Lecture 6

Question 1

Axis change triggered by Neurulation

Answer 1

Medio-Lateral axis exists with the single cell thick neural plate.
- Induction of Neurulation -
Lateral edges round up & fuse to form the dorsal neural tube - this is where the roof plate lies. Neural tube pinches off.

This process transforms the axis from M-L to Dorsal-Ventral

Question 2

Diagram/ Layers post-neuralation

Answer 2

Surface Ectoderm
Neural tube
Notochord lies just below

Question 3

Floor Plate

Answer 3

As neurulation occurs the notochord and prechordal mesoderm come to lie just under the neural tube.
- The floor plate is another specialised group of cells that lie in the neural tube’s ventral midline -> immediately above the notochord.

Question 4

Bilateral Symmetry

Answer 4

Neurons develop around the midline with bilateral symmetry - this occurs across the whole D-V axis
- i.e. same neurons form on both sides up the axis.
- could secreted factors from the floor plate be responsible…

Question 5

Gain Of Function: Floor Plate

Answer 5

Ectopic floor plate or notochord induces formation of ventral neurons (motor).
(floor plate is sufficient).

Question 6

Sonic Hedgehog (Shh)

Answer 6

As Axial Mesoderm forms –> TF’s upregulate expression of Shh gene.
- first identified in the notochord & prechordal mesoderm
- mRNA is then also found in the floor plate

Shh encodes a secreted protein.

Question 7

Shh Morphogen Gradient

Answer 7

Shh protein is secreted by floor plate –> diffuses to create a gradient in the ventral neural tube
- visualised by immunohistochemistry.

Shh acts as a morphogen -> inducing bands of different progenitors along the D-V axis (with bilateral symmetry).

Question 9

Morphogen

(one of most important concepts in dev biology).

Answer 9

Molecule secreted from a fixed source forms a concentration gradient which provides positional information.
- Morphogens are capable of inducing at least 2 distinct cell fates
- They’re instructive (=directly determine cell fate.)

Positional information idea: differentiation of a cell is dependent on where it’s positioned relative to the signalling molecule (due to its conc gradient).

Question 10

2 Models for Morphogens.

Answer 10

• Reaction-diffusion (Alan Turing)
• Positional Information (Lew Wolpert) - We focus on this one.

Question 11

Difference between Generic Signal Transduction vs Morphogen ST

Answer 11

(Extrinsic signal activates signalling pathway –> activating TF –> enters nucleus to elicit new Transcriptional profile -> responds by differentiating)

Morphogen: STRENGTH of original signal influences the transcriptional profile elicited -> makes different progenitor cell fates.

Question 12

Transcription Factor Code

Answer 12

Shh gradient generates a code of different TF’s along D-V axis.
-This code of TFs are the ‘master regulators’ of neuronal identity.
(they influence differentiation)

• PV3 region codes for interneurons
• PV2 region codes for motor neurons

(PV= Progenitor Ventral region)

Question 13

Lateral Migration

Answer 13

As nerve cells differentiate & form neurons they migrate laterally (radially), (in -> out).
- they extend their axons & form the defined neurons of the spinal cord

Question 14

TF Regionalisation along D-V axis

Answer 14

Dorsally- generally find Pax transcription factors
Ventrally- generally find Nkx (found in PV2 & PV3)

Question 15

BMP & Shh pathways Antagonise each other
(counter gradients)

Answer 15

Shh=morphogen: floor plate is fixed source of secretion so always a low conc of Shh dorsally

BMP: the axial mesoderm is derived from organiser cells remember - it retains the BMP antagonists - so these antagonise BMP action ventrally.

Question 16

Putting it all together…

Answer 16

BMP antagonism induces neural fate by upregulating Sox2+ to induce the neural plate.

High Wnt signalling meant the derivation of spinal cord progenitors compared to low Wnt (& Dkk antagonists) inducing brain progenitor cells.

• Dorsal spinal cord progenitors form due to BMP signalling
• Ventral spinal cord progenitors form due to Shh morphogen.
  ‘’’

Question 17

Last few slides to link Furley lectures to hers - come back and review these. ( sorry).