Embryology: Early Blastocyst Stage and Development
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Questions and Answers

What is the process of oocyte production called?

  • Spermatogenesis
  • Cleavage
  • Fertilization
  • Oogenesis (correct)

    • How long does it generally take for fertilization to occur after ovulation?

  • 24 to 48 hours
  • 12 to 24 hours (correct)
  • 1 to 3 hours
  • 48 to 72 hours

    • At what stage does the morula consist of 16 blastomeres?

  • 4 days of age
  • 30 hours of age
  • 1 day of development
  • 3 days of age (correct)

    • What role do the fimbriae of the uterine tubes play in fertilization?

    <p>Sweeping the oocyte into the ampulla</p> Signup and view all the answers

    What marks the beginning of implantation of the blastocyst?

    <p>Disappearance of the zona pellucida</p> Signup and view all the answers

    Which stage occurs approximately 4.5 days after fertilization?

    <p>Development of the blastocyst</p> Signup and view all the answers

    What happens to the zona pellucida during the early stages of embryo development?

    <p>It degenerates</p> Signup and view all the answers

    What is the primary process that produces a single blastomere from a fertilized oocyte?

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

    What occurs during the compaction stage of embryo development?

    <p>Blastomeres decrease in size with mitotic divisions.</p> Signup and view all the answers

    Which structure persists and ultimately degenerates to allow implantation?

    <p>Zona pellucida</p> Signup and view all the answers

    What is the role of the trophoblast during implantation?

    <p>To invade the endometrium and mediate attachment.</p> Signup and view all the answers

    What significant change occurs in the cell size during cleavage divisions?

    <p>Cells become smaller with each cleavage division.</p> Signup and view all the answers

    What happens to the inner cell mass during the morula stage?

    <p>It becomes the embryoblast from which the embryo develops.</p> Signup and view all the answers

    Which of the following processes is initiated by selectins on the trophoblast cells?

    <p>Initial attachment of the blastocyst to the endometrium.</p> Signup and view all the answers

    At what embryonic stage does the embryo reach the 16-cell stage?

    <p>Morula stage.</p> Signup and view all the answers

    How do cells communicate during compaction?

    <p>Through extensive gap junctions.</p> Signup and view all the answers

    At what stage is the embryo considered to be at the early blastocyst stage?

    <p>Day 4.5</p> Signup and view all the answers

    What structure covers the point of entry of the blastocyst into the endometrial wall?

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

    Which cell mass is pushed to one pole in the blastocyst cavity?

    <p>Inner cell mass (embryoblast)</p> Signup and view all the answers

    Which layer forms the wall of the blastocyst?

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

    What is formed from the hypoblast layer during the blastocyst stage?

    <p>Exocoelomic (Heuser's) membrane</p> Signup and view all the answers

    Where is the embryo suspended during the early blastocyst stage?

    <p>Connecting stalk</p> Signup and view all the answers

    What is the term for the extraembryonic mesoderm forming the connection between the embryo and the trophoblast layer?

    <p>Connecting stalk</p> Signup and view all the answers

    Which structure forms the primitive/primary yolk sac in the blastocyst?

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

    What happens to the zona pellucida as the blastocyst enlarges?

    <p>It degenerates to allow expansion of the blastocyst.</p> Signup and view all the answers

    What is formed when multiple divisions of the zygote occur?

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

    During which stage does the inner cell mass get pushed to one pole?

    <p>Early blastocyst stage</p> Signup and view all the answers

    What do the outer cells of the blastocyst become?

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

    What is the fate of the morula as it continues to develop?

    <p>It develops into a blastocyst.</p> Signup and view all the answers

    What indicates that a blastocyst is ready for implantation?

    <p>Degeneration of the zona pellucida</p> Signup and view all the answers

    How long after fertilization does the blastocyst typically reach the uterine cavity?

    <p>3-4 days</p> Signup and view all the answers

    What structure remains intact as the zygote travels to the uterine cavity?

    <p>Zona pellucida</p> Signup and view all the answers

    What occurs after the third cleavage during embryo development?

    <p>Blastomeres maximize their contact with each other</p> Signup and view all the answers

    What is the role of selectins on trophoblast cells during implantation?

    <p>They mediate the attachment of the blastocyst to the endometrium</p> Signup and view all the answers

    What is the main feature of the morula stage?

    <p>Development of gap junctions for communication</p> Signup and view all the answers

    What happens to the zona pellucida by day 5?

    <p>It completely degenerates, allowing the blastocyst to hatch</p> Signup and view all the answers

    Which statement is true regarding the blastocyst during the implantation phase?

