Implantation And Fertelization of Bilaminar Germ Disk Flashcards
What happens as implantations of the blastocysts occur ?
Morphologic changes in the embryoblast produce a bilaminar emberyonic disc composed of epiblast and hypoblast.
What is the function of the emberyonic disc ?
It gives a rise to the germ layers that form all the
tissues and organs of the embryo.
What are the 4 emberyonic structures that form during the second week ?
- amniotic cavity
- amnion
- umbilical vesicle connecting -stalk
- chorionic sac.
What happens during implantation ( day 6-8 )?
• Approximately 6 days after fertilization (around day 20 of a typical 28-day menstrual cycle), the blastocyst attaches to the endometrial epithelium, usually near the embryonic pole.
• At the eighth day of development, the blastocyst is partially embedded in the endometrial stroma. In the area over the embryoblast, the trophoblast has differentiated into two layers:
1. an inner layer of mononucleated cells, the cytotrophoblast.
2. an outer multinucleated zone without distinct cell boundaries, the syncytiotrophoblas .
Where are mitotic figures are found in ?
They are found in the the cytotrophoblast but not in the syncytiotrophoblast. Thus, cells in the cytotrophoblast divide and migrate into the syncytiotrophoblast, where they fuse and lose their individual cell membranes.
What is a syncytiotrophoblast ?
Its a multinucleated mass without distinct cell
boundaries, extends finger-like processes through the endometrial epithelium and produces enzymes that further erode maternal tissues, allowing the blastocyst to embed itself more deeply into the endometrium.
What are the the two different layers that differentiate the cells of the inner cell mass or embryoblasts ?
- a layer of small cuboidal cells adjacent to the blastocyst cavity, known as the hypoblast layer,
- a layer of high columnar cells adjacent to the amniotic cavity, the epiblast layer.
What dose the two layers that differentiate the cells of the inner cell mass or embryoblast form ?
They form a flat disc at the same time a small cavity appears within the epiblast .
What happens to the small cavity that appears within the epiblast when it enlarges ?
It becomes the amniotic cavity. Epiblast cells adjacent to the cytotrophoblast are called amnioblasts; together with the rest of the epiblast, they line the amniotic cavity.
What are the characteristics of the endometrial stroma ?
The endometrial stroma adjacent to the implantation site is edematous and it is highly vascular. The large tortuous glands secrete abundant glycogen and mucus.
What happens in day 9 of fertilization ?
• The blastocyst is more deeply embedded in the endometrium, and the penetration defect in the surface epithelium is closed by a fibrin coagulum.
• The trophoblast shows considerable progress in development, particularly at the embryonic pole, where vacuoles appear in the syncytium.
What happens when the vacuoles fuse together on day 9 ?
they form large lacunae, and this phase of trophoblast development is thus known as the lacunar stage.
What dose the flattened cells originating from the hypoblast form ?
form a thin membrane where the exocoelomic (Heuser) 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.
What happens in day 11-12 of fertelization ?
• By the 11th to the 12th day of development, the blastocyst is completely embedded in the endometrial stroma.
• Initially, there is a surface defect in the endometrial epithelium that is soon closed by a closing plug of a fibrin coagulum of blood.
• By day 12, an almost completely regenerated uterine epithelium covers the closing plug.
• The blastocyst now produces a slight protrusion into the lumen of the uterus
• Concurrently, cells of the syncytiotrophoblast penetrate deeper into the stroma and erode the endothelial lining of the maternal capillaries.
• These capillaries, which are congested and dilated, are known as sinusoids. The syncytial lacunae become continuous with the sinusoids, and maternal blood enters the lacunar system.
• As the trophoblast continues to erode more and more sinusoids, maternal blood begins to flow through the trophoblastic system, establishing the uteroplacental circulation.
What dose the early circulation allow ?
the exchange of oxygen and nutrients between the mother and embryo where Oxygenated blood enters the lacunae from spiral endometrial arteries, while poorly oxygenated blood is removed through endometrial veins. This circulation system ensures that the embryo receives the necessary oxygen and nutrients for its development.
• In the meantime, a new population of cells appears between the inner surface of the cytotrophoblast and the outer surface of the exocoelomic cavity.
• These cells, derived from yolk sac cells, form a fine, loose connective tissue, the extraembryonic mesoderm, which eventually fills all of the space between the trophoblast externally and the amnion and exocoelomic membrane internally.
• Soon, large cavities develop in the extraembryonic mesoderm, and when these become confluent, 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 germ disc is connected to the trophoblast by the connecting stalk.
