Lecture 7 Flashcards
Wnt signalling mediates a secondary symmetry breaking event but unlike nodal, it is activated on one side of the embryo. Explain how this is achieved and where wnt activity is seen in the early embryo
The Wnt signalling pathway is activated on one side of the embryo, opposite to the point of sperm entry. This region is referred to as the dorsal embryo but also represents the site where gastrulation movements will begin, i.e. the posterior embryo.
Nodal is activated throughout the animal cap of the embryo, T or F
F – it is activated throughout the vegetal hemisphere
Which gene product accumulates in the nuclei of cells in the posterior/dorsal embryo as a result of increase Wtn signalling and promotes the expression of Wnt target genes
?-catenin
What is the Nieuwkoop centre
The region of the embryo where ?-catenin has accumulated in the nuclei and there is activation of Nodal signalling
How does ?-catenin and nodal interact
?-catenin and nodal interact directly and result in enhanced activation of Wnt signalling in regions of the embryo where ?-catenin is also expressed. The causes the conversion of what was a uniform gradient of nodal in stage 8 to a clear concentration gradient from anterior to posterior in stage 9 embryos.
Where is the highest nodal activity seen as a result of its interaction with ?-catenin
Highest nodal in the dorsal/posterior embryo
What is the role of the Nieuwkoop centre
Region of high nodal and ?-catenin signalling that induces the formation of the organiser
What happens to regions with low nodal activity
Induction of ventral mesoderm
What are the two crucially different types of mesoderm induced in the early embryo
Organiser mesoderm, ventral/lateral mesoderm
The mechanism of ?-catenin and nodal interactions is poorly understood, T or F
T
What two factors are required for goosecoid expression
Nodal downstream effector known as Smad2/4 binding to its distal region of the promoter and a Wnt/?-catenin downstream effector called Xtwn binding to the proximal element of the promoter.
Brachyury is induced in response to high levels of Nodal in the dorsal/posterior embryo, T or F
F – brachyury expression is induced in response to low Nodal signalling in the ventral/anterior embryo
What levels of nodal signalling are required for expression of T, another transcription factor involved in anterior-posterior patterning
T is expressed at low Nodal signalling levels
Chordin, Xnot and Xlim1 are other genes expressed alongside goosecoid that act as inducers of various mesodermal fates, what is required for their induction
High levels of nodal signalling and wnt signalling activation and subsequent ?-catenin presence in the nucleus
Different types of mesoderm have different abilities in terms of proliferation, migration and differentiation etc, T or F
T
The combinatorial expression of T, Gsc, Chordin and Tbx6 accounts for the differences seen in the mesodermal tissue, T or F
T
What rod-like structure does the organiser/node differentiate into
Axial mesoderm
Explain how goosecoid and siamois act in a cell-autonomous way to direct the differentiation of the node/organiser
Siamois and goosecoid expressed in the organiser/node in turn act on the cells that express them in a cell-autonomous, or intrinsic manner to alter their fate. They begin induce the cells of the node/organiser to differentiate into axial mesoderm
What is one key property of axial mesoderm and accounts for rolling up of the neural plate into the neural tube during neurulation
Axial mesoderm is able to undergo convergent extension
Siamois and goosecoid are expressed at uniform levels but only transiently in different cells. This accounts for the formation of the 3 different types of mesoderm derived from the axial mesoderm, T or F
F – whilst gsc and siamois expression is only transient, they are also expressed at marginally different levels in the cells too
What are the three different types of mesoderm derived from the axial mesoderm induced by the transient expression of goosecoid and siamois
Prechordal mesendoderm, prechordal endoderm and notochord
Which of the three mesodermal tissues derived from the axial mesoderm are most anterior and most posterior
Prechordal mesendoderm are most anterior and the notochord lies posteriorly
Explain the two hypotheses by which Hensen’s node is thought to express chordin and goosecoid
Either cells in Hensen’s node co-express both gsc and chordin or there are individual cells that express each transcription factor
Explain how the expression of Gsc and chordin differs between notochord and prechordal mesoderm
Gsc is only expressed in the prechordal mesoderm (and not in the notochord) whereas chordin is only expressed in the more posterior, notochord and not at all in prechordal mesoderm in the anterior embryo
Explain how anterior-posterior axis extension is mediated and refined in the posterior embryo
Signals that were initially close together become spatially separated through the anterior-posterior axis. High FGF signalling and the presence of RA in the posterior embryo keep cells in a proliferative state and confers them to a posterior identity. The proliferation of posterior cells causes the addition to the axis as it elongates posteriorly
What happens in the anterior embryo
BMP and Wnt antagonises maintain the anterior identities
Anterior-posterior identity is through to be realised through the Hox code, T or F
T
Neurulation of ectoderm and ventralisation of the mesoderm are both mediated by BMP antagonists, T or F
F – neurulation of ectoderm and dorsalisation are mediated by BMP antagonists
BMP antagonists are secreted by which structure
The organiser/node
Other than chordin, name four other BMP antagonists
Follistatin, frizbee, cerberus and noggin
Where do the BMP antagonists end up and prevent the BMP signal in
Top layer of epiblast
How do BMP antagonists act
Block the interaction of BMPs with their receptors
What happens to the cells where the BMP signal is inhibited
They will become neural plate
Explain how BMP antagonists lead to the refinement of cell fates in the mesoderm
Initially, low levels of Nodal give ventral mesodermal fate. Subsequent inhibition of BMPs in the adjacent mesoderm to the neural tube induces the dorsalisation of what was ventral mesoderm initially. By this process the most proximal mesoderm to the organiser become the somites
What feature of the mesodermal tissue leads to the rolling up of the neural plate during neurulation and how does this occur
Convergent extension of the axial mesoderm creates a force in the anterior-posterior direction that is translated to one which drives curling up of the neural plate with somites residing either side.
