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Volgograd state medical university
Department of histology, embryology, cytology
for the 1st course
English medium students
Volgograd,
Volgograd, 2013
2013
1
Objectives:
Objectives:
1. To describe development, structure and functions of
placenta and umbilical cord.
2. To characterize the fetal membranes – chorion,
chorion,
amnion, yolk sac, allantois.
allantois.
3. To evaluate the stages of the fetal period.
2
1
FUNCTIONS
FUNCTIONS OF
OF PLACENTA
PLACENTA
AND
AND FETAL
FETAL MEMBRANES:
MEMBRANES:
Before birth:
Protection
Nutrition
Respiration
Excretion
Hormone production
At birth:
Placenta and fetal membranes separate from the fetus
and are expelled from the uterus as afterbirth.
3
FULL-TERM
FULL
FULL-TERM
PLACENTA
PLACENTA
Placenta is a fetomaternal
organ with fetal and
maternal portion.
It permits the exchange of
materials carried in the
bloodstreams
of
the
mother and embryo/fetus.
The chorion,
chorion, amnion, yolk
sac
and
allantois
constitute
the
fetal
membranes.
They
develop from a zygote but
do not form embryonic
structures, except for
portions of the yolk sac
and allantois.
allantois.
4
2
DEVELOPMENT
DEVELOPMENT OF
OF PLACENTA
PLACENTA
inner cell mass
degenerating
zona
pellucida
blastocyst
cavity
44 days
days
trophoblast
55 days
days
The morula passes down the uterine tube, enters the uterus
and fluid passes into the morula from the uterine cavity and collects
between the cells. As the fluid increases, it separates these cells
cells into
two parts: an outer cell mass called trophoblast and the inner cell
mass called embryoblast.
embryoblast. Trophoblast is the 1st extraembryonic organ
of the conceptus to be formed. It will give rise to chorion 5and
placenta.
Early stage of
implantation (the
6th
day).
The
trophoblast
is
attached to the
endometrial
epithelium at the
embryonic pole
of the blastocyst.
blastocyst.
On about 5th day zona pellucida disappears and the blastocyst
6
attaches to the maternal uterine epithelium on about 6th day.
3
IMPLANTATION
IMPLANTATION
In humans trophoblastic cells over the embryoblast pole begin
to penetrate between the epithelial cells of the uterine mucosa
7
about the 6th day.
Early
Early Stage
Stage of
of Implantation
Implantation (7th
(7th day).
day).
As invasion of the
trophoblast proceeds,
two cell layer form in it:
an
inner
cytotrophoblast
(cellular) and an outer
syncytiotrophoblast.
syncytiotrophoblast.
The
fingerfinger-like
processes
of
syncytiotrophoblast
penetrate
the
endometrial epithelium,
begin
to
destroy
adjacent
endometrial
cells
and
invade
endometrial stroma.
stroma. By
the end of the 1st week
the
blastocyst
is
superficially implanted
in the endometrium.
endometrium.
8
4
Week
Week 2.
2. Day
Day 8.
8. Size
Size of
of the
the conceptus
conceptus –– 0.1
0.1 mm
mm
At the 8th day
the blastocyst
is partially emembedded in the
endometrial
stroma.
stroma. Small
spaces appear
between
the
cells
of
the
inner cell mass
(embryoblast),
embryoblast),
these
spaces
soon coalesce
to form a slitslitlike
amniotic
cavity.
9
Blastocyst
Blastocyst partially
partially embedded
embedded at
at 8th
8th day,
day,
showing
showing embryoblast
embryoblast and
and trophoblast
trophoblast
10
As amniotic cavity forms, changes occur in the embryoblast resulting
in the formation of the flattened, essentially circular, embryonic
embryonic disk.
5
Human
Human
blastocyst.
blastocyst.
88 days.
days.
Epiblast cells adjacent to the cytotrophoblast are called
amnioblasts.
amnioblasts. They comprise amnion. Together with the rest of
the epiblast they line the amniotic cavity. The epiblast forms
the floor of the amniotic cavity and is continuous peripherally
11
with the amnion.
