Formation of the neural plate and neurulation Flashcards
Which organism was it first indicated how the neurogenic region arose?
Drosophila
What is cell differentiation?
Process in which cells become different to each other and acquire specialised fates
How does differentiation occur?
By CHANGES in the gene expression in a cell, which dictate the repertoire of protein synthesise - determines shape, function and behaviour
What is gene expression governed by?
Extrinsic factors (morphogens) and intrinsic factors (transcription factors) which determine if a particular TF is expressed in the cell - can bind to enhancers/promotors
What happens to cell fate and potency as a cell differentiates
Potency decreases - cell gets more committed to cell fate
What is the homologue of BMP7 in drosophila?
Screw
What is the homologue of BMP4 in drosophila?
dpp (decapentaplergic)
What is the homologue of chordin in drosophila?
sog (short gastrulation)
What is the function of chordin and what type of protein is it?
Secreted factor which prevents BMP signalling
What type of molecules are BMPs?
Secreted molecules
When does neurogenic fate occur?
When BMP/dpp signalling is in inhibited - by chordin/sog
Originally, all cells have BMP signalling
What happens when BMP/dpp binds to a receptor?
1) Phosphorylation of SMAD
2) Phosphorylated SMAD can enter the nucleus - as a key part of the transcription machinery
3) Dictates if a gene is transcribed
How does chordin/sog prevent BMP/dpp signalling?
- Mop up the BMP/dpp (act as ‘sponges’)
OR - By competitive inhibition
Where is chordin/sog expressed in invertebrates and vertebrate and why?
Vertebrates - Dorsal side
Invertebrates - Ventral side
Where chordin/sog is expressed, BMP/dpp signalling is inhibited and a neural fate is induced
Why does the nervous system of the vertebrates lie dorsally?
Vertebrates and invertebrates developed from the same common ancestor, but as vertebrates develop, their head twisted
What is the molecular pathway for the development of epidermal cells?
(in a blastoderm??)
1) Ectodermal cell - BMP signalling pathway, induced by BMP release from neighbouring cells.
Releases BMP also.
2) BMP signalling - Phosphorylate SMAD (High SMAD 1)
3) Switch on TFs which dictate epidermal fate - MSX1, GATA1 and vent
4) These TFs act in an autonomous manner, switching on higher level TF LEF1
5) LEF1 commits cells to skin epidermal fate by restricting potency over time
What is the molecular pathway for the development of neurogenic cells?
(in a blastoderm??)
1) BMP signalling is prevented in these cells by chordin
2) A different SMAD is phosphorylated (High SMAD 7)
3) Upregulates a set of TF (Sox TF)
4) Autonomously upregulate NEUROGENIN
5) Autonomously upregulate NeruoD which commits the cell to neural differentiation
What are the levels of SMAD 1 and 7 in cells that have BMP signalling?
High SMAD 1, low SMAD 7
What are the levels of SMAD 1 and 7 in cells that DON’T have BMP signalling?
Low SMAD 1, high SMAD 7
What is neurulation?
The folding up of the neural plate to form the neural tube
What is caused if the neural tube doesn’t close properly in development?
Spinal bifida
Where did the understanding of the neural plate and neurulation come from
Studies in Xenopus
How do we know that the 3 germ layers in the early stages of Xenopus development, are different to each other?
Using in situ hybridisation, can see that the 3 layers express different transcription factors and proteins
How is the neural plate established in the xenopus?
1) After fertilisation - 3 germ layers form
2) Subset of cells in the mesoderm start to become specialised - called the ‘organiser’
3) The organiser expresses specialised TF (siamois and goosegoid)
4) TF act in an autonomous manner - upregulate transcription of chordin, noggin, follistatin
6) These TFs diffuse into the immediate ectoderm (directly next to the organiser) and antagonise BMP signalling
7) This part of the ectoderm no longer has active BMP signalling and therefore becomes the future CNS (neural plate)
What is the ‘organiser’ called in humans and chicks?
The node
What are the transcription factors which upregulate chordin, noggin and follistatin?
Siamois
Goosecoid (Gsc)
Xnot
Xlim1
What are chordin, noggin and follistatin?
Secreted molecules which antagonise BMP signalling - causing a neural fate
What is ‘neural induction’?
The process through which the neural plate forms when BMP antagonists prevent the BMP ligand from stimulating its receptor
What are BMPs?
Bone morphogenic proteins
What are BMP receptors?
