Second Week of Development
Bilaminar Germ Disc
c h a p t e r 3
By. Abiib
4/1/2023 Awdal college 1

8 cell
stage
Fertilization
ampulla
Male &
female
pronucleus
2 cell
stage
morula blastocyst

BILAMINAR GERM DISC
• The bilaminar embryonic disc is formed
when the inner cell mass forms two
layers of cells, separated by an
extracellular basement membrane.
• The external layer is called the epiblast
and the internal layer is called the
hypoblast.
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DAY 8 TROPHOBLAST
• At the eighth day of development, the
blastocyst is partially embedded in the
endometrial stroma.
• The trophoblast has differentiated into two
layers:
– An inner layer of mononucleated cells, the
cytotrophoblast,
– An outer multinucleated zone without distinct cell
boundaries, the syncytiotrophoblast
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Day 8 Trophoblastic

DAY 8 EMBRYOBLAST
• Cells of the inner cell mass or embryoblast
also differentiate into two layers:
– A layer of small cuboidal cells adjacent to the
blastocyst cavity, known as the hypoblast layer.
– A layer of high columnar cells adjacent to the
amniotic cavity, the epiblast layer
• At the same time, a small cavity appears
within the epiblast. This cavity enlarges to
become the amniotic cavity.
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DAY 8 EMBRYOBLAST

DAY 9
• Blastocyst more deeply embedded in the endometrium,
• Fibrin coagulum closes defect in surface epithelium
Vacuoles appear in syncitium→fuse → lacunar stage
Abembryonic pole flattened cells from hypoblast form
thin exocoelomic (Heuser’s)membrane inside
cytotrophoblast
Hypoblast + Heuser’s membrane= primitive yolk sac
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DAY 11 AND 12
• By the 11th to 12th day of development, the surface
epithelium almost entirely covers the original defect
in the uterine wall
• Lacunar spaces in syncytium that form an
intercommunicating network - particularly
embryonic pole
• Synciotrophoblast erode maternal capillaries are
known as sinusoids.
• As maternal blood begins to flow through the
trophoblastic system - the uteroplacental
circulation.
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DAY 11 AND 12

DAY 11 AND 12
• Cells derived yolk sac fill inner surface of
cytotrophoblast and outer surface of exocoelomic
cavity
• They form Fine, loose connective tissue-
extraembryonic mesoderm
• when these Cavities become
confluent→extraembryonic coelom or chorionic cavity
• Extraembryonic coelom surrounds primitive yolk sac
and amniotic cavity except where trophoblast is
connected by connecting stalk
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Extraembryonic
somatopleuric
mesoderm
Lining cytotrophoblast and
amnion
Extraembryoni
c
splanchnopleu
ric mesoderm
Lining yolk sac
Day 11 and 12
Connecting stalk

• The extraembryonic mesoderm lining the cytotrophoblast
and amnion is called the extraembryonic somatopleuric
mesoderm
• The lining covering the yolk sac is known as the
extraembryonic splanchnopleuric mesoderm
• Cells of the endometrium become polyhedral and loaded
with glycogen and lipids; intercellular spaces are filled with
extravasate, and the tissue is edematous. These changes,
known as the decidua reaction
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DAY 13
• By the 13th day of development, the surface
defect in the endometrium has usually healed.
• Cells of cytotrophoblast proliferate and
penetrate syncytiotrophoblast forming cellular
columns surrounded by syncytium.
• Cellular column+ syncytial covering= primary villi
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Day 13
•Cells from hypoblast migrate
inside exocoelomic
membrane proloferate and
form a new cavity:
secondary yolk sac or
definitive yolk sac
•Pinched out portions of
exocoelomic cavity:
exocoelomic cyst

Day 13
•Extraembryonic mesoderm lining inside
of cytotrophoblast is chorionic plate.
•Extraembryonic mesoderm and two
layers of trophoblast form chorion
•Chorion forms the wall of chorionic sac
in which embryo, its amniotic and yolk
sacs are suspended by connecting stalk.

Day 13
•Extraembryonic
mesoderm traverse
chorionic cavity in
connecting stalk→ with
development of blood
vessels → the stalk
become umbilical cord

Ectopic pregnancy
• A woman with a tubal pregnancy has signs and
symptoms of pregnancy (e.g., misses her
menstrual period). She may also experience:
Abdominal pain
Abnormal bleeding,
Produce human chorionic gonadotropin (HCG) at a
slower rate than normal pregnancies.
Some times ectopic pregnancy characterized
abdominal pregnancy

Placenta Previa
• Implantation of a blastocyst in the inferior
segment of the uterus near the internal os
results in placenta previa.
• A placenta partially or completely covers the
os.
• Placenta previa may cause bleeding because of
premature separation of the placenta during
pregnancy or at delivery of the fetus.

Day 13
•By end of 2nd week germ
disc : 2 apposed cell discs-
•Epiblast: floor of amniotic
cavity
•Hypoblast: roof of
secondary yolk sac