Amniotic
Amniotic Cavity
Cavity
On day 8 of development, embryoblast
differentiates into epiblast and hypoblast forming
bilaminar germ disc.
A small cavity appears within the epiblast. This
cavity becomes amniotic cavity and the covering
layer is amnion.
Amniotic cavity contains amniotic fluid.
12
6
Human
Human
Blastocyst.
.
Blastocyst
Blastocyst.
Day
9.
Day 9.
Concurrently other cells probably originating from hypoblast
form a thin exocelomic membrane (Heuser
(Heuser’’s membrane) which
encloses, together with the hypoblast, a cavity known as the primary
primary
13
(primitive) yolk sac or exocoelomic cavity.
Implanted
. 10
blastocyst
Implanted human
human blastocyst.
blastocyst.
10 days.
days.
A new population of cells appears between the inner surface
of the cytotrophoblast and the outer surface of the exocoelomic
cavity. These cells, derived from the yolk sac cells, form a fine
fine loose
14
connective tissue, the extraembryonic mesoderm.
7
Implanted
. 12
blastocyst
Implanted human
human blastocyst.
blastocyst.
12 days.
days.
Extraembryonic mesoderm eventually fills all the space
between the trophoblast externally and the amnion and exocoelomic
membrane internally. Soon large cavities appear in it, and when these
become confluent, they form a new space known as extraembryonic
15
coelom or chorionic cavity.
Formation
of
primary
or
extraembyonic
mesoderm
between
Heuser’
Heuser’s
membrane and
cytotrophoblast
at 12th day.
The extraembryonic mesoderm lining the cytotrophoblast and amnion
is called the extraembryonic somatopleuric mesoderm; that covering
16
the yolk sac is called extraembryonic splanchnopleuric mesoderm.
8
Human
Human embryo,
embryo,
13
13 days
days old.
old.
After implantation the
endometrium
transforms into decidua
starting from implantation
site. By the end of the 2nd
week the endometrium is
completely replaced by
the decidua.
decidua.
The extraembryonic somatic mesoderm and the trophoblast
together constitute the chorion.
Extraembryonic
mesoderm
chorion.
lining the inside of the trophoblast is known as chorionic plate.
A small secondary yolk sac has formed inside the primitive
17
yolk sac as it is pinched off thus decreasing the primitive yolk sac.
Human
Human embryo,
embryo,
14
days
14 days old.
old.
The definite yolk sac is much smaller than the primitive yolk
sac or original exocoelomic cavity. During its formation large
portions of the exocoelomic cavity are pinched off. These portions
are represented by exocoelomic cysts, which are often found in the
18
extraembryonic coelom or chorionic cavity.
9
Yolk
Yolk Sac
Sac
The cavity related to the hypoblast is
exocoelomic cavity which will be modified into
primary yolk sac.
Embryonic disc lies between amniotic cavity and
primary yolk sac.
ExtraExtra-embryonic mesoderm is developed from
the lining cells of the yolk sac.
Due to the development of extraembryonic
coelom in the extraembryonic mesoderm, a large
part of the primary yolk sac is pinched off
forming the secondary yolk sac.
19
Human Embryo, 14 days old. Bilaminar germ disc lined with tall
columnar ectoderm cells and cuboidal endoderm cells. It is
connected to the overlying trophoblast by means 20 of
connecting stalk.
10
Germ disk of a 1616-day embryo indicating the movement of
surface epiblast cells through primitive streak and
primitive node.
21
When the cloacal membrane appears, the posterior wall of
the yolk sac forms a small finger-like outpouching or diverticulum
from its caudal wall that extends into the connecting stalk. This
diverticulum is called allantoenteric diverticulum or allantois
(Greek “allantos”, “sausage”. In some lower vertebrates it serves
as a reservoir for excretion products of the renal system.
17 days
22
11
Allantois
Allantois
It is formed about the 16th day of development
when the yolk sac forms a small diverticulum
that extends into the connecting stalk.
