Lecture 7

Question 1

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

Answer 1

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.

Question 2

Nodal is activated throughout the animal cap of the embryo, T or F

Answer 2

F – it is activated throughout the vegetal hemisphere

Question 3

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

Answer 3

?-catenin

Question 4

What is the Nieuwkoop centre

Answer 4

The region of the embryo where ?-catenin has accumulated in the nuclei and there is activation of Nodal signalling

Question 5

How does ?-catenin and nodal interact

Answer 5

?-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.

Question 6

Where is the highest nodal activity seen as a result of its interaction with ?-catenin

Answer 6

Highest nodal in the dorsal/posterior embryo

Question 7

What is the role of the Nieuwkoop centre

Answer 7

Region of high nodal and ?-catenin signalling that induces the formation of the organiser

Question 8

What happens to regions with low nodal activity

Answer 8

Induction of ventral mesoderm

Question 9

What are the two crucially different types of mesoderm induced in the early embryo

Answer 9

Organiser mesoderm, ventral/lateral mesoderm

Question 10

The mechanism of ?-catenin and nodal interactions is poorly understood, T or F

Answer 10

T

Question 11

What two factors are required for goosecoid expression

Answer 11

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.

Question 12

Brachyury is induced in response to high levels of Nodal in the dorsal/posterior embryo, T or F

Answer 12

F – brachyury expression is induced in response to low Nodal signalling in the ventral/anterior embryo

Question 13

What levels of nodal signalling are required for expression of T, another transcription factor involved in anterior-posterior patterning

Answer 13

T is expressed at low Nodal signalling levels

Question 14

Chordin, Xnot and Xlim1 are other genes expressed alongside goosecoid that act as inducers of various mesodermal fates, what is required for their induction

Answer 14

High levels of nodal signalling and wnt signalling activation and subsequent ?-catenin presence in the nucleus

Question 15

Different types of mesoderm have different abilities in terms of proliferation, migration and differentiation etc, T or F

Answer 15

T

Question 16

The combinatorial expression of T, Gsc, Chordin and Tbx6 accounts for the differences seen in the mesodermal tissue, T or F

Answer 16

T

Question 17

What rod-like structure does the organiser/node differentiate into

Answer 17

Axial mesoderm

Question 18

Explain how goosecoid and siamois act in a cell-autonomous way to direct the differentiation of the node/organiser

Answer 18

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

Question 19

What is one key property of axial mesoderm and accounts for rolling up of the neural plate into the neural tube during neurulation

Answer 19

Axial mesoderm is able to undergo convergent extension

Question 20

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

Answer 20

F – whilst gsc and siamois expression is only transient, they are also expressed at marginally different levels in the cells too

Question 21

What are the three different types of mesoderm derived from the axial mesoderm induced by the transient expression of goosecoid and siamois

Answer 21

Prechordal mesendoderm, prechordal endoderm and notochord

Question 22

Which of the three mesodermal tissues derived from the axial mesoderm are most anterior and most posterior

Answer 22

Prechordal mesendoderm are most anterior and the notochord lies posteriorly

Question 23

Explain the two hypotheses by which Hensen’s node is thought to express chordin and goosecoid

Answer 23

Either cells in Hensen’s node co-express both gsc and chordin or there are individual cells that express each transcription factor

Question 24

Explain how the expression of Gsc and chordin differs between notochord and prechordal mesoderm

Answer 24

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

Question 25

Explain how anterior-posterior axis extension is mediated and refined in the posterior embryo

Answer 25

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

Question 26

What happens in the anterior embryo

Answer 26

BMP and Wnt antagonises maintain the anterior identities

Question 27

Anterior-posterior identity is through to be realised through the Hox code, T or F

Answer 27

T

Question 28

Neurulation of ectoderm and ventralisation of the mesoderm are both mediated by BMP antagonists, T or F

Answer 28

F – neurulation of ectoderm and dorsalisation are mediated by BMP antagonists

Question 29

BMP antagonists are secreted by which structure

Answer 29

The organiser/node

Question 30

Other than chordin, name four other BMP antagonists

Answer 30

Follistatin, frizbee, cerberus and noggin

Question 31

Where do the BMP antagonists end up and prevent the BMP signal in

Answer 31

Top layer of epiblast

Question 32

How do BMP antagonists act

Answer 32

Block the interaction of BMPs with their receptors

Question 33

What happens to the cells where the BMP signal is inhibited

Answer 33

They will become neural plate

Question 34

Explain how BMP antagonists lead to the refinement of cell fates in the mesoderm

Answer 34

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

Question 35

What feature of the mesodermal tissue leads to the rolling up of the neural plate during neurulation and how does this occur

Answer 35

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.