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
capsulris
Decidua
paritalis
Decidua
basalis
Fetal membrane:
•
•
•
•
•
Chorion
Amnion
Yolk sac
allantois
Unbiliad cord
Lacuna
syncytiotrophoblast
cytotrophoblast
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 discshaped mass which is called the placenta.
The part of the chorion that helps form the
placenta is called the chorion frondosum.
Amnion
Connecting stilk
Germ disc
Yolk sac
chorion
Amnion
Connecting stilk
Yolk sac
Extraembryonic
cavity
Extraembryonic
cavity
Amniotic
cavity
Umbilical
cord
Yolk sac
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
Cell shell
decidua
Syncytiotrophoblast
Connective tissue
capillary
cytotiotrophoblast
• 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.
Aminiotic
cavity
Epiblast
Hypioblast
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.
Amnion
Connecting stilk
Germ disc
Yolk sac
chorion
Amnion
Connecting stalk
Yolk sac
Extraembryonic
cavity
Extraembryonic
cavity
Amniotic
cavity
Umbilical
cord
Yolk sac
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
Aminiotic
cavity
Epiblast
Hypioblast
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
-
• Thickness
- 3cm.(at the center).
• Weight
15 to 20 cm.
- 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 PlacentaMain 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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