Mod.A Embryology Lec4

Question 1

During implantation, the trophoblasts forms two

layers:

Answer 1

syncytiotrophoblast

cytotrophoblast

Question 2

Outer syncytiotrophoblasts:

Answer 2

multiple nuclei in a common cytoplasm with on cell boundaries.

Question 3

Inner cytotrophoblast:

Answer 3

on layer of cells .

Question 4

The cytotrophoblast proliferates forming a new layer

beneath it called

Answer 4

extraembryonic (1 ry ) mesoderm.

Question 5

Clefts appear in the mesoderm & fuse forming a cavity

called

Answer 5

extraembryonic coelom or chorionic cavity.

Question 6

extraembryonic coelom or chorionic cavity.

• It divides the mesoderm into

Answer 6

outer and inner layers except
in a region called connecting stalk which is the future
umbilical cord

Question 7

Chorion: is formed of

Answer 7

the syncytiotrophoblasts +
the cytotrophoblasts + the outer layer of the
extraembryonic mesoderm.

Question 8

The chorion sends finger – processes called

Answer 8

chorionic villi

which invade the decidua. The villi erode the decidual
blood vessels forming LACUNAE in the intervillous spaces (in the syncytiotrophoblast) filled with the maternal blood.

Question 9

3 stages of chorionic villi

Answer 9

1-primary
2-secondary
3- tertiary

Question 10

Primary chorionic villi

Answer 10

The end of the second week is characterized by the

appearance of primary chorionic villi, the first stage in the development of the chorionic villi of the placenta

Question 11

Secondary chorionic villi

Answer 11

Early in the third week, secondary chorionic villi are formed

which are composed of three layers of the chorion as
extraembryonic mesoderm or mesenchyme grows into the primary villi, forming a core of loose mesenchymal
(connective) tissue, which surrounderd by a layer of
cytotrophoblast then a layer of syncytiotrophoblast.

Question 12

Tertiary chorionic villi

Answer 12

Mesenchymal cells in the secondary villi soon differentiate

into both capillaries and blood cells. When capillaries are present, the villi are called tertiary chorionic villi.

Question 13

Types of tertiary villi:

Answer 13

1- Stem (anchoring villi): fix the embryonic vesicle to the
decidua.

2- Free ( absorbing villi ): increases the surface area for
exchange between the feta and maternal blood.

Question 14

Formation of a cytotrophoblastic shell

Answer 14

Cytotrophoblastic cells of the chorionic villi proliferate
and extend through the syncytiotrophoblast to form a
cytotrophoblastic shell, which gradually surrounds the
chorionic sac and attaches it to the endometrium to
prevent further penetration of decidua by
syncytiotrophoblast.

Question 15

Placental barrier

Answer 15

Maternal blood (in the intervillous spaces) and the fetal
blood in the capillaries of the villi) do not mix, they are
separated by what is called placental barrier which is
formed of :
1- Syncytiotrophoblast. 2- Cytotrophoblast.
3- Extraembryonic mesoderm 4- Endothelial lining of
the fetal capillaries.

• By the 4th month, the 2 & 3 layers disappear and the
barrier becomes relatively leaky.

Question 16

degenerating villi may form
cystic swellings called

what do they produce?

Answer 16

hydatidiform moles/vesicular moles( benign)

These moles produce excessive
amounts of human chorionic
gonadotropin.

In 3% to 5% of such cases, these
moles develop into malignant
trophoblastic lesions, called
choriocarcinomas.

Question 17

Syncytial Knots

Answer 17

Pieces of syncytium
occasionally detach into the
intervillous spaces & enter the
maternal circulation. They usually
degenerates & cause no harm .

Question 18

The size of the chorionic sac is useful in determining

Answer 18

the gestational age of embryos in pregnant women.

Question 19

Growth of the chorionic sac is extremely rapid between the

Answer 19

5-10 week

Question 20

Modern ultrasound devices permit detection of the

chorionic sac when it has a median diameter of

Answer 20

2-3mm

Chorionic sacs with this diameter indicate a gestational
age of approximately 32 days.