Questions and Answers
What is the name of the cavity that forms between the extra-embryonic mesodermal cells?
What is the name of the structure that develops from the extraembryonic somatic mesoderm and trophoblast?
What is the name of the process by which the hypoblast cells migrate to line the inner surface of the cavity?
What is the name of the structure that forms from the fluid space within the extra-embryonic mesoderm?
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What is the name of the layer of cells that forms from the hypoblast and surrounds the umbilical vesicle?
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What is the name of the structure that develops from the connecting stalk?
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What is the name of the process by which the oxygenated maternal blood and nutrients nourish the embryo?
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What is the name of the thickened area of columnar cells that acts as an organization center?
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What is the function of the prechordal plate?
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When does implantation of the blastocyst in the uterine endometrium begin and end?
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What is the primary function of the lytic enzymes released from the acrosomes of the sperms?
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What is the result of the fusion of adjacent lacunae?
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During which day does the blastocyst sink beneath the endometrial epithelium and the defect is filled by a closing plug?
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What is the name of the cavity that develops from spaces in the extraembryonic mesoderm?
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What is the term for the endometrial changes that result from the adaptation of these tissues in preparation for implantation?
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What is the function of the syncytiotrophoblast?
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During which day does the trophoblast differentiate into two layers: the syncytiotrophoblast and cytotrophoblast?
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What is the result of the erosion of endometrial blood vessels by the syncytiotrophoblast?
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What is the origin of mesoblastic cells?
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During week 3, what happens to the hypoblast?
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What is the function of the notochord?
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During which week do the cells from the primitive streak migrate widely and differentiate into diverse cell types?
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What is the fate of the primitive streak by the end of week 4?
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What is the characteristic of the embryonic disk at the end of week 2?
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What is the size of the embryo at the end of week 1?
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During which week does gastrulation occur?
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What is the fate of the hypoblast cells during week 3?
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What is the outcome of the formation of the notochord during week 3?
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What is the name of the structure that forms a thick column of mesoderm on either side of the notochord during the 3rd week?
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What is the name of the structure that gives rise to the axial skeleton and associated musculature, as well as the dermis?
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What is the name of the layer of mesoderm that forms the embryonic gut?
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What is the name of the process by which the intraembryonic coelom appears as isolated spaces in the lateral mesoderm and cardiogenic mesoderm?
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What is the name of the structure that forms the body wall?
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During which week do somites develop adjacent to the neural tube?
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What is the name of the structure that gives rise to the embryonic heart primordia?
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How many main body cavities does the intraembryonic coelom develop into during the 2nd month?
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What is the name of the structure that forms the horseshoe-shaped intraembryonic coelom?
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During which process is the bilaminar embryonic disc converted into a trilaminar embryonic disc?
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What is the primary function of the pharyngeal arches?
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What forms the cervical sinus during the fifth week?
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What is the number of pharyngeal pouches?
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What gives rise to the maxilla, zygomatic bone, and a portion of the vomer?
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When do the pharyngeal arches begin to develop?
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What is the name of the structure that forms from the primordial pharynx and joins the stomodeum?
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What is the function of the pharyngeal membrane?
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What is the number of pharyngeal arches visible externally by the end of the fourth week?
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What is the primary function of the decidual cells?
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What is the term for the process of forming three embryonic germ layers?
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What is the result of the erosion of endometrial blood vessels by the syncytiotrophoblast?
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What is the origin of the epiblast?
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What is the function of the amnioblasts?
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What is the direction referred to as cephalad?
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What is the process by which the syncytiotrophoblast invades into the stroma?
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What is the term for the fluid-filled cavity that forms between the extra-embryonic mesodermal cells?
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What is the purpose of the neurenteric canal?
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What occurs to the notochordal process after it approaches the prechordal plate?
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What is the outcome of the notochordal plate transition?
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What occurs to the notochordal plate cells during the transition to the notochord?
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What is the direction of the notochordal plate transition?
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What is the final fate of the neurenteric canal?
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What is the significance of the neural crest in the development of the CNS?
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What is the result of the coalescence of coelomic vesicles?
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In which structures do blood vessels first appear?
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What is the significance of the paraxial mesoderm?
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What is the purpose of embryonic folding?
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What is the result of the fusion of the neural folds?
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What is the significance of the heart tubes?
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What is the significance of the somites?
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What forms the cuboidal masses of mesoderm on either side of the notochord?
