Embrylogy notes made by Jón Kolbeinn Guðmundsson for University of Debrecen Medicine and Dentistry students.

1. Embryology part 11
Jon Kolbeinn Gudmundsson

2. The formation of the placenta. The
structure of the matured placenta
Topic 38

3. The formation of placenta.
When talking about the placenta, we divide it into two parts.
The fetal part and maternal part.
1. The fetal part of the placenta derives from trophoblast and extraembryonic
mesoderm called chorion.
2. The maternal part derives from the uterine endometrium. With the spiral
arteries bringing oxygen rich blood to the placenta. This part of the
endometrium is called decidua, and is the part that is shed during birth.
Towards the second month (after 4th week) the fetal part is characterized by
secondary and tertiary villi.

4. The maternal blood is
supplied by the spiral
arteries.
The ends of the spiral
arteries are invaded by the
cytotrophoblast cells which
undergo epithelial-to-
endothelial transition,
making the spiral arteries
continous with the
intervillous space (lacunae).

5. Early stages.
Each villi creates a barrier between the fetal and maternal
circulation, and is initially composed of 4 layers.
1. Syncytium (syncytiotrophoblast) layer
2. Cytotrophoblast layer
3. Vascularized extraembryonic mesoderm layer.
4. Endothelium of fetal capillaries.

6. Later stages.
During the following months in the fetal period, the barrier between the
maternal blood and fetal blood becomes thinner still, which allows for
greater exchange of gases and nutrients.
The villi branch out more and form tertiary villi with a thin barrier
composed only of:
1. Syncytium
2. Endothelium

7. The Chorion.
In the early weeks of development the villi cover the entire chorion.
As pregnancy continues the villi on the embryonic side continue to grow and
form chorion frondosum (bushy chorion).
The opposite side, which faces away from the embryo, the villi degenerate
and form the chorion laeve (smooth chorion).

8. The decidua (endometrium) which faces the chorion frondosum is called decidua
basalis and is composed of a dense layer of decidual cells.
The decidua covering the opposite side is called decidua capsularis.
As the placenta grows in size, the decidua capsularis is stretched an compressed,
eventually degenerating, leaving the chorion laeve to come into contact with the
decidua on the other side of the uterine cavity, called the decidua parietalis.
The chorion laeve
fuses with the
decidua parietalis,
obliterating the
uterine lumen.

9. The mature placenta.
During 4th-5th month.
The decidua forms a number of decidual septa which project into the intervillous
spaces but don’t reach the chorionic plate.
The septa form from decidua but are covered by a layer of syncytial cells, which
separates maternal blood in the the intervillous lakes (lacunae) from the fetal
tissue of the villi.

10. Due to the decidual septa, the placenta is devided into a number
of compartments called cotyledons.
The increased thickness of the placenta is caused by increased
branching of the existing villi, not due to further penetration into
maternal tissues.

13. Fetal membranes. Umbilical cord.
Amniotic fluid. Fetal circulation
Topic 40

14. The umbilical cord.
The umbilical cord originates from the connecting stalk, which connects the early
embryonic disc to the chorion.
At 5th week of development, the connecting stalk contains 2 umbilical arteries and
1 umbilical vein, The vitelline duct (yolk stalk) and its accompanying vein and
artery.
During further development, the
amniotic sac will fill out the
chorionic cavity and push the
connecting stalk and yolk stalk
together to form the primitive
umbilical cord.
With the amnion covering the
umbilical cord.

15. The primitive umbilical cord contains intestinal loops, yolk stalk and
allantois initially, but those structures will eventually be obliterated
and we are left with the final umbilical cord containing 2 arteries and
1 vein surrounded by loose mesenchyme called Wharton’s jelly.
The wharton’s jelly is rich in proteoglycans and functions as a
protective layer for the vessels.

