Lecture 11 Flashcards
Which axis is the first to be laid down during embryonic development
Anterior-posterior axis
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
Which mesodermal tissue gives rise to the somites
Paraxial mesoderm
Which germ layer forms at the primitive streak during gastrulation
Mesoderm
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 is meant by the determination front
Wavefront that travels from the anterior part of the presomitic mesoderm towards the posterior embryo
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
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 transcription factor do RA and FGF8 regulate the expression of
Mesp2
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
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
How do the number of somites relate to segmentation in vertebrates such as humans
Somite number dictates the number of vertebrae
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)
Cells that ingress posteriorly to the paraxial mesoderm give rise to which tissue
Intermediate mesoderm
RA and FGF8 agonise eachother, T or F
F – they antagonise each other
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
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
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
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
Somites are the earliest evidence of segmentation in vertebrates, T or F
T
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
List the structures that the paraxial mesoderm give rise to
Axial skeleton, heart, somites, cartilage and tendons
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
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 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
What structure separates the left and right somites
Spinal cord
The formation of somites occurs in a sequential manner on both sides with the size of somites being preserved throughout, T or F
T
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
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
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.
What structures do the intermediate mesoderm give rise to
Kidneys and gonads
Where is retinoic acid produced to dictate determination front positioning
Somites
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
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
What aspect of the paraxial mesoderm determines somite boundary formation
Position of the somite minus II (S-II)
What experimental method lead to the identification of the origin of mesodermal tissue development
Lineage tracing
