Fetal membrane and placenta

• Decidua:

• After the implantation of the embryo, the

uterine endometrium is called the decidua.

The stramal cells enlrge,become vacuolated

and lipids.This change in the stromal cells

is called the decidua reaction.

• The decidua is divided into three parts according to the association with the embryo:

Decidua basalis: deep to the embryo

Decidua capsulris: over the embryo

Decidua paritalis: the left part of decidua

Decidua basalis

Decidua paritalis

Decidua capsulris

• Chorion

• Amnion

• Yolk sac

• allantois

• Unbiliad cord

Fetal m

emb

rane:

Lacuna

cytotrophoblast

syncytiotrophoblast

Extraembryonic mesoderm

• Chorion:

Formation of chorion:The cytotrophoblast

differentiates internally into a layer of primary

mesoderm. Trophoblast and primary mesoderm

together form the chorion.They give off

numerous process called villi or chorionic

villi.These villi are surrounded by maternal blood

.

• The chorionic villi are first formed all over

the trophoblast and grow into the

surrounding decidua.those related to the

decidua capsularis are transitory. After

some time they degenerate.This part of the

chorion becomes smooth and is called the

chorion laevae.

• In contrast,decidua undergo considerable

development. Along with the tissues of the

decidua basalis these villi form a disc-

shaped mass which is called the placenta.

The part of the chorion that helps form the

placenta is called the chorion frondosum.

Germ disc

Amnion

Yolk sac

chorion

Connecting stilk

Amnion

Connecting stilk

Extraembryonic cavity

Yolk sac

Extraembryonic cavity

Yolk sac

Umbilical cord

Amniotic cavity

Amniotic cavity

Umbilical cord

amnion

Chorion laevae

• All elements (syncytium, cytotrophoblast

and mesoderm) take part in forming

chorionic villi.Three stages in formation of

chorionic villi are seen:

A primary villus:cytotrophoblast and is

covered by the cells of syncytiotrophoblast.

• A secondary villus:primary mesoderm and is covered successively by cyto-and syncytiotrophoblasts.

• A tertiary villus contains in the center the

foetal blood vessels which are surrounded

successively from within outwards by primary

mesoderm, cyto and syncytiotrophoblasts.

capillary

Intervilli space

decidua

Syncytiotrophoblast

cytotiotrophoblast capillary Connective tissue

Cell shell

• From each tertiary stem villus numerous

branching villi project into the intervillous

space. the intervillous space is converted

into a sponge-like network of villous type

of labyrinthine structure and is filled with

maternal blood.

• THE AMNIOTIC CAVITY

• A fluid-filled amniotic cavity appears in the second week of development between the germ disc and the trophoblast.

• The roof of the cavity: a layer of flattened cells, the amnioblast, which lines the inner aspect of the cytotrophoblast.

• The floor of the cavity:the tall columnar cells of the epiblast of the germ disc.

Epiblast

Hypioblast

Aminiotic cavity

Connecting stalk

二胚层的羊膜

三胚层的胎膜

• With the extension of the extra-embryonic

coelom the outer surfaces of the amniotic

cavity and the yolk sac are covered with a

layer of primary mesoderm, which is

continuous with the primary mesoderm of

the chorion at the caudal end of the germ

disc through the connecting stalk.

• The formation of the embryonic folds

allows the amniotic cavity to surround the

outer surface of the cylindrical embryo. As

a result the amnio-ectodermal junction

converges towards the ventral surface of the

embryo to form the umbilical cord

• The amniotic cavity gradually increases in

size at the expense of the extra-embryonic

coelom, and eventually the amnion and

chorion leave are fused. Finally the extra-

embryonic coelom is completely obliterated,

except a small part which is contained in the

proximal part of the umbilical cord up to the

10th week of intra-uterine life.

Germ disc

Amnion

Yolk sac

chorion

Connecting stilk

Amnion

Connecting stalk

Extraembryonic cavity

Yolk sac

Extraembryonic cavity

Yolk sac

Umbilical cord

Amniotic cavity

Amniotic cavity

Umbilical cord

amnion

Chorion laevae

• The Amniotic Fluid

• The amniotic fluid, also called liquor

amnii, is clear and watery, containing about

2% solids which include inorganic salts,

urea, proteins and a trace of sugar. The

source of the fluid still remains unsettled-it

may be foetal from the amniotic cells,

maternal or both.

• Functions of the liquor amnii-

• 1.It acts as a protective cushion for the embryo against shock, blows or pressure. The embryo is suspended by the umbilical cord and literally swims in the fluid.

• 2.The fluid maintains a uniform pressure for the proper growth and differentiation of the delicate tissues of the embryo.

• 3.It allows foetal movements and maintains a constant environmental temperature.

• 4.During parturition, the amniotic sac forms a hydrostatic wedge and helps to dilate the cervical canal.

