Mod.A Embryology Lec6

Question 1

Placenta (source)

Answer 1

The placenta is a fetomaternal organ that has two components:
A fetal part that develops from chorion frodosum
A maternal part that is derived from the endometrium (inner layer
of uterine wall)- deciduas basalis.

Question 2

Placenta (dimensions) (weight, diameter, thickness)

Answer 2

Weight= 500 gm
Diameter= 15 – 25 c
Thickness= 3 – 4 cm in the
center

Question 3

Shape of placenta

Answer 3

Circular (cake) or oval
disc shape, having two
surfaces

Question 4

Surfaces of placenta:

Answer 4

1- Fetal surface.
Smooth, covered by
amnion, the umbilical
cord attached to it’s
center.

2- Maternal surface.
Irregular, divided into15 –
20 lobes (cotyledons)
separated by placental
septa.

Question 5

Several partitions formed by the decidua basalis called the

Answer 5

placental septa

Question 6

The placental septa divide the fetal part of the placenta into irregular convex areas

Answer 6

cotyledons

Question 7

Maternal blood in the intervillous spaces is always kept separated
from the fetal blood in villi by

Answer 7

placental barrier

Question 8

placental barrier is composed of four layers:

Answer 8

syncytiotrophoblast, cytotrophoblast,

connective tissue of the villus, and endothelium of the fetal capillaries

Question 9

when does the cytotrophoblast disappear?

Answer 9

after the 20th week

Question 10

absence of cytotrophoblast allows

Answer 10

more passage of nutritive

material across the barrier from the maternal to the fetal blood.

Question 11

Maternal-Placental circulation

Answer 11

When maternal blood flows into
the lacunae, oxygen and nutritive
substances become available to
the extraembryonic tissues over
the large surface of the
syncytiotrophoblast.

Oxygenated
blood passes into the lacunae
from the spiral endometrial
arteries in the endometrium;
deoxygenated blood is removed
from the lacunae through
endometrial veins.

Question 12

Feto-placental Circulation

Answer 12

Poorly oxygenated blood
leaves the fetus via the umbilical
arteries.

• The well-oxygenated blood
reach to the fetus via
the umbilical vein.

Question 13

Functions of the placenta

Answer 13

1- Respiration: exchange of O2 &CO2.
2- Excretion of the waste products.
3- Nutrition
4- Placental barrier protects the fetus.

5- Endocrine function:
a- chorionic gonadotrophines.
b- progesterone &estrogen.
c- placental lactogen.
d- relaxine.

Question 14

Substances which can cross the placenta barrier

Answer 14

1-Organisms. Spirochetes of syphilis, virus of German measles and
AIDS.
2- Drugs: as Thalidomide→ phocomelia.
3- Chemicals: insecticides.
4- Maternal antibodies: transplacental immunity.
5- Rhesus factor: It may leads to erythroblastosis fetalis.

Question 15

Rh-incompatibility/Hemolytic disease of the 11

newborn

Answer 15

• This condition occurs when the blood of
the foetus is Rh. positive while the blood of
the mother is Rh. negative.

• At labour, fetal blood are transfused into
the maternal part. The Rh (Rhesus) antigens
in the transfused fetal blood induce
production of Rh. antibodies in the maternal
blood.

• In the subsequent pregnancies these
antibodies pass across the placental barrier
to the blood of the foetus.

• The antigen-antibody reaction occurring
in the blood of the foetus may lead to foetal
death or hemolysis of fetal Rh-positive blood
cells and anemia in the fetus.

Question 16

Anomalies of the placenta

Answer 16

1- Placenta accreta
•2- Placenta percreta

3- Placenta previa
• Placenta previa might be
marginalis, centralis and
lateralis.
Placenta previa causes
antepartum haemorrhage.

4- Placenta membranecia : thine like membrane
􀂃5-Bipartite or Tripartite placenta
􀂃6-Filamentous insertion of the cord.

Question 17

how is the amniochrionic membrane formed

Answer 17

The amnion is expanded greatly to surround all aspects of the
embryo.
• The chorionic cavity is gradually obliterated until it practically
disappears- forming amniochrionic membrane.

Question 18

amnichrionic membrane ruptures during

Answer 18

labor

Question 19

The amniochrionic membrane adheres to

Answer 19

decidua parietalis

+The amniotic fluid is a watery fluid containing only 2% of solids. It is
formed by secretion of the amniogenic cells and the foetal urine

Question 20

Significance of the amniotic fluid

Answer 20

Permits uniform external growth of the embryo
• Acts as a barrier to infection
• Permits fetal lung development
• Amniotic fluid forms a water jacket around the fetus and protects it
from external shocks.
• Helps to control embryonic body temperature by maintaining a
relatively constant temperature
• Enables the fetus to move freely, thereby aiding muscular
development (e.g., in the limbs)
• The amniotic fluid is a good dilator of the cervix at birth and is
aseptic medium surrounding the foetus. When the membranes
rupture, the amniotic fluid flushes the vagina before delivery of child.

Question 21

Disorders of amniotic fluid volume

Answer 21

The amount of amniotic fluid at full term is normally ¾ to 1.5 letters.
If the amount is less than half or third of a letter the condition is known
as oligohydramnios. If the amount exceeds 2 letters, the condition is
called hydramnios or polyhydramnios.

Question 22

Oligohydramnios

Answer 22

results, in some cases, from placental
insufficiency, with diminished placental blood flow.

• Preterm rupture of the amniochorionic membrane is the most common cause of oligohydramnios.

• Renal agenesis (failure of kidney formation), which lead to the lack
of fetal urine in the amniotic fluid.

Complications of oligohydramnios include fetal abnormalities
(pulmonary hypoplasia, facial defects, and limb defects) caused by
fetal compression by the uterine wall.

Question 23

polyhydramnios

Answer 23

Most cases of polyhydramnios (60%) are idiopathic (of unknown cause).

• Polyhydramnios may be associated with severe anomalies of the
central nervous system, such as meroencephaly (anencephaly) and
other anomalies, such as esophageal atresia, amniotic fluid
accumulates because it cannot pass to the fetal stomach and the
intestines for absorption.

Question 24

Umbilical Cord

Answer 24

The umbilical cord is the connecting stalk, which is a band of extraembryonic
mesoderm connecting the caudal end of the embryonic
plate to the chorion.
• After folding, the stalk is much elongated and is now attached to
the ventral aspect of the embryo in the position of the future
umbilicus.
• As result of expansion of the amniotic cavity, the stalk becomes
covered by the amnion.
• The umbilical cord usually has two arteries and one
vein surrounded by mucous connective tissue (Wharton jelly).

Question 25

Umbilical Cord Abnormalities

Answer 25

At birth, the umbilical cord is approximately 2 cm in diameter and 50 to 60
cm long. It is tortuous, causing false knots.
• An extremely long cord may encircle the neck of the fetus, usually without
increased risk, whereas a short one may cause difficulties during delivery
by pulling the placenta from its attachment in the uterus.
• Normally there are two arteries and one vein in the umbilical cord.
• In 1 in 200 newborns, however, only one artery is present, and these
babies have approximately a 20% chance of having cardiac and other
vascular defects.
• The missing artery either fails to form (agenesis) or degenerates early in
development.