    <p>It begins to invade the endometrial epithelium</p> Signup and view all the answers

    What characterizes the cleavage division stages of the embryo?

    <p>Progressive decrease in cell size after each division</p> Signup and view all the answers

    What is the inner cell mass of the blastocyst known as?

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

    What occurs during the compaction of the morula?

    <p>Blastomeres form a tightly packed structure</p> Signup and view all the answers

    Which mesodermal layer continues with the mesoderm covering the amnion?

    <p>Somatic/Parietal Mesoderm</p> Signup and view all the answers

    What is the primary role of the myotome from the ventrolateral region?

    <p>Gives rise to skeletal muscle of the lateral body wall</p> Signup and view all the answers

    What does the remaining dermatome of the dorsal portion form?

    <p>Dermis of the neck and trunk</p> Signup and view all the answers

    Which pathway involves migration beneath the ectoderm?

    <p>Dorsal pathway</p> Signup and view all the answers

    At what stage does the paraxial mesoderm start to organize into somitomeres?

    <p>By the beginning of the third week</p> Signup and view all the answers

    Which body structures do the epimere myotome give rise to?

    <p>Extensors of the vertebral column</p> Signup and view all the answers

    Which cavity is lined by the two layers of mesoderm?

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

    What type of cells do the myotomes become as they migrate?

    <p>Mesenchymal cells</p> Signup and view all the answers

    What occurs during the closure of the neural tube?

    <p>Cells from the neural crest form a new cell population.</p> Signup and view all the answers

    What is the primary contribution of the neural crest cells?

    <p>Formation of the peripheral nervous system.</p> Signup and view all the answers

    During which week of embryonic development does neurulation primarily occur?

    <p>Third week.</p> Signup and view all the answers

    What marks the beginning of central nervous system formation?

    <p>Formation of the neural plate.</p> Signup and view all the answers

    What indicates the completion of the neurulation process?

    <p>Complete fusion of the neural tube.</p> Signup and view all the answers

    What happens to the neural folds during the transformation into the neural tube?

    <p>They fuse together.</p> Signup and view all the answers

    Which of the following is not a step in the neurulation process?

    <p>Producing new dermal cells.</p> Signup and view all the answers

    What is the significance of the neural crest during embryonic development?

    <p>It plays a crucial role in forming the peripheral nervous system.</p> Signup and view all the answers

    What do myotomes do when they migrate to the trunk and limbs?

    <p>They fuse and merge to form skeletal muscle.</p> Signup and view all the answers

    What is the effect of the plexus formation on nerve fibers?

    <p>They make up different peripheral nerves from various spinal levels.</p> Signup and view all the answers

    Why might different peripheral nerves innervate muscles responsible for the same function?

    <p>Muscle groups may be derived from the same myotome.</p> Signup and view all the answers

    What do nerve fibers from a myotome maintain during migration?

    <p>Connection to their original somitic segment.</p> Signup and view all the answers

    What do the peripheral nerves innervate?

    <p>Different muscle groups.</p> Signup and view all the answers

    What does fusion of myotomes enable them to do?

    <p>Form skeletal muscle at various joints.</p> Signup and view all the answers

    What is a significant characteristic of peripheral nerves in relation to myotomes?

    <p>They mix fibers from several spinal levels.</p> Signup and view all the answers

    What type of muscle do myotomes contribute to during their development?

    <p>Striated (skeletal) muscle.</p> Signup and view all the answers

    What primarily separates the extraembryonic mesoderm into two layers during the implantation phase?

    <p>Appearance of extraembryonic coelom</p> Signup and view all the answers

    By the end of Day 8 of implantation, which component of the blastocyst has penetrated the endometrium of the uterus?

    <p>Outer cell mass</p> Signup and view all the answers

    Which structure forms the initial contact point with the maternal uterine wall during implantation?

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

    What is the key role of the lacunae formed in the syncytiotrophoblast layer during the establishment of uteroplacental circulation?

    <p>Enable maternal blood to nourish the embryo</p> Signup and view all the answers

    What significant development occurs between Days 11 and 12 following fertilization?

    <p>Establishment of uteroplacental circulation</p> Signup and view all the answers

    What main structures do the embryoblast form at the embryonic pole?

    <p>Epiblast and hypoblast</p> Signup and view all the answers

    What does the zona pellucida prevent during the cleavage stages of the zygote?

    <p>Increase in cell size</p> Signup and view all the answers

    At what stage does the zygote reach the 4-cell stage of development?