In humans it remains rudimentary, but may be
involved with early blood cells and blood vessels
formation and is associated with development of
the urinary bladder and may be involved in
abnormalities of bladder development.
As the bladder enlarges, the allantois becomes
the urachus.
23
Extraembryonic
Extraembryonic Membane
Membane Formation
Formation
The trophoblast develops the two layers: outer
syncytiotrophoblast and inner cytotrophoblast.
cytotrophoblast. The
epiblast cell mass becomes hollow to form a fluidfluid-filled
primitive amniotic cavity. The hypoblast cells form a
simple squamous primitive yolk sac.
24
12
A second wave of hypoblast cell migration displaces the primitive
primitive yolk sac.
Extraembryonic mesoderm coats the old blastocyst cavity to complete the
extraembryonic membranes. The trophoblast is now the threethree-layered chorion.
chorion.
Mesoderm and endoderm (former epiblast cells) form the definite yolk sac.
Mesoderm and ectoderm (former epiblast cells) form the definite amnion. 25
Chorion
Chorion
Extraembryonic coelom becomes expanded to
form a chorionic cavity.
The extraembryonic mesoderm lining
cytotrophoblast is known as chorionic plate.
the
Connecting
stalk
is
extraembryonic mesoderm.
the
formed
by
The placenta forms of chorion.
chorion.
The connecting stalk between embryo and
placenta shifts to the tail end of the embryo to
form the umbilical cord.
26
13
Blastocyst
Blastocyst of
of
14
days
14 days
showing
showing
chorionic
chorionic
cavity
cavity and
and
endoderm
endoderm
lined
lined
secondary
secondary
yolk
yolk sac
sac
The trophoblast is characterized by villous structure. Cells of
cytotrophoblast proliferate locally and penetrate into the syncytiotrophoblast,
syncytiotrophoblast,
forming cellular columns surrounded by syncytium.
syncytium. Cellular columns with the
syncytial covering are known as primary villi.
villi.
By the beginning of the 3rd week the trophoblast is characterized by
27
primary villi that consist of cytotrophoblastic core covered by a syncytional
layer.
The
The Human
Human
Blastocyst,
,
Blastocyst
Blastocyst,
th
99th day.
day.
Bilaminar
Bilaminar
germ
germ disk
disk in
in
the
the lacunar
lacunar
stage.
stage.
The trophoblast shows considerable progress in development,
particularly at the embryonic pole, where vacuoles appear in the
syncytium.
syncytium. When these vacuoles fuse, they form large lacunae, and
this phase of trophoblast development is known as the lacunar stage.
The lacunar networks are the primodia of the intervillous spaces of
28 the
placenta.
14
Implanted
Implanted human
human
blastocyst.
.
blastocyst
blastocyst. 10
10 days.
days.
Trophoblastic lacunae at the
embryonic pole are in open
connection with maternal
sinusoids in the endometrial
stroma.
stroma.
The endometrial capillaries around the implanted embryo
become dilated to form sinusoids and some of them become eroded
by syncytiotrophoblast.
syncytiotrophoblast. Maternal blood now seeps into the lacunar
networks and soon begins to flow slowly through the lacunar system,
establishing a primitive uteroplacental circulation. As both arterial
and venous vessels come into communication with 29 the
syncytiotrophoblastic lacunae, blood circulation is established.
DEVELOPMENT
DEVELOPMENT OF
OF THE
THE FETAL
FETAL MEMBRANES
MEMBRANES
During further development mesodermal cells penetrate in
the core of the villi and grow toward endometrium.
newly
endometrium. The 30
formed villi are called secondary villi.
villi.
15
By the end of the 3rd week these cells begin to differentiate
into blood cells and small blood vessels, forming the villous capillary
capillary
system. These villi are known as tertiary or definite villi.
villi. Capillaries in
them make contact with the capillaries developing in the mesoderm
mesoderm
of the chorionic plate and in the connecting stalk. These vessels in
turn establish contacts with the intraembryonic circulatory system,
connecting future placenta and the embryo. Hence, when the heart
begins to beat in the 4th week of development, the villous system is
ready to supply the embryo with oxygen and nutrients.
