_bilaminar & trilaminar discs

7/30/2019 _Bilaminar & Trilaminar Discs

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Dr. Ahmed Fathalla Ibrahim


2/29

SECOND WEEK

Completion of implantation of theblastocyst

Production of a bilaminar embryonic

disc Formation of extraembryonic

structures: amniotic cavity & amnion,

yolk sac, chorionic sac & connectingstalk

Appearance of primary chorionic villi

Appearance of the prechordal plate


3/29

THIRD WEEK

Appearance of the primitive streak

Development of the notochord

Differentiation of the 3 germ layers(formation of a trilaminar germ disc)

Beginning of formation of neural tube

Completion of formation of chorionicvilli


4/29

BILAMINAR DISC

A flat, almost circular bilaminar plateof cells composed of 2 layers:

1. Epiblast a layer of high columnar cells

2. Hypoblast: a layer of small cuboidalcells


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7/29

AMNIOTIC CAVITY & AMNION

Amniotic cavity: a space appears dorsal tothe epiblast (primordium of amniotic cavity)

Amniogenic cells (amnioblasts) separate

from epiblast & form the amnion whichencloses the amniotic cavity

N.B.:

The amnion forms the roof of amniotic cavity The epiblast forms the floor of amniotic

cavity & is continuous peripherally withamnion


8/29

YOLK SAC

The hypoblast becomes continuousperipherally with the exocoelomicmembrane to enclose the

exocoelomic cavityPRIMARY YOLK SAC: has:1. A roof: formed by hypoblast2. A floor: formed by exocoelomic

membrane3. A cavity: formed by exocoelomic

cavity


9/29

YOLK SAC

SECONDARY YOLK SAC:

Smaller than primary yolk sac

Formed by migration ofendodermal cells from hypoblastto enclose a portion of primaryyolk sac

The rest of primary yolk sac ispinched off


10/29

YOLK SAC

SECONDARY YOLK SAC: has:

1. A roof: formed by hypoblast

2. A floor: formed by endodermalcells migrating from hypoblast

3. A cavity: formed by a portion of

the cavity of primary yolk sac


11/29

EXTRAEMBRYONICMESODERM

Formed by:1. Endodermal cells of yolk sac

2. later on by primitive streak

Surrounds amnion & yolk sac


12/29

CHORION

It is formed of:

1. Trophoblast

2. Extraembryonic mesoderm


13/29

EXTRAEMBRYONIC COELOM

Isolated spaces filled with fluid appearwithin the extraembryonic mesoderm

The spaces fuse to form the coelom The coelom divides the meosderm into

2 layers:

1.Extraembryonic somatic mesoderm:between trophoblast & coelom

2.Extraembryonic splanchnic mesoderm:between coelom & yolk sac


14/29

CONNECTING STALK

It is a part of extraembryonicmesoderm that does not contain

coelom It attaches the amnion & yolk sac

to the chorion

It indicates the future site ofumbilical cord


15/29

CHORIONIC SAC

It containsembryo +amniotic sac +

yolk sacsuspended bythe connectiong

stalk It is enclosed by

the chorion


16/29

PRIMARY CHORIONIC VILLI


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SECONDARY CHORIONIC VILLI


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TERTIARY CHORIONIC VILLI


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CHORIONIC VILLI

DEVELOPMENT: During implantation,the trophoblast thickens at region ofcontact with endometrium &

differentiates into:

1.Outer syncytiotrophoblast:a layercontaining multiple nuclei in a

common cytoplasm with no cellboundaries (secretes HCG)

2.Inner cytotrophoblast:a layer of

mononucleated cells


20/29

STAGES OF CHORIONIC VILLI

PRIMARY: At the end of 2nd week,finger-likeprocesses formed of outersyncytiotrophoblast & inner cytotrophoblastappear

SECONDARY: Early in 3rd week,extraembryonic mesoderm extends insidethe villi

TERTIARY: During 3rd week, arterioles,

venules & capillaries develop in themesenchyme of villi & join umbilical vessels

By the end of 3rd week, embryonic bloodbegins to flow slowly through capillaries in

chorionic villi


21/29

PRECHORDAL PLATE


22/29

PRECHORDAL PLATE

In a localized area in the medianplane, the cuboidal cells of thehypoblast become columnar &

form a thickening: prechordal plate The prechordal plate indicates the

future cranial region & future siteof mouth

It is an important organizer of the

head region


23/29

PRIMITIVE STREAK


24/29

PRIMITIVE STREAK & NODE


25/29

BILAMINAR & TRILAMINARDISCS


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PRIMITIVE STREAK

It appears as a linear opacity in theepiblast of the caudal part of theembryo, in the median plane

It results from proliferation of epiblast Primitive node: the enlarged cranial

end of primitive streak

Primitive Pit: a depression in primitivenode

Primitive groove: a narrow groove inprimitive streak that is continuous with

the primitive pit


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PRIMITIVE STREAK

IMPORTANCE:

1. It forms embryonic mesoderm

2. It replaces cells of hypoblast from the

roof of yolk sac & forms embryonicendoderm

3. The primitive node forms the

notochordal process4. It shares in the formation of

extraembryonic mesoderm


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_bilaminar & trilaminar discs

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