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Questions and Answers
What two layers are formed by the differentiation of the trophoblast during implantation?
What is the primary function of the syncytiotrophoblast?
At what stage does the blastocyst begin to embed itself into the endometrial stroma?
Which layer of the embryonic disc is formed by small cuboidal cells?
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What process allows the blastocyst to become partly embedded in the endometrium by the eighth day of development?
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What is the composition of the bilaminar embryonic disc?
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What does the amniotic cavity develop from?
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What kind of environment surrounds the endometrial stroma adjacent to the implantation site?
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What is formed when the vacuoles in the trophoblast's syncytium fuse?
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What primarily covers the closing plug of the endometrial epithelial surface defect by day 12?
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Which of the following structures forms the lining of the primitive yolk sac?
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What are the congested and dilated maternal capillaries called, which are involved in the establishment of uteroplacental circulation?
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What type of cells derive the fine, loose connective tissue known as extraembryonic mesoderm?
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On which day does the blastocyst become completely embedded in the endometrial stroma?
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Which process begins the establishment of uteroplacental circulation?
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What is the function of the trophoblastic system once maternal blood begins to flow through it?
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What is a common symptom associated with tubal pregnancies?
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Which implantation site commonly results in early rupture of a tubal pregnancy?
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What is a potential consequence of abdominal pregnancies?
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What factors contribute to the occurrence of tubal pregnancies?
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What can lead to a false-negative result in early pregnancy tests for tubal pregnancies?
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What are primary villi formed from?
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What is the definitive yolk sac also known as?
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What structure becomes the umbilical cord during development?
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What is the primary function of the hormone hCG during early pregnancy?
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Which of the following is true regarding ectopic pregnancies?
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During the second week of development, which two layers does the trophoblast differentiate into?
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What are the exocoelomic cysts a remnant of?
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What does the extraembryonic mesoderm lining the cytotrophoblast become known as?
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What is the term for the new space formed by the confluent large cavities in the extraembryonic mesoderm?
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Which type of mesoderm lines the cytotrophoblast and amnion?
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What significant change occurs in the endometrium during the decidua reaction?
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What substances do decidual cells produce to inhibit natural killer cell activation?
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By day 13 of development, what happens at the implantation site regarding the surface defect?
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What is a characteristic of syncytiotrophoblast cells regarding major histocompatibility complex?
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What role do infiltrating leukocytes in the endometrial stroma play during the decidua reaction?
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Why might bleeding occur at the implantation site around the 28th day of the menstrual cycle?
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Study Notes
Implantation
- Occurs around day 6 after fertilization, near the embryonic pole of the blastocyst
- By day 8, the blastocyst is partially embedded in the endometrial stroma
- Syncytiotrophoblast, the outer layer of the trophoblast, erodes the endometrial epithelium, allowing the blastocyst to embed deeper
- Maternal blood enters the syncytial lacunae, establishing the uteroplacental circulation
- By day 9, the blastocyst is more deeply embedded, with a fibrin coagulum closing the surface defect