44 weeks
weeks
Developme
Developme
nt
nt of
of the
the
Fetal
Fetal
Membranes
Membranes
31
Development
Development of
of Chorion.
Chorion.
Meanwhile cytotrophoblastic cells in the villi penetrate
progressively into the overlying syncytium until they reach the
maternal endometrium.
endometrium. Here they establish contact with similar
extensions of neighboring villous stems, forming a thin outer
32
trophoblastic shell.
16
This
shell
gradually
surrounds the trophoblast
entirely and attaches the
chorionic sac firmly to the
maternal endometrial tissue.
Tertiary and secondary villi
give trophoblast a characteristic
radial
appearance.
Intervillous spaces are lined
with
syncytium.
Cytotrophoblastic
cells
surround
the
trophoblast
entirely and are in direct
contact with endometrium. The
chorionic cavity meanwhile
becomes larger and larger. By
the 20th day embryo is attached
to its trophoblastic shell by
only a narrow connecting
stalk. The connecting stalk
later develops into umbilical
cord.
Presomite
Presomite Embryo.
Embryo.
20 days
33
By the beginning of the 2nd month the trophoblast is
characterized by a great number of secondary and tertiary villi that
give it a radial appearance. The villi are anchored in the mesoderm of
the chorionic plate and are attached peripherally to the maternal
decidua by way of outer cytotrophoblastic shell. At abembryonic pole
the villi are few and poorly developed.
End
End of
of the
the
nd
22nd month
month
34
17
Fetomaternal
Fetomaternal Junction
Junction
Fetal part of the placenta (villous
chorion)
chorion) is separated from the
maternal part (decidua
(decidua basalis)
basalis) by
the cytotrophoblastic shell.
35
36
18
FORMATION
FORMATION OF
OF DECIDUA
DECIDUA
th week
4th
The
term
decidua
(Latin “falling off”
off”): is applied
to the functional layer of
gravid (pregnant) endometendometrium indicating that it is shed
at parturition. Three regions of
decidua
are
designated
according to their relation to
the implantation site: decidua
basalis – the part underlying
the conceptus and forming
the maternal component of
placenta; decidua capsularis –
the
superficial
portion
overlying the conceptus and
decidua parietalis – the
remaining uterine mucosa.
Until the 8th week chorionic
villi cover the entire surface of
chorionic sac.
37
DEVELOPMENT
DEVELOPMENT OF
OF THE
THE DECIDUA
DECIDUA
As the sac grows, the villi associated with the decidua capsularis
become compressed and their blood supply is reduced. Subsequently,
Subsequently,
these villi degenerate, producing by the end of the 2nd month a bare
area known as the chorion laeve (Latin “levis”
levis”=smooth). As it occurs,
the chorionic villi associated with the decidua basalis,
basalis, rapidly increase
in number, branch profusely and enlarge. This portion of the of the
38
chorionic sac, known as the chorion frondosum (Latin
“frondosus”
frondosus”=leafy, bushy), forms the fetal component of the placenta.
19
Decidua basalis consists of a compact layer of large decidual
cells, with abundant amount of lipid and glycogen. With a growth of
the chorionic vesicle, decidua capsularis becomes stretched and
degenerates.
End
End of
of the
the
nd
nd
22 Month
Month
39
Subsequently the chorion laeve comes into contact with the
uterine wall (decidua
(decidua parietalis)
parietalis) on the opposite side of the
uterus, and the two fuse obliterating the uterine lumen.
40
20
Formation
Formation of
of the
the Amnio-Chorionic
Amnio-Chorionic Membrane.
Membrane.
As amniotic sac enlarges
faster than the chorionic sac,
the amnion and smooth
chorion fuse to form amnioamniochorionic membrane. This is
the membrane that ruptures
during labour.
labour.
