EXTRAEMBRYONIC MEMBRANES,

PLACENTATION

Dr Iram Iqbal

THE FETAL MEMBRANES

In tracing the development of any vertebrate

embryo, it becomes apparent that only part of the

egg (or of its cleavage cell mass) forms the actual

embryo, whereas other parts lie outside the

embryonic territory and for this reason are called

extra embryonic. These later regions are concerned

with the elaboration of so called embryonic or

fetal membranes. 22 jan 2010 2

FUNDAMENTAL SET OF FETAL MEMBRANES

Amnion Chorion Yolk sac Allantois

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FUNCTIONS OF FETAL

MEMBRANES

Protection Respiration Excretion Placenta is a specialized membrane for which

major part is contributed by chorion and allantois while uterine wall provides the rest.

Umbilical cord is a vascular cable between fetus and placenta.

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YOLK SAC Structure located ventral to the

bilaminar germ disc derived from hypoblast

Endodermal roof of the yolk sac is composed of taller cells from where tubular gut is fashioned by folding, also forms blood cells and vessels

With growth of embryo’s body there is progressive constriction of embryo from the yolk sac forming yolk stalk

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YOLK SAC The yolk stalk which soon is incorporated into

the umbilical cord. Yolk stalk detaches from the gut by the end of

5th week and presently degenrates The yolk sac usually persists throughout

pregnancy and sometimes can be found after birth beneath amnion near the attachment of umbilical cord to placenta

Not functional as store of yolk but plays a significant role in transfer of nutritive fluid to embryo from trophoblast

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YOLK SAC

Origin Fashions from early endodermal layer

Composition Endoderm and splanchnic mesoderm

Location Yolk stalk: within umbilical cord. Yolk sac: between amnion & placenta

Fate Disconnects from gut early. Stalk disappears early. Sac may persist

Function Roof forms the gut. Forms blood cells and vessels. Early absorptive function

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13 DAY EMBRYO

14 DAY EMBRYO22 jan 2010 10

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AMNION

The Amnion is a thin (but tough), transparent, nonvascular membrane containing amniotic fluid

The margins of the early amnion is attached to the periphery of the embryonic disc which serves as a floor to the amniotic cavity 22 jan 2010 13

AMNION With further development of the embryonic disc the

embryo takes a tubular form and the amniotic margins also show underfolding

Now the line of attachment becomes limited to the ventral body wall and then becomes confined to a constricting umbilical area

At the end of 2nd month it fills the chorionic sac and fuses loosely with chorion

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AMNIONOrigin Cavitation where inner cell mass joins

trophoblast

Composition Ectoderm and somatic mesoderm

Location Encloses embryo and umbilical cord. Attaches to embryo at the umbilicus

Fate Persists until birthFuses with chorionCovers fetal surface of placenta & after birth

Function Contains fetus immersed in amniotic fluid.

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FUNCTIONS OF AMNIOTIC FLUID:

Protective cushion which absorbs jolts

Prevents adherence of the amnionPermits changes of fetal postureAt childbirth the amnio-chorionic sacas act as a hydrostatic wedge to help ditate the neck of uterus

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DAY 15--16

DAY 2122 jan 2010 17

CHORION The primitive capsule of

trophoblastic tissue forms a sac which encloses the embryo and all other fetal membranes

The chorion is associated to the lining of the uterus which becomes highly specialized in the region of placenta

At the end of 2nd month the amnion fuses loosely with the chorion

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CHORIONOrigin Trophoblastic capsule of blastocyst

Composition

Trophoblast and somatic mesoderm of trophoblastic origin

Location Encloses embryo and all other fetal membranes

Fate Frondose part becomes fetal placenta. Smooth part fuses with decidua parietalis. Castoff after birth

Function Placental area is the fetal organ for nutrition, respiration and excretion

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UMBILICAL CORDOrigin Amnion wraps about yolk stalk &

body stalkComposition Chiefly allantoic vessels and C.T.