16. Amniotic Fluid
Amniotic fluid is a clear and watery fluid that is produced by the amnioblasts
and maternal blood.
Initially the amount is low, roughly 30 ml.
At 10th week it is 450 ml, which rises to 800-1000 ml by 20th week onwards.
The amniotic fluid acts to:
1. Absorb jolts
2. Prevent the embryo from adhering to the amnion
3. Allow for fetal movement
The volume is replaced every 3 hours.
In the beginning of the 5th month, the fetus starts to swallow its own amniotic
fluid (400ml per day). At this time fetal urine is added to the amniotic fluid
every day, but technically this urine is mostly water.
** Oligohydramnios is a condition where there is too little amniotic fluid,
this can result in pulmonary hypoplasia, since amniotic fluid is required for
proper development of the lungs**

17. Fetal membranes.
Fetal membranes are composed of:
1. Placenta (trophoblast derived)
2. Chorion (extraembryonic mesoderm)
3. Amnion
Once the amniotic sac starts to fill out the chorinic space, the two
fuse to form the amniochorionic membrane (this is the layer that
ruptures when the “water breaks”)

18. Fetal circulation – before birth.
1. Oxygenated placental blood
returns to fetus via the umbilical
vein.
2. Most of the blood is shunted into
the IVC via the ductus venosus.
3. In the IVC oxygenated blood
mixes with deoxygenated blood
returning from the lower limbs.
4. The blood enters into the right
atrium where it is guided through
the foramen ovale into the left
atrium, where it mixes with
deoxygenated blood from the
lungs.
5. Blood passes into the left
ventricle where it is pumped into
the ascending aorta and from
there into systemic circulation

19. Deoxygenated blood returning from the SVC passes into the right atrium, right
ventricle and through the pulmonic trunk.
In the pulmonic trunk the deoxygenated blood is shunted via the ductus
arteriosus into the aortic arch.
All the blood is then returned to the placenta via the umbilical arteries.

20. Fetal circulation – after birth
Cessation of placental blood flow causes closure of several vessels.
1. Closure of umbilical arteries – they become the medial umbilical
ligaments, except for their most proximal part which remains open as the
superior vesical arteries.
2. Closure of the umbilical vein and ductus venosus – form ligamentum
teres and ligamentum venosum.
3. Closure of ductus arteriosus – forms the ligamentum arteriosum.
4. Closure of foramen ovale – closure is caused by increased pressure in the l.
atrium causing the septum primum to press against the septum secundum,
making them adhere and fuse.

22. Development of external features of the fetus.
External features of a matured new born. Twin
pregnancy. Fetal membranes in twins.
Topic 20

23. The fetal period.
The fetal period is defined as the 9th week until birth.
It is characterized by maturation of organs and rapid growth of the fetus, which is
measured in crown-rump length (CRL)
At 9th week the fetus is 10-45 g and 5-8 cm in length.
At 20th week it weighs 500-820g and is 20-23 cm in length.
At 37-38th week it reaches normally 3000-3400g in weight and 35 cm in length.

24. External changes.
3rd month
1. During the 3rd month the fetus becomes more
human looking, with having moved from the
lateral position towards the front. The
external genitalia can be distinguished by this
time and the sex determined. The limbs are
proportional to the size of the body.
2. The head is still 50% of the length of the
entire body, very large in proportion to the
rest.

25. 5th-6th month
1. During the 5-6th month the fetus lengthens
rapidly and the boy becomes more proportional.
The weight doesn’t increase very much at this
stage.
2. The baby can be felt moving at this stage.

26. 37-38th week
1. During the last 2 months before birth collects
large amounts of subcutaneous fat.
2. The contours of the face are well pronounced
3. The fetus has hair and eyebrows.
4. The head is 1/4th the total length of the fetus.

27. Twin pregnancy and twin fetal membranes.
Twins can be dizygotic (90%) or monozygotic.
1. Dizygotic twins are also called fraternal twins, and don’t have the same
DNA, because they develop from separate eggs and sperm.
2. Monozygotic twins have identical DNA, since they are the result of the
fertilized zygote or blastocyst dividing in two.

28. Dizygotic twins.
Dizygotic twins develop from separate
egg and sperm. Therefore each implants
in its own place in the uterus and usually
each develops its own placenta, amnion
and chorionic sac.

29. Monozygotic twins
Develop from a single fertilized ovum or zygote. There they are called
identical twins, since they have identical DNA.
The splitting of the zygote can happen anywhere on the way to the uterus.

30. 1. The zygote splits creating two
separate blastocysts which
implant separately and each
creates their own placenta,
chorion and amnion.
2. The inner cell mass of the
blastocyst could split into 2
separate embryoblasts with
each forming their own
amniotic sac but sharing a
placenta and chorion.
3. Rarely the 2 separate
embryoblasts become so
closely associated fusing their
amniotic sacs into a single
one. These twins will share a
placenta, chorion and
amniotic sac. This greatly
increases the risk of
conjoined twins.