Placenta Dr.Sumeya
Obgective
The lecture show normal placentation and abnormal placentation
that cause pregnancy complication such as placenta previa, accrete
and IUGR.
Placenta
• This is a fetomaternal organ.
• It has two components:
– Fetal part and Maternal part
• The placenta and the umbilical cord are a transport system for substances between
the mother and the fetus.( vessels in umbilical cord )
Further Development of Chorionic Villi
Early in the 3rd week, mesenchyme
growth into the primary villi forming a
core of mesenchymal tissue. Thus the
Secondary Chorionic Villi are formed over
the entire surface of the chorionic sac.
Some
mesenchymal
cells
in
the
secondary
villi
differentiate
into
capillaries and blood cells forming the
Tertiary Chorionic Villi.
The capillaries in the villi fuse to form
arteriocapillary networks.
The fetal component of placenta is derived
from the trophoplast and extraembryonic
mesoderm(chorionic plate);
the maternal component is derived from the
uterine endometrium.
Maternal blood is delivered to the placenta
by spiral arteries in the uterus. Erosion of
these maternal vessels to release blood into
intervillous spaces is accomplished by
endovascular invasion by cytotrophoblast
cells.
• These cells, released from the ends of anchoring villi ,invade the terminal ends of spiral
arteries, where they replace maternal endothelial cells in the vessels walls, creating hybrid
vessels containing both fetal and maternal cells.
• To accomplish this process, cytotrophoblast cells undergo an epithelial-to-endothelial
transition. Invasion of the spiral arteries by cytotrophoblast cells transforms these vessels
from small-diameter, high-resistance vessels to larger-diameter, low resistance vessels that
can provide increased quantities of maternal blood to intervillous spaces .
• During the following months, numerous
small extensions grow out from existing
stem villi and extend as free villi into the
surrounding lacunar or intervillous spaces.
• Initially, these newly formed free villi are
primitive, but by the beginning of the
fourth month, cytotrophoblastic cells and
some connective tissue cells disappear. The
syncytium and endothelial wall of the
blood vessels are then the only layers that
separate
the
maternal
and
fetal
circulations.
DECIDUA
• This is the endometrium of the gravid
(pregnant) uterus.
• It has four parts:
– Decidua basalis: it forms the maternal part of
the placenta, consists of a compact layer of
large cells, decidual cells, with abundant
amounts of lipids and glycogen. This layer, the
decidual plate, is tightly connected to the
chorion.
– Decidua capsularis: it covers the conceptus.With growth of
the chorionic vesicle, this layer becomes stretched and
degenerates. Subsequently, the chorion laeve comes into
contact with the uterine wall (decidua parietalis) on the
opposite side of the uterus, and the two fuse to obliterating
the uterine lumen.
– Decidua parietalis: the rest of the endometrium.
– Similarly, fusion of the amnion and chorion to form the
amniochorionic membrane obliterates the chorionic cavity
DEVELOPMENT OF PLACENTA
– In the early weeks ofdevelopment villi cover the entire surface of the chorion .As
pregnancy advances villi on the emberionic pole continue to grow and expand ,giving rise
to the chorion frondosum (bushy chorion ). Villi on the abemberyonic pole degenerate
,and by the third month , this side of the chorion ,now known as the chorion leave , is
smooth.
– Chorion frondosum (villous chorion)
– Chorion laeve – bare (smooth) chorion
– About 18 weeks old, it covers 15-30% of the decidua and weights about 1\ 6 of fetus
• The villous chorion ( increase in number, enlarge and branch ) will form the fetal part of
the placenta.
• The decidua basalis will form the maternal part of the placenta.
• The placenta will grow rapidly.
• By the end of the 4
th
month, the decidua basalis is almost entirely replaced by the fetal
part of the placenta.
Structure of placenta
• By the beginning of the fourth month, the placenta has two components:
• (1) a fetal portion, formed by the chorion frondosum and
• (2) a maternal portion, formed by the decidua basalis
• On the fetal side, the placenta is bordered by the chorionic plate
• on its maternal side, it is bordered by the decidua basalis, of which the decidual plate is
most intimately incorporated into the placenta.
• In the junctional zone, trophoblast and decidual cells intermingle.