    <p>Around 40 hours post-fertilization</p> Signup and view all the answers

    Which layer is responsible for the invasion of the endometrium during implantation?

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

    In which structure is the inner cell mass pushed to one pole during early development?

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

    What happens to the size of each blastomere with successive mitotic divisions?

    <p>They decrease in size gradually</p> Signup and view all the answers

    What structures do neural crest cells typically not contribute to?

    <p>Skeletal muscle tissue</p> Signup and view all the answers

    Which layer forms a flat disc between the amniotic cavity and the blastocyst cavity?

    <p>Epiblast and hypoblast</p> Signup and view all the answers

    Which of the following is NOT derived from the ectodermal germ layer?

    <p>Smooth muscle tissue</p> Signup and view all the answers

    What is the role of the cytotrophoblast in early embryo development?

    <p>It contributes to the placenta formation</p> Signup and view all the answers

    The paraxial mesoderm primarily gives rise to which of the following structures?

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

    Which layer does the lateral plate mesoderm remain thin compared to?

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

    Which of the following structures is formed from both oral and neural ectoderm?

    <p>Pituitary gland</p> Signup and view all the answers

    What is the function of the mesenchyme during early embryonic development?

    <p>Migration of neural crest cells</p> Signup and view all the answers

    How do the derivatives of the neural crest differ from those of the mesodermal germ layer?

    <p>Mesodermal derivatives are often associated with muscle and bone</p> Signup and view all the answers

    What is the primary distinction between the ectodermal germ layer and the mesodermal germ layer?

    <p>Ectoderm forms structures directly interfacing with the environment</p> Signup and view all the answers

    What is the relationship between myotomes and peripheral nerves during muscle innervation?

    <p>Myotomes maintain nerve supply from their spinal segment while forming various peripheral nerves.</p> Signup and view all the answers

    How do the myoblast cells behave in relation to their original somitic segment?

    <p>They maintain nerve connections to their original somitic segment despite migration.</p> Signup and view all the answers

    What statement best describes muscle innervation in relation to spinal cord levels?

    <p>Muscles may have different peripheral nerves yet still perform identical functions.</p> Signup and view all the answers

    What does the process of myotome migration primarily result in?

    <p>Development of skeletal muscle while retaining nerve supply.</p> Signup and view all the answers

    What occurs due to plexus formation in relation to spinal nerves?

    <p>Nerve fibers from various spinal levels combine to form peripheral nerves.</p> Signup and view all the answers

    How do different myotomes affect the nerve supply to muscles?

    <p>Multiple myotomes can innervate the same muscle for functional purposes.</p> Signup and view all the answers

    In what manner can muscles that perform similar functions be characterized?

    <p>They may have different peripheral nerves but perform similar functions.</p> Signup and view all the answers

    What characteristic is true about myoblast cells originating from the same myotome?

    <p>They maintain their nerve supply while migrating to various areas.</p> Signup and view all the answers

    What characterizes the layer that surrounds the entire embryo during early development?

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

    During the development of the bilaminar embryonic disc, which layer is formed first?

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

    Which structure is formed from the remnants of the primitive yolk sac?

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

    How is the chorionic cavity formed during embryonic development?

    <p>By pinching off of the primitive yolk sac</p> Signup and view all the answers

    What is the significance of the forming lacunar networks in the syncytiotrophoblast?

    <p>They allow for direct blood circulation to the embryo.</p> Signup and view all the answers

    What condition results from the interaction of syncytiotrophoblast and maternal endometrial tissue?

    <p>Establishment of uteroplacental circulation</p> Signup and view all the answers

    Which component does NOT belong to the bilaminar embryonic disc?

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

    What does the term 'week of twos' refer to in embryology?

    <p>The development of the bilaminar embryonic disc layers</p> Signup and view all the answers

    Which structure connects the developing embryo to the trophoblast?

    <p>Connecting stalk</p> Signup and view all the answers

    What type of cells line the inside of the cystotrophoblast?

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

    What process is initiated by the differentiation of trophoblast into cytotrophoblast and syncytiotrophoblast?

    <p>Lacunar network establishment</p> Signup and view all the answers

    What structure forms the walls of the primitive yolk sac?

    <p>Hypoblast and cytotrophoblast</p> Signup and view all the answers

    Which element participates in the true definition of 'bilaminar germ disc'?

    <p>It exhibits a disc shape made of two layers.</p> Signup and view all the answers

    What critical functional role does the syncytiotrophoblast play in relation to maternal tissues?