enveloped by the amnionLocation Connects belly wall with the fetal

side of placenta

Fate Cut of after birth and lost with placenta. Stump withers and detaches

Function Vascular pathway between fetus and placenta

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ALLANTOIS

Diverticulum of hind gut region of the yolk sac

Composed of Endoderm and splanchnic mesoderm

First lies within body stalk later within umbilical cord

Fate--epithelium disappears early (except traces)

Blood vessels persist which connect fetal circulation

with placenta

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ALLANTOISOrigin Diverticulum of hind gut region of

yolk sac

Composition Endoderm and splanchnic mesodermLocation First lies within body stalk later

within umbilical cord

Fate Epithelium disappears early (except traces). Blood vessels persist

Function Vessels connect fetal circulation with placenta

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REPTILES & BIRDSPresence of an enormous mass of yolkEmbryonic life spent within a shellBlastoderm

Small disc, spreads by peripheral growth, covers entire surface of egg

Embryo proper Extra embryonic blastoderm

Somatopleure (ectoderm and somatic mesoderm) Splanchopleure (endoderm and splanchnic

mesoderm)

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This image shows the embryo getting larger and some rippling texture beginning to form in the yolk

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YOLK SACAs the embryo enlarges, its circular connection with the extraembryonic blastoderm grows at a slower rate

This produces a constriction of splanchnopleure where it joins the rapidly elongating gut.

Yolk stalk

Yolk sac

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Vitelline blood vessels

arising in splanchnic mesoderm ramify on the surface of yolk sac.

Through these the absorbed yolk substance and oxygen are conveyed to the chick

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AMNION AND CHORIONConcentric sacs which arise by folding of extra embryonic somatopleure

The doubled layered somatopleure is thrown up into two crescentic folds.

Inner membrane is amnion Lined with ectoderm

Covered externally with somatic mesoderm

Thin transparent sac containing amniotic fluid

Lacks blood vessels

Muscle fibres differentiate in its mesodermal layer

Produce rythmic contractions

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Outer sac of somatopleure is chorion (serosa)

Component layers are in

reverse order to those of

amnion

Lies next to shell, encloses

both the embryo and all its

other fetal membranes and is

separated from them by extra

embryonic coelom22 jan 2010 32

ALLANTOISThis accessory organ was primarily evolved by reptiles

and birds as a temporary sac for urinary storage

Arises as an out pouching of ventral floor of the gut,

near its hind end

Layers of allantois (endoderm &splanchnic mesoderm)

Allantois sac and its expansion (in to extra-embryonic

coelom)

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ALLANTOIS

Fusion with chorion

Allantoic blood vessels

Functional lung of the embryo

Reservoir for the excreta of kidney

Assist in absorption of egg albumen

Detachment of allantois(shortly before hatching)

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MAMMALSYOLK SAC

Marsupial and placental mammals lack an actual yolk mass, yet a typical, stalked yolk sac appears and produces a complete vitelline circulation in quite young embryos

Early history and relation of this organ vary

Splanchnic mesoderm, surfacing the yolk sac is the layer that bears the vitelline blood vessels

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AMNION AND CHORIONMany mammals produce an amnion by folding but the details of the process vary

In some (rabbit, carnivores) Amnion arises by the simple folding of somatopleure

Chorion is merely all of the original trophoblastic capsule and its lining of extra embryonic mesoderm (other than those portions used in making the amnion)

Chorion has a diverse history but in all mammals above marsupials it becomes functionally important by differentiating chorionic villi and participating in the production of placenta

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In certain other mammals (guinea pig, hedge hog, anthropoids)

Primitive amnion cavity arises as a cleft that

separates a inner cell mass into two parts

Chorion is merely a later stage of original

trophoblastic capsule to which a lining of somatic

mesoderm has been added

In other mammals (pig, deer, rat)

Combination of the two methods of amnion

formation described above

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ALLANTOIS

Many mammals like reptiles and birds produce a

prominent allantois by the sacculation of gut

splanchnopleure into extra embryonic coelom

In carnivores it become very large, lines the chorionic sac

and fuses with it. Urinary wastes collect in it & are not

excreted through placenta

In rodents and primates it is vestigial

In anthropoids a tiny endodermal tube pushes into body

stalk even before the hind gut develops22 jan 2010 39

UMBILICAL CORD

Except for the persistence of allantoic stalk and

allantois throughout the fetal life of many

mammals, the formation and history of umbilical

cord do not differ significantly from man.

