Dr. FARHAT AAMIR
Lecturer of Anatomy and Embryology
Objectives
a)Discuss the changes and formation of bilayer
differentiation of trophoblast
b)Describe lacunar phase of trophoblast and formation of
primitive yolk sac
c)Discuss the developmental change in trophoblast and
blastocyst in day 11 and 12 of gestation
d)Describe the formation of definitive yolk sac, chorionic
cavity and chorionic plate
e)Discuss Abnormal Implantation
DAY 8
- The blastocyst is formed of trophoblast
inner cell mass and blastocyst cavity.
It is partially embedded in the
endometrial stroma.
A - The trophoblast in the area over the
embryoblast, has differentiated into 2
layers:
1. An inner layer of mononucleated cells:
The cytotrophoblast.
2. An outer multinucleated zone without
definite cell boundaries:
The syncytiotrophoblast.
- Mitotic figures are found in the
cytotrophoblast: So cells in the
cytotrophoblast divide and migrate into
the syncytiotrophoblast, where they fuse
and lose their individual cell membranes.
- Mitotic figures are not found in the
syncytiotrophoblast.
B - Cells of the inner cell mass is
differentiated into 2 layers:
1. A layer of small cuboidal cells:
Adjacent to the blastocyst cavity, known
as the hypoblast layer.
2. A layer of high columnar cells:
Adjacent to the amniotic cavity, the
Epiblast layer.
- Together, the 2 layers form a flat disc.
- At the same time: A small cavity appears
within the Epiblast and it enlarges to
become the amniotic cavity.
- Epiblast cells adjacent to cytotrophoblast
layer called Amnioblasts.
Amnioblasts with the rest of the
Epiblast they line the amniotic cavity.
- The endometrial stroma: Adjacent to
the site of implantation is edematous and
highly vascular. The large, tortuous glands
secretes abundant glycogen and mucus.
DAY 9
The blastocyst: It is more deeply embedded
in the endometrium and the penetration
defect in the epithelium is closed by fibrin
clot.
1. At the embryonic pole of trophoblast: It
shows progress in the development and
multiple vacuoles appear in the syncytium. The vacuoles fuses to form large lacunae and
this phase is known as (lacunar stage).
2. At the abembryonic pole of trophoblast:
- Flattened cells originating from the
hypoblast to form thin membrane called the
exocoelomic membrane that lines the inner
surface of the cytotrophoblast.
- This membrane together with the hypoblast,
forms the lining of the exocoelomic cavity,
or primitive yolk sac.
DAYS 11 AND 12
The blastocyst: It is completely embedded
in the endometrial stroma and the surface
epithelium covers the original defect in the
uterine wall.
1. At the embryonic pole: The trophoblast
is characterized by lacunar spaces in the
syncytium that forms intercommunicating
network.
- Cells of the syncytiotrophoblast penetrates
the endothelial lining of the maternal
capillaries.
- These capillaries are congested, dilated
and known as sinusoids.
- The lacunae become continuous with the
sinusoids and maternal blood enters the
lacunar system to share in the
uteroplacental circulation.
- At the abembryonic pole:
- A new population of cells derived from
the yolk sac cells appears between the
cytotrophoblast and the exocoelomic
cavity.
- These cells forms the extraembryonic
mesoderm that fills all of the space
between the trophoblast externally and
the amnion and exocoelomic membrane
internally.
- Large cavities develops in extraembryonic
mesoderm then fuses to form the
extraembryonic or chorionic cavity .
- This space surrounds the primitive yolk
sac and amniotic cavity, except where the
germ disc is connected to the trophoblast
by the connecting stalk.
- The extraembryonic mesoderm lining
the cytotrophoblast and amnion is
called the extraembryonic somatic
mesoderm.
- The lining covering the yolk sac is
known as the extraembryonic
splanchnic mesoderm.
- Growth of the bilaminar disc is
relatively slow compared with that of
the trophoblast.
DAY 13
- The surface defect in the endometrium has
usually healed.
- Bleeding occurs at the site of
implantation as a result of increased blood
flow into the lacunar spaces.
- The trophoblast is characterized by
villous structure: Cells of the
cytotrophoblast proliferate locally and
penetrate into the syncytiotrophoblast to
form primary villi.
- The hypoblast produces additional cells
that migrates inside the exocoelomic
membrane.
- These cells proliferate and gradually form
new cavity within the exocoelomic cavity
called secondary yolk sac or definitive
yolk sac.
- The yolk sac is much smaller than
the original exocoelomic cavity or
primitive yolk sac.
- The extraembryonic coelom
expands to form chorionic cavity.
- The extraembryonic mesoderm
lining the cytotrophoblast is known as
the chorionic plate.
- The extraembryonic mesoderm
traverse the chorionic cavity at the
connecting stalk (Future umblical
cord).
Clinical application
Normal site of implantation:
The blastocyst implants along the anterior
and posterior wall of the body of the
uterus.
Abnormal site of implantation:
A. Intrauterine: Outside the normal site
of implantation as Placenta Previa:
1. Complete Placenta Previa.
2. Partial Placenta Previa.
3. Marginal Placenta Previa.
4. Low lying Placenta Previa.
B. Extra uterine (Ectopic pregnancy):
Outside the uterine cavity.
1. In the fallopian tube: It represent 95%
of Ectopic pregnancy.
2. In the ovary.
3. Abdominal cavity.