Creating the Body Plan

Question 1

1. name the structures that the ectoderm gives rise to. (2)
2. name the structures that the mesoderm gives rise to (6)
3. name the structures that the endoderm gives rise to (3)

Answer 1

1. epidermis and nervous system
2. skeleton, muscle, connective tissue, kidneys, heart and blood
3. gut, liver and lungs.

Question 2

1. How does the blastula appear at 5 days post fertilisation. What structures do the 2 cell types give rise to?
2. What occurs at 12 days post fertilisation? What structures are clearly defined at this stage?
3. what forms 16 days post fertilisation?

Answer 2

1. an inner cell mass surrounded by trophoblastic cells. The trophoblasts form the extra-embryonic structures, while the inner cell mass becomes the embryo.
2. implantation. Animal/epiblast and vegetal/holoblast regions
3. formation of the primitive streak and node.

Question 3

1. how does the chick embryo appear before gastrulation?
2. what is the name of the thickening along the midline of the embryo? What process is responsible for its formation
3. what is the name of the most anterior part of the thickening?
4. Which part of the embryo is responsible for inducing the 3 germ layers? How does it do so?
5. What is the process by which the 3 germ layers form?

Answer 3

1. a sheet of epiblast on top of a sheet of holoblast
2. primitive streak. the migration of cells towards the midline in response to inductive signals
3. hensen’s node
4. the holoblast signals to the overlying epiblast, forming a gradient of the signal. Cells which become the ectoderm don’t recieve any signal thys dont migrate. Cells which become mesoderm receive a low concentration thus migrate forming a middle layer. Cells which become endoderm recieve a high concentration therefore migrate forming an inner layer.
5. epithelial to mesenchymal transition

Question 4

1. how do amphibian embryos differ from the chick embryo?
2. which hemisphere signals to the other?
3. where does the invagination of cells occur? What is this the site of?
4. how do the 3 different germ layer cells migrate in respect to the rest of the embryo?

Answer 4

1. they are hollow balls of cells rather than flat sheets
2. cells of the vegetal hemisphere signal to cells of the overlying hemisphere
3. at the blastopore; site of the spemann organiser
4. endoderm and mesoderm move into the gastrula forming inner and middle layers respectively, while the ectoderm spreads downwards to cover the whole embryo.

Question 5

1. Why is the xenopus embryo polarised before fertilisation?
2. Along which plane does the first cell division occur? What does this result in?
3. What is VegT? What signalling molecule does it induce the expression of?
4. What process results in the non-uniform expression of cytoplasmic factors (such as Q3) in the vegetal hemisphere?

Answer 5

1. under the influence of gravity, heavy cytoplasmic components are found vegetally
2. the first cell division occurs along the horizontal plane, creating animal and vegetal cells which are fundamentally different due to the asymmetric segregation of cytoplasmic factors
3. a transcription factor which induces the expression of nodal.
4. cortical rotation that is stimulated by fertilisation causes the cytoplasmic factors to be moved to the site opposite sperm entry.

Question 6

1. what does the accumulation of nodal at the point opposite sperm entry lead to?
2. what is the name of the vegetal hemisphere where nodal and beta-catenin expression overlap?
3. The concentration of nodal patterns the mesoderm. What does [high] induce and what does [low] induce?

Answer 6

1. wnt signalling thus beta-catenin accumulation in this part of the embryo
2. nieuwkoop centre
3. organiser/dorsal mesoderm | ventral mesoderm.

Question 7

1. Name a TF that is expressed in response to low nodal. In what type of mesoderm is this expressed?
2. name 4 TFs that are expressed in reponse to high nodal and wnt signalling. In what type of mesoderm are they expressed?

Answer 7

1. brachyury
2. siamois, goosecoid, Xlim and Xnot

Question 8

1. What are siamois and goosecoid?
2. In what manner do they act? What do they cause
3. name the 3 types of axial mesoderm
4. name the process in which the axial mesoderm undergoes. What does this result in.
5. What pathway does the axial mesoderm follow during the process mentioned in Q4. How does it do so?

Answer 8

1. transcription factors
2. cell autonomous manner (i.e. on the cells in which they are expressed); they cause the self differentiation of the organiser.
3. prechordal plate, prechordal mesoderm, notochord.
4. convergent extension. result in the formation of a rod which underlies the ectoderm on the dorsal side of the embryo.
5. follows a fibronectin rich pathway in response to expressing integrins on the cell surface, a result of them adopting an organiser fate.

