Early Embryology 2 Flashcards
Describe the formation of the bilaminar disc & amnion
Week 2
As implantation of the blastocyst progresses, a small space appears in the embryoblast, which forms the amniotic cavity
- At the same time, morphologic changes occur in the embryoblast that result in the formation of a flat, circular bilaminar plate of cells, the embryonic disc, consisting of two layers
- Epiblast- the thicker layer, consisting of high columnar cells related to the amniotic cavity
Soon, amniogenic(amnion-forming) cells, amnioblasts, separate from the epiblast and form the amnion, which forms the roof of the amniotic cavity (the remaining epiblast form the floor of the cavity)
Describe formation of the primary umbilical vesicle and extraembryonic mesoderm
Week 2
The hypoblast forms the roof of the exocolemic cavity, and it’s endoderm all cells migrate to form the thin exocoelomic (Heuser’s) membrane
- This membrane, together within the hypoblast, lines the exocoelomic cavity which is now called the primary umbilical vesicle (primitive yolk sac)
- The bilaminar embryonic disc now lies between the amniotic cavity above and the primary umbilical vesicle below
- A second migration of cells from hypo last migrate to low between the primitive vesicle and the trophoblast, forming a layer connective tissue called the extraembryonic mesoderm. This surrounds the amnion and umbilical vesicle
Explain the formation of the extraembryonic coelem & primordial uteroplacental circulation
Spaces begin to appear in the extraembryonic mesoderm which rapidly enlarged to form the extraembryonic coelem. This grows to surround the amnion and the primitive umbilical vesicle
At the same time, lacunae (small spaces) appear in the syntiotrophoblast. These become filled with maternal blood from maternal capillaries that were eroded during the implantation process. This establishes the primordial uterosacral circulation as maternal blood in the lucanae allows oxygen and nutrition to pass into the embryo
What are the divisions of the extraembryonic mesoderm?
The extraembryonic coelom splits the extraembryonic mesoderm into two layers:
-extraembryonic somatic mesoderm lining the trophoblast and covering the amnion
-extraembryonic splachnic mesoderm surrounding the primitive umbilical vesicle
The extraembryonic somatic mesoderm and the 2 layers of trophoblast form the walls of the chorionic sac (chorion)
The extraembryonic coelom eventually becomes the chorionic cavity
Explain the formation of the secondary umbilical vesicle
The amnion and the primitive umbilical vesicle are suspended in the extraembryonic coelom (chorionic cavity) and attached to the chorion at the connecting stalk
As the cavity grows, the primary umbilical vesicle decreases in size and a smaller secondary umbilical vesicle forms
Eventually the primary umbilical vesicle regresses, leaving only a remnant
At the end of the second is still a flat bilaminar disc, but hypoblaastic cells in the cranial end of the embryo, thicken into columnar cells forming the prechoral plate. Thus indicates the site of the future mouth
What is gastrulation?
The process by which gastrulation becomes the blastula
Explain the first phase of gastrulation
Week 3: day 15/16
Bilaminar disc—> trilaminar
Primitive streak- beginning of the third week, a thickened linear band of epiblast appears caudally in the middle plane of the disallowed aspect of the embryonic
Left/right, caudal/cranial and dorsal/ventral parts can now be identified
-The cranial end of the streak proliferates to form the primitive node
Concurrently, a narrow groove, the primitive groove, develops in the primitive streak that is continuous with a small depression in the primitive node, the primitive pit.
Explain the second phase of gastrulation
The first wave of the epiblast cells displace the hypoblast to form endoderm
Intraembryonic mesoderm is then formed from epiblast cells which migrate laterally between the new endoderm and the epiblast
The remaining epiblast now constitutes the ectoderm which will later differentiate into neuriectoderm & surface ectoderm
In summary, cells of the epiblast, through the process of gastrulation, give rise to all three germ layers in the embryo, the primordia of all its tissues and organs
What is a sacrococcygeal teratoma?
Most common tumors in newborn infants
More common in female infants than male
Persistence of remnants of the pluripotent primitive steak in the caudal region of the embryo
Derivatives from all three germ layers may be found (teeth, hair, muscle, glands etc)
Explain the first phase of the formation of the notochord
Ectoderm and endoderm remain firmly adhered to each other at two points in the embryonic disc
-Cranially, at the prechordal plate where the oropharyngeal membrane forms
-At the cloacal membrane which is caudal to the origin of the primitive streak
Mesodermal cells at the primitive node migrate cranially from the primitive node and pit, forming a median cellular cord, the notochord process
This process soon acquires a lumen, the notichordal canal. The notichordal process grows cranially between ectoderm and endoderm until it reaches the prechordal plate
Explain the second phase of the notochord formation
The floor of the notochord process fuses with the underlying endoderm becoming the notichordal plate
The notochord plate detaches from the endoderm and it’s free edges fuse as it rolls up to the intraembryonic mesoderm, changing into a solid rod called the notochord
The intraembryonic mesoderm on either side of the notochord differentiates into three parts:
- paraxial mesoderm
- intermediate mesoderm
- lateral mesoderm
The notochord induces the overlying ectoderm to thicken and form the neural plate and eventually the CNS.
The notochord eventually regresses. Adult remnants of the notochord are pulposus of intervertebral discs and Apical ligament
Persistence of additional remnants may result in a chordoma, a slowly growing malignant tumor which invades bone. Occurs at base of skull or lumbosacral region
What happens to the intraembryonic mesoderm?
- Notochord
- Cells which migrate cranial around the prechordal plate—> carcinogenic mesoderm
- Paraxial, intermediate and lateral mesoderm
What happens with the paraxial mesoderm?
Directly adjacent to the developing notochord
-In the head forms loose mesenchyme—> skeletal muscles of the face, jaw and throat
In the future trunk and neck region on either side of the notochord forms paired segmented bodies—> somites
The sequential formation of somites can be used to approximate embryonic age
What are the subdivisions of the somite?
Somites form most of the axial skeleton, dermis, voluntary muscles of the head, limbs and body wall
3 main derivatives
- dermatome—> dermis(fibroblasts)
- Sclerotome—> vertebrae and ribs
- Myotome—> primordial muscle cells
What is the function of the dorsal epiaxial of the somite?
Extensors of the neck, vertebral column and lumbar region
