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Learn the BASIC CONCEPTS ABOUT THE SECOND WEEK OF GESTATION in an easy, fast and concise way.

#EMBRYOLOGY #MEDICINE #SUMMARY

⌚Time stamps:

Introduction 00:00
Review 00:50
Gestation Day 8 1:40
Gestation Day 9 3:10
Day 11 12 of Gestation 4:07
Gestation Day 13 7:00
Gestation Day 14 8:48
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Resume:
By the 8th day of development, the blastocyst appears partially submerged in the maternal endometrium. The trophoblast is differentiated into 2 layers:
In cytotrophoblast
Y in syncytiotrophoblast
The embryoblast differs into 2 layers:
In Hypoblast
And in Epiblast
The Hypoblast and the Epiblast constitute the Bilaminar Germinative Disc.
In the Epiblast a small cavity appears that increases in size and becomes the Amniotic Cavity.
Towards the 9th day of development, the blastocyst is implanted deeper in the maternal endometrium, and as a consequence of its immersion, an occlusion is generated on the surface of the epithelium by a fibrin clot.
Spaces called vacuoles appear in the syncytiotrophoblast. These vacuoles then merge and form large spaces called trophoblastic lagoons.
A thin membrane begins to form on the inner surface of the cytotrophoblast, known as the exocoelomic or Heuser membrane.
This membrane, together with the hypoblast, generates the lining of the exocoelomic cavity or primitive yolk sac.
For days 11 and 12 of development. The blastocyst is fully immersed in the endometrial stroma, and the surface epithelium closes completely, causing only a small bulge in the lumen of the uterus.
The trophoblastic lagoons of the syncytiotrophoblast form a remarkable intercommunicating network and at the same time, the cells of the syncytium penetrate deeper into the stroma and erode the endothelial lining of the maternal capillaries, causing the entry of maternal blood into the lagoon system.
These capillaries, which are congested and dilated, are known as Maternal Sinusoids.
In this way, as the trophoblast continues to erode more sinusoids, maternal blood begins to flow through the trophoblastic system, establishing the 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 exocoelomic cavity, forming a thin and loose connective tissue. This tissue is called the Extraembryonic Mesoderm.
Cavities soon develop that end up merging into 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 unfolds into 2 sheets.
Extraembryonic somatic mesoderm
Extraembryonic splanchnic mesoderm.
The endometrium that is adjacent to this product, becomes rich in glycogen and lipids. And also this tissue becomes edematous. These changes are known as the decidual reaction.
By the 13th,
The scar on the superficial epithelium has already disappeared.
However, bleeding at the implantation site occasionally occurs as a consequence of increased blood flow to the lacunar spaces.
As this bleeding takes place around day 28 of the menstrual cycle, it can be mistaken for a normal menstrual bleeding and in some cases it can make us fail to calculate the probable date of delivery.
Cytotrophoblast cells show local proliferation and penetrate the syncytiotrophoblast to organize cell columns surrounded by syncytium. These cell columns with their syncytial covering are known as primary villi.
Additional cells are produced in the hypoblast and migrate along the interior of the exocoelomic membrane. These cells proliferate and give rise to a new cavity within the exocoelomic cavity. This new space is known as the secondary yolk sac or the definitive yolk sac.
This yolk sac is much smaller than the primitive yolk sac. And during its formation large portions are detached in the chorionic cavity called exocoelomic cysts.
The extraembryonic mesoderm that lines the interior of the cytotrophoblast changes its name and is known as the chorionic plaque.