L11 - Mesoderm Patterning and Somite Formation Flashcards
What are the four mesodermal tissues
Axial mesoderm
Paraxial mesoderm
Intermediate mesoderm
Lateral mesoderm
What is the mesoderm closest to the neural tube (either side)
Paraxial mesoderm
What is the mesdoerm which sits under the flat ectoderm
Lateral plate mesoderm
What is the mesoderm which sits between the paraxial mesoderm and the lateral mesoderm
Intermediate mesoderm
What is the mesoderm which also makes up the notochord
Axial mesoderm
Intermediate mesoderm gives rise to
Gonads and Kidney
Paraxial mesoderm gives rise to
Head and the somites
Somites give rise to
Skeletal muscle
Endothelila cells
Dermatome
Sclerotome
Sclerotome gives rise to
Cartilage and tendons
Lateral plate mesoderm gives rise to
Extraembryonic
Splanchnic
Somatic
Somatic meso gives rise to
Body cavity - pelivis - limbs and bones
Splanchnic meso gives rise to the
Circulatory system
What are somites
Segmented paraxial mesoderm tissues
Where do somites form
Pre-somitic mesoderm
What are earliest evidence for segmentation in vertebrates
Somites
How can meosderm segmentation be visualised
Engrailed 1 expression
Can see clear segmentation in the different organisms
Humans born with how many vertebrae
33
Humans adult how many vertebra and how many fused
24 vetebraes and 9 fused ones
How many somites in humans
38-44
How many somites in chick
55
How mnay somites in the mouse
65
How many somites in zebrafish
33
Paraxial mesoderm forms in a continuous manner until
Proper somite number is reached
The primitive streak is present until
Somites no longer form
Presomitic mesoderm prefigures
Future segmentation of the somites
Size of the presomitic mesoderm is equivilant to
12 somites
Formation of somites preceeds in a
A –> P
Why does the length of the paraxial mesoderm remain the same
Processes are cordinated
Formation of new somites and the extension of the paraxial mesoderm
What must cells within the presomitic mesoderm respond to
Positional information Mechanism coordinating L and R somites Mechanism generating anterior boundary Mechanism generating posterior boundary Formation of the cleft
Describe the clock and wavefront model
Predicts a clock ticks in the posterior presomitic mesoderm - drives a molecular oscilatory that dictates the periodicity of somites
Where cells hit the travelling wavefront, an abrupt change of property occurs leading to the decision to form somites
Hairy gene
IN CHICK
IN MOUSE
IN ZEBRAFISH
c-Hairy-1
Hes
her
Describe the expression of c-hairy
At 0 mins high
Then falls
Then increases and peaks
Hairy/hes/her are targets of
Notch signalling
Hairy/hes/her proteins are
bHLH transcriptional repressors
Describe the feedback of Hes
Notch activates Hes1
Hes1 mRNA interacts with Hes1 protein
Caused poly ubiq of Hes1 protein and degredation
Hes1 protein represseses Hes1 ttranscription
This negative feedback on Hes1 means
Hes1 protein and Hes1 mRNA are expressed at different levels at different times
Effect of Hes1 mRNA
Poly ubiqu and degredation of of Hes1 protein
Effect of Hes1 protein
Repression of Hes1 transcriptin
Clock genes are targets of which signalling pathway
Notch
Wnt
FGF
Travelling wavefront tells cells that
They now need to form somites
After how many oscilations will a cell find itself in the anterior part of the presomitic mesoderm
12
Which was does the travelling wavefront move
Posteriorly
Determination front positioned where
At interface of RA gradient and FGF8 gradient
what is posteiror
Pasraxial mesoerm
Why is RA high anteriorly
Somites produce an enzyme required for the synthesis of RA
What is used to ensure there is very little overlap of the FGF8 and RA gradient
Negative regulation
Effect of RA of FGF8
RA inhibits FGF8
What is the effect of cyp26
Cyp26 activated by FGF8
Cyp26 represses synthesis of RA
What is the effect of Raldh2
Inhibited by FGF8 - activates Ra
Where Hi FGF8 ___ Ra
Lo
Where Hi RA ____ FGF8
Hi
FGF8 expression induced ______ which acts with ______ to cause expression of Mesp2
TBX6
Notch
Describe the initial expression of Mesp2
Describe where Mesp2 becomes localised to
What is the mechanism for this localisation
Whole of the prospective somite
Mesp2 becomes localised to the anterior
Mesp2 drives expression of ripply2 - which is a negative regulator of Mesp2