• Between the chorionic and decidual plates are the intervillous spaces, which are filled with
maternal blood. They are derived from lacunae in the syncytiotrophoblast and are lined
with syncytium of fetal origin. The villous trees grow into the intervillous blood lakes
• During the fourth and fifth months, the decidua forms a number of decidual septa, which
project into intervillous spaces but do not reach the chorionic plate . These septa have a
core of maternal tissue, but their surface is covered by a layer of syncytial cells, so that at
all times, a syncytial layer separates maternal blood in intervillous lakes from fetal tissue of
the villi.
• As a result of this septum formation, the placenta is divided into a number of
compartments, or cotyledons.
• Because the decidual septa do not reach the chorionic plate, contact between intervillous
spaces in the various cotyledons is maintained.
• As a result of the continuous growth of the fetus and expansion of the uterus, the placenta
also enlarges.
• Its increase in surface area roughly parallels that of the expanding uterus, and throughout
pregnancy, it covers approximately 15% to 30% of the internal surface of the uterus. The
increase in thickness of the placenta results from arborization of existing villi and is not
caused by further penetration into maternal tissues
FULL-TERM PLACENTA
• Cotyledons –about 15 to 20 slightly bulging
villous areas. Their surface is covered by shreds
of decidua basalis from the uterine wall.
• After birth, the placenta is always inspeced for
missing cotyledons. Cotyledons remaining
attached to the uterine wall after birth may
cause severe bleeding.
• Grooves – formerly occupied by placental septa
FULL-TERM PLACENTA
( Discoid shape -500- 600 gm- Diameter 15-20 cm – Thickness of 2-3 cm)
• Fetal surface:
• This side is smooth and shiny. It is covered entirely by the chorionic plate,on it amnion.
• The umbilical cord is attached close to the center of the placenta.
• The umbilical vessels radiate from the umbilical cord.
• They branch on the fetal surface to form chorionic vessels.
• They enter the chorionic villi to form arteriocapillary-venous system.
PLACENTAL CIRCULATION
Circulation of placenta
• Cotyledons receive their blood through 80 to 100 spiral arteries that pierce the decidual
plate and enter the intervillous spaces at more or less regular intervals.
• Pressure in these arteries forces the blood deep into the intervillous spaces and bathes the
numerous small villi of the villous tree in oxygenated blood. As the pressure decreases,
blood fl ows back from the chorionic plate toward the decidua, where it enters the
endometrial veins .
• Hence, blood from the intervillous lakes drains back into the maternal circulation through
the endometrial veins. Collectively, the intervillous spaces of a mature placenta contain
approximately 150 mL.
Placental barrier
• Placental barrier(placental membrane) which is formed by endothelial layer of vessel and
syncytium is not true barrier ,as many substances pass through it freely .
• Normaly ,there is no mixing of maternal and fetal blood. However ,small numbers of fetal
blood cells occasionally escape across microscopic defects in the placental membrane.
STRUCTURE OF STEM CHORIONIC VILLUS
Functions Of The Placenta
1. Exchange of gases
2. Exchange of nutrition and electrolytes
3. Transmission of maternal antibodies
4. Excretion
5. Hormone production
Placental endocrine synthesis
• The syncytiotrophoblast synthesizes protein &steroid hormones
• The protein homones
• 1- human chorionic gonadotropin
• 2- h.c. somatomammotropin
• 3-h.c. thyrotropin
• 4-h.c. corticotropin
• The steroid hormones
• Progesterone & Estrogens
FULL-TERM UMBILICAL CORD
• Usually it is attached near the center of the
fetal surface of placenta.
• Length: about 50 cm
• Diameter: 1-2 cm
• Contains two arteries and one vein,
surrounded by mucoid connective tissue
(Wharton jelly)
• The vessels are longer than the cord and may
have loops (false knots).
• Rarely, however, does it insert into the chorionic
membranes outside the placenta (velamentous
insertion).
Bilobate placentae
• Bilobate placentae consist of two lobes of approximately equal size Can rarely have three or
more lobes).
Succenturiate lobes
• One or more accessory lobes are developed in the membranes at a distance from the
periphery of the main placental mass.
Placenta praevia
• Placenta is inserted partially or wholly in the lower uterine segment.
• It is classified as either major, in which the placenta is covering the internal cervical os, or
minor, when the placenta is sited within the lower segment of the uterus, but does not
cover the cervical os.
Abnormal implantation
Placenta accreta
Three grades of abnormal placental attachment are defined according to the depth of
invasion:
• Accreta — chorionic villi attach to the myometrium, rather than being restricted
within the decidua basalis.
• Increta — chorionic villi invade into the myometrium.
• Percreta — chorionic villi invade through the myometrium.
Thank You