Embryonic Development Day 8
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Questions and Answers

What is the outer layer of the trophoblast?

  • Syncytiotrophoblast (correct)
  • Epiblast
  • Cytotrophoblast
  • Hypoblast

    • Where are mitotic figures found?

  • Cytotrophoblast (correct)
  • Hypoblast
  • Epiblast
  • Syncytiotrophoblast

    • What is the layer of small cuboidal cells adjacent to the blastocyst cavity?

  • Cytotrophoblast
  • Syncytiotrophoblast
  • Epiblast layer
  • Hypoblast layer (correct)

    • What is the function of the large, tortuous glands in the endometrial stroma?

    <p>Secretion of glycogen and mucus</p> Signup and view all the answers

    What is the term for the phase of trophoblast development characterized by the formation of vacuoles that fuse to form large lacunae?

    <p>Lacunar stage</p> Signup and view all the answers

    What is the term for the thin membrane formed by flattened cells originating from the hypoblast?

    <p>Exocoelomic membrane</p> Signup and view all the answers

    What is the shape of the blastocyst at Day 8?

    <p>Flat disc</p> Signup and view all the answers

    What closes the penetration defect in the surface epithelium at Day 9?

    <p>Fibrin coagulum</p> Signup and view all the answers

    What forms the lining of the exocoelonic cavity, or primitive yolk sac?

    <p>This membrane, together with the hypoblast</p> Signup and view all the answers

    By what day of development is the blastocyst completely embedded in the endometrial stroma?

    <p>11th to 12th day</p> Signup and view all the answers

    What is characterized by lacunar spaces in the syncytium that form an intercommunicating network?

    <p>Syncytiotrophoblast</p> Signup and view all the answers

    What is the name of the capillaries that are congested and dilated?

    <p>Sinusoids</p> Signup and view all the answers

    What is the name of the circulation that is established when maternal blood begins to flow through the trophoblastic system?

    <p>Uteroplacental circulation</p> Signup and view all the answers

    What is the name of the fine, loose connective tissue that forms between the inner surface of the cytotrophoblast and the outer surface of the exocoelomic cavity?

    <p>Extraembryonic mesoderm</p> Signup and view all the answers

    What is the name of the space that develops in the extraembryonic mesoderm and surrounds the primitive yolk sac and amniotic cavity?

    <p>Extraembryonic cavity</p> Signup and view all the answers

    What is the name of the lining that covers the yolk sac?

    <p>Extraembryonic splanchnic mesoderm</p> Signup and view all the answers

    Study Notes

    Blastocyst Development

    • By the 8th day, the blastocyst is partially embedded in the endometrial stroma.
    • The trophoblast differentiates into two layers: the cytotrophoblast (inner layer of mononucleated cells) and the syncytiotrophoblast (outer multinucleated zone).
    • Mitotic figures are found in the cytotrophoblast, but not in the syncytiotrophoblast.
    • Cells in the cytotrophoblast divide and migrate into the syncytiotrophoblast, where they fuse and lose their individual cell membranes.

    Inner Cell Mass Differentiation

    • The inner cell mass or embryoblast differentiates into two layers: the hypoblast layer (small cuboidal cells) and the epiblast layer (high columnar cells).
    • Together, these layers form a flat disc.
    • A small cavity appears within the epiblast, which enlarges to become the amniotic cavity.

    Amniotic Cavity and Epiblast

    • Epiblast cells adjacent to the cytotrophoblast are called amnioblasts; they line the amniotic cavity.
    • The endometrial stroma adjacent to the implantation site is edematous and highly vascular.

    Development by Day 9

    • 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.
    • These vacuoles fuse to form large lacunae, marking the lacunar stage of trophoblast development.

    Exocoelomic Membrane

    • At the abembryonic pole, flattened cells from the hypoblast form a thin membrane, the exocoelomic (Heuser's) membrane, which lines the inner surface of the cytotrophoblast.
    • This membrane, together with the hypoblast, forms the lining of the exocoelomic cavity or primitive yolk sac.

    Development by Day 11-12

    • The blastocyst is completely embedded in the endometrial stroma, and the surface epithelium almost entirely covers the original defect in the uterine wall.
    • The trophoblast is characterized by lacunar spaces in the syncytium that form an intercommunicating network.
    • The syncytial lacunae become continuous with the sinusoids, and maternal blood enters the lacunar system, establishing the uteroplacental circulation.

    Extraembryonic Mesoderm

    • 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.
    • The extraembryonic mesoderm eventually fills the space between the trophoblast and the amnion and exocoelomic membrane.
    • Large cavities develop in the extraembryonic mesoderm, forming the extraembryonic cavity or chorionic cavity.

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    Description

    Learn about the development of the blastocyst on the 8th day, including the differentiation of the trophoblast into cytotrophoblast and syncytiotrophoblast layers. Understand the role of mitosis in these layers.

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