Lecture 4

Question 1

What are the 2 types of axial mesoderm cells?

Answer 1

Prechordal Mesoderm (anterior)

Notochord (posterior)

Question 2

Wnt Signalling

Answer 2

• 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

Question 3

Location of Wnt signalling

Answer 3

Released in the mesoderm –> but also determines neural identity (ectoderm)

Question 4

Are all organiser cells uniform?

Answer 4

No
- only a subset secrete both BMP antagonists & Wnt antagonists…

This subset are the first to involute & differentiate into prechordal mesoderm.

Question 5

name of Wnt Antagonists?

Answer 5

Dkk (Dickkofp)

Question 6

Key features of later-involuting organiser cells…

Answer 6

Do not express Dkk (Wnt antagonist)

Differentiate into notochord.

Question 7

Explaining the Wnt Gradient

Answer 7

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.

Question 8

How do we know the Neural Plate is being Patterned or Regionalised.

(step 1= to look)

Answer 8

Sox2 expressed across the whole neural plate/tube

Otx2 is only expressed in brain progenitors.

(in situ hybridisation to detect mRNA )
(immunohistochemistry for proteins)

Question 9

TFs of different Progenitor Cells

Answer 9

(Neural Plate= Sox2)

• Brain Progenitors= OTX2
• Spinal Cord Progenitors= HOX

Question 10

How to prove the PM contains signals for regionalising part of the neural plate into a brain.

Answer 10

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)

Question 11

Manipulating Wnt & Dkk to understand their importance of Brain Formation…

Answer 11

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.)

Question 12

So, what confers A-P regional Identity.

Answer 12

Gradient of Wnt Signalling - determining neural identity.

Question 13

Termination of the Prechordal Mesoderm

Answer 13

Transient Structure: after its role in the Wnt signal axis it disperses and can’t be seen in the adult CNS.

Question 14

Difference between Early & Late Neural Tube

Answer 14

Initial Tube is straighter, maintained within the one axis

• Quickly begins to bend more with a more distinguishable hindbrain, midbrain & forebrain.

Question 15

What section of the Nervous System is regionalised by Wnt & Wnt antagonists?

Answer 15

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.)

Question 16

So what is the bottom line?

Answer 16

Brain & Spinal chord progenitors form in Space & Time.

Question 17

Spinal Chord is Segmented along the A-P axis:

Answer 17

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

Question 18

What areas do HOX genes influence?

Answer 18

Spinal Chord & Hindbrain.

Question 19

How HOX Profiles influencing Nerve Formation

Answer 19

• 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.

Question 20

Can we use Loss of Function Studies for determining HOX genes in conferring regional identity.

Answer 20

Yes

But very Difficult & Expensive due to amount of HOX genes

Question 21

Example of a HOX gene Loss of Function Study:

Answer 21

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’

Question 22

Last 3 Slides needed…