Patterning Flashcards
What is patterning?
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.
Describe signalling to tell a cell its location
Diffusible signals produce different responses based on concentration.
Explain how concentration of a signal affects its effect
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.
When does patterning begin?
While the CNS is still a simple sheet of cells
How does patterning occur?
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.
What are the 4 stages of patterning in the CNS?
- Simple sheet of cells
- Patterned into broad areas
- Broad areas are refined by further patterning
- Functioning CNS
What regulates expression of transcription factors?
Long-range signalling.
As a result of this, sharp gene expression boundaries begin to be established.
Describe long-range signalling along the anteroposterior axis
- 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
Where is segmentation along the anteroposterior axis most pronounced?
Hindbrain - formation of rhombomeres
Describe what Hox genes are and their role in segmentation along the AP axis
- Hox genes = highly conserved family of segmentally expressed genes.
- Each segment expresses a specific subset of Hox genes
What establishes the rhombomeres in the hind brain?
- Hox code
- Each rhombomere has its own unique gene expression pattern
Describe the phenotype of the rhombomeres and why they are like that
- Rhombomeres are all physically distinct
- This is because there is no cell mixing between adjacent segments
What prevents mixing of rhombomere segments?
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.
What is the boundary in local patterning between the midbrain and the hindbrain?
Rhombic isthmus
What is the boundary in local patterning within the developing diencephalon?
Zona limitans intrathalamica
Where does patterning of the dorsoventral axis in the neuroectoderm begin?
In the neural plate
Which signal is involved in the dorsal neural plate?
BMP
BMP induces the formation of neural crest precursors and roof plate cells.
Which signal is involved in the ventral neural plate?
SHH
Induces formation of floor plate cells at the midline.
What kind of gradient is produced by BMP and SHH signals when expressed together in patterning of the dorsocentral axis?
Opposing gradient
What does dorsoventral signalling do?
Defines different progenitor regions which give rise to different, region-specific cell types.
What does the floor plate do?
It is a big inducer of ventral fates
Where is Shh produced?
Shh is produced in the notochord and the floor plate diffuses dorsally.
Describe the concentration of Shh in the floor plate and dorsally
Cells near the floor plate = high levels of Shh.
Concentration of Shh decreases dorsally giving rise to different cell types.
What happens when Shh is introduced in a dorsal position?
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.
How are different cell types specified along the DV axis?
Opposing actions of BMP and Shh specifies different cell types along the DV axis by inducing / repressing expression of specific transcription factors.
Describe the set up of patterning in the telencephalon
- 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
