Podcast
Questions and Answers
What structure forms during the differentiation of the inner cell mass of the blastocyst?
What structure forms during the differentiation of the inner cell mass of the blastocyst?
Which two components differentiate from the trophoblast during the implantation process?
Which two components differentiate from the trophoblast during the implantation process?
During which stage of implantation does the blastocyst begin to exit the zona pellucida?
During which stage of implantation does the blastocyst begin to exit the zona pellucida?
Which of the following best describes the role of the syncytiotrophoblast?
Which of the following best describes the role of the syncytiotrophoblast?
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What structure develops below the hypoblast during embryonic development?
What structure develops below the hypoblast during embryonic development?
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What structures are formed by cytotrophoblasts during the development of the placenta?
What structures are formed by cytotrophoblasts during the development of the placenta?
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During gastrulation, which germ layer is first formed by invaginating cells replacing hypoblast cells?
During gastrulation, which germ layer is first formed by invaginating cells replacing hypoblast cells?
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What is the primary role of syncytiotrophoblasts in the implantation stage?
What is the primary role of syncytiotrophoblasts in the implantation stage?
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Which extra-embryonic structures are primarily involved in gas and nutrient exchange with maternal blood?
Which extra-embryonic structures are primarily involved in gas and nutrient exchange with maternal blood?
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What is the final structure formed after the proliferation of extra-embryonic mesoderm into secondary villi?
What is the final structure formed after the proliferation of extra-embryonic mesoderm into secondary villi?
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How does the mesoderm contribute to the formation of bodily structures?
How does the mesoderm contribute to the formation of bodily structures?
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What is the first embryonic structure formed during the transition from a bilaminar disc to a trilaminar disc?
What is the first embryonic structure formed during the transition from a bilaminar disc to a trilaminar disc?
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What is the role of the umbilical arteries in the connecting stalk?
What is the role of the umbilical arteries in the connecting stalk?
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What is the main role of the trophoblast during implantation?
What is the main role of the trophoblast during implantation?
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During which embryonic stage does the intraembryonic cavity first become distinct and separate from the extraembryonic cavity?
During which embryonic stage does the intraembryonic cavity first become distinct and separate from the extraembryonic cavity?
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Which process is primarily responsible for the transformation of the bilaminar disc into the three germ layers?
Which process is primarily responsible for the transformation of the bilaminar disc into the three germ layers?
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Which anatomical structure is formed at the cranial end of the embryo during longitudinal folding?
Which anatomical structure is formed at the cranial end of the embryo during longitudinal folding?
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What is a major consequence of the lateral folding of embryonic sheets?
What is a major consequence of the lateral folding of embryonic sheets?
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Which of the following is NOT a structure formed from the endoderm layer?
Which of the following is NOT a structure formed from the endoderm layer?
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What type of membrane is the cloacal membrane, and where is it located during embryonic development?
What type of membrane is the cloacal membrane, and where is it located during embryonic development?
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At which developmental stage do the heart field cells rearrange themselves to lie caudally to the mouth?
At which developmental stage do the heart field cells rearrange themselves to lie caudally to the mouth?
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Which embryonic layer forms the gastrointestinal tract and related structures?
Which embryonic layer forms the gastrointestinal tract and related structures?
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What marks the transition from a flat embryo to a three-dimensional structure during development?
What marks the transition from a flat embryo to a three-dimensional structure during development?