TFb
Where are BMP receptors present?
In all ectodermal cells
As long as the BMP signalling pathway is activated, what fate will ectodermal cells assume?
Epidermis
If the BMP signalling pathway is inhibited, what fate will ectodermal cells assume and why?
Neural tissue, due to the upregulation of transcription factors which direct the cell down the neural pathway
What do BMP antagonists do?
Diffuse into the extracellular spaces of ectodermal tissue and compete with BMPs for the TGFb receptor - inhibiting BMP signalling
What do chicks and humans develop from?
Flattened sheet with 3 layers
Where there is high levels of Nodal, what develops?
The organiser
Where there is low levels of Nodal, what develops?
Ventral mesoderm
Where does the organiser develop?
Dorsal mesoderm
What is expressed in the ventral mesoderm?
BMP
Tolloid
Where does the node develop in vertebrates?
In the centre of the flattened sheet, above the primitive streak
What is sox2 and where is it expressed?
TF in cells which are going to aquire neural fates
What happens at the same time as gastrulation?
NERULATION
Neural plate gets extends along the AP axis and folds up into a neural tube
What do the master regulators (siamois and goosegoid) cause the cells in the organiser to do?
Develop into 3 different cell types:
1) Anterior endoderm
2) Pre-chordal mesoderm
3) Chordamesoderm (notochord)
What are is the axial mesoderm?
The collective name for the pre-chordal mesoderm and the notochord
What happens to the organiser later on in development?
1) Differentiates into 3 cell types under the influence of the master regulators (siamois, goosegoid)
2) Cells involute, intercalate and undergo convergent extension, come to lie beneath the ectoderm
From the organiser, which cells involute in which order?
1) Anterior endoderm
2) Pre-chordal mesoderm
3) Notochord
What is involution?
Sheet of cells roll inwards to form an underlying layer
What is invagination?
Sheet of cells bend inwards
What is epiboly?
Sheet of cels spreading out by thinning
What is intercalation?
Rows of cells move between one another - create array of cells which is longer but thinner
What is ingression?
Individual cells leave an epithelial sheet and become freely migrating mesenchyme cells
What is convergent extension?
Rows of cells intercalate with high directionality
Where do the cells of the pre-chordal mesoderm come to lie after involution and convergent extension?
Underneath the developing forebrain and are followed by cells of the notochord
In close contact with the developing mouth
What is the shape of the prechordal mesoderm?
Fan-like
Describe the experimental proof for neural induction
‘Organiser graft’ by Spemann and Mangold
- Graft organiser from donor to host newt (with different pigment) in an ECTOPIC location
- Donor organiser produced BMP agaonists and induced the cells of the host to become neural (will become the neural plate and then neural tube)
- Self-differentiated into anterior endoderm and axial mesoderm
- This underwent involution and convergent extension to lie underneath the neural plate
- Created a second ‘twinned’ embryo, with a secondary neural tube, underlied by a second axial mesoderm
What were the 2 conclusions of the ‘organiser graft’?
1) Host cells were induced to a neural identity - due to signals from the grafted organiser
2) Axial mesoderm and anterior endoderm all came from the donor organiser self-differentiating
How was the ‘organiser graft’ done similarly in the chick?
Quail node grafted into a host chick
How were BMP antagonists discovered?
1) Dissecting out the organiser from xenopus
2) Extract all mRNA from Xenopus cells and reverse transcribe to cDNA
3) Tested each to look for gene/protein that would mimic organisers ability to induces a second neural plate and ectopic axis
What experiments could be done to prove that a molecule is a ‘neural inducer’?
1) Make cDNA (antisense mRNA)
2) Probe for the complimentary mRNA - expressed in the right time and place?
3) Ectopically express the molecule - if cause a 2nd axis to form, it is sufficient
4) Knock-out the molecule - if no development, it is necessary
5) Is the molecule secreted?
How is neurulation driven?
By the change in cell shape:
1) Band of F-actin (filementous) at the apical domain of neuroepithelial cells contracts
2) Pulls the actin tight, causing cells to be constricted at that edge - change shape
What causes F-actin to contract?
Interaction with many molecules
Eg. folate in pregnant women
What can happen if pregnant women don’t get enough folate?
F-actin cannot contract
Neural tube cannot close
Cause spinal bifida
What TF commits cells to an epidermal fate?
LEF1
What TF commits cells to a neural fate?
NeuroD
