Forming Embryonic Axes Flashcards
What is important in establishing the AP axis in avian embryos like chick embryos?
Gravity
How many times does the egg spin in the hen?
10-15 times/hour for 20 hours
What lies under one side of the blastoderm of a chick?
Lighter yolk components
Where does the primitive streak in chick embryos form?
Posterior and extends anteriorly
Can each wedge of the cut chick embryo form its own primitive streak?
Yes
What molecule is important for primitive streak formation?
Vg1
What experiment was done to observe the expression of cNodal in the primitive streak?
Transplant cells expressing Vg1 at the opposite side of the embryo from Koller’s sickle
Allow embryos to develop until the primitive streak is formed
Perform whole mount in situ hybridization experiment to monitor expression of cNodal
If Vg1 pellet is placed at the anterior end, what happens?
Ectopic and endogenous primitive streaks form
If Vg-1 pellet is placed on lateral margin, what happens?
Only ectopic primitive streak forms
Formation of endogenous primitive streak is inhibited
What happens if 2 Vg-1 pellets are implanted?
Only one primitive streak forms
What was suggested from the observation of one primitive streak forming in the presence of two Vg-1 pellets?
Vg-1 induces both the primitive streak and the expression of a molecule that inhibits induction of the other primitive streak
What is the first definitive axis in the mouse embryo?
AP axis
What are the signaling centers required to pattern the anterior end of the mouse embryo?
Node (organizer) and anterior visceral endoderm (AVE)
Where is AVE derived from?
Distal visceral endoderm (DVE)
The position of AVE is determined by what?
Physical constraints of uterus
Primitive streak and node form on ___________ side of the embryo from AVE
Opposite
There are (high/low) levels of Nodal in distal epiblast and (high/low) levels of Nodal in proximal epiblast of the mouse embryo
Low; high
What is required for patterning the anterior end of the embryo?
AVE
What is node responsible for?
Creating the body plan
Where are 3’ and 5’ genes more expressed in Hox gene clusters?
3’ genes more anteriorly and 5’ genes more posteriorly
What do mutations in Hox genes cause?
Homeotic transformations
Loss-of-function mutations in Hox genes cause _____________ to ______________ transformations
Posterior; anterior
Gain-of-function mutations in Hox genes cause _______________ to ________________ transformations
Anterior; posterior
How many clusters of Hox genes do vertebrates have?
Four
What left-right asymmetry is associated with the heart on the left, left lung bilobed and right lung trilobed, liver on the right, stomach and spleen on the left?
Situs solitus
What kind of asymmetry is typically inherited?
Directional asymmetry
What is the first step in LR patterning?
Initiation
When does initiation occur?
Prior to node formation and at the level of the node
What is another critical early step in establishing LR axis?
Asymmetric cell movement at Hensen’s node
Which molecules of Hensen’s node drive asymmetric gene expression in chicks?
Shh on the left and Cldn10 on the right
What is found at the node of mouse, rabbit, zebrafish and Xenopus?
Cilia
What happens if Kif3B is knocked out in embryos?
LR patterning defects, stunted and immotile cilia
True or false: Monociliary movement is important for left-right patterning
True
What is the second step in LR patterning?
Stabilization of the signal
What are the events of the second step?
Asymmetric expression of nodal in chick (at the node and/or in the perinodal region)
What is the third step of LR patterning?
Propagation of signal
What are the events of the third step?
Transfer and maintenance of information on left and right sides of the embryo
Midline (Notochord) acts as a barrier
Asymmetric gene expression turned on (Lateral plate mesoderm LPM)
In chick, Nodal is expressed on which side of the perinodal region?
Left
What is the purpose of Lefty?
Feedback inhibitor of Nodal signaling
The diffusion model that hypothesizes that interactions between an activator and an inhibitor control signaling gradients within a field of cells and subsequently pattern them is referred to as what?
The Turing reaction
What are the principles of the reaction-diffusion model?
Activator activates its own production
Activator activates its inhibitor
Inhibitor blocks autoactivation of the activator
Inhibitor acts at a long range to restrict the effective range of the activator
Asymmetric Pitx2 expression occurs on which side of the LPM?
Left side
Asymmetric Pitx2 expression is induced by what signaling pathway?
Nodal
Bilateral Pitx2 expression randomizes the direction of heart looping in which organisms?
Chick and Xenopus
What is the fourth step of LR patterning?
Translation of signal
Where does the translation of signal occur?
Along the entire AP axis and at the level of individual organs
What event occurs during the fourth step?
Pitx2 is asymmetrically expressed in organs that exhibit asymmetry
Which steps are species-specific or conserved in all species?
Steps 1 and 2 are species-specific and steps 3 and 4 are conserved in all species
What is the cellular basis of asymmetric organogenesis?
Differential cell death, differential proliferation, differential cell elongation, differential migration, rearrangement of actin-cytoskeleton, intrinsic/extrinsic forces
