Mesoderm Induction
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Name the three vertebrate body axes
Anterior-posterior (head to tail), dorsal-ventral (back to belly), left-right.
Which axis is the main body axis?
A-P.
When are the axes determined?
During cleavage and gastrulation.
Are the axes set the same way in all species?
No, at different times and by different mechanisms.
What sets the A-P axis in Xenopus?
Maternal determinants, with the anterior end developing from the animal pole.
What sets the A-P axis in the chick?
Gravity, through the egg’s rotation in the oviduct.
Which side of the chick blastoderm becomes anterior?
The side pointing in the direction of rotation.
How is the A-P axis set in the mouse?
By cell-cell interactions with no maternal cue.
Which signaling centers pattern the mouse embryo?
DVE (distal visceral endoderm) and AVE (anterior visceral endoderm).
What sets the D-V axis in Xenopus?
Sperm entry point and cortical rotation.
Where does the dorsal side form in Xenopus?
On the side opposite the sperm entry point.
Where does the Nieuwkoop center form?
On the dorsal side, opposite the sperm entry point.
What defines the D-V axis in the chick?
The position of the blastoderm on top of the yolk.
What defines the D-V axis in the mouse?
Interaction between the ICM and trophoblast. The ICM attachment site becomes dorsal.
What does dorsal mesoderm become in Xenopus?
Notochord and somites.
What does ventral mesoderm become in Xenopus?
Mesenchyme and blood cells.
What is a ventralized embryo?
One with excess mesenchyme and blood-forming mesoderm but deficient dorsal structures.
What is a dorsalized embryo?
The opposite of a ventralized embryo.
Are vertebrate embryos symmetric inside and out?
Symmetric externally, but most internal organs are asymmetric.
What is situs inversus?
Complete mirror-image reversal of the internal organs’ positions.
What is heterotaxia?
One or more organs develop with reversed L-R asymmetry (unlivable).
What is isomerism?
Failure to establish L-R asymmetry in individual organs (unlivable).
Name genes involved in L-R axis formation
Shh, Nodal, Activin, Pitx2, Vg-1.
About how many genes affect the L-R axis?
More than 100.
Name the four mesoderm regions from dorsal to ventral.
Chordamesoderm, somitic mesoderm, intermediate mesoderm, lateral plate mesoderm.
What does chordamesoderm form?
The notochord.
What are the notochord’s major functions?
Inducing the neural tube and establishing the A-P body axis.
What are somites?
Blocks of mesoderm along both sides of the neural tube.
What do somites produce?
Bone, muscle, cartilage, and dermis of the back.
What is “dorsal mesoderm”?
Chordamesoderm plus somitic mesoderm.
Where is intermediate mesoderm, and what does it form?
Lateral to the somites. It forms the urinary system and genital ducts.
What does lateral plate mesoderm form
Heart, blood vessels, and blood cells.
What is another name for lateral plate mesoderm?
Ventral mesoderm.
When does mesoderm induction occur relative to gastrulation?
Much earlier than gastrulation, even though mesoderm is laid down during gastrulation.
Define embryonic induction
One tissue interacts with an adjacent tissue to influence its fate, differentiation, and behavior, via diffusible factors.
Describe the Spemann-Mangold experiment.
A dorsal lip from a pigmented newt gastrula was grafted into the presumptive belly skin of an unpigmented newt gastrula.
Where did most cells in the secondary axis come from in Spemann’s experiment?
The host embryo.
What does Spemann’s result tell us about the dorsal lip?
It secretes signals that recruit and instruct neighboring host cells.
What molecule families are the signals from?
FGFs and TGF-β.
What is the process in Spemann’s experiment process traditionally called?
Primary embryonic induction.
What is the dorsal lip also called?
Primary organizer or Spemann’s organizer.
Is the organizer found in all vertebrates?
Yes.
What is the organizer in amphibians?
The dorsal lip
What is the organizer in birds?
Hensen’s node.
What is the organizer in mammals?
The primitive knot (primitive node)
What happens if you transplant Hensen’s node into a host?
It induces a secondary body axis.
Four functions of the organizer?
Initiate gastrulation movements
become dorsal mesoderm
dorsalize flanking mesoderm
induce dorsal ectoderm to form the neural tube.
What does “dorsalize the flanking mesoderm” mean?
Convert neighboring ventral mesoderm into dorsal mesoderm, setting up the A-P axis.
