Third week of Development Flashcards
What is the source of all 3 germ layers?
Epiblast
Where do each of the 3 germ layers come from?
Ectoderm- cells that remain in the epiblast
Mesoderm- cells that lie b/w the epiblast and the new endoderm
Endoderm- formed from cells that displace the hypoblast
What controls cell migration and cell specification?
fibroblast growth factor 8 (FGF8) which is synthesized in the streak cells
What does FGF8 regulate?
It down regulates E-cadherin (which keeps epiblast cells together) and Brachyury which controls specification in the mesoderm
Where does the prechordal plate form?
between the tip of the buccupharyngeal membrane and the notochord and is important for the induction of the forebrain
How is the notochord formed?
prenotochordal cells move to the prechordal plate and insert into the hypoblast to make up the notochordal plate; they then proliferate and detach as a solid cord
When the cloacal membrane (future anus) appears, what happens?
the posterior wall of the yolk sac forms a small appendage (allantois) into the connecting stalk
How is the anteroposterior axis established?
by the anterior visceral endoderm which secretes:
cerberus
lefty
3 transcription factors
Nodal (initiates formation and maintains streak)
How is mesoderm ventralized?
BMP4 (bone morphogenetic protein 4) and FGF
What down regulates BMP4
chordin (activated by goosecoid), noggin, and Follistatin
What does HNF- 3beta do?
maintains the node and induces regional specificity in the forebrain and midbrain
What does the Brachyury (T) gene do?
controls the regulation of dorsal mesoderm formation in the middle and caudal regions and is expressed in the node notochord, and notochord precursor cells
What does an absence of Brachyury (T) gene cause?
sirenomelia (mermaid disease)
What is the transcription factor responsible for establishing left sidedness?
PITX2
Which factor is expressed on the left side and is a barrier to prevent cross over?
Lefty-1
what is the cascade the upregulates PITX2?
primitive streak appears and produces FGF8
induces Nodal on left side
FGF8 maintains Nodal and Lefty-2
These 2 upregulate PITX2
What is SHH (sonic hedge hog)?
prevents left-sided gene expression on the right
How is Brachyury T gene also needed for establishment of left-right sidedness?
is it essential for the expression of Nodal, Lefty-1, and Lefty-2
How is serotonin (5HT) important in lef/right sidedness?
It is broken down on the right by MAO and concentrated on the left
Alterations in this are bad
What is snail?
it is transcription factor that is restricted to the right side and regulates genes there
Cells that enter through the cranial region of the node become what?
notochord
Cells that migrate at the lateral edges of the node and from the cranial end of the streak become what?
paraxial mesoderm
Cells that migrate through the midstreak region become what?
intermediate mesoderm
cells that migrate through the more caudal part of the streak become what?
lateral plate mesoderm
