Lecture 9- Patterning of the Nervous System: Dorsal-Ventral patterning Flashcards
When does dorsal ventral patterning occur?
- early in in neurogenesis
- as the neural plate starts to fold, it marks the beginning of dorsal ventral patterning
- picture: 23-24 hr old chick embryo, closure of neural tube: even here have specified dorsal and ventral patterning, not yet commited though, as dividing neuroepthilial cells are still active
What is the structure of the neural tube along the dorsal-ventral axis?
- neural tube with underlying notochord forms
- by the time the neural tube closes there is a clear dorsal-ventral pattern
-the node extends under and becomes the notochord, this influences the cells close to it, midline cells= to become floor plate
(dorsal)
-the neural fold cells= also influenced by surrounding ectoderm (ventral)
What is the development of the node into the notochord over time in a mouse embryo?
- early on (early streak stage) it is the gastrula organiser (GO)
- mid-streak stage it is the node
- it later extends anteriorally and becomes the notochord
(red is the notochord)
Does the mouse embryo turn during development?
- yes
- the picture shows embryo 8.5 (left) and 9.5 (right)
What is this?
- saggital section through a 9.5 days old mouse embryo
- shows the dorsal-ventral patterning occuring
- can see the influence of morphogens in both the dorsal and ventral region
What are the primary organizers in dorsal-ventral patterning?
- Overlying Ectoderm
- Underlying notochord
What does the Overlying Ectoderm secrete?
- dorsalizing morphogens
- BMPs, FGF, Wnts
What does the Underlying Notochord secrete?
- ventralizing morphogens
- Sonic hedgehog (Shh)
What is the specification of neural crest cells in the dorsal neural tube mediated by?
-mediated by BMP and Wnt signals from surrounding ectoderm
How do neural crest cells arise?
- neural crest cells are specified in the neuroepithelium
- go from being neuroectoderm to mesenchyme-like cells
- delaminate from the neural tube prior to neural tube closure
- first derived from neuroepithelial cells
- arise due to Wnt and BMP signalling
- will become PNS, melanocytes, dentine structures etc.
What are the secondary organisers in dorsal-ventral patterning?
- Roof plate
- Floor plate
When do the secondary organizers appear?
-once closure of the neural tube is finished
What does the roof plate develop from and what does it secrete?
- neural tube becomes roof plate
- secretes dorsalizing BMP and Wnt morphogens
What is the floor plate derived from and what does it secrete?
- neural tube becomes floor plate
- secretes ventralizing Sonic hedgehog
- Floor plate is induced by the notochord
What is the relationship between BMP and Shh?
- BMP inhibits Shh signalling
- Shh inhibits BMP signalling
- this helps to induce the cells to be ventral or dorsal
What does BMP do?
-induces dorsal cell types and represses ventral cell types
What does sonic hedgehog induce and repress?
- induces ventral cell types and represses dorsal cell types
- very sensitive gradient system, finesse!
Why is the expression of different transcription factors important in dorsal-ventral patterning?
-the expression of different transcription factors along the dorsal-ventral neural tube gives rise to various cell types, including dorsal and ventral interneurons and motor neurons
Where in the neural tube will be different types of neurons induced (dorsal-ventral pattern)?
- have Shh being released from the floor plate and BMPs and Wnts from the roof plate, differing concentration of these will induce various neuron types
- going from the ventral (Shh, floor plate) to the dorsal side (BMP,Wnt, roof plate)
a) floor plate
b) V3 neurons
c) motor neurons
d) V2 interneurons
e) V1 interneurons
f) V0 interneurons
g) D2 interneurons
h) D1 interneurons
i) roof plate - green represents Shh, blue BMPs and Wnts
What are the two gradients most important for induction of neurons and interneurons in the neural tube?
- Gradient of Shh
- Gradient of TGF-ß family factors (BMP4, BMP7, BMP5, Dorsalin, Activin)
How is the infuence of TGF-ß family factors distributed in the neural tube?
- BMP4,BMP7,BMP5, Dorsalin and Activin are all present in the very dorsal region of the neural tube
- moving ventrally only BMP7, Dorsalin and Activin remain
- moving even further in the ventral direction only Dorsalin and Activin remain
What are the V3 neurons for?
-visual system
What is cell division like in development (short summary)?