10 weeks
20 weeks
41
Decidua
Decidua
is a functional layer of the endometrium of a
pregnant women,
decidual cells contain glycogen and lipids,
falls off after parturition
has three regions:
Decidua basalis - maternal part of placenta
Decidua capsularis - overlying conceptus
Decidua parietalis - remaining parts
42
21
Placenta
Placenta
is a fetomaternal organ,
fetal
portion
develops
frondosum of chorionic sac,
from
chorionic
maternal portion develops from decidua basalis
of gravid endometrium,
endometrium,
provides protection, nutrition,
excretion, hormone production.
respiration,
43
Full-term
Full-term placenta:
placenta:
is a discoid with a diameter of 15 to 25 cm,
3 cm thick,
weighs 500500-600 g,
at birth it is torn from the uterus wall,
approximately 30 minutes after birth of a
child it is expelled from the uterus.
44
22
ПОСЛЕД
ПОСЛЕД
45
Full-Term
Full-Term Placenta
Placenta
The
final
shape of placenta is
determined by the
form of the persistent
area of chorionic villi
giving the placenta a
discoid shape. As the
villi
erode
the
decidua basalis, they
leave several wedgeshaped
areas
of
decidual tissue called
placental
septa.
These septa divide
the fetal part of
placenta into 10-38
irregular areas called
cotyledons.
46
23
Each cotyledon consists of two or more main stem villi and their
many branches. The fetal portion of the placenta (or villous chorion)
is anchored to the maternal portion of the placenta (decidua basalis)
by anchoring villi. Blood-filled intervillous spaces are derived from
the lacunae developed in the syncitiotrophoblast during 2nd week.
Development
Development
of
of the
the
Placentic
Placentic
Chorionic
Chorionic
Villi
Villi
47
Collectively the
spaces form a
large
blood
sinus,
the
intervillous
space, which is
bound by the
chorionic plate
and
decidua
basalis.
48
24
Chorion
Chorion
Formation of chorionic villi:
villi:
Primary villi - consist of cytotrophoblastic core
covered by a syncytial layer.
Secondary villi = primary villi with mesodermal
core.
Tertiary villi = secondary villi with capillaries.
Stem villi –
arise from chorionic frondosum
and project into the intervillous space
containing maternal blood.
49
Chorion
Chorion
Chorionic plate - is the part of the
chorionic wall related to the placenta.
Chorionic frondosum (villous chorion)
chorion) is
formed by the chorion related to the
decidua basalis.
basalis.
Chorion leave (smooth chorion)
chorion) is related
to the decidua capsularis degenerate and
become smooth.
50
25
PLACENTA
PLACENTA
The intervillous
space is divided
into compartments
by the placental
septa, but because
the septa do not
reach the chorionic
plate,
there
is
communication
between the interintervillous spaces of
different compartcompartments.
ments. The intervilintervillous
space
is
drained by endoendometrial veins which
open over the entire
surface of decidua
basalis.
basalis.
51
Intervillous
Intervillous Space
Space
Derives from the lacunae that developed in the
syncytotrophoblast,
syncytotrophoblast,
Contains maternal blood,
Spiral endometrial arteries pour blood in these
spaces,
Blood from these
endometrial veins,
spaces
is
drained
by
Branch villi float in the intervillous spaces.
52
26
The chorionic plate is on the
fetal surface of the placenta.
Extending
from
it
with
placental branches of the
umbilical vessels are stem villi
that branch extensively to form
the free villi,
villi, where metabolic
exchange
occurs.
The
syntrophoblast is the outer
layer of the villi except at the
ends of the anchoring villi that
connect the villous chorion to
the decidua basalis.
basalis. Here the
cytotrophoblast layer extends
through the syntrophoblast to
form the cytotrophoblastic
shell at the interface between
fetal and maternal component
of the placenta. Decidual spiral
arteries open through this shell
to directly bathe the chorionic
villi in maternal blood.
Placental
Placental
Structure
Structure
53
PLACENTAL
PLACENTAL CIRCULATION
CIRCULATION
Placenta essentially provides a large area where
materials may be exchanged across the placental
membrane interposed between the fetal and maternal
circulation.