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The subject of placentation includes all the

events that are related to the following:

1. Implantation and establishment of the embryo within the uterus of the mother

2. The differentiation of the uterine lining into a specialized decidual membrane

3. The development of a placenta

4. The fetal – maternal association throughout pregnancy

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TRANSPORT OF THE OVUM & BLASTOCYST

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PREPARATION OF THE UTERUS FOR THE EMBRYO

Lining membrane undergoes: Thickening

Specialization

Vascularization

Glands Dilated

Contain glycogen

Compact layer is the site where embryo embeds & placenta develops

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IMPLANTATION OF EMBRYO Implantation includes the attachment of the blastocyst to

the epithelial lining of the uterus, the penetration of the blastocyst through the epithelium and its invasion and embedding in the compact layer of endometrium

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ATTACHMENT AND PENETRATION

Loss of corona radiata during the journey down the tubeShortly before implantation begins, zona pellucida disappears as wellHuman blastocyst begins to attach late in the 6th day after ovulationAdherence of blastocyst to uterine epitheliumThickening of trophoblastic wall of blastocyst in the region of contact Breakdown of cells of uterine epithelium in the area of attachmentCompletion of implantation

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ESTABLISHMENT OF THE EMBRYO

Even as the blastocyst is becoming implanted, its

trophoblastic wall (future chorion) starts on a course of

specialization which will put the invader in intimate

relation with the uterine blood

Establishment is not a one sided adjustment

Location of implanted blastocyst

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CHANGES IN TROPHOBLAST

Trophoblastic lacunae

Vascular relations

Endometrial erosionEndovascular invasion by cytotrophoblastic cells

Hybrid vessels

Development of villiPrimary villi

Secondary villi

Tertiary villi

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A 13 DAYS HUMAN BLASTOCYST ---- Primary villi22 jan 2010 50

TROPHOBLAST AT THE

END OF THIRD WEEK

OF DEVELOPMENT

Radial appearance

Intervillous spaces lined

with syncytiotrophoblast

Outer cytotrophoblast shell

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TROPHOBLAST AT THE BEGININING OF 2ND MONTH OF DEVELOPMENT

Secondary & Tertiary villi

Chorion Frondosum

Chorion Laeve

Endovascular invasion

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DEVELOPMENT OF A VILLUS

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THE DECICUAThe mucosal lining (endometrium) of the uterus,

already specialized in anticipation of pregnancy

and utilized as a nesting place by the implanting

embryo, rapidly acquires some additional

characteristics distinctive of pregnancy and then

persists throughout the gestation period.

Endometrium of pregnant uterus is named decicua

(that which falls off)22 jan 2010 56

DECIDUA BASALIS is the part of the decidua deep

to the conceptus that forms the maternal part of the placenta.

DECIDUA CAPSULARIS is the superficial part of the decidua overlying the conceptus.

DECIDUA PARIETALIS all the remaining parts of

decidua.22 jan 2010 57

The portions of chorion

participating in exchange process

are:

Chorion Frondosum

Decidua Basalis

Fusion of amnion and chorion

together form amniochorionic

membrane which rupture during

labour

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THE PLACENTABy the beginning of fourth month, Placenta has two components.

Fetal portion (Chorion Frondosum)

Maternal portion (Decidua basalis)

Fetal side placenta is bordered by chorionic plate while maternal side is bordered by decidua basalis.

Junctional zone

Intervillous space22 jan 2010 59

During 4th to 5th month decidual septa project into intervillous spaces but do not reach chorionic plate.

Septa have a core of maternal tissue but surface is covered by syncytial cells which separates the maternal blood from fetal tissue of villi.

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Placenta is divided into cotyledons by septa however contact between them is maintained.Placenta enlarge with advancement of pregnancy & may occupy 15 to 30% of uterine space.