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Study Notes
Blastocyst
- Fluid-filled core: blastocoele
- Outer cell mass: trophoblast
- Inner cell mass: embryoblast
- Four stages of implantation: hatching, apposition, adhesion, invasion
- Blastocyst becomes sealed off in the uterus
- Inner cell mass aligns with uterus
- Trophoblast contacts the decidual cells of uterus
- Inner cell mass rotates to the point of contact - apposition
Day 6-7
- Embryoblast becomes bilaminar disc
- Epiblast
- Hypoblast
- Trophoblast develops into cytotrophoblast and syncytiotrophoblast
- Syncytiotrophoblast:
- Proliferation of cells into uterine lining
- Cell membranes disintegrate and cytoplasm mixes
- Eventually becomes the placenta
- Cytotrophoblasts have well-defined cell margins
Day 8-9
- Epiblast forms the embryo
- Hypoblast forms extraembryonic membranes
- Amnion
- Yolk sac
- Chorion
- Allantoin
- Exocoelomic membrane
- Primitive yolk sac develops below hypoblast
- Provides nutrients, red blood cells, primitive germ cells
- Amniotic cavity forms above epiblast
- Embryo becomes fully embedded into the uterus
Day 12-13
- Extraembryonic mesoderm forms between cytotrophoblast cells and amnion/yolk sac
- Spaces develop in extra-embryonic cavities
- Spaces merge to form chorionic (extra-embryonic) cavity
- Chorionic cavity surrounds amnion and yolk sac except at the connecting stalk
- Extraembryonic mesoderm
- Splanchnic
- Somatic
Placenta
- Syncytiotrophoblasts spread into the endometrium
- Syncytiotrophoblasts form lacunae (spaces)
- Syncytiotrophoblasts release proteolytic enzymes
- Proteolytic enzymes break down endometrial blood vessels
- Blood collects in lacunae
Chorionic Villi
- Primary villi: Cytotrophoblasts proliferate into syncytiotrophoblast projections.
- Secondary villi: Extra-embryonic mesoderm projects up within the cytotrophoblasts.
- Tertiary villi: Blood vessels form and pass through the connecting stalk into the chorionic villi.
- Blood vessels in the villi form capillaries
- Exchange occurs between chorionic villi capillaries and maternal blood in lacunae.
Tertiary Villi
- Inner vascularized mesoderm
- Cytotrophoblast
- Syncytiotrophoblast layer
Connecting Stalk
- 2 umbilical arteries
- 1 umbilical vein
Gastrulation
- Transition from bilaminar disc (epiblast and hypoblast) to trilaminar disc (ectoderm, mesoderm, endoderm)
- Formation of axes:
- Primitive node
- Primitive streak
- Cells in epiblast migrate towards and into the primitive streak
- Cells move under epiblast to replace or create new cells
Gastrulation (Epiblast)
- Invaginating cells replace hypoblast cells to form the endoderm
- Cells move between epiblast and hypoblast to form mesoderm
- Paraxial
- Intermediate
- Lateral Plate
Ectoderm
- Outermost layer
- Dorsal
- External surface of embryo
- Forms:
- Epidermis of skin
- Nervous system (via neurulation)
- Sensory structures
- Neural crest cells
- PNS
- Skeletal facial structures
- Melanocytes
Mesoderm
- Forms:
- Bones, cartilage, and connective tissue of skeleton
- Striated skeletal and smooth muscle
- Cardiovascular system (majority)
- Lymphatic system
- Reproductive system
- Parts of renal system
- Spleen
- Linings of cavities (peritoneum)
- Dermis of skin
- Cells of CV and immune system (from marrow)
Endoderm
- Forms:
- Gastrointestinal tract
- Epithelia lining systems exposed to external substances
- Respiratory tract, GI tract
- Bladder and urethra
- Respiratory system (derived from gut tube)
- Organs:
- Liver
- Gall bladder
- Pancreas
- Thyroid
- Tonsils
- Thymus
- Parathyroid glands
Embryonic Folding: Longitudinal
- Cranial-caudal (longitudinal) folding
- Heart tube located at cranial end
- Folding of “flat sheet” begins at each end
- Oropharyngeal membrane (will become mouth) is pushed caudally, below the brain
- Heart field cells move around to lie caudal to mouth
- Cloacal membrane moves underneath
- Connecting stalk is pulled towards umbilical region
Embryonic Folding: Lateral
- Left and right sides grow down and under, meet in midline
- Germ layers meet and fuse
- Endoderm forms an internal tube
- Mesoderm surrounds the intraembryonic cavity
Formation of Body Cavities
- Coronal section
- Cavities arise in the lateral plate region
- Intraembryonic cavity continuous with extraembryonic cavity at this stage
Formation of Body Cavities (Continued)
- 21-day embryo: lateral folding brings left and right sides close together on ventral surface
- 28-day embryo: germ layers fuse
Overview (Key Learning Outcomes)
- Understand fertilization, zygote formation, and cleavage
- Explain blastulation, implantation, and placenta formation
- Explain gastrulation, germ layer formation, and significance of germ layers for organ system development
- Understand how longitudinal and lateral folding forms the basic body structure
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