- Syncytial lacunae become continuous with maternal sinusoids, allowing oxygen and nutrients exchange between mother and embryo
- Extraembryonic mesoderm forms, filling the space between the trophoblast, amnion, and exocoelomic membrane
- Large cavities develop in the extraembryonic mesoderm, forming the extraembryonic cavity (chorionic cavity), surrounding the yolk sac and amniotic cavity
- The extraembryonic mesoderm lining the cytotrophoblast and amnion is called the extraembryonic somatic mesoderm
- The lining covering the yolk sac is called the extraembryonic splanchnic mesoderm
- By day 12, the uterine epithelium almost completely regenerates, closing the initial defect
- The blastocyst protrudes slightly into the lumen of the uterus
- By day 13, the surface defect in the endometrium usually heals
Decidual Reaction
- Endometrial cells become polyhedral and loaded with glycogen and lipids
- Intercellular spaces are filled with extravasate, making the tissue edematous
- Decidual cells degenerate adjacent to the penetrating syncytiotrophoblast
- Provides an immuneprotective environment for the embryo by:
- Producing immunosuppressive substances (mainly prostaglandins)
- Infiltrating leukocytes in the endometrial stroma that secrete interleukin-2
- Syncytiotrophoblast cells do not express major histocompatibility complex class II, preventing maternal tissue rejection of the implanting embryo
Bilaminar Germ Disk Formation
- The embryoblast differentiates into two layers:
- Hypoblast: small cuboidal cells adjacent to the blastocyst cavity
- Epiblast: high columnar cells adjacent to the amniotic cavity
- A cavity appears within the epiblast, enlarging to become the amniotic cavity
- Epiblast cells adjacent to the cytotrophoblast are called amnioblasts, lining the amniotic cavity
Second Week of Development: Week of 2's
- Two layers of trophoblast: cytotrophoblast and syncytiotrophoblast
- Two layers of embryoblast: epiblast and hypoblast
- Two layers of extraembryonic mesoderm: somatic and splanchnic
- Two cavities: amniotic and yolk sac cavities
Primary Villi
- Cytotrophoblast cells proliferate and penetrate the syncytiotrophoblast, forming cellular columns surrounded by syncytium
- These cellular columns, with the syncytial covering, are known as primary villi
Secondary Yolk Sac
- Hypoblast cells form a new cavity within the exocoelomic cavity, known as the secondary yolk sac
- The secondary yolk sac is much smaller than the original exocoelomic cavity (primitive yolk sac)
- Exocoelomic cysts, pinched-off portions of the original cavity, can be found in the chorionic cavity
Chorionic Cavity
- The extraembryonic coelome expands, forming the chorionic cavity, surrounding the yolk sac and amniotic cavity
- The extraembryonic mesoderm lining the cytotrophoblast is called the chorionic plate
- The connecting stalk, the only point where extraembryonic mesoderm crosses the chorionic cavity, develops into the umbilical cord
Human Chorionic Gonadotropin (hCG)
- Produced by the syncytiotrophoblast
- Enters the maternal blood via the lacunar system
- Maintains the corpus luteum in the ovary, ensuring the production of estrogen and progesterone to sustain pregnancy
- Detectable in maternal blood by the end of the second week
Ectopic Pregnancies
- Implantation of a blastocyst outside the uterus
- 95-98% occur in the uterine tubes, mostly in the ampulla and isthmus
- Tubal pregnancies are the leading cause of maternal deaths during the first trimester
- Symptoms include typical pregnancy symptoms, abdominal pain, tenderness, abnormal bleeding, and sometimes peritonitis
- The rate of β-human chorionic gonadotropin production is slower, leading to possible false-negative results in early tests
- Transvaginal ultrasonography is crucial for early detection
Causes of Ectopic Pregnancies
- Factors delaying or preventing the zygote's transport into the uterus, such as mucosal adhesions or scarring from pelvic inflammatory disease
Types of Ectopic Pregnancies
- Isthmus of the Uterine Tube: Narrow and unexpandable, often rupturing early with extensive bleeding
- Intramural Uterine Tube: Can develop beyond 8 weeks before rupture, causing significant bleeding
- Ampulla or Fimbriae of the Uterine Tube: Can be expelled into the peritoneal cavity, sometimes leading to abdominal pregnancies, which may go to full term but involve high maternal risk from hemorrhage
- Abdominal Pregnancies: High risk of maternal death; rare instances of the fetus surviving, or forming a "stone fetus" (lithopedion) if undetected
- Heterotopic Pregnancies: Rare simultaneous intrauterine and ectopic pregnancies, more common with assisted reproductive technologies.
- Cervical Implantations: Rare, often leading to heavy bleeding and requiring surgical interventions like hysterectomy.
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Description
This quiz delves into the intricate process of implantation that occurs around day 6 after fertilization. It explores the embedding of the blastocyst, the role of syncytiotrophoblast, and the establishment of uteroplacental circulation. Test your understanding of these foundational concepts in embryology.