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During which week do the cells from the primitive streak migrate widely and differentiate into diverse cell types?
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What is the outcome of the formation of the notochord during week 3?
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What forms the embryonic gut?
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During which week does the intraembryonic coelom develop into three main body cavities?
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What gives rise to the axial skeleton and associated musculature, as well as the dermis?
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What forms the body wall?
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During which week do the cells from the primitive streak migrate widely and differentiate into diverse cell types?
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What is the name of the process by which the intraembryonic coelom appears as isolated spaces in the lateral mesoderm and cardiogenic mesoderm?
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During which process is the bilaminar embryonic disc converted into a trilaminar embryonic disc?
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Which pharyngeal arches give rise to the laryngeal cartilage?
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Which muscle is derived from the first pharyngeal arch?
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What is the fate of the fifth pharyngeal arch?
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Which bone is formed from the proximal mandibular prominence?
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Which pharyngeal arches form the ossicles of the middle ear?
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What is the role of the first pharyngeal arch in the formation of the face?
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What is the primary role of the notochord in embryonic development?
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What is the outcome of the formation of the notochord during week 3?
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What is the function of the notochordal process?
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What is the relationship between the notochord and the development of axial MSK structures?
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What is the significance of the notochord in relation to the CNS?
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During which week does the notochord develop a lumen known as the notochordal canal?
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What is the result of the notochordal process approaching the prechordal plate?
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What happens to the notochordal plate cells?
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What is the function of the neurenteric canal?
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What happens to the neurenteric canal after the notochord is fully developed?
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What is the direction of the notochordal plate !notochord transition?
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What is the primary function of the notochord?
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What is the significance of the prechordal plate?
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What is the function of the oropharyngeal membrane?
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What is the outcome of the formation of the neural tube?
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What is the characteristic of the neural crest cells?
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What is the fate of the neural crest cells?
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What is the primary function of the extra-embryonic mesoderm?
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What is the derivative of the neural crest cells?
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During which week does neurulation occur?
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What is the significance of the prechordal plate in embryonic development?
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What is the role of the syncytiotrophoblast in the development of the embryo?
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What is the function of the allantois?
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What is the result of the accumulation of fluid spaces within the extra-embryonic mesoderm?
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What is the significance of the neural plate?
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What is the relationship between the extra-embryonic coelom and the chorionic cavity?
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What is the significance of the hypoblast cells in the development of the embryo?
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What is the result of the erosion of endometrial blood vessels by the syncytiotrophoblast?
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What is the primary function of the cytotrophoblast?
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What is the significance of the chorionic cavity in embryonic development?
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What is the primary function of the amniotic cavity?
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What is the primary source of mesenchymal cells during the third week of embryonic development?
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What is the result of the infolding of the notochordal plate during the third week?
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Which of the following structures is NOT present in the embryonic disc at the end of the third week?
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What is the role of the mesenchymal cells produced by the primitive streak?
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What is the location of the notochordal process during the third week?
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What is the outcome of the displacement of the hypoblast by epiblastic cells?
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What is the relationship between the embryonic mesoderm and the extraembryonic mesoderm?
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What is the significance of the neural plate during the third week?
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Study Notes
Embryonic Development
- At the end of week 1, the blastocyst consists of the zona pellucida, blastocoel, and two distinct groups of cells: the inner cell mass (embryoblast) and the trophectoderm.
- During week 2, the trophoblast differentiates into two layers: the syncytiotrophoblast and cytotrophoblast.
- The blastocyst adheres to the endometrial epithelium, and the syncytiotrophoblast erodes endometrial tissues, allowing the blastocyst to embed in the endometrium.
Extraembryonic Mesoderm and Coelom
- Extraembryonic mesoderm develops between the cytotrophoblast and the cells derived from the hypoblast (yolk sac).
- Fluid accumulates between the extraembryonic mesodermal cells to form the extraembryonic coelom.
- By the end of week 2, there are three distinct fluid-filled cavities: the umbilical vesicle (yolk sac), amniotic cavity, and extraembryonic coelom.
Chorion and Amnion
- The chorion consists of the extraembryonic somatic mesoderm and the trophoblast, forming the wall of the chorionic sac.
- The chorionic sac encloses the embryo and its cavities, and is surrounded by the syncytiotrophoblast.
Prechordal Plate and Embryonic Mesoderm
- At the end of week 2, the prechordal plate appears as a thickened area of columnar cells that acts as an organizing center.