    <p>It ensures efficient nutrient transfer to the embryo.</p> Signup and view all the answers

    Study Notes

    Early Blastocyst Stage

    • Reached at around day 4.5
    • Inner cell mass (embryoblast) is pushed to one pole by the fluid within the blastocyst cavity
    • Outer cell mass (trophoblast) forms the wall of the blastocyst
    • Blastocyst cavity becomes the primitive/primary yolk sac
    • Exocoelomic (Heuser's) membrane forms from the hypoblast layer, lining the inner surface of the cytotrophoblast
    • Embryo is suspended by the connecting stalk, which will become the umbilical cord
    • The connecting stalk is the only place where the developing embryo communicates with the trophoblast layer

    Day 4 - Uterine fluid and zona pellucida

    • Uterine fluid begins to penetrate through the zona pellucida and into the intercellular spaces of the inner cell mass

    Day 5 - Blastocyst growth and implantation

    • Zona pellucida degenerates and the trophoblast cells flatten
    • Blastocyst rapidly enlarges and becomes ready for implantation

    Second Week of Development

    Formation of Bilaminar Germ Disc

    • Both the embryoblast and trophoblast cells differentiate into two layers:
      • Embryoblast divides into epiblast and hypoblast
      • trophoblast divides into cytotrophoblast and syncytiotrophoblast

    Day 8 - Differentiation of cells

    • Embryoblast cells differentiate into the epiblast (upper layer) and hypoblast (lower layer)
    • Trophoblast cells differentiate into the cytotrophoblast (inner layer) and syncytiotrophoblast (outer layer)

    Day 6-9 - Implantation

    • Blastocyst attaches to the endometrial epithelium
    • Trophoblast layers start to invade the endometrium, penetrating between the endometrial mucosal cells
    • Selectins on the trophoblast cells interact with carbohydrate receptors on the endometrium, mediating the initial attachment and further invasion of the blastocyst
    • Zona pellucida degenerates and completely disappears by day 5, allowing the blastocyst to hatch and rapidly increase in size
    • The blastocyst now interacts directly with the endometrium for implantation

    Blastocyst Formation and Implantation

    • Begins as a loosely arranged clump of cells, forming a compact ball of cells at the eight-cell stage.
    • Blastomeres maximize contact with each other, forming tight junctions.
    • Zona pellucida persists.
    • Morula forms at the 16-cell stage (approx. day 3 from fertilization).
    • Cells segregate into two groups:
      • Inner cell mass (embryoblast) - develops into an embryo
      • Outer cell mass (trophoblast) - contributes to the placenta
    • By day 4-6, the morula undergoes further divisions and forms a blastocyst.
    • The zona pellucida degenerates, allowing the blastocyst to rapidly increase in size.
    • The blastocyst hatches and interacts with the endometrium for implantation.
    • Implantation occurs on day 6-9.
    • The blastocyst attaches to the endometrial epithelium.
    • The trophoblast layers invade the endometrium.
    • Selectins on trophoblast cells interact with carbohydrate receptors on the endometrium, mediating the blastocyst’s attachment.
    • Fluid enters the blastocyst, creating a blastocoel cavity.
    • The inner cell mass is pushed to one pole.
    • The outer cell mass (trophoblast) becomes a thin, flattened wall.

    Neurulation

    • Occurs during the third week of embryonic development.
    • Neural plate formation is the beginning of the central nervous system formation.
    • The neural tube is formed from the neural plate.
    • Neural crest cells detach from the neural folds during closure of the neural tube and form a new cell population.
    • Neural crest cells contribute to the formation of the peripheral nervous system.
    • Neurulation is complete once the neural tube has completely fused.

    Somite Formation

    • By the beginning of the third week, paraxial mesoderm forms segments called somitomeres.
    • Somitomeres further organize into somites.
    • Somites are further divided into:
      • Sclerotome: forms the vertebral column
      • Dermatome: forms the dermis
      • Myotome: forms skeletal muscle
    • The remaining dorsal portion of the dermatome becomes mesenchymal cells and forms the dermis of the neck and trunk.

    Myotome Nerve Formation

    • Myotomes migrate to the trunk and limbs, fusing and merging to form skeletal muscle at joints.
    • Myotomes maintain their nerve supply from their spinal segment of origin.
    • Nerve fibers from different spinal cord levels make up different peripheral nerves, which innervate different muscle groups.
    • Myoblast cells from the same myotome migrate to different areas, but still maintain their nerve connection to their original somitic segment.

