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Gastrulation
Definition: The process by which theinner cell mass is changed into trilaminardisc( ectoderm, mesoderm and
endoderm). It is the beginning ofmorphogensis.
Formation of primitive streak, germlayers (ectoderm., mesoderm, andendoderm), and notochord are theimportant processes occuring during
gastrulation. The embryo is reffered as
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Third week of
Development
Gastrulation: is theprocess of formation ofthe 3 germ layers(ectoderm, mesoderm
& endoderm).It begins by the
formation of a midline groove on the
epiblast calledprimitive streak.The cephalic end of
this streak is calledprimitive node. In thecenter of this node
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The epiblastic cells migrate & reach theprimitive streak.
They slip under the primitive streak(invaginate) displace the hypoblast formingendoderm.
Some of invaginated epiblastic cells forms
the intra-embryonic mesoderm. Thus the epiblast is the source of the 3 germ
layers, which are ectoderm (dorsally),endoderm (ventrally) and in-between these2 layers is the mesoderm.
From these 3 germ layers arise all organs &tissues of the body.
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Amnioticcavity
Primitive
streak
Ectoderm
Endoderm
Mesoderm
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Trilaminar Embryonic
Disc
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Formation of the
Notochord Epiblastic cells invaginatethe primitive pit. They
migrate cranially in themidline forming thenotochordal plate. This platelies between the endoderm& ectoderm. This plate will
form solid cord called thenotochord.
The notochord is a solid midline axis, so it forms fetalskeleton. Formation of the 3germ layers occurred in a
cephalocaudal direction, thismeans that the 3 germ layerare establish first in thehead region then the tailregion. Also tissues andorgans develop in acephalocaudal direction.
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as ru a on
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Functions of thenotochord1. It forms thebasis of theaxial skeleton (bones ofthe head and vertebralcolumn).
2. It induces the overlyingectoderm to thicken andform the neural plate;the primordium of
thecentral nervoussystem. It is theorganizer for theformation of the neural
tube of C.N.S.
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Early DevelopmentNeurulation
the development of the nervous system.
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Early Development
Neural crest Neural fold
Notochord
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Early Development Gradually, these folds move together to create
a neural tube from which the spinal cord andthe nervous system develop.
Neural crest Neural tube
Ectoderm
Notochord
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Functions of thenotochord3. The notochorddegenerates and
disappears as thebodies of the vertebrae
form. Its remnant is thenucleus pulposusof the
intervertebral discs.4. It functions as the
primary inducerin the
early embryo i.e. it is aprime mover in a series ofsignal-calling episodes thatultimately transformunspecialized embryoniccells into definitive adult
tissues and organs
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Intraembryonic
mesodermThe cells of the primitivestreak& notochordalprocess proliferate 2rymesoderm migratelaterally & craniallybetween ectoderm&endoderm except:
oral membrane
( cranially).cloacal membrane(caudally).
The median planecranial to primitive
Dorsal aspect of pear shapedembryo
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Clinical application
Third week of development is avery sensitive period in fetal
development. Many factorssuch as drugs, alcohol orirradiation to the mother may
cause congenital anomalies toher embryo.
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Mesoderm:It differentiates into:
1-Paraxial mesoderm givesfinal count of approximately35 pairs of somites. Thesesomites further differentiateinto 3 components:
a- Sclerotome (cartilage andbone)
b- Myotome (muscles)
c- Dermatome (dermis ofskin)2-Intermediate cell mass(gives kidneys& genitalsystem)
3-Lateral plate of mesoderm.