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