Neurulation (everything up until Neural Crest section) Flashcards
What model organism do we use to study the formation of the neural tube itself?
The Xenopus
what flanks either side of the neural tube?
The somites
What is neurulation?
The step by which the ectoderm divides into three distinct (functionally and physically) tissues - the epidermis, the neural plate, and the neural crest
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
- Epidermis = High levels of BMP
- Neural crest cells (NCCs) = moderate levels of BMP
- Neural plate/Neural cells = low levels of BMP
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)?
The neural plate cells
Which of the three ectodermal tissues - epidermis, neural plate, neural crest cells - shows high Sox2 staining?
Neural plate cells, neural cells express the gene Sox2.
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.
True
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.
In the Xenopus:
anterior ends of neural tube - primary neurulation
posterior ends of neural tube - secondary neurulation
Describe the steps of primary neurulation.
- Cells of the neural plate proliferate, and the neural plate thickens. This thickening happens along the entire A-P axis.
- 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.
- Neural folds elevate, and epidermal ectoderm start to migrate to the midpoint.
- DLHPs form on either side of the neural tube, causing the neural folds to bend and face one another.
- The neural folds fuse (mechanism unknown, filopedia may be involved). The overlying epidermis also fuses on top.
- Neural cells and epidermal cells separate from one another, causing the neural tube to separate from the overlying epidermis.
Describe the steps of secondary neurulation.
- Migrating mesenchymal neural cells form a solid cord at the dorsal region of the body
- The centre of this solid cord hollows out as development continues.
When discussing the morphogens involved in neural tube formation, will we be using the xenopus model?
No, we will be using the chick or mammalian model to discuss the MORPHOGENS involved in primary neurulation
The MHP anchors to the _______ while the DLHPs (Dorsal-lateral hinge points) anchor to the _________
MHP anchors to the notochord
DLHPs anchor to the epidermis
The cells of the MHP and DLHPs change shape in order to create those bend points using what process?
Apical constriction
The MHP and DLHPs are analogous to what structure in the Xenopus?
The bottle cells.
Describe how apical constriction happens in MHP and DLHPs
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.
What three morphogens are involved in the formation of the MHP and DLHPs?
BMP, shh (Sonic HedgeHog) and noggin.
In the developing neural tube, where is noggin, the BMP inhibitor, localized?
it is in the dorsal-most regions of the neural tube.
Describe the steps of shh involvement in MHP formation.
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.
True or false. Without shh we wouldn’t have the MHP.
True.
If we manipulatively removed Shh, or the floorplate, or the notochord, the MHP won’t form.
What happens to the neural tube if we experimentally remove the notochord? What happens to the MHP?
- The floorplate won’t form
2. The MHP therefore won’t form
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?
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).
What does BMP do to MHPs and DLHPs? Is the effect symmetrical?
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.
Imagine you’re looking at a neural tube before fusion.
What do you predict will happen if noggin is over-expressed?
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.
Imagine you’re looking at a neural tube before fusion.
What do you predict will happen if noggin is under-expressed?
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.
