Lecture 10: Organogensis Flashcards
What is organogenesis?
The process of formation of specific organs and related structures during development
Organogenesis can be used as a model system to study what developmental processes?
Specification
Axis and pattern formation
Induction
Morphogenesis
Cellular differentiation
The mechanisms involved are very similar to the ones in earlier development
Specification of germ layers
An unfertilized amphibian egg has polarity among animal and vegetal poles.
What do each of these give rise to?
Animal pole blastomere
Vegetal pole blastomere
Equatorial blastomere
Animal pole blastomere = ectoderm
Vegetal pole blastomere = endoderm
Equatorial blastomere = mesoderm
Specification of germ layers
What are the two major functions of vegetal cells?
What is this process called?
Differentiate into endoderm
Induce equatorial cells to become mesoderm
BOTTOM-UP SPECIFICATION
Specification of germ layers
Sperm entry sets up what axis?
the D-V axis
Where is the dorsal side relative to the site of sperm entry?
On the opposite side
What are dorsalizing factors?
form a signalling centre in DV axis
Generating A-P axis in Drosophila
Summary of all the genes
Pattern established by MATERNAL EFFECT GENES —> form gradients and regions of MORPHOGENS (transcription factors)
These morphogens activate the GAP GENES –> define broad territories of the embryo
The gap genes lead to expression of PAIR RULE GENES –> divide the embryo into periodic units
Transcription factors encoded by the pair-rule genes SEGMENT POLARITY GENES –> divide the embryo into segment-sized units along the anterior-posterior axis
All combined = regulate HOMEOTIC GENES that define identities of each of the segments
Generating A-P axis in Drosophila
What is an example of maternal effect?
Bicoid
Generating A-P axis in Drosophila
What is an example of gap genes?
Hunchback and Kruppel
Generating A-P axis in Drosophila
What is an example of pair-rule gene?
Fushi tarazu
Generating A-P axis in Drosophila
What is an example of segment polarity gene?
Engrailed
Generating A-P axis in Mammals
The mammalian embryo has how many two signaling centers. What are they?
The node
Anterior visceral endoderm (AVE)
Generating A-P axis in Mammals
The mammalian embryo has how many two signaling centers.
One of them is the node. What is it responsible for?
The node is responsible for neural induction and patterning of AP axis
Generating A-P axis in Mammals
The mammalian embryo has how many two signaling centers.
One of them is the Anterior Visceral Endoderm (AVE). What is it responsible for?
position of the primitive streak
similar to the head portion of the amphibian organizer
Generating A-P axis in Mammals
AVE and therefore the A-P axis are likely generated by
the shape of the uterus such that the growth occurs in one direction
Generating A-P axis in Mammals
A-P axis is dependent on what to regulate the activity of Hox genes?
concentration gradients of BMPs, Wnts, FGFs, Retinoic acid etc
Generating A-P axis in Mammals
______ are homologous to the homeotic genes of Drosophila (highly conserved)
Hox genes
Generating left-right axis in mammals
Placement of the organs in humans is determined along the left-right axis
It works through what?
works through gradients of BMP, Shh, Nodal and Pitx2 etc.
What did Spemann’s experiment show the concept of?
nuclear equivalence
we know a lot more about the DV axis because this was one of the earliest studies
Spemann-Mangold organizer
Describe the experiment with newts.
Fertilized egg and tied along the first plane of cleavage –> nuclear division was constricted to one side (only dividing on one side bc other side had no nucleus)
The nucleus could migrate to the other side
As soon as the nucleus went there, cleavage would start and growth
They found development of twin larvae (one was more developed than the other)
Spemann-Mangold organizer
Describe the experiment with amphibians.
Showed the concept of asymmetry!
A. Made constriction on amphibian egg so that each cell gets some gray crescent and then separate two isolated cells
B. Made constriction on amphibian egg so that one cell gets all the gray crescent and then separate the isolated cells
RESULT: if there was no gray crescent, formation of a belly piece resulted (lack of dorsal structures)
Spemann-Mangold organizer
Later fate mapping experiments showed that gray crescent cells give rise to what?
the dorsal lip of the blastopore
Spemann-Mangold organizer
Dorsal lip cells are ________ to invaginate into the blastula to initiate gastrulation
committed
Spemann-Mangold organizer
Further experimentation: cells of early gastrula were uncommitted and went through conditional specification whereas cells of late gastrula
were determined and went through autonomous specification
Spemann-Mangold organizer
Describe the experiment that showed the only the dorsal lip of the blastopore has autonomous determination int eh early gastrula?
Experiment: Transplant dorsal lip between embryos of two differently pigmented species (dorsal lip –> ventral epidermis (belly skin))
Spemann-Mangold organizer
Dorsal lip cells and their derivatives (notochord and head endo-mesoderm) were called
organizer
established the basic body plan
Spemann-Mangold organizer
What happens when cells are at the right place at the right time?
convergence of two signals
Spemann-Mangold organizer
Cell right place, right time
Convergence of two signals
First signal tells the cells that they are _____
Second signal tells the cells that they are _____
Interaction of these signals does what?
First signal tells cells they are dorsal (dorsal signal)
Second signal tells cells they are mesodermal (vegetal signal)
Interaction of these signals creates polarity within mesoderm –> basis for specifying the organizer and forming D-V polarity
What is the organizer induce by?
The Nieuwkoop Centre
What is the Nieuwkoop Centre?
It induces the organizer
The dorsal vegetal cells of the blastula that can induce the overlying tissue to become the organizer
The dorsal signal – role of β-catenin
Upon sperm entry, what happens?
Upon sperm entry,
microtubules get organized and corticol rotation takes place
Enables translocation of Dsh protein and Wnt11 RNA
- slow by emans of cortical roation (Wnt11)
Fast transport by means of motor protein kinesin and microtubules (Dsh-GBP complex)
Wnt11, Dsh and GBP accumulate on the opposite side of the sperm entry, leading to
stabilization / accumulation of β-catenin on dorsal side
Where does β-catenin accumulate?
on the dorsal side
What activates the genes that are necessary for the formation of the D-V axis?
β-catenin and Tcf3 complex
The Vegetal Signal
Vegetal region has an accumulation of maternal mRNA for TFs called
VegT and Vg1
What is the role of VegT?
VegT –> Sox17 –> activates genes necessary to specify the cells to become the endoderm
What does VegT also activate?
VegT also activates Nodal (TGF-β family paracrine factors) –> induce the cells above to become mesoderm
What does Nodal activate?
accumulation of phosphorylated Smad2 —> activates eomesodermin (eomes) and Brachyury (Xbra) genes —> specification as mesoderm
Eomes + Smad2 —> activate zygotic VegT i.e. switch to zygotic mRNA from maternal
Combination of β-catenin (Wnt signalling) and VegT/Vg1 (TGF-β / Smad signalling) activates
Xnr (Xenopus nodal-related) genes —> gradient of Xnr proteins (highest in the dorsal region)