    Neural Crest Cell Migration

    • Neural crest cells migrate along two pathways:
      • Dorsal pathway:
        • Migrate beneath the ectoderm.
      • Ventral pathway:
        • Migrate along the anterior portion of somites and the neural tube.
        • Contribute to the formation of the peripheral nervous system.
        • Form smooth muscle (vessels, GI tract), melanocytes, adrenal medulla cells, and craniofacial cartilage and bones.

    Cleavage and Blastocyst Formation

    • The fertilized zygote undergoes a series of divisions, starting with the 2-cell stage at approximately 30 hours after fertilization, followed by the 4-cell stage at around 40 hours, and the 8-cell stage at about 2.5 days.
    • Each daughter cell resulting from these divisions is called a blastomere, and the zygote maintains its size due to the presence of the zona pellucida.
    • The morula stage is formed around Day 3, marking the arrival of the embryo at the opening of the uterine cavity.

    Embryoblast Differentiation

    • The embryoblast, an inner mass of cells within the blastocyst, gives rise to the superficial epiblast and the deeper hypoblast layers at the embryonic pole.
    • The epiblast forms the amniotic cavity, while the hypoblast and epiblast layers create a flat disc positioned between the amniotic cavity and the blastocyst cavity.

    Trophoblast Differentiation

    • The trophoblast, the outer cell mass, differentiates into an outer syncytiotrophoblast layer and an inner cytotrophoblast layer.
    • The syncytiotrophoblast is responsible for invading the endometrium and the underlying connective tissue.

    Implantation

    • Implantation of the blastocyst typically occurs 6-8 days after fertilization, embedding itself in the endometrium of the uterus by Day 8.

    Lacunae and Lacunar Networks

    • Lacunae, small spaces within the syncytiotrophoblast, form as it expands.
    • By Day 12, these lacunae merge to create larger, interconnected spaces called lacunar networks.

    Uteroplacental Circulation

    • Maternal sinusoids, dilated capillaries in the endometrium surrounding the developing embryo, form.
    • Enzymes released by the expanding syncytiotrophoblast erode the lining of the sinusoids and uterine glands, allowing maternal blood and secretions to flow into the lacunar networks, establishing uteroplacental circulation.

    Extraembryonic Mesoderm

    • Extraembryonic mesoderm forms between the inner surface of the cytotrophoblast and the outer surface of the exocoelomic cavity.
    • This layer proliferates, filling the space between the cytotrophoblast externally and the amnion and exocoelomic membrane internally.
    • The extraembryonic mesoderm does not contribute to any embryonic structure.

    Extraembryonic Coelom

    • Large cavities form within the extraembryonic mesoderm, eventually merging to create a single cavity known as the extraembryonic cavity or coelom.

    Extraembryonic Mesoderm Layers

    • The extraembryonic coelom divides the extraembryonic mesoderm into two layers:
      • Splanchnic (or splanchnopleure): Inner extraembryonic
      • Somatic (or somatopleure): Outer extraembryonic

    Primitive Yolk Sac Formation

    • Cells migrating from the hypoblast create a thin membrane, the exocoelomic membrane, which covers the inner surface of the cytotrophoblast.
    • The exocoelomic membrane and hypoblast cells together form the walls of the primitive yolk sac.

    Secondary Yolk Sac Formation

    • Around Day 13, a portion of the exocoelomic cavity pinches off, creating a smaller cavity called the secondary yolk sac.
    • Remnants of the primitive yolk sac become known as exocoelomic cysts.

    Chorionic Plate and Cavity

    • The extraembryonic somatic mesoderm lines the inside of the cytotrophoblast and becomes the chorionic plate.
    • The extraembryonic coelom forms the chorionic cavity.

    Connecting Stalk

    • By the end of the second week, the chorionic cavity enlarges, and the bilaminar embryonic disc connects to the trophoblast via a band of extraembryonic mesoderm called the connecting stalk (the future umbilical cord).

    Bilaminar Germ Disc

    • The bilaminar germ disc refers to the embryoblast and consists of:
      • Epiblast
      • Hypoblast
    • This "Week of Twos" is characterized by:
      • Trophoblast: Cytotrophoblast & Syncytiotrophoblast
      • Embryoblast: Epiblast & Hypoblast
      • Extraembryonic Mesoderm: Somatic & Splanchnic
      • Cavities: Amniotic & Yolk sac

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    Description

    This quiz covers the early blastocyst stage, which occurs around day 4.5 of development, and key processes leading to implantation. Explore crucial concepts such as the inner and outer cell masses, growth dynamics, and the changes in the zona pellucida. Test your understanding of embryonic development in its early stages.

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