- The prechordal plate induces the formation of structures in the head and mouth region and prevents the formation of structures in the caudal aspect of the embryo.
Week 3 - Embryonic Mesoderm
- During week 3, cells from the epiblast, as well as from the primitive node and other parts of the primitive streak, displace the hypoblast to form the embryonic endoderm.
- The cells remaining in the epiblast form the embryonic ectoderm.
- Mesenchymal cells derived from the primitive streak migrate widely and differentiate into diverse types of cells.
Notochord and Somites
- The notochord forms and then gives rise to the neural groove, neural plate, and early neural tube.
- Paraxial mesoderm + somites develop adjacent to the neural tube.
Intraembryonic Mesoderm and Coelom
- During the 3rd week, the intraembryonic mesoderm proliferates to form a thick column of mesoderm on either side of the notochord.
- The intermediate mesoderm is found just lateral to the paraxial mesoderm, and the lateral mesoderm is lateral to the intermediate mesoderm.
- The primordium of the intraembryonic coelom appears as isolated spaces in the lateral mesoderm and cardiogenic mesoderm.
Pharyngeal Arches and Pouches
- The pharyngeal arches begin to develop early in the 4th week.
- Four pairs of pharyngeal pouches develop, and the endoderm of the pouches contacts the ectoderm of the pharyngeal grooves to form a double-layered pharyngeal membrane.
Pharyngeal Apparatus
- The pharyngeal arches are separated from each other by the pharyngeal grooves.
- The first pharyngeal arch (mandibular arch) separates into two prominences: the maxillary prominence and the mandibular prominence.
Bony and Cartilaginous Derivatives
- The pharyngeal arches give rise to bony and cartilaginous derivatives, including the maxilla, zygomatic bone, and portion of the vomer.
Week 1-2: Embryonic Development
- hCG is the hormone detected in pregnancy tests
- At the end of Week 1 (days 6-7), the syncytiotrophoblast invades into the stroma, and the inner cell mass (embryoblast) differentiates into two distinct layers:
- Epiblast: will become the embryo proper
- Hypoblast: will line the blastocystic cavity (coelom) and form the primary yolk sac, also known as the primary endoderm
Week 2: Embryologic Terms
- Coelom: a fluid-filled cavity
- Gastrulation: the process of forming three embryonic germ layers:
- Ectoderm: typically found on the “exterior” of the organism
- Endoderm: typically found on the “interior” of the organism
- Mesoderm (intraembryonic mesoderm): found between the ectoderm and endoderm
- Cephalad: towards the head region (anatomical term)
- Caudad: towards the “tail” region (anatomical term)
Week 2: Implantation and Epiblast-Hypoblast Development
- Implantation of the embryo is complete at about day 10:
- The embryo is completely embedded within the endometrium
- Surrounded by syncytiotrophoblast cells
- The stromal cells (below the epithelium) undergo decidualization:
- Now known as decidual cells
- Decidual cells accumulate glycogen and lipids throughout the uterus
- The epiblast enlarges and gives rise to amnioblasts:
- Amnioblast = cells that surround the developing amniotic cavity
- The hypoblast extends around the entire interior surface of the blastocoel:
- New class of cells begin to form and migrate between the hypoblast-derived cells (yolk sac or extraembryonic endoderm) and the cytotrophoblast
Week 3: Development of the Notochord
- Development of the notochord:
- Mesenchymal cells dive into the primitive pit and migrate cephalad
- They form a cord called the notochordal process
- The notochordal process develops a lumen known as the notochordal canal
- After the notochordal process approaches the prechordal plate, the floor of the process “fuses” with the endoderm:
- The notochordal process is now the notochordal plate
- The amniotic cavity and the umbilical vesicle can communicate through an opening (the neurenteric canal)
- At this point, the notochordal plate cells proliferate and fold inwards, forming the fully-developed notochord:
- No canal is present
- Notochordal plate ! notochord transition starts cranially and progresses caudally
- The notochord gives rise to:
- Intervertebral discs
Week 3: Intraembryonic Mesoderm and Coelom
- During the 3rd week:
- Intraembryonic mesoderm proliferates to form a thick column of mesoderm on either side of the notochord
- Beside the axis of the organism (as defined by the notochord) = paraxial mesoderm
- The intermediate mesoderm is found just lateral to the paraxial mesoderm
- The lateral mesoderm is lateral to the intermediate mesoderm
- The primordium of the intraembryonic coelom (embryonic body cavity) appears as isolated spaces in the lateral mesoderm and cardiogenic mesoderm:
- These spaces soon coalesce (join together) and form a single horseshoe-shaped intraembryonic coelom
- Intra-embryonic coelom divides the lateral mesoderm into two layers:
- A somatic or parietal layer of lateral mesoderm located beneath the ectodermal epithelium and continuous with the extraembryonic mesoderm covering the amnion
- A splanchnic or visceral layer of lateral mesoderm next to the endoderm and continuous with the extraembryonic mesoderm covering the umbilical vesicle
Week 3: Somites and Neural Tube
- During the 3rd – 5th week, somites develop adjacent to the neural tube:
- Somite = cuboidal masses of mesoderm on either side of the notochord, visible along the dorso-lateral surface of the embryo on each side of the neural tube
- Formed from the paraxial mesoderm
- Somites give rise to most of the axial skeleton and associated musculature, as well as the dermis in those areas
- The neural tube forms from the neural plate:
- A longitudinal neural groove develops in the neural plate
- Fusion of the folds forms the neural tube, the primordium of the CNS
Week 3: Summary
- The bilaminar embryonic disc is converted into a trilaminar embryonic disc during gastrulation
- A longitudinal neural groove develops in the neural plate, which is flanked by neural folds
- Fusion of the folds forms the neural tube, the primordium of the CNS
- The mesoderm on each side of the notochord condenses to form longitudinal columns of paraxial mesoderm, which, by the end of the third week, give rise to somites
- The coelom (cavity) within the embryo arises as isolated spaces in the lateral mesoderm and cardiogenic mesoderm
- Blood vessels first appear in the wall of the umbilical vesicle (yolk sac), allantois, and chorion
- Fetal and adult erythrocytes develop from different hematopoietic precursors
- The primordial heart is represented by paired endocardial heart tubes
- By the end of the third week, the heart tubes have fused to form a tubular heart that is joined to vessels in the embryo, umbilical vesicle, chorion, and connecting stalk to form a primordial cardiovascular system
Development of Extraembryonic Structures
- The hypoblast and epiblast form the bilaminar disk.
- The hypoblast cells migrate to line the inner surface of the cavity, forming amnioblasts.
- Extraembryonic mesoderm develops between the cytotrophoblast and the cells derived from the hypoblast (umbilical vesicle/yolk sac).
- Fluid accumulation between the extraembryonic mesodermal cells forms the extraembryonic coelom.
- Three distinct fluid-filled cavities develop in the embryo:
- Umbilical vesicle (yolk sac)
- Amniotic cavity
- Extraembryonic coelom
Development of the Notochord
- The notochord develops from mesenchymal cells that dive into the primitive pit and migrate cephalad.
- The notochordal process develops a lumen known as the notochordal canal.
- Roles of the notochord:
- Establishes the longitudinal axis of the embryo and gives it rigidity.
- Provides signals for the development of axial MSK structures and the CNS.
- Contributes to the intervertebral discs.
Neurulation and Folding
- The notochord induces the overlying ectoderm to form the neural plate.
- The neural plate invaginates to form the neural groove, and the neural folds eventually fuse together to form the neural tube.
- The neural tube is the primordium of the CNS.
- Neurulation is the process by which the neural tube is formed, and it is complete at the end of the 4th week.
Neural Crest Cells
- Neural crest cells originate from the "crest" at the apex of the neural folds.
- They migrate dorso-laterally on either side of the tube and many migrate widely throughout the mesenchyme.
- Derivatives of the neural crest include:
- Ganglia of CN V, VII, IX, X
- Spinal ganglia
Oropharyngeal and Cloacal Membranes
- The prechordal plate develops into the oropharyngeal membrane.
- The cloacal membrane forms caudal to the primitive streak.
- Both membranes are two-layered, consisting of ectoderm and endoderm, with no mesoderm.
Allantois
- The allantois is a small, vascularized diverticulum from the caudal wall of the umbilical vesicle, extending into the connecting stalk.
- Functions in early blood formation and bladder development.
- Blood vessels become umbilical arteries, and a small portion persists as the urachus that extends from the bladder to the umbilical region, eventually becoming the median umbilical ligament in adults.
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Description
Learn about the development of the blastocyst during weeks 1-2, including the formation of the inner cell mass and trophectoderm, and the differentiation of the trophoblast into two layers.