What occurs if you transplant cells from a prospective somite boundary region into a non boundary regions
Leads to the formation of a boundary
What do boundary cells do
Instruct cells that are anterior to form a boundary
Lunatic frinfe AKA
what is the ffect of lunatic fringe
Glycosyltransferase
Inhibitor of notch
Describe what happens when you force expression of lunatic fringe
Inducing lunatic fringe expression induces a new boundary
what two things does notch drive
Boundary formation
Control Mesp2 production
Describe the phenotype for a delat-like 3 mutant
Skeletal abnormalities since there isnt periodic segmentation of the somite
Delta, notch and Mesp2 control
Expression of cell adhesion molecules
What is an example of a cell adhesion moleucle controled by delta/notch and Mesp2
Ephron
The changing in cell adhesion is coupled to
Epithelial to mesenchymal transition
Describe the appearacne of the presomitic mesoderm - what does it contain
Unsegmented
Contains potential for 12 prospective somites to form
What is the process which forms more paraxial mesoderm
Gastrulation
Preiodicicity is dictated by
A clock which generates a molecular oscilator
What is periodicity coupled to
The travelling wavefront
Period of oscialtion is equivilant to
The time taken for a new somite pair to form
What do cells in the position somite -1 decide to form
Somites
Which axis is the first to be laid down during embryonic development
Anterior-posterior axis
Which germ layer forms at the primitive streak during gastrulation
Mesoderm
What experimental method lead to the identification of the origin of mesodermal tissue development
Lineage tracing
What determines the adoption of different mesodermal fates in the developing embryo
Where the cells ingress along the anterior-posterior axis into the primitive streak
Put these different mesodermal tissue types in order from most anterior to most posterior. Intermediate mesoderm, axial mesoderm, lateral mesoderm, paraxial mesoderm
Axial mesoderm, paraxial mesoderm, intermediate mesoderm, lateral mesoderm
The axial mesoderm is one of the mesodermal tissues formed by the ingression of cell into the primitive streak, what two subtypes of mesoderm does it give rise to and where
Prechordal mesoderm (anteriorly) and the notochord (posteriorly)
Which mesodermal tissue gives rise to the somites
Paraxial mesoderm
Cells that ingress posteriorly to the paraxial mesoderm give rise to which tissue
Intermediate mesoderm
The more posterior a cell ingresses into the primitive streak, the more posterior the mesoderm it will give rise to will be, T or F
F – more posterior ingress, the more lateral the mesoderm
Alike the axial mesoderm, the posterior paraxial mesoderm is also subdivided, what are these subdivisions
Unsegmented posterior paraxial mesoderm and the segmented posterior paraxial mesoderm that give rise to somites
What structures do the intermediate mesoderm give rise to
Kidneys and gonads
How many components is the lateral mesoderm divided into and what do these divisions give rise to
The lateral mesoderm divides into 3 components, two of which give rise to the circulatory system and the other which contributes to extraembryonic structures and limb bones
List the structures that the paraxial mesoderm give rise to
Axial skeleton, heart, somites, cartilage and tendons
The formation of somites occurs in a sequential manner on both sides with the size of somites being preserved throughout, T or F
T
The pre-somitic mesoderm is unsegmented, T or F
T
Describe the rough structure of the somites viewed as a horizontal cross section
The somites show a clear metameric structure with a clear repeating pattern and defined anterior and posterior boundaries
Somites are the earliest evidence of segmentation in vertebrates, T or F
T
How do the number of somites relate to segmentation in vertebrates such as humans
Somite number dictates the number of vertebrae
How many somites are present in humans and how does this relate to their segmentation
The human embryo has between 38 and 44 somites, this correlates to the 33 vertebrae which we are born with
The number of somites is fixed for any given species and the timing of somite formation remains constant, T or F
