Guang-xi medical university
chen weiping
Components of the placenta and
fetal membranes :
• The chorion, amnion,
yolk sac, allantois, and
umbilical cord
constitute the fetal
membranes.
• All these membranes
develop from the
Zygote.
The decidua
The endometrium that are filled with extravasate and the
tissue is edematous, cells of the endometrium become
polyhedral and loaded with glycogen and lipids , is named
the decidua. (decidua reaction)
Three regions of the decidua
(1) Decidua Basalis: the part of the decidua deep to the
conceptus that forms the maternal component of the
placenta.
(2) Decidua Capsular: the superficial portion overlying the
conceptus.
(3) Decidua Parietalis: all the remaining mucosal lining of
the uterus.
Changes in the trophoblast 1
By the beginning of the second month the trophoblast
is characterized by a great number of villi , so it is
named the chorion.
The surface of the villi is formed by the syncytrophoblast ,
resting on a layer of cytotrophoblastic cells , with a core of
vascular mesoderm.
Changes in the trophoblast 2
1.The villi on the embryonic pole
continue to grow and expand,
giving rise to the chorion frondosum
(bushy chorion).
2.The villi on the abembryonic
pole degenerete , known as the
chorion laeve (smooth chorion).
3.The decidua over the chorion
frondosum( bushy chorion) is the
decidua basalis
Structure of the placenta
The chorion frondosum (bushy chorion) and the decidua basalis
makes up the placenta.
The placenta has two components:
a fetal portion
formed by chorion frondosum
a maternal portion formed by decidua basalis
The decidual septa grow from the decidua ,project into the intervillous
spaces , separate the villi into cotyledons ( about 15-25 group).
Full-term placenta
The placenta is a discoid with
Diameter
15-25 cm
thick
3 cm
Weigh
500-600 g
Viewed from the maternal side:
cotyledons
15-25 slightly bulging areas
Viewed from the fetal surface:
the chorionic vessels
the umbilical cord
the amnion
Circulation of the placenta
The spiral arteries
The fetal
circulation
The
intervillo
us spaces
The
endometrial
veins
The placental
membrane
The maternal
circulation
The placental membrane
The placental membrane (placental barrier)
composed of four layers:
the endothelial of fetal vessels
the connective tissue
the cytotrophoblast
the syncytium
After fourth month the placenta
membrane thins , consist of the
endothelial lining of the vessels
and the syncytial membrane.
syncytiotrophoblast
The
intervillous
spaces
cytotrophoblast
endothelium
of the fetal
capillaries
connective
tissue
Transverse section of a villus
Fuction of the placenta 1
The placenta has three main activities:
(1) transfer
(2) endocrine secretion.
All these activities are essential for
maintaining pregnancy and promoting
normal embryonic and fetal development.
O2
H2O, Ion
Monosaccharides
Free fatty acids
Amino acids
Vitamins
Hormones
Drugs
Viruses
CO2
Urea
Uric acid
Bilirubin
H2O, Ion
Hormones
maternal blood Placental Membrane fetal blood
Fuction of the placenta 2
The syncytiotrophoblast synthesizes various hormones.
Protein Hormones
The protein hormone products of the placenta are
(1) human chorionic gonadotropin (hCG),
(2) human placental lactogen (hPL).
Steroid Hormones
The placenta plays a major role in the production of
(1) progestins
(2) estrogens.
The Umbilical Cord
The attachment of the umbilical cord is
usually near the center of the placenta, but
it may be found at any point.
The umbilical cord is usually 1 to 2 cm in
diameter and 30 to 90 cm in length (average
55 cm).
The umbilical cord usually contains two
arteries and one vein that are surrounded
by mucoid connective tissue,.
amnion
amniotic sac
ectoderm
allantois
mesoderm
endoderm
yolk sac Foregut
midgut
hindgut
The Amnion And Amniotic Fluid
The amnion forms a fluid-filled membranous
amniotic sac that surrounds the embryo and,
later the fetus.
Because the amnion is attached to the margins
of the embryonic disc, its junction with the
embryo is located on the ventral surface after
folding of the embryo. As the amnion enlarges,
it gradually obliterates the chorionic cavity and
sheathes the umbilical cord.
Origin of the Amniotic Fluid
Initially some fluid may be secreted by the
amniotic cells, but most amniotic fluid is
derived from the maternal blood by
transport across the amnion from the
decidua parietalis and the intervillous
spaces of the placenta.
The fetus also makes a contribution to the
amniotic fluid by excreting urine into the
amniotic cavity.
Volume of the Amniotic Fluid
The volume of amniotic fluid normally
increases slowly, reaching about 30 ml
at 10 weeks, 350 ml at 20 weeks, and
1000 ml by 37 weeks.
Circulation of the Amniotic Fluid
The water content of amniotic fluid changes
every three hours. Large amounts of amniotic
fluid pass through the amnion into the
maternal blood, particularly where the
amnion is adherent to the fetal surface of the
placenta.