From the maternal blood fetal blood acquires
nutrients and oxygen.
Waste products formed within the embryo are carried
to the placenta and transferred to the maternal blood.
Within the placenta, the maternal and fetal
bloodstreams flow close to each other
BUT THEY DO NOT NORMALLY MIX!!!
54
27
Placenta
Placenta
Maternal blood circulates
through the intervillous
space
(C),
bringing
nutrients
and
other
substances necessary for
embryonic
and
fetal
development, and taking
away the waste products
of fetal metabolism.
A – villus
D – anchoring villus
G – decidua basalis
H – glands
55
Placental
Placental Membrane
Membrane (( Barrier)
Barrier)
is interposed between fetal and maternal
circulation.
is formed by extraextra-fetal tissues
until 20 weeks consists of four layers —
a) Syncytotrophoblast
b) Cytotrophoblast
c) Connective tissue of villi
d) Endothelium of fetal capillaries
56
28
Chorionic
Chorionic Villi
Villi
Bathed
by
Bathed by
Maternal
Maternal
Blood
Blood
As pregnancy advances the placental membrane (barrier)
becomes progressively thinner and many capillaries come to lie very
very
close to the syncytiotrophoblast sharing basal lamina (B). Syncitial
cell mass may become trophoblastic embolus (A). Towards the end of
pregnancy fibrinoid material forms on the surface of peripheral
trophoblast in the junctional zone (Rohr’
Langhans’
Rohr’s fibrinoid.),
fibrinoid.), Langhans’
subchorionic fibrinoid,
fibrinoid, and Nitabuch’
Nitabuch’s basal fibrinoid layer. 57The
cytotrophoblast disappears. These changes result from aging.
Collectively the spaces form a large blood sinus, the
intervillous space, which is bound by the chorionic plate
and decidua basalis.
58
29
59
CLINICAL
CLINICAL CORRELATIONS
CORRELATIONS
Maternal antibodies (IgG
(IgG class) are transported to fetal capillaries by
syncitiotrophoblast pinocytosis,
pinocytosis, providing passive immunity against
diphteria,
diphteria, smallpox, measles, etc, but not against chickenpox and
wooping cough. Passive immunity is important because the fetus
has little capacity to produce own antibodies until after birth.
Fetal red blood cells invading the maternal blood stream (through
(through
small bleeds at the surface of the villi)
villi) may elicit antibody response
in the mother if fetus is RhRh-positive while mother is RhRh-negative.
Maternal antibodies return to the fetus causing hemolytic disease
disease of
the newborn which may lead to the intrauterine death.
Many viruses traverse the placenta without difficulty (rubella,
cytomegalovirus,
Coxsackie,
variola,
varicella,
measles,
variola,
varicella,
poliomyelitis) causing fetal infection resulting in death or
malformations.
Most drugs and drug metabolites traverse placenta easily causing
damage (antibiotics, anticoagulants, anticonvulsants, antitumor
agents).
Maternal use of heroine and cocaine can cause habituation in
the
60
fetus.
30
Newborn male infant with typically malformed limbs
(meromelia)
meromelia) caused by thalidomide
61
Hormone
Hormone Production
Production by
by the
the Placenta
Placenta (Syntropoblast):
(Syntropoblast):
progesterone (by the end of the 4th month in
sufficient amount);
estrogens (with maximum level by fullfull-term);
human chorionic gonadotropin (during the first two
months);
somatomammotropin (gives the fetus priority on
maternal blood glucose making the mother
somewhat diabetogenic).
diabetogenic).
62
31
Placental
Placental Variations
Variations
The placenta may
have
accessory
lobes
with
vascular
connections
between
them
(succenturiate
placenta) or there
can be no vascular
connections
(placenta spuria).
spuria).
The umbilical cord
may be inserted at
the margin of the
placenta to give it
a clubclub-like appearappearance
(battledore
placenta).
