Chapter 5 - 3rd Week of Development: Trilaminar Germ Disk Flashcards
When does the gastrulation begin?
It begins with the formation of the primitive streak on the surface of epiblast
What happens during gastrulation?
All the three germ layers (ectoderm, mesoderm, endoderm) are established.
When is the streak vaguely defined?
day 15-16
*Which proteins controls cell invagination in gastrulation?
Fibroblast growth factor 8 (FGF8)
Where is fibroblast growth factor 8 (FGF8) synthesized?
It is synthesized by the streak cells
How does the FGF8 regulate the cell specifitcation into the mesoderm?
By regulating Brachyury (T) expression
What is the source for all germ layers?
The epiblast
What happens when the cloacal membrane dissapears?
The posterior wall of the yolk sac forms a small diverticulum that extends into the connecting stalk
Primitive node
- Is the cephalic end of the steak.
- Consists of a slightly elevtaed area surrounding the small primitive pit
Invagination
Is the migration of the epiblast towards the primitive streak. Upon arrival in the region of the streak, they become flask-shaped, detach from epiblast, and slip beneath it.
When does the allantoenteric diverticulum, or allantois, appear?
Around the 16th day of development
The cloacal membrane
- Formed at the caudal end of the embryonic disk
- Consists of an endoderm and ectoderm adherent
- No intervenning mesoderm
Anterior visceral endoderm (AVE)
- Express genes essential for head formation
- The genes essential for head formation is:
- The transcription factors
> OTX2
> LIM1
> HESX1 - secreted factors
> Cerebrus
> Lefty
- The transcription factors
What inhibits the nodal activity in the cranial end of the embryo?
Cerebru and lefty
What maintains and initiate the primitive streak?
The expression of Nodal
Nodal is a member of….
Transforming growth factor-B
The nodal is responsible….
for formation of dorsal and ventral mesoderm adn head and tail structures
Which are members of transforming growth factor?
- Nodal
- Bone morphogenic protein 4 (BMP4)
Where can we find Bone morphogenic protein 4 (BMP4)
BMP4 is secreted throughout the embryonic disk
Which part of the embryo is the organizer?
The node is the organizer, becuase BMP4 is not blocked by other genes expressed in the node.
HNF-3B
Maintains the node adn later induces regional specifity in the forebrain and the midbrain areas
Goosecoid
Activates inhibitors of BMP4 contributes to regulation of head development.
Brachyury (T) gene
- Control the regulation of dorsal mesoderm formation in middle and caudal regions of the embryo.
- Is essential for cell migration through the primitive streak
Where is Brachyury (T) gene expressed?
It is expressed in the node, notochord precusor cells, and notochord.
Brachyury
Encodes a sequence-specific DNA binding protein that function DNA binding protein that functions as a transcription factor
the T-box
- Is the DNA binding domain.
- Hvae more than 20 genes
FG8 maintains…
nodal expression as well as LEFTY-2. And both of these genes upregulate PITX
Which homeobox-containing transcription factor is responsible for establishing left sideness?
PITX2
Situs inversus and dextrocardia (placement of the heart to the right side) is caused by…
the gene PITX2 being expressed ectopically (e.g on the right side)
LEFTY and Sonic hedgehog (SHH)
prevent left sided signals from crossing over
Brachyury (T) is essential for expression
of Nodal, LEFTY-1, and LEFTY-2
Monoamine oxidase (MAO)
- Breaks down serotonin (5HT)
- Is upstream form FGF8
Changes in 5HT signaling result in:
- Setus inversus
- Dextrocardia
- Other heart defects
What does cells that ingress through the cranial region of the node become?
They become the prechordal plate and the notochord
What does those cell who migrate at the lateral edges of the node and form the cranial end of the streak become?
They become paraxial mesoderm
Cells who migrate trough the midstreak region become…
Intermediated mesoderm
Those cells who migrate through the more caudal part of the streak form….
lateral plate mesoderm
Where does the cells migrating through the caudalmost part of the streak contribute to?
They contribute to extraembryonic mesoderm
What characterize the trophoblast by the beginning of the thrid week?
The trophoblast is characterized by primary villi that cosists of a cytotrophoblastic core covered by a syncytial layer.
Secondary villus
Is the structure we get when mesodermal cells penetrate the core of primary villi and grow toward the decidua.
When does the mesodermal cells in the core of the villus begin to differentiate into blood cells and small blood vessles?
By the end of third week
When do we get the villus capillary system?
By the end of third week, when the mesodermal cells in the core of the villus differentiate into blood cells and small blood vessles.
Other names for the villus?
- Tertiary villus
- Definitive placental villus
What connects the placenta to the embryo?
- Capillaries in tertiary villi make contact with capillaries developing in the mesoderm of the chorionic plate and in the connecting stalk.
- These vessels, in turn, establish contact with the intraembryonic circulatory system
When does the heart begins to beat?
- In the 4th week of development
When the heart begins to beat….
the villous system is ready to supply the embryo proper essential neutriens and oxygen.
