Third week of Embryological
development
EVENTS:
1. GASTRULATION
2. NEURULATION
3. FOLDING OF THE EMBRYO
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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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Other events in third week
 Formation of neural
 -plate
 -tube
 -crest
 Formation of somites
 Formation of intraembryonic coelom
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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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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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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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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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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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Gastrulation (Significance…)
1. Converts the bilaminar to trilaminar disc
2. Establishes the cranio-caudal axis and
bilateral symmetry(side by side) of the
embryo
3. Induces embryonic anlagen cells to form
organ systems (organogenesis) thus
gastrulation
 Signals morphogenesis
 Establishes 3 germ layers
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Mesoderm differentiation
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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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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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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 craniocaudal 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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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 extraembryonic coelom.
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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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Neural crest derivatives
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Folding of the Embryo
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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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Folding of the Embryo...
1. Head fold 2. Tail fold 3. Lateral folds
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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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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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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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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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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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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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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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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
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