gastrulation maha

8/3/2019 Gastrulation maha

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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.

gastrulation maha

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