Lecture 4 Flashcards
What are the 2 types of axial mesoderm cells?
Prechordal Mesoderm (anterior)
Notochord (posterior)
Wnt Signalling
- Wnt is expressed Posteriorly
- Antagonists (of Wnt) are expressed anteriorly
(This A-P gradient of Wnt is set up after gastrulation)
Neural cells exposed to Low Wnt = become the brain = posterior
High Wnt triggers neural cells to become the spinal chord = anterior.
Anterior: Forebrain - Midbrain - Hindbrain - Spinal Cord :Posterior
Location of Wnt signalling
Released in the mesoderm –> but also determines neural identity (ectoderm)
Are all organiser cells uniform?
No
- only a subset secrete both BMP antagonists & Wnt antagonists…
This subset are the first to involute & differentiate into prechordal mesoderm.
name of Wnt Antagonists?
Dkk (Dickkofp)
Key features of later-involuting organiser cells…
Do not express Dkk (Wnt antagonist)
Differentiate into notochord.
Explaining the Wnt Gradient
Anterior= low because Wnt antagonists are expressed in the prechordal mesoderm
Posterior= High
Notochord doesn’t express Wnt antagonists.
Wnt signals up-regulate genes of the posterior mesoderm i.e. the notochord.
How do we know the Neural Plate is being Patterned or Regionalised.
(step 1= to look)
Sox2 expressed across the whole neural plate/tube
Otx2 is only expressed in brain progenitors.
(in situ hybridisation to detect mRNA )
(immunohistochemistry for proteins)
TFs of different Progenitor Cells
(Neural Plate= Sox2)
- Brain Progenitors= OTX2
- Spinal Cord Progenitors= HOX
How to prove the PM contains signals for regionalising part of the neural plate into a brain.
Gain of Function Study:
Implanting the anterior mesoderm (prechordal mesoderm) into an early embryo… induces an extra head & brain.
(prechordal mesoderm –> brain)
Vs
Implanting posterior mesoderm produces an extra trunk
(notochord –> spinal chord)
Manipulating Wnt & Dkk to understand their importance of Brain Formation…
Dkk is a Wnt antagonist so regions of the neural plate exposed to high amounts will develop into parts of the brain. Regions exposed to low amounts will develop into the spinal chord - as is shown in the A-P axis…
Over-expressing Dkk leads to a bigger forebrain (G.O.F study)
Knock-out of Dkk leads to the loss of forebrain (L.O.F study.)
So, what confers A-P regional Identity.
Gradient of Wnt Signalling - determining neural identity.
Termination of the Prechordal Mesoderm
Transient Structure: after its role in the Wnt signal axis it disperses and can’t be seen in the adult CNS.
Difference between Early & Late Neural Tube
Initial Tube is straighter, maintained within the one axis
- Quickly begins to bend more with a more distinguishable hindbrain, midbrain & forebrain.
What section of the Nervous System is regionalised by Wnt & Wnt antagonists?
Anterior Nervous System= forebrain, midbrain, hindbrain & Spinal Chord
(Posterior Nervous System via a different mechanism but we are not being tested on.) (generated by bipotent progenitors.)
So what is the bottom line?
Brain & Spinal chord progenitors form in Space & Time.
Spinal Chord is Segmented along the A-P axis:
Different nerves arise along the spinal chord due to varying levels of the HOX genes along the A-P axis.
example= long motor nerves form specifically to reach down to our hands
What areas do HOX genes influence?
Spinal Chord & Hindbrain.
How HOX Profiles influencing Nerve Formation
- Around 30 HOX genes
Unique HOX combinations responsible for differentiating different parts of the spinal chord & its nerves.
I.e. Patterns of Gene Expression Specify Identity of Distinct Regions.
Can we use Loss of Function Studies for determining HOX genes in conferring regional identity.
Yes
But very Difficult & Expensive due to amount of HOX genes
Example of a HOX gene Loss of Function Study:
Part of the hindbrain: r4 & r5 are characterised by transcription factors HOXa1 & HOXb1.
Knocking out these 2 HOX genes means rhombomere 4&5 regions never form –> leading to an abesence of the ‘abducens nerve’
Last 3 Slides needed…