What does the organizer induce the dorsal ectoderm to become?
The neural tube.
What is the vegetal half of the blastula?
Presumptive endoderm.
What happened when animal cap was combined with the whole vegetal half?
Induction: the animal cap formed notochord, muscle, intermediate mesoderm, blood cells, and neural tube.
What happened with animal cap + dorsal vegetal portion?
Notochord, muscle, and neural tube formed.
What happened with animal cap + ventral vegetal portion?
Only mesenchyme and blood cells (ventral mesoderm) formed.
What did these results show about the vegetal pole?
It contains mesoderm-inducing factors, and different regions contain different factors.
Which vegetal cells induce dorsal mesoderm and the organizer?
The dorsalmost vegetal cells (the Nieuwkoop center).
What is the Nieuwkoop center?
The dorsalmost vegetal cells of the blastula, which induce dorsal mesoderm including the organizer.
What does the Nieuwkoop center induce?
The organizer (dorsal mesoderm).
What does the Nieuwkoop center itself become?
Endoderm.
Is Nieuwkoop center induction cell autonomous?
No, it is mediated by diffusible factors.
How does the Nieuwkoop center relate to the organizer?
It induces the organizer, which then builds the dorsal axis.
What determines the position of the Nieuwkoop center?
The site of sperm entry at fertilization.
Where does the Nieuwkoop center form relative to sperm entry?
On the opposite side.
What is the cortex?
A gel-like layer of actin filaments and associated materials under the egg surface.
What happens during cortical rotation?
The cortex rotates, moving specific signaling molecules to the side opposite sperm entry.
What else appears because of cortical rotation?
The gray crescent.
What is the dorsal lip derived from?
The gray crescent cytoplasm.
Chain of events from sperm entry to dorsal axis?
Sperm entry → cortical rotation → Nieuwkoop center → organizer → dorsal axis.
Which signaling pathway is critical for establishing the Nieuwkoop center?
The Wnt pathway
What is the key component of the WNT pathway?
β-catenin.
Is maternal β-catenin localized in the unfertilized egg?
No, it is present with no localization.
What destroys β-catenin?
GSK3, through phosphorylation.
What moves from the vegetal pole to the dorsal side at fertilization?
Dsh, GBP, and Wnt11 mRNA, along microtubules.
What do Dsh and GBP do?
Repress GSK3, stabilizing β-catenin.
What does Wnt11 mRNA do on the dorsal side?
It is translated into protein that activates Wnt signaling.
Where does β-catenin accumulate?
In the nuclei of dorsal cells covering the Nieuwkoop center and the organizer.
Which two homeobox genes does β-catenin activate?
Siamois and twin.
What do Siamois and twin plus TGF-β signals activate in the organizer region?
Goosecoid, Noggin, Chordin, and Cerberus.
What do the same factors do in the Nieuwkoop center?
Work with vegetal transcription factors to activate endodermal genes.
Which signals likely come from the Nieuwkoop center?
Vg-1 and nodal-related proteins.
What protein superfamily are Vg-1 and nodal-related proteins in?
TGF-β
Where are Vg-1 mRNAs localized?
The vegetal pole (maternally expressed).
What induces Derriere?
VegT
What induces Xnr1, Xnr2, and Xnr4?
The synergistic action of β-catenin and VegT (and possibly Vg-1).
What is VegT?
A transcription factor made from maternal mRNA tethered to the vegetal cortex.
What does the Nodal-related protein pattern form along the D-V axis?
A gradient.
What does the hypothesis say about high vs. low Xnr?
Highest Xnr induces the organizer. Lower Xnr specifies ventral mesoderm.
What does the Nieuwkoop center induce the dorsal marginal region to become?
The organizer.
What do ventral vegetal cells induce the marginal region to become?
Ventral mesoderm.
How many steps and how many signals are in the model?
Three steps, four signals.
Name the steps of patterning of the mesoderm
Induction of ventral mesoderm by the vegetal region.
Induction of the organizer by the Nieuwkoop center.
Induction of the lateral marginal cells by the adjacent organizer to become intermediate mesoderm.
What does the general mesoderm inducer from the vegetal region specify?
Ventral-type mesoderm.
What does the Nieuwkoop center signal specify?
The dorsalmost mesoderm (organizer, notochord).
Which mesoderm type is the default?
Ventral mesoderm.