The document discusses fetal membranes including the amnion, chorion, yolk sac, allantois, and umbilical cord. It describes their development, structure, and functions. The amniotic fluid is mainly composed of water and plays various roles in fetal growth. The chorion develops from the trophoblast and contains villi that contribute to placental formation. The placenta forms from the chorionic villi and decidua basalis and functions to exchange gases, nutrients, and waste between the maternal and fetal blood without mixing.

1. Fetal membranes , placenta & twins
Dr. Mohamed El fiky
Professor of anatomy and embryology

2. Early Human Development
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3. Early Human Development
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4. Early Human Development
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5. Early Human Development
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6. Embryo after folding
Head swelling
Cardiac swelling
Umbilical cord
Y.S
G
U
T
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7. • The term fetal membrane is applied to those structures derived from
the blastocyst which do not contribute to the embryo.
ØThe amnion,
Øthe chorion,
Øthe yolk sac
ØAllantois
ØUmbilical cord
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8. Amnion
•Amniotic membrane :
amniotic epi. + extraembryonic
mesoderm
•Amniotic fluid:
Produce:1)amniotic cells
2) infusion of fluid from
maternal blood
3) urine output from the fetus
4) pulmonary secretions
Output: 1) absorbed by amniotic cells
2) fetus swallow
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9. Amniotic Fluid
• Plays a major role in fetal growth and development.
• Daily contribution of fluid from respiratory tract is 300-400 ml.
• 500 ml of urine is added daily during the late pregnancy.
• Amniotic fluid volume is 30 ml at 10 weeks, 350 ml at 20 weeks,
700-1000 ml at 37 weeks.
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10. Composition of Amniotic Fluid
• 99 % is water
• Desquamated fetal epithelial cells
• Organic & inorganic salts
• Protein, carbohydrates, fats, enzymes, hormones
• Meconium & urine in the late stage
Abnormalities of amniotic fluid
• Oligo-hydramnios: the volume of the amniotic fluid is less than ½ litre. This may lead to
adhesions between the embryo and the amnion.
• Poly-hydramnios: the volume of the amniotic fluid is more than 2 litres. This may lead to
premature rupture of the amnion.
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11. Significance of Amniotic Fluid
• Permits symmetrical external growth of the embryo and fetus
• Acts as a barrier to infection
• During labor it help dilatation of the cervix of the uterus and It wash birth canal and
protect the fetus against infections
• Prevents adherence of amnion to fetus
• Cushions & protects the embryo and fetus
• Helps maintain the body temperature
• Enables the fetus to move freely
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12. Yolk Sac
• It is large at 32 days
• Shrinks to 5mm pear shaped
remnant by 10th week &
connected to the midgut by a
narrow yolk stalk
• Becomes very small at 20
weeks
• Usually not visible thereafter
Primary yolk sac
secondary yolk sac
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13. Significance of Yolk Sac
• Has a role in transfer of nutrients during the 2nd and
3rd weeks
• Blood development first occurs here
• Incorporate into the endoderm of embryo as a
primordial gut
• Primordial germ cells appear in the endodermal lining
of the wall of the yolk sac in the 3rd week
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14. Fate of Yolk Sac
• At 10 weeks lies in the chorionic cavity between chorionic and amniotic sac
• Atrophies as pregnancy advances
• Sometimes it persists throughout the pregnancy but of no significance
• In about 2% of adults the proximal intra-abdominal part of yolk stalk persists
as an ileal diverticulum or Meckel diverticulum
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15. Allantois
• In the 3rd week it appears as a
sausagelike diverticulum from the
caudal wall of yolk sac that extends
into the connecting stalk
• During the 2nd month, the
extraembryonic part of the allantois
degenerates Mohamed el fiky

16. Functions of Allantois
• Blood formation occurs in the wall during the 3rd to 5th
week
• Its blood vessels persist as the umbilical vein and
arteries
• Becomes Urachus and after birth is transformed into
median umbilical ligament extends from the apex of
the bladder to the umbilicus
Anomalies of allantois:
• Urachal fistula: The urachus remains patent, urine
discharges from the umbilicus.
• Urachal sinus: The upper end of the urachus remains
patent.
• Urachal cyst :The middle part of the urachus remains
patent.
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17. The Umbilical Cord
Anatomy
•Origin : It develops
from the connecting
stalk.
•Length:
At term, it measures
about 50 cm.
•Diameter: 2 cm.
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18. Structure: It consists of mesodermal
connective tissue called Wharton's
jelly, covered by amnion.
It contains:
1. One umbilical vein carries
oxygenated blood from the placenta
to the foetus
2. Two umbilical arteries carry
deoxygenated blood from the foetus
to the placenta,
3. Remnants of the yolk sac and
allantois.
The Umbilical Cord
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19. Insertion:
•The cord is inserted in
the foetal surface of the
placenta near the center
"eccentric insertion"
(70%)
• Or at the center "central
insertion" (30%).
The Umbilical Cord
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20. Abnormalities Of The
Umbilical Cord

