Patterning

Question 1

What is patterning?

Answer 1

The neural plate is a simple sheet of cells. Patterning divides this simple plate into different brain regions. These regions are present as different gene expression domains before any morphological differences can be seen - transcription factors are very important in this.

Question 2

Describe signalling to tell a cell its location

Answer 2

Diffusible signals produce different responses based on concentration.

Question 3

Explain how concentration of a signal affects its effect

Answer 3

Near the source of the signal there will be lots of that signal.

Cells respond not only to the presence of the signal but also to te quantity.

So, the cells responding to the signal above a certain threshold can become green cells (strongest signal), then decreasing signal strength to certain lower thresholds causes blue, white and red cells respectively.

Question 4

When does patterning begin?

Answer 4

While the CNS is still a simple sheet of cells

Question 5

How does patterning occur?

Answer 5

2 morphogens perpendicular to one another give precise positional information (like map coordinates).

The neural plate uses anteroposterior and mediolateral in the same way that a map uses longitude and latitude.

Question 6

What are the 4 stages of patterning in the CNS?

Answer 6

1. Simple sheet of cells
2. Patterned into broad areas
3. Broad areas are refined by further patterning
4. Functioning CNS

Question 7

What regulates expression of transcription factors?

Answer 7

Long-range signalling.

As a result of this, sharp gene expression boundaries begin to be established.

Question 8

Describe long-range signalling along the anteroposterior axis

Answer 8

• BMP / Wnt antagonist cerebrus expressed strongly in the forebrain
• RA antagonist Cyp26 expressed strongly in the forebrain and midbrain
• Wnt antagonists expressed weakly in the hindbrain
• Wnt, RA, FGF and TGFβ expressed strongly in the spinal cord

Question 9

Where is segmentation along the anteroposterior axis most pronounced?

Answer 9

Hindbrain - formation of rhombomeres

Question 10

Describe what Hox genes are and their role in segmentation along the AP axis

Answer 10

• Hox genes = highly conserved family of segmentally expressed genes.
• Each segment expresses a specific subset of Hox genes

Question 11

What establishes the rhombomeres in the hind brain?

Answer 11

• Hox code
• Each rhombomere has its own unique gene expression pattern

Question 12

Describe the phenotype of the rhombomeres and why they are like that

Answer 12

• Rhombomeres are all physically distinct
• This is because there is no cell mixing between adjacent segments

Question 13

What prevents mixing of rhombomere segments?

Answer 13

Eph-Ephrin signalling.

The receptors don’t like one another - they push each other apart and over time a very distinct boundary forms between the segments.

Question 14

What is the boundary in local patterning between the midbrain and the hindbrain?

Answer 14

Rhombic isthmus

Question 15

What is the boundary in local patterning within the developing diencephalon?

Answer 15

Zona limitans intrathalamica

Question 16

Where does patterning of the dorsoventral axis in the neuroectoderm begin?

Answer 16

In the neural plate

Question 17

Which signal is involved in the dorsal neural plate?

Answer 17

BMP

BMP induces the formation of neural crest precursors and roof plate cells.

Question 18

Which signal is involved in the ventral neural plate?

Answer 18

SHH

Induces formation of floor plate cells at the midline.

Question 19

What kind of gradient is produced by BMP and SHH signals when expressed together in patterning of the dorsocentral axis?

Answer 19

Opposing gradient

Question 20

What does dorsoventral signalling do?

Answer 20

Defines different progenitor regions which give rise to different, region-specific cell types.

Question 21

What does the floor plate do?

Answer 21

It is a big inducer of ventral fates

Question 22

Where is Shh produced?

Answer 22

Shh is produced in the notochord and the floor plate diffuses dorsally.

Question 23

Describe the concentration of Shh in the floor plate and dorsally

Answer 23

Cells near the floor plate = high levels of Shh.

Concentration of Shh decreases dorsally giving rise to different cell types.

Question 24

What happens when Shh is introduced in a dorsal position?

Answer 24

The addition of a source of Shh in a more dorsal position causes a change in cell fate due to and increase in the concentraion of Shh.

Question 25

How are different cell types specified along the DV axis?

Answer 25

Opposing actions of BMP and Shh specifies different cell types along the DV axis by inducing / repressing expression of specific transcription factors.

Question 26

Describe the set up of patterning in the telencephalon

Answer 26

• Dorsal - BMPs and Wnts from cortical horn
• Ventral - Shh
• Invagination causes what was the dorsal part to invaginate into the midline - dorsal signalling centre dives down between the two telencephalic lobes