This process is called gastrulation.
The first sign of gastrulation is the formation in the epiblast of the primitive streak.
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5. Events in Gastrulation
1.    Cell proliferation – cells from the epiblast in the caudal half proliferate at the future median plane causing heaping up of the cells and is the source of a new layer of cells
2.    Cell migration by amoeboid movement – the cells insinuate themselves between the epiblast and hypoblast
3.     Cell determination - the cells arising from the primitive streak are determined to give rise to different tissues
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6. Gastrulation…
At the cranial end of the primitive streak, cells proliferate to form the primitive node, which finally undergo apoptosis to form the primitive pit
Within the primitive streak there is apoptosis to form the primitive groove
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9. Gastrulation…
At this point a trilaminar disc is formed with the epiblast giving rise to all three germ layers: 1. the epiblast that remains becomes the ectoderm, the cells that replaced the hypoblast become the endoderm, the cells in between becoming the mesoderm (intra-embryonic mesoderm).
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G.LUFUKUJA

10. Gastrulation
The phenomenon under the influence of maternal effect & zygotic genes, thus:
Midline mesoderm forms notochord
Paraxial mesoderm gives rise to myotomes (skeletal muscle); sclerotomes (vertebra); and dermatomes (dermis).
Ectoderm develops into CNS (induced by mesoderm and skin)
Endoderm forms the lining of the Alimentary System and RS
Mesoderm forms the CVS (induced by endoderm)
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11. Gastrulation (Significance…)
Converts the bilaminar to trilaminar disc
Establishes the cranio-caudal axis and bilateral symmetry(side by side) of the embryo
Induces embryonic anlagen cells to form organ systems (organogenesis) thus gastrulation
Signals morphogenesis
Establishes 3 germ layers
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12. Mesoderm differentiation
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14. Formation of the notochord
The notochordal process grows out from the primitive node growing as a rod of cells cranially along the midline.
Its growth is limited by the buccopharyngeal membrane.
The most cranial part of the notochord is at the prochordal plate.
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G.LUFUKUJA

15. Notochord… Found in all chordate embryos - flexible, rod-shaped body
Develops from axial mesoderm
Defines the primitive axes of the embryo
Replaced by vertebral column in most vertebrates
Position: Ventral to the neural tube
Persists throughout life as the main axial support of the body in some chordates
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16. Segmentation of the paraxial mesoderm
The segments are termed somites.
The first somite appears on day 20 at the cranial end close to the prochordal plate.
Somites develop in cranio-caudal sequence to form 42 to 44 somites by day 30.
The somites are: 4 occipital, 8 cervical, 12 thoracic, 5 lumbar, 5 sacral, and 3 coccygeal.
A few other somites at the caudal end degenerate
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17. The intra-embryonic cavities
Intraembryonic coelomic spaces appear in lateral & cardiogenic mesoderm; they coalesce to form horseshe-shaped intraembryonic coelom
The intra-embryonic coelom is shaped like an inverted U-tube, and is divided into three parts:pericardial cavity, pleuro-peritoneal canals, intra-embryonic coelom communicates laterally with the extra-embryonic coelom.
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18. Formation of the Neural plate & neural tube
The neurectoderm is now termed the neural plate.
Like the notochord it is limited craniallly by the buccopharyngeal membrane and caudally by the cloacal membrane.
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20. Neural crest derivatives
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21. Folding of the Embryo
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22. Changes resulting from folding
Note that drastic and important changes occur in the embryonic cavities as a consequence of folding:
1. The amniotic cavity surrounds the embryo completely on all aspects and becomes the predominant cavity. It enlarges progressively.
2. The yolk sac becomes constricted on all sides, and becomes a small sac connected to the midgut by a narrow vitelline duct. It becomes progressively smaller.
3. The extra-embryonic coelom is gradually obliterated by the expanding amnion and eventually disappears completely
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23. Folding of the Embryo... 1. Head fold 2. Tail fold 3. Lateral folds
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26. Head Fold… As a result of the formation of the head fold:
a. The foregut is formed by folding of the endoderm
b. The stomodaeum is an invagination of ectoderm, and the buccopharyngeal membrane separates it from the foregut
It opens into the amniotic cavity.
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27. Head Fold…
c. The pericardial cavity and cardiogenic mesoderm are shifted to the ventral aspect of the embryo and lie ventral to the foregut.
d. The part of the transverse mesoderm between the pericardial cavity and the yolk sac is the septum transversum. The liver will develop in it and it forms the diaphragm.
e. The amniotic cavity extends ventral to the cranial end of the embryo.
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28. Head Fold…
f. The yolk sac is constricted and will later form the primitive foregut.
As a result of the formation of the tail fold: The hindgut is formed & The cloaca is an invagination of ectoderm and has the cloacal membrane separating it from the hindgut.
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G.LUFUKUJA

29. Tail fold…
e. The amniotic cavity extends ventral to the caudal end of the embryo.
f. The yolk sac is constricted from the caudal end and will form the primitive hindgut
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G.LUFUKUJA

30. Somites A somite is divided into two parts:
The sclerotome is the ventro-medial part of the somite.
Cells from the sclerotome migrate medially to surround the notochord and neural tube and form the axial skeleton.
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G.LUFUKUJA

31. Somites The dermomyotome is the dorso-lateral part of the somite.
Cells from the dermomyotome migrate laterally and, as its name implies, gives rise to
(i) skeletal muscle and
(ii) the dermis of the skin.
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32. Somite…
Each anatomical myotome is derived from the embryological dermomyotome that is innervated by a segmental nerve and forms a group of skeletal muscle cells and the dermis of the corresponding segment of ectoderm.
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33. Neural arches
The neural tube induces the formation of the neural arches and their fusion across the midline.
Defects of closure of the neural tube will also cause failure of fusion of the overlying neural arches.
This anomaly has various terms depending on the degree of the defects e.g meningomyelocoele
7/3/2018
G.LUFUKUJA