✅ SECOND WEEK : EMBRYOLOGY 📚 EMBRYONIC DEVELOPMENT : PREGNANCY : BILAMINAR DISK

Summary:
At the 8th day of development, the blastocyst appears partially submerged in maternal endometrium. The trophoblast is differentiated into 2 layers:
- In cytotrophoblast
- And into syncytiotrophoblast.
The embryoblast, is differentiated into 2 layers:
- In Hypoblast
- And Epiblast
The Hypoblast and the Epiblast constitute the Bilaminar Germinative Disc.
A small cavity appears in the Epiblast, which increases in size and becomes the Amniotic Cavity.
By the 9th day of development, the Blastocyst is implanted deeper in the maternal endometrium, and as a consequence of its immersion, an occlusion by a fibrin clot is generated on the surface of the epithelium.
In the syncytiotrophoblast, spaces called vacuoles appear. These vacuoles then fuse and form large spaces called trophoblast lacunae.
On the inner surface of the cytotrophoblast, a thin membrane known as the exocellular or Heuser membrane begins to form.
This membrane, together with the hypoblast, forms the lining of the exocellomic cavity or primitive yolk sac. 
By days 11 and 12 of development. The blastocyst is completely immersed in the endometrial stroma, and the surface epithelium closes completely, causing only a small bulge in the lumen of the uterus.
The trophoblastic lacunae of the syncytiotrophoblast form a remarkable intercommunicating network, and at the same time the syncytial cells penetrate deeper into the stroma and erode the endothelial lining of the maternal capillaries, causing maternal blood to enter the lacunar system.
These capillaries, which are congested and dilated, are known as maternal sinusoids.
Thus, as the trophoblast continues to erode more sinusoids, maternal blood begins to flow through the trophoblast system establishing uteroplacental circulation.
At the same time, a new population of cells appears between the inner surface of the cytotrophoblast and the outer surface of the exocellular cavity, forming a thin, loose connective tissue. This tissue is called extraembryonic mesoderm.
Soon cavities develop that eventually converge into a single one and create a new space known as the Extraembryonic Cavity or Chorionic Cavity.
As a consequence of the appearance of the Chorionic Cavity, the extraembryonic mesoderm splits into 2 sheets.
- Extra-embryonic somatic mesoderm.
- Extraembryonic splanchnic mesoderm.
The endometrium adjacent to this product becomes rich in glycogen and lipids. In addition, this tissue becomes edematous. These changes are known as the decidual reaction.
By day 13,
the scar on the surface epithelium has disappeared.
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