Development of the hypothalamus Flashcards
Where are neurons found in most regions of the CNS?
What does this reflect?
In COLUMNS and LAYERS
Reflects the development along the AP and DV axes
How are the layers of neurons developed in the spinal cord?
Motor neurons differentiate in a STEREOTYPED pattern along the DV axis
How are the layers of neurons developed in the cortex and retina?
Neurons are born in a specific ORDER –> layers
How are the neurons arranged differently in the hypothalamus compared to the spinal cord or retina/cortex?
Retina/cortex/spinal cord –> UNIFORM manner
In the hypothalamus –> arranged in clusters (nuclei) that are arranged in a PATCHWORK manner
Why do we need to know how the hypothalamus develops?
Important if we are to understand hypothalamus FUNCTIONS
Why do we know so little about the development of the hypothalamus?
1) Doesn’t follow the same principles as in other regions (neurons not in columns/layers)
2) So tiny - VERY HARD to know where it develops from in the neural tube and HOW it develops
What are the first clues as to how the hypothalamus develops?
Shh studies in the MOUSE:
- DELETION of Shh –> failure of hypothalamic development
What are the symptoms of failed hypothalamic development? (3)
1) Cyclopia - sinfle eye
2) Holoprocencephaly - no bilateralisation of the forebrain
3) Head - abnormal
How is the structure of the brain altered in shh-/-?
Normally: ventral midline separates the brain into 2 halves
Shh-/-:
ABSENCE of the ventral midline –> no separation of the eye field
Eye fields and lateral mesenchyme fail to resolve bilaterally
How do we know that the shh pathway is conserved in cyclopia/holoprocencephaly?
What did this finding focus attention on for the development of the hypothalamus? Why?
In 2000:
- Genetic mutations in shh or shh pathway found to underlie cyclopia/holoprecencephaly (same phenotype as KO studies in the mouse)
Focused attention on:
The prechordal mesoderm (PM) - this is the part of the axial mesoderm that expresses Shh and underlies the anterior neural tube
Why need to determine the first tissue where shh is expressed?
Probably important in ventral forebrain development