• Abnormalities of the liquor amnii-

• 1.Hydramnios This is a condition when the volume of the amniotic fluid exceeds two litres.

• 2.Oligamnios- In this condition the fluid is scanty in amount.

• Yolk sac

Primary Yolk sac

secondary yolk sac

Epiblast

Hypioblast

Aminiotic cavity

allantois

Primitive female sex cell

Yolk sac

• .Vitello-intestinal duct –It communicates

the mid gut with extra-embryonic part of

the yolk sac (umbilical vesicle). In the later

part of foetal life the duct disappears. On

rare occasions, the proximal part of the duct

persists as the Meckel’s diverticulum which

is attached to the antimesenteric border of

the ileum.

allantois

• Allanto-enteric diverticulum or allantois:

• Associated with the formation of the cloacal

membrane, a tubular endodermal outgrowth

known as allanto-enteric diverticulum or

allantois arises from the dorsi-caudal end of

the yolk sac extends into the primary

mesoderm of the connecting stalk.

• Probably in man the allantois helps to

vascularise the chorion and its villi with the

allantoic or umbilical blood vessels.

• The distal part of the diverticulum is fibrosed to form the urachus and the proximal part incorporates with the apex of the urinary bladder.

• If the lumen of the diverticulum persists entirely after the falling off of the cord, a urinary fistula takes place at the umbilicus.

• Placenta

• Human placenta is a discoid, chorio-

deciduate organ which connects the foetus

with the uterine wall of the mother. It is a

structure where maternal and foetal tissues

come in direct contact without rejection,

suggesting immunological acceptance of

the foetal graft by the mother.

• Gross Anatomy- At full term the placenta

is disc like, and presents after separation

from the uterine wall foetal and maternal

surfaces, and peripheral margi

• Foetal surface is smooth, covered by

amnion and presents the attachment of the

umbilical cord close to its center. Beneath

the amnion umbilical vessels radiate from

the cord.

• Maternal surface is rough and irregular,

and is mapped out into 15-30 polygonal

areas known as the cotyledons which are

limited by fissures. Each fissure is occupied

by a placental septum.

• Peripheral margin is continuous with the

foetal membrane which consists from

outside inwards of fused deciduas parietalis

and capsularis, chorion laeve and amnion.

Measurements –At full term the placenta

presents the following measurements:

• Diameter - 15 to 20 cm.

• Thickness - 3cm.(at the center).

• Weight - 500gms.

Structure of the placenta

• By the beginning of the fourth month, the placenta has two components:

• (a) a fetal portion, formed by the chorion frondosum

• (b) a maternal portion, formed by the decidua basalis

• Placenta is developed from two sources- foetal part

from chorion frondosum and maternal part from

deciduas basalis.

• The placenta consists of chorionic plate on the

foetal side, basal plate on the maternal side, stem

villi extending between the plates, and intervillous

space between the stem villi filled with the maternal

blood.

• On the fetal side, the placenta is bordered

by the chorion plate; On the fetal side, the

placenta is bordered by the decidua basalis

of which the decidual plate is most

intimately incorporated into the placenta.

• Chorionic plate:• (1)Villi: Fixed stem villi: cytotrophoblast

emerges through the syncytium of each villus,and attached to the decidua

Free stem villi: can not contact with the decidua,just float in the blood of the intervillus space.

• (2)Intervillous sapce:surrounding the villi filled in the maternal blood.

• The basal plate is perforated by the spiral

branches of uterine arteries and veins;

eventually the intervillous space is filled

with maternal blood. The portions of the

basal plate in between the stem villi project

into the intervillous space as placental

septa. Numerous placental septa project

from the basal plate into the intervillous

space but they fail to reach the chorionic

plate.

• Placental circulation:

• Maternal blood in the intervillous space-

• Foetal Blood in the villi of the Placenta

• The placental barrier consists of tissues

which intervene between foetal blood in the

chorionic villi and maternal blood in the

intervillous space. Through this barrier

exchange of gaseous and metabolic

products takes place between the foetus and

the mother.

• the barrier consists of the following four

layers from foetus to mother-endothelium

of foetal capillaries resting on a basement

membrane, a core of primary mesodermal

cells, a basement membrane upon which

rest cytotrophoblast and

syncytiotrophoblast.

• Functions of the Placenta-

Main function of placenta are exchange of metabolic and gaseous products between maternal and fetal bloodstream and production of hormones

•

• 1)Placenta acts in the exchange of gaseous and metabolic products between the maternal and foetal blood streams across the placental barrier.

• a)Oxygen intake and Carbon dioxide output-

• Intake of glucose, fatty acids, sodium, potassium, chloride and water in the foetal blood; Excretion of urea,

• uric acid and creatinine from the foetal to the maternal blood.

• Hormone production:

progesterone

estrogenic

human chorionic gonadotropin(HCG)

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