Transcript Document
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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