In a more extreme
type of marginal
insertion
the
umbilical cord is
attached
to
amnion
and
chorion instead of
the
placenta
(velamentous
insertion of the
cord).
In
circumvallate
p.
the
membranes
extend over the p.
to form a ring
before
doubling
back toward the
margin.
63
may result in bleeding
64
32
Placenta
Placenta Previa
Previa
If
implantaimplantation is in lower
part
of
the
uterus,
the
placenta
will
partially or totally
cover the internal
os of the cervix.
It can block the
birth canal and is
a common cause
of bleeding in the
3rd
trimester.
Hemorrhage from
placenta previa
can be fatal to
the fetus or even
the mother.
65
20-weeks
On the right – two monozygotic twins (boys),
On the left – dizygotic girl.
66
33
67
Development of fetal membranes in twins
(A) Cleavage was before trophoblast formation – different amnion & chorion.
(B) Cleavage was after trophoblast formation,but before amnion development –
same chorion but different amnions.
68
(C) Cleavage was after amnion formation – same amnion & chorion.
(Langman 1981)
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Further
Further Development
Development of
of the
the Fetal
Fetal Membranes
Membranes
STAGE 12
End of the 4th week
foregut
midgut
hindgut
allantois
Vitelline
duct
Connecting stalk
After head and tail folding the connecting stalk is attached to the
ventral surface of the embryo and allantois is partially incorporated
into the embryo. By the end of the 5th week the yolk stalk and the
connecting stalk merge to form umbilical cord. The midgut
communicates with the yolk sac by way of a broad stalk – vitelline
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duct.
In 33-week embryo the
yolk sac found in the
chorionic cavity. During
further development the
amniotic
cavity
enlarges rapidly at the
expense
of
the
chorionic cavity, and
the amnion begins to
envelop the connecting
stalk and yolk sac stalk,
crowding them together
and giving rise to the
primitive umbilical cord.
3 weeks
4 weeks
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35
Distally
the
cord
contains vitelline duct and
umbilical vessels. Proximally it
contains intestinal loops and the
remnants of allantois.
allantois.
By the 10th week the yolk
sac is still found in the chorionic
cavity, it is connected to the
umbilical cord by the vitelline
duct. At the end of the 3rd month
the amnion has expanded so
much that it comes in contact
with chorion obliterating the
chorionic cavity. The yolk sac
then usually shrinks to a pearpearshaped remnant, 5 mm in
diameter, connected to the
midgut by the narrow yolk stalk,
and is gradually obliterated.
10 weeks
20 weeks
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The
The Yolk
Yolk Sac:
Sac:
In humans is nonfunctional as far as yolk storage is
concerned,
Is essential in humans as an organ of primitive
hemopoiesis (between the 3rd and 6th week) and sourse
of primodial germ cells (in the 3rd week).
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36
Umbilical
Umbilical Cord
Cord
Develops from the connecting stalk which is
formed by the extraembryonic mesoderm.
It connects fetus and placenta
It’
It’s diameter is of 11-2 cm
It’
It’s length varies from 39 to 90 cm
It contains two arteries and one vein (left)
It contains mucoid connective tissue
(Wharton jelly)
The remnants of allantois and yolk sac
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persist in it almost until fullfull-term.
20-weeks fetus with true knot of umbilical chord
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37
12-weeks fetus inside amniotic cavity
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Baby with amputated right foot
in reason of amniotic tapes formation
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38
Amniotic
Amniotic cavity:
cavity:
Is filled with a watery fluid produced by the amniotic cells,
Contains around 30 ml of amniotic fluid at 10 weeks of gestation,
gestation,
350 ml at 20 weeks, 800800-1000 ml at 37 weeks,
The fluid absorbs jolts, prevents adherence of the embryo to the
amnion, allows fetal movements.
The volume of fluid is replaced every 3 hours.
From the beginning of the 5th month the fetus swallows its
amniotic fluid (400 ml per day).
Fetal urine is added to the amniotic fluid in the 5th month.
This urine is mostly water as placenta is functioning as an
exchange for metabolic wastes.
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39