Question 9

1. Name another molecule that the organiser expresses and secretes. What is its function?
2. What does the gradient of the Q1 molecule do?

Answer 9

1. BMP antagonists. They prevent BMP signalling (BMPS are expressed in all cells)
2. Patterning of the mesoderm and neurulation of the ectoderm

ventral mesoderm; intermediate mesoderm and dorsal mesoderm.

Question 10

1. How does convergent extension result in AP patterning?
2. what type of axial mesoderm marks the anterior | what type of axial mesoderm marks the posterior?
3. Name 2 signalling molecules that give a posterior identity and where they are produced from
4. Name 2 signalling molecules which maintain anterior identities and where they are produced from.

Answer 10

1. signals that were close together become spacially separated, thus they can have differential effects on overlying tissue
2. prechordal plate | notochord
3. FGF and RA
4. BMP antagonists and Wnt antagonists.

Question 11

1. how is crude DV patterning established?
2. Name 2 signalling molecules involved in DV patterning of the neural tube. Where are they produced?
3. how are defined territories of cell fates created?
4. Describe how one of the molecules induces expression.

Answer 11

1. convergent extension, as the axial mesoderm rod lies dorsally
2. BMPs (from surface ectoderm) and Shh (from axial mesoderm)
3. BMPs form a dorsal gradient while Shh forms a ventral gradient. The opposing gradients create different thresholds which lead to the development of different territories
4. Shh induces expression by derepression. Shh represses class 1 genes. Class 1 genes repress Class 2 genes. Therefore by repressing class 1 genes so that they are confined to dorsal regions, the repression of class 2 genes is lifted so that they can be expressed in ventral regions.

Question 12

1. from which type of mesoderm do the somites arise from
2. how are the somites formed?
3. what needs to be present until the correct number of somites has been formed
4. in which direction does the formation of somites occur?
5. Somite formaton is the first example of what in development?
6. How often is a new pair of somites formed?

Answer 12

1. paraxial
2. in a continous manner by the budding off of the paraxial mesoderm
3. primitive streak
4. posteriorly
5. metamerism
6. 90 mins

Question 13

Name 4 things that cells must respond to in the formation of somites

Answer 13

1. positional information
2. mechanisms that co-ordinate the left and right sides
3. mechanisms that create the left and right sides
4. formation of the cleft.

Question 14

1. Describe the c-hairy gene expression pattern in the formation of a new somite.
2. what are the c-hairy oscilations under the influence of?

Answer 14

1. c-hairy is expressed in the caudal region of a new somite, in a thin band that will form the caudal half of the next somite, plus the posterior region of the presomitic mesoderm

the posterior region of expression extends anteriorly as the new somite begins to separate from the presomitic mesoderm

c-hairy is maintained in the caudal half of the newly formed somite

the posterior band of expression continues to move anteriorly and shorten until it represents the caudal domain of the next somite

2. a molecular clock, whick tells cells to express c-hairy in this oscilatory manner.

Question 15

1. What is the determination front.
2. where is it set?
3. What do boundary cells do?
4. what does ectopic expression of a prospective boundary region lead to?
5. What signals are expressed at the boundary?
6. Where is the determination front set (interface of opposing gradients of what?

Answer 15

1. the point at which cells of the presomitic mesoderm become determined to form somites
2. S-II (2 somites prior to somite formation)
3. instruct cells that are anterior to them to form a boundary
4. the creation of an ectopic boundary
5. Notch and Lunatic Fringe
6. At the interface of opposing FGF8 and RA gradients. Oscilations are maintained at high [FGF8] and arrest when RA is met

Question 16

1. what type of molecule is lunatic fringe and what is its function?
2. what are delta 3 mutations associated with?
3. What molecules probably act downstream of notch?
4. What is the role of these molecules (Q3)

Answer 16

1. glycosyltransferase. It enables the potentiation of specific binding between delta and notch, and prevents binding between notch and serrate
2. Jarcho Levin Syndrome, a disorder characterised by vertebral and rib malformations
3. ephs and ephrins.
4. they enable changes in cell adhesion which allow for the formation of the cleft.