F – whilst the number of somites does differ between species, so too does the timing of somite generation
What five factors must cells in the paraxial mesoderm be able to respond to
Positional information, mechanisms that coordinate left and right, anterior and posterior boundary formation and the formation of the cleft
What structure separates the left and right somites
Spinal cord
What model describes the periodicity of somite formation and how is this achieved
Clock and wavefront model. The clock explains the temporal component whilst the wavefront provides spatial information to drive somite formation. Where cells hit the travelling wavefront an abrupt change of property leads to the decision to form somites
Explain how a molecular oscillator drives mesodermal segmentation in chick embryos
In the embryo levels of the helix-loop-helix transcription factor C-hairy was found to fluctuate at different embryonic stages. Later genes were discovered that regulate the timing of this clock oscillation and are members of the notch, wnt or FGF signalling pathways
What is meant by the determination front
Wavefront that travels from the anterior part of the presomitic mesoderm towards the posterior embryo
How does the determination front dictate somitigenesis
When the cells of the paraxial mesoderm encounter oscillations from the molecular clock an abrupt change determines their formation of the next somite pair
What aspect of the paraxial mesoderm determines somite boundary formation
Position of the somite minus II (S-II)
Explain the results of ectopic grafting of boundary cells
Somite boundary cells isolated from one embryo transplanted into another embryo is sufficient to induce the formation of a new boundary. Where you’d expect to see one somite you would now get two. This shows that boundary cells instruct cells that are anterior to it to form a boundary
What family of genes were discovered to be expressed at the somite boundaries
Notch family genes. They are selectively expressed in the anterior or posterior part of the somite
Explain the results of forced lunatic fringe expression in the paraxial mesoderm and the significance of this
Lunatic fringe is a gene that blocks notch activity and thus forced expression results in an inhibition of notch signalling. This results in the formation of a new boundary and hence an additional somite
Give an example of a human disease that proves a role of notch signalling in mesodermal segmentation
Jarcho Lewin syndrome causes spondylocostal dysplasia due to problems with the segmentation of the axial skeleton. This occurs due to a mutation in the delta 3 ligand that alters notch signalling and problems with somite segmentation
Explain how the determination front is positioned
The determination front is determined at the interface of two opposing gradients. Retinoid acid which is high anteriorly and fibroblast growth factor 8 which is high posteriorly. When these gradients are equal the determination front forms.
Where is retinoic acid produced to dictate determination front positioning
Somites
RA and FGF8 agonise eachother, T or F
F – they antagonise each other
Explain how RA and FGF8 interact to dictate determination front positioning
High levels of FGF8 result in high levels of Cyp26 which inhibits RA synthesis. High levels of FGF8 also inhibit the production of the Rhald2 enzyme that is normally required for RA synthesis
What transcription factor do RA and FGF8 regulate the expression of
Mesp2
How do RA and FGF8 interact to regulate expression of transcription factor(s) involved in somite boundary formation
RA activates Mesp2 expression whilst FGF8 inhibits it. Mesp2 expression in turn blocks local Notch signalling. Lower Notch signalling on one side of the border results in high notch signalling in adjacent cells on the other side of the boundary.
What are the downstream effects of opposing notch signalling in cells either side of the boundary
High notch acitivity in one side and low activity in the cells opposite leads to formation of the somite boundary. The boundary itself forms from physical formation of cleft within the mesenchymal tissue. Downstream extensive changes in cell morphology and adhesion leads to creation of this cleft and is mediated by ephs and ephrins