21. (A) Abnormal cord insertion
1. Marginal insertion : in the placenta ( battledore insertion).
2. Velamentous insertion: in the membranes and vessels
connect the cord to the edge of the placenta.
• If these vessels pass at the region of the internal os , the
condition is called " Vasa praevia".
1. Short cord which may lead to :
i-Intrapartum haemorrhage due to premature separation of the
placenta,
ii-Delayed descent of the foetus druing labour,
iii-Inversion of the uterus.
2. Long cord which may lead to
i-Cord presentation and cord prolapse,
ii-Coiling of the cord around the neck,
iii-True knots of the cord. Mohamed el fiky
Velamentous insertion(B) Abnormal cord length

22. Chorion
Chorion
1- extraembryonic mesoderm
2- cytotrophoblast
3- Syncytiotrophoblast
Chorion
• Definition : Chorion is the name given
to the trophoblast after the formation
of the extraembryonic mesoderm
from its inner surface.
• The chorion is composed of :
• Syncito-trophoblast (outer layer).
• Cytotrophoblast (middle layer).
• Extra-embryonic mesoderm (inner
layer).
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23. CHORION FRONDOSUM AND DECIDUA BASALIS
• In the early weeks of development, villi
cover the entire surface of the chorion .
As pregnancy advances, villi on the
embryonic pole continue to grow and
expand, giving rise to the chorion
frondosum (bushy chorion). Villi on the
abembryonic pole degenerate and by the
third month this side of the chorion, now
known as the chorion laeve, is smooth
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24. Chorion
• Chorionic villi cover the entire chorionic sac until the beginning of 8th week
• As this sac grows, the villi associated with decidua capsularis are compressed, reducing the
blood supply to them
• These villi soon degenerates producing an avascular bare area smooth chorion (chorion
laeve)
• As the villi disappear, those associated with the decidua basalis rapidly increase in number
• Branch profusely and enlarge
• This bushy part of the chorionic sac is villous chorion
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25. CHORIONIC VELLI
• By the beginning of the third week, the trophoblast is characterized by
primary villi that consist of a cytotrophoblastic core covered by a syncytial
layer. During further development, mesodermal cells penetrate the core of
primary villi and grow toward the decidua. The newly formed structure is
known as a secondary villus .
• By the end of the third week, mesodermal cells in the core of the villus begin
to differentiate into blood cells and small blood vessels, forming the villous
capillary system . The villus is now known as a tertiary villus or definitive
placental villus. Mohamed el fiky

26. PRIMARY villous
•Growth of these
extensions are
caused by
underlying
extraembryonic
somatic mesoderm
•The cellular
projections form
primary chorionic
villi Mohamed el fiky

27. SECONDARY CHORIONIC VILLI
Early in 3rd
week,
extraembryonic
mesoderm
extends inside
the villi
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28. Tertiary villus
During 3rd week, arterioles,
venules & capillaries
develop in the mesenchyme
of villi & join umbilical
vessels
By the end of 3rd week,
embryonic blood begins to
flow slowly through
capillaries in chorionic villi
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29. Decidua
• The gravid endometrium is known as
decidua
• It is the functional layer of endometrium in
a pregnant woman
• This part of the endometrium separates
from the rest of the uterus after
parturition
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30. Parts of decidua
• Decidua basalis: It is the part of decidua
between blastocyst and myometrium. It
forms the fetal part of placenta.
• Decidua capsularis: It covers the
blastocyst except embryonic pole and
separates it from uterine cavity.
• Decidua parietalis: It is the rest of
endometrium that lines the rest of
uterine cavity. Mohamed el fiky

31. Fate of decidua
• Decidua basalis shares in the
formation of placenta.
• Decidua capsularis and
parietalis fuse together and
shedded with placenta after
delivery.
Decidua basalis
Amniotic cavity
Fused decidua paritalis ,
chorion laeve and amnion
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32. 32
PLACENTA
• This is a fetomaternal organ.
• It has two components:
• Fetal part – develops from the chorion
frondosum )
• Maternal part – derived from the decidua
basalis )
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33. Mohamed el fiky