- at first have proliferation of neuroepithelial cells within the ventricular zone of neural tube
- then have differentiation to progenitor, exit ventricular zone and migration to intermediate zone (or beyond)
- Differentiation to specific neuron or glial cell
What type of cell division is there during development?
-initially in development just symmetric division later asymmetric, more and more asymmetric as development progresses
What does the ventricular zone eventually become?
-eventually the ventricular zone no longer proliferates and becomes epidermal
What are the three regions of the developing spinal cord called and what is their main function?
- Ventricular zone: Dviding germinal neuroepithelial cells (in an adult= ependymal layer)
- Intermediate/Mantle zone: Grey matter (cell bodies of neurons and glia)
- Marginal zone: White matter (axonal connections)
What does the fully developed spinal cord look like? (cross-section)
- mantle layer (the intermediate zone during development)
- ependymal layer (the ventricular zone during development)
What is the key piece of information to have about cells that have migrated?
- where they migrated from
- they can change in form but where they originate during early development is key to their function
What gives rise to specific expression factors in the neural tube? (along the dorsal-ventral axis)
-it is the gradient concentration of morphogens (BMP, Shh) in the dorsal and ventral neural tube
- the morphogens switch on numerous transcription factors
- the mix or different mixes do different things = cells will commit differently even when there is a small difference in concentration
for example: cells that express Nkx6.1 are repressed from expressing Dbx2
= also helps with cell specification
What are the Gli proteins?
- Shh effectors
- Gli transcription factors activate/inhibit transcription by binding to Gli responsive genes and by interacting with the transcription complex
- regulate Shh signalling
What Gli transcription factors are involved in regulation of Shh signalling?
- Gli1= the key one, activator of Shh signalling
- Gli2= can mean on/off (activator/repressor)depending on the specific Shh concentration
- Gli3= repressor of Shh signalling
How does activation of Shh signalling work?
- when Shh reaches target cell it binds to PTCH1 receptor (In the absence of a ligand PTCH inhibits SMO (Smoothened))
- It seems PTCH removes sterols from SMO which makes it active, so SMO inhibition is lifted which leads to activation of Gli transcription factors
- activated Gli accumulates in the nucleus and controls the transcription of the Shh target genes
- what happens after Shh binding to PTCH1 depends on which Gli transcription factor the cell has
- in a lab you can get around using Shh by directly putting in SAG or Puromorphamine, those will also activate SMO
What are the progenitors transcription factors involved in dorsal-ventral patterning?
What are the post-mitotic transcription factors involved in dorsal-ventral patterning and what do they do?
-once asymmetric division is taking place the expression of transcription factors changes, now they have new ones
- have specific ones for each cell type
- these factors determine where the cells migrate
How does development of neuronal cell types continue beyond their specification?
- expression of receptors and cell surface proteins involved in axon guidance are part of the cell program during patterning and specification of neuronal cell types
e. g. there are lot of different motor neuron types, the specific transcription factors in each cell are there depending on where the cell came from in the neural tube
What does specification of different motor neurons along the anterior-posterior and dorsal-ventral axes depend on?
-classes of motor neurons within the spinal cord express specific transcription factors and will innervate specific muscle groups
Why is timing important?
- neuroepithelial cells within the ventricular zone are influences by morphogen concentration (what and how)
- although cells in the ventricular zone express specific genes along the neural tube axes, they are still not completely committed. Cells become more committed when they leave the ventricular zone and enter into the intermediate zone and are postmitotic
- different array of transcription factors are expressd by neural and glial progenitors within the intermediate zone of the neural tube. These progenitors are more committed to their fate, no longer influenced by morphogen exposure.
Why do motor neurons arise from ventral regions of the neural tube?
- because of exposure to Sonic hedgehog at a specific concentration
- however this induction will not occur at later stages of development, when cells have exited the ventricular zone and become committed
What do oligodendrocytes and astrocytes arise from?
- both dorsal and ventral regions of the neural tube
- are specified at slightly later stages than neural progenitors
How similar are oligodendrocytes to motor neurons?
-oligodendrocytes are similar to motor neurons, only at the end they are different, depends if the cell expresses olig2 (oligodendrocyte) or olig1(motor neuron)
- also have oligodendoryctes from dorsal parts but later
- at first ventral develop
What does dorsal-ventral patterning look like in Telencephalon?
- sonic hedgehog is secreted from underlying prechordal plate
- prechordal plate= also called medendoderm in later stages, secretes Shh that switches on Nox2.1