Amniotic fluid is also swallowed by the fetus
and absorbed by the fetus’s digestive tract.
胎盘
脐带
子宫腔
Exchange of the Amniotic Fluid
Large volumes of fluid move in both directions
between the fetal and maternal circulations,
mainly via the placental membrane.
Fetal swallowing of amniotic fluid is a normal
occurrence. Most of the fluid passes into the
gastrointestinal tract.
The fluid is absorbed into the fetal circulation
and then passes into the maternal circulation
via the placental membrane.
Composition of the Amniotic Fluid
The fluid in the amniotic cavity is a
solution in which undissolved material is
suspended. It consists of desquamated
fetal epithelial cells.
Studies of cells in the amniotic fluid
permit diagnosis of the sex of the fetus
and the detection of fetuses with
chromosomal abnormalities.
Significance of the Amniotic Fluid
The embryo, suspended by the umbilical cord, floats freely
in the amniotic fluid This buoyant medium
(1) permits symmetrical external growth of the embryo;
(2) prevents adherence of the amnion to the embryo;
(3) cushions the embryo against injuries by distributing
impacts the mother may receive;
(4) helps control the embryo’s body temperature by
maintaining a relatively constant temperature;
(5) enables the fetus to move freely, thus aiding
musculoskeletal development.
Oligohydramnios
Low volumes of amniotic fluid (e.g., 400
ml in the third trimester) results in most
cases from placental insufficiency with
diminished placental blood flow.
In renal agenesis (failure of the kidneys to
form), the absence of the fetal urine is the
main cause of oligohydramnios.
Polyhydramnios
An accumulation of amniotic fluid in excess of
2000 ml may occur when the fetus does not
drink the usual amount of amniotic fluid.
This condition is often associated with severe
malformations of the central nervous system.
In other malformations, such as esophageal
atresia, amniotic fluid accumulates because it
is unable to pass to the stomach and the
intestines for absorption.
The Yolk Sac
By nine weeks, along with the folding of
embryo the yolk sac has shrunk to a
pear-shaped remnant.
It is connected to the midgut by the
narrow yolk stalk.
By 20 weeks, the yolk
sac is very small;
thereafter, it is usually
not visible.
amnion
amniotic sac
ectoderm
allantois
mesoderm
endoderm
Foregut
midgut
hindgut
yolk sac
Significance of the Yolk Sac
Although the human yolk sac is nonfunctional, it plays
an important rule in making hematopoiesis and
forming the germ cells during the embryo development.
Blood development occurs in the wall of the yolk sac
beginning in the third week and continues to form there
until hematopoietic activity begins in the liver during
the sixth week.
The primordial germ cells appear in the wall of the yolk
sac in the third week and subsequently migrate to the
developing sex glands or gonads. Here, they become the
germ cells (spermatogonia or oogonia).
Fate of the Yolk Sac
At 10 weeks, the small yolk sac lies in the
chorionic cavity between the amnion and the
chorionic sac.
The yolk sac shrinks as pregnancy advances,
eventually becoming very small.
The yolk stalk usually detaches from the
midgut loop by the end of the sixth week.
The Allantois
Significance of the Allantois
Although the allantois does not function
in human embryos, it is important for
two reasons:
(1)blood formation occurs in its wall
during the third to fifth weeks; and
(2) its blood vessels become the umbilical
vein and arteries
amnion
amniotic sac
ectoderm
allantois
mesoderm
endoderm
Foregut
midgut
hindgut
yolk sac
Fate of the Allantois
The intraembryonic portion of the allantois runs
from the umbilicus to the urinary bladder, with
which it is continuous.
As the bladder enlarges, the allantois involutes
to form a thick tube called the urachus.
After birth, the urachus becomes a fibrous cord,
called the median umbilical ligament , that
extends from the apex of the urinary bladder to
the umbilicus.
Twins
Twins may originate from two zygotes, in which case
they are dizygotic twins or fraternal twins, or from
one zygote, i.e., monozygotic twins or identical twins.
The fetal membranes and placenta(s) vary according
to the origin of the twins.
In the case of monozygotic twinning, the type of
membranes formed depends on when the twinning
occurs. If division of the inner cell mass occurs after
the amniotic cavity forms (about eight days), the
identical embryos will be within the same amniotic
and chorionic sacs.
Dizygotic Twins
Because they result from the fertilization of two ova
by two different sperms, dizygotic twins may be of the
same sex or different sexes.
Dizygotic twins always have two amnions and two
chorions, but the chorions and placentas may be
fused.
Dizygotic twinning shows a hereditary tendency.
Monozygotic Twins
Because they result from the fertilization of one ovum,
monozygotic twins are of the same sex, genetically
identical, and very similar in physical appearance.
Physical differences between identical twins are
environmentally induced
Monozygotic twinning usually begins in the blastocyst
stage, around the end of the first week, and results
from division of the inner cell mass into two
embryonic primordia. Subsequently two embryos,
each in its own amniotic sac, develop within one
chorionic sac and have a common placenta.