34. • During the 4th and 5th month, the decidua forms a number of
decidual septa, which project into the intervillous space.
• As a result of this septum formation, the placenta is divided
into a number of compartments (cotyledons).
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35. PLACENTAL MEMBRANE (placental barrier)
• This is a composite structure that separating the fetal blood from the
maternal blood.
• Early placental barrier : (It has four layers):
• Syncytiotrophoblast
• Cytotrophoblast
• Connective tissue of villus
• Endothelium of fetal capillaries
• Late placental barrier : After the 20th week, the cytotrophoblastic cells
disappear and the placental membrane consists only of 2 layer :
• Syncytiotrophoblast
• Endothelium of fetal capillaries 35Mohamed el fiky

36. 36
It separates fetal from maternal
blood.
It prevents mixing of them.
It is an incomplete barrier as it
only prevents large molecules to
pass ( heparin & bacteria)
But cannot prevents passage of
viruses(e.g. rubella), micro-
organisms(toxoplama, treponema
pallidum) drugs and hormones.
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37. is discoid in shape.
n Diameter = 15-25 cm,
n 2-3 cm thick,
n Weight = 0.5 kg.
n Umbilical cord is attached to its center.
nPosition : in the upper uterine
segment (99.5%), either in the posterior
surface (2/3) or the anterior surface (1/3).
The full term placenta
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38. 1- Fetal surface: which is smooth and
shinny because it is covered by an
amniotic membrane. The umbilical cord
is attached centrally to this surface.
2- Maternal surface: which is rough,
reddish, and has 15 – 20 elevated areas
called cotyledons with deep grooves in
between made by the decidual septa.
Surfaces
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39. Function of placenta:-
1. Respiratory function
2. Excretory function
3. Nutritional function
4. Endocrine function:- placenta acts as endocrine
gland
5. Barrier function:- prevents transfer of maternal
infection.
6. Enzymatic action-
7. Immunological function:- ig G.
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40. Abnormalities Of The Placenta
(A) Abnormal Shape
(B) Abnormal Diameter
(C) Abnormal Weight
(D) Abnormal Position
(E) Abnormal Adhesion
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41. Abnormalities of placenta
1- Abnormal position: Placenta Previa
the placenta is attached to the lower uterine segment (due to low level of
implantation of the blastocyst). It causes severe antepartum haemorrhage.
There are three types:
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Marginalis
Lateralis
(parietalis)
Centralis
the placenta does not reach the
internal os of the cervix.
the margin of the placenta overlies
the internal os of the cervix.
the center of the placenta
overlies the internal os
of the cervix .

42. 2- Abnormal adhesion:
1- Placenta accreta: due to abnormal
adhesion between the chorionic villi and
the uterine wall.
2- Placenta percreta: The chorionic villi
penetrate the myometrium all the way to
the perimetrium.
- the placenta fails to separate from the
uterus after birth and may cause severe
postpartum hemorrhage.
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43. 3- Abnormal attachment of umbilical cord:
a- Velamentous attachment:
The cord does not reach the placenta itself but is attached to amniotic membrane over the
fetal surface of placenta. The umbilical vessels pass in the membrane to reach the placenta.
It is easly torn.
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44. (4) Abnormal Shape
A- Accessory placenta: one or two lobes are completely separated from the main
placenta. Bilobate , Bipartite
B- Placenta Fenestrate
(5) Abnormal Diameter : Placenta membranacea : The placenta is large, thin and may
measure 30-40 cm in diameter. may encroach on the lower uterine segment
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45. Twins
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46. Twins
• Di-zygotic twins
• Incidece : more common (70%).
• Mechanism of development : results from fertilization of 2 ova (from 2
ovaries ) during an overian cycle by 2 separate sperms.
• Placenta : 2 separate placentae.
• Chorion : 2 separate chorionic sacs.
• Sex : may be of the same or different sex.
• Features : different fetures. Mohamed el fiky

47. Twins
•zygotic -Mono
• Incidece : less common. results from fertilization of one ova by one sperms.
• Mechanism of development : results from division of a single fertilized ovum
during one of the following stages of development:
• Two cell stage.
• Stage of blastocyst, where the inner cell mass divides into 2 masses.
• Placenta : single common placenta.
• Chorion : single common chorionic sac.
• Sex : always of the same sex.
• Features : exactly identical.
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