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FULL TERM PLACENTA

Discoid in shapeDiameter 15-25cm3cm thickWeight 500-600gExpulsion about 30 minutes after child birth

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No. of cotyledons 15-20 visible on maternal side after child birth.

Cotyledons are covered by thin layer of decidua basalis.

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Fetal surface of placenta is covered by chorionic plate

Chorionic vessels converge towards umbilical cord

Attachment of umbilical cord is eccentric

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CIRCULATION OF THE PLACENTA

Cotyledons receive their blood through 80-100 spiral arteries

Intervillous spaces of a mature placenta contain approx 150ml of blood which is replenished 3-4 times / min

This blood moves along chorionic villi which have a surface area of 4-14 meter square

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Separates maternal and fetal blood

Initially consists of four layersEndothelial lining of fetal vessels

Connective tissue in the villus core

Cytotrophoblastic layer

Syncytium

From fourth month

Endothelial lining come in close contact with syncytium increasing rate of exchange greatly

PLACENTAL MEMBRANE

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APLACENTAL MAMMALS EGG LAYING MONOTREMES

MARSUPIALS After a brief gestation period give birth to immature young.

Yolk sac is large and unites with chorion to form, yolk sac-placenta

With longer gestation periods chorio -allantoic placental relation.

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HIGHER MAMMALS (PLACENTALIA)

Chorio- allantoic placentaAllantois fuses with chorion

Chorionic placentaWhen allantois is insignificant or lacking

Semi placenta --- Non deciduateChorionic villi lie in opposition with the uterine lining but do not fuse with it.

Placenta vera (True placenta) --- deciduate

Chorionic villi fuse with the eroded uterine mucosa

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TYPES OF IMPLANTATION Relation of the chorionic sac to uterine wall varies

greatly among placental mammals. In general there are three types of implantation

Superficial / Central implantation Carnivores, Monkey

Eccentric Rat, Squirrel

Interstitial Guniae pig, some bats, ape, man

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PLACENTAL SHAPEBased on the distribution pattern of villi

Diffuse Lemurs (among primates), ungulates (sow, mare)

Cotyledonary Cattle, Sheep and Deer

Zonary Carnivores

Discoid Insectivores, bats, rodents

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STRUCTURAL TYPESBased partly on the degree of contact between chorion

and uterus but more particularly upon the histological

relations established at the zone of junction of these two

components

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1.EPITHELIO – CHORIAL

Least modified placental condition Illustrated by lemurs, sow and mare The allantois and chorion unite and become jointly

vascularized by allantoic vessels, this composite membrane comes in apposition with uterine lining

Uterine milk Simple chorionic villi fit into corresponding pits in

the mucosa of uterus

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2. SYNDESMO – CHORIALSlight modificationProminent villi which occupy deeper pits in uterine liningIn some portions of uterine mucosa between the villi, there is local destruction of uterine epithelium which allows the chorionic ectoderm to come into direct contact with vascular maternal connective tissue Example: General type of ungulate

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3. ENDOTHELIO - CHORIALErosion of uterine mucosa practically bares the endothelium of its blood vessels and the syncytial chorionic epithelium then packs about these maternal vesselsAt birth there is destructive separation of placentaExample: Carnivores

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4. HEMO – CHORIALCharacterized by more thorough erosion of superficial uterine mucosaLabyrinthine type

Like endothelio-chorial except that endothelium of uterine vessels lost.

Villous typeEach chorionic villus is, in large measure, a freely branching tuft; these dangle in cavernous spaces and a directly bathed by maternal blood issuing from opened vessels

Example: Lower rodents, insectivores, bats and anthropoids

5. HEMO – ENDOTHELIALNearest approach to actual intermingling of blood of the two circulationsExample: Higher rodents (rat, guinea pig, rabbits)

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REFERENCES Arey’s Developmental Anatomy, 7th

edition. Langman’s medical embryology, 10th

edition The developing human, Keith L.

Moore, 8th edition22 jan 2010 78

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