Neurulation (everything up until Neural Crest section)

Question 1

What model organism do we use to study the formation of the neural tube itself?

Answer 1

The Xenopus

Question 2

what flanks either side of the neural tube?

Answer 2

The somites

Question 3

What is neurulation?

Answer 3

The step by which the ectoderm divides into three distinct (functionally and physically) tissues - the epidermis, the neural plate, and the neural crest

Question 4

The levels of BMP determine what type of ectodermal tissue you get:

1) The epidermis /surface ectoderm = ____ levels of BMP
2) The neural crest cells = ______ levels of BMP
3) Neural plate cells/neural tube = _____BMP levels

Answer 4

1. Epidermis = High levels of BMP
2. Neural crest cells (NCCs) = moderate levels of BMP
3. Neural plate/Neural cells = low levels of BMP

Question 5

Which of the three ectodermal tissues - epidermis, neural plate, neural crest cells - give rise to the neural tube and therefore give rise to the CNS (brain + spinal cord)?

Answer 5

The neural plate cells

Question 6

Which of the three ectodermal tissues - epidermis, neural plate, neural crest cells - shows high Sox2 staining?

Answer 6

Neural plate cells, neural cells express the gene Sox2.

Question 7

True or false. It is only after the neural tube fully fuses along the entire anterior-posterior axis, that the anterior-most region of the neural tube begins to swell to form the brain.

Answer 7

True

Question 8

In the Xenopus embryo, the more anterior ends of the neural tube are formed via _______ neurulation. As you go towards the caudal/posterior end, neural tube formation is done via _______ neurulation.

Answer 8

In the Xenopus:

anterior ends of neural tube - primary neurulation

posterior ends of neural tube - secondary neurulation

Question 9

Describe the steps of primary neurulation.

Answer 9

1. Cells of the neural plate proliferate, and the neural plate thickens. This thickening happens along the entire A-P axis.
2. Formation of the MHP (medial hinge point) causes the neural tube to fold inwards like an invagination, though Stella never used the word “invagination.” This forms the neural groove.
3. Neural folds elevate, and epidermal ectoderm start to migrate to the midpoint.
4. DLHPs form on either side of the neural tube, causing the neural folds to bend and face one another.
5. The neural folds fuse (mechanism unknown, filopedia may be involved). The overlying epidermis also fuses on top.
6. Neural cells and epidermal cells separate from one another, causing the neural tube to separate from the overlying epidermis.

Question 10

Describe the steps of secondary neurulation.

Answer 10

1. Migrating mesenchymal neural cells form a solid cord at the dorsal region of the body
2. The centre of this solid cord hollows out as development continues.

Question 11

When discussing the morphogens involved in neural tube formation, will we be using the xenopus model?

Answer 11

No, we will be using the chick or mammalian model to discuss the MORPHOGENS involved in primary neurulation

Question 12

The MHP anchors to the _______ while the DLHPs (Dorsal-lateral hinge points) anchor to the _________

Answer 12

MHP anchors to the notochord

DLHPs anchor to the epidermis

Question 13

The cells of the MHP and DLHPs change shape in order to create those bend points using what process?

Answer 13

Apical constriction

Question 14

The MHP and DLHPs are analogous to what structure in the Xenopus?

Answer 14

The bottle cells.

Question 15

Describe how apical constriction happens in MHP and DLHPs

Answer 15

A concentration of actin and myosin concentrates at the apical end of the cell, causes a reduction in the size of the apical side of the cell. The cell ends up changing shape into a wedge-shaped cell.

There will be increased expression of adherens junctions and tight junctions between cells at the apical end, forcing neighbouring cells to also constrict.

Question 16

What three morphogens are involved in the formation of the MHP and DLHPs?

Answer 16

BMP, shh (Sonic HedgeHog) and noggin.

Question 17

In the developing neural tube, where is noggin, the BMP inhibitor, localized?

Answer 17

it is in the dorsal-most regions of the neural tube.

Question 18

Describe the steps of shh involvement in MHP formation.

Answer 18

Shh is first expressed at the notochord.

It induces the neural plate cells above the notochord to form the “floorplate.” Floorplate cells also express shh

This will trigger apical constriction in the floorplate cells, to form the MHP.

Question 19

True or false. Without shh we wouldn’t have the MHP.

Answer 19

True.

If we manipulatively removed Shh, or the floorplate, or the notochord, the MHP won’t form.

Question 20

What happens to the neural tube if we experimentally remove the notochord? What happens to the MHP?

Answer 20

1. The floorplate won’t form

2. The MHP therefore won’t form

Question 21

Imagine you’re looking at a neural tube before fusion. How will noggin levels change as you from neural to epidermal tissue (medial to lateral)? What about BMP?

Answer 21

noggin levels decrease as you get to epidermal tissue (as you go laterally).

BMP levels increase as you get to epidermal tissue (as you go laterally).

Question 22

What does BMP do to MHPs and DLHPs? Is the effect symmetrical?

Answer 22

BMP inhibits the formation of the MHPs and the DLHPs. Its effect on the DLHPs is more severe, because of the distance it would need to travel to affect MHPs.

Question 23

Imagine you’re looking at a neural tube before fusion.

What do you predict will happen if noggin is over-expressed?

Answer 23

Too much noggin = too much BMP inhibition = too little BMP activity = not enough inhibition of MHP and DLHP formation = exaggerated bending in the MHP and DLHP, but more obvious in DLHPs.

Question 24

Imagine you’re looking at a neural tube before fusion.

What do you predict will happen if noggin is under-expressed?

Answer 24

Too little noggin = too little BMP inhibition = too much BMP activity = too much inhibition of MHP and DLHP formation = DLHPs and possibly MHPs (if BMP activity is high enough) will not form.

Question 25

True or false. The proper amount of BMP signalling is essential for proper formation of DLHPs and MHPs

Answer 25

True

Question 26

Shh is an inhibitor of what morphogen?

Answer 26

Noggin

Question 27

True or false. The concentration of shh is important because it determines the concentration of noggin, which is essential to determine the concentration of BMP, which is essential for proper formation of the hinge points

Answer 27

True

Question 28

Imagine you’re looking at a neural tube before fusion.

What do you predict will happen if Shh is over-expressed?

Answer 28

Too much Shh = too much inhibition of noggin = too little noggin activity = too little inhibition of BMP = high BMP activity = too much inhibition of the formation of DLHPs = DLHPs won’t form BUT MHP will have exaggerated bending because of a lot of Shh.

Question 29

Imagine you’re looking at a neural tube before fusion.

What do you predict will happen if Shh is under-expressed?

Answer 29

Too little shh = too little noggin inhibition = too much noggin activity = too much BMP inhibition = too little BMP activity = too little inhibition of DLHPs = DLHPs will have exaggerated bending, but also, too little Shh means the MHP doesn’t form.

Question 30

True or false. Right at the midline between the two neural folds, the cells extend out filopodial processes (filopedia) to form a cytoplasmic bridge between the two folds.

Answer 30

true

Question 31

Epidermal ectoderm specifically expresses __-cadherins, and the neural ectoderm specifically express ___-cadherins.

Answer 31

Epidermal ectoderm specifically express E-cadherins

Neural ectoderm specifically express N-cadherins

Question 32

What process separates the neural cells from the epidermal cells? specifically the neural tube from the overlying epidermis?

Answer 32

The differential expression of different forms of cadherins.

Question 33

True or false. In lab we have seen that cells expressing the same type of cadherin will associate with one another and segregate away from cells expressing a different form of cadherin. It appears that cadherin expression is one popular way of cell-sorting!

Answer 33

true

Question 34

What would happen if you disrupted N-cadherin formation? (By either introducing a morpholino for it or by mutating the gene to lose function)

Answer 34

1. there will be an unclear distinction between the cells of the neural plate and cells of the epidermis
2. the neural tube will fail to separate from the epidermis.

Question 35

In the Xenopus, one gene responsible for the downregulation of E-cadherins in the neural folds is the __________ gene

Answer 35

Grainyhead-like gene

Question 36

True or false. The failure of epidermal ectoderm separating from neural ectoderm can lead to parts of the brain and parts of the spinal cord not being within the skull or the vertebral column.

Answer 36

True