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Second Week of Development
Bilaminar Germ Disc In this lecture, we are going to trace the development of the embryo day by day throughout
the second week …
Before going through the contents, it would be great to smile and have a look at some names
which will be mentioned later:
Blastocyst: Simply, the blastocyst is a structure formed in the early development of
mammals. It possesses an inner cell mass (ICM) which subsequently forms the
embryo. The outer layer of the blastocyst consists of cells collectively called the
trophoblast.
Embryoblast: simply, embryoblast is the same as the inner cell mass.
*** Let’s start:
Day 8:
At the eighth day of development, the blastocyst is partially embedded in endometrial
stroma (epithelial cells of the uterine mucosa).
Main events:
1- Trophoblast cells differentiate into 2 layers:
a- Inner layer (cytotrophoblast); which are mono-nucleated individual cells.
b- Outer layer (syncytiotrophoblast); which are multinucleated protoplasmic mass
lacking cells.
*Both mentioned layers become part of the chorion; which is the fetal part of the placenta.
2- The embryoblast differentiates into 2 layers:
a- Epiblast: columnar cells.
b- Hypoblast: cuboidal cells.
** both the epiblast & the hypoblast form a thick plate called: bilaminar
embryonic disc.
However, a small cavity appears within Epiblast and this cavity enlarges to form
what is called: Amniotic cavity
Epiblast cells adjacent to cytotrophoblast is called: Amnioblast
I bet that this picture will help us to imagine the mentioned stuff:
: Amnioblast
X: Epiblast
: Amniotic Cavity
: Hypoblast
: blastocyst cavity
: cytotrophoblast
Now let’s have a closer look at the implantation:
Syncytiotrophoblastic cells of the implanted blastocyst secretes HCG hormone; Human
Chorionic Gonadotropin, to the blood of the mother; “Maternal “blood which it’s an
indication for a positive pregnancy test.
This hormone also prevents the degeneration of the corpus luteum and maintains it as
Corpus Luteum Of Pregnancy.
** endometrium after pregnancy is called Decidua; which means falling off; it falls off
with fetus at delivery.
Day 9:
The blastocyst is more deeply embedded in the endometrium.
Syncytium: a multinucleate mass of protoplasm produced by the merging of cells which
we can easily notice in the syncytiotrphoblast.
However, Syncytium shows a small fluid filled cavities which contain mixture of maternal
blood & endometrial glands’ secretions, these cavities fuse together to form trophoblastic
Lacunae.
Using this picture -> -> we can
notice the following:
1- Embryonic disc lies between
Amniotic cavity & primitive yolk
sac.
2- Because the amniotic cavity
enlarges, the
cytotrophoblast
differentiates into amino
blast; which forms a thin
membrane called Amnion.
3- The amnion surrounds the
amniotic cavity except its floor
which is formed by Epiblast.
4- The Hypoblast cells also migrate down to form a thin extraembryonic membrane that
surrounds the primitive yolk sac (cavity); this membrane is called: Exocoelomic
(Heuser) membrane.
Days 11 & 12:
By the 11th and 12th day of development, the blastocyst is completely embedded in the
endometrial stroma.
Remember that Trophoblastic lacunae fuse together to form lacunar network which
communicate with the mother’s blood vessels. However, these capillaries which are
congested and dilated are called Sinusoids.
Then the maternal blood seeps into the lacunar networks to establish →→→ The
Uteroplacental Circulation.
Using this picture↓↓, we can notice the Connecting Stalk (Umbilical Cord to be); which
appears between the Embryonic disc and the Trophoblast:
*** We can also
notice that a new
population of cells
appear between the
inner surface of the
cytotrophoblast and
the outer surface of
the Exocoelomic
cavity. These cells,
derived from the
yolk sac cells, form a
fine, loose
connective tissue,
the Extraembryonic
Mesoderm, which
eventually fills all
the spaces between
the trophoblast
“externally” & the amnion and Exocoelomic membrane internally.
Soon, large cavities will develop in the extraembryonic mesoderm, and when they
become confluent(merged), they form a new space known as the extraembryonic
cavity OR Chorionic Cavity →→ This space surrounds the primitive yolk sac and
amniotic cavity, EXCEPT Where the embryonic disc is connected to the trophoblast by
the connecting stalk.
Day 13:
Main events:
1- A finger like processes from cytotrophoblast invade syncytiotrphoblast to give Primary
Chorionic Villi
2- Endometrial implantation defect will be healed.
3- Bleeding occasionally occurs at the implantation site.
4- Connecting stalk is clearly visible.
Love Is Wisdom … Hatred Is Foolish