L7 - Ventralisation of the Neural Tube

Question 1

Where does the axial mesoderm lie?

Answer 1

Just below the ventral midline of the neural tube

Question 2

What is bilateral symmetry?

Answer 2

Neurones develop with bilateral symmetry, around the midline, throughout entire dorso-ventral axis

Question 3

Notochord and floor plate cells are located?

Answer 3

At the ventral midline

Question 4

Where do floor plate cells differentiate?

Answer 4

Immediately above the notochord

Question 5

Notochord and floor plate cells both secrete?

Answer 5

A morphogen

• Conserved in evolution
• Has a homologue in Drosophila

Question 6

Bilateral symmetry is tested by

Answer 6

1. Grafting a donor notochord or floor plate to an ectopic position in a host embryo
   - Between neural tube and sclerotome
2. Cells in host neural tube change their fate due to the secreted signal by the donor graft
3. Ectopic floor plate and ventral neurones are induced

Question 7

Where is Shh mRNA expressed?

Answer 7

In notochord and then in the floor plate

• Shh protein is detected more widely than the mRNA
• Appears in a gradient – diffuses through ventral neural tube from ventral to dorsal

Question 8

Shh signalling induces expression of?

Answer 8

Transcription factors in progenitor cells

• The transcription factors confer central neural tube identities
• Progenitors will give rise to cells that differentiate into ventral neurones

Question 9

Role of Shh is tested by

Answer 9

1. Take Shh soaked bead and transplant into host

2. Ectopic floor plate and central neurones induced

Question 10

Threshold of Shh induces?

Answer 10

Transcription factors induced in bands of progenitor cells along ventral half of
neural tube due to threshold of Shh
- Readouts of a Shh morphogen gradient along the D-V axis
- These transcription factors are upstream regulators of particular neural fate

Question 11

Which signalling molecules pattern the DV axis?

Answer 11

Opposing gradients of BMPs and Shh

Question 12

Roof of neural tube is exposed to?

Answer 12

BMPs/Wnts from the epidermis

- Cascade of TGF-β related factors spreading ventrally

Question 13

Floor of neural tube is exposed to?

Answer 13

Exposed to Hh/Shh protein from the notochord

- Diffuses as a gradient from the floor plate cells dorsally

Question 14

Neurones are given their identities by?

Answer 14

Their exposure to gradients of paracrine factors

Amount and type of paracrine factors cause different transcription factors to be activated

Question 15

After differentiation neurones either?

Answer 15

Migrate laterally to form the defined neurones at mantle zone and form axons
Stay in ventricular zone

Question 16

If no Hh

Answer 16

Ptc represses Smo
Smo cant interact with downstream cytoplasmic machinery
Transcription factors remain in their repressor form stopping transcription of Hh target genes

Question 17

Why is it important to study the Hh signalling pathway?

Answer 17

For drug discovery, disease understanding and regenerative medicine

Question 18

Where is Shh expressed in the embryo?

Answer 18

Expressed along the entre AP axis ventrally
It governs ventralisation along the entire rostro-caudal axis
- Caudal – is what was initially posterior
- Rostral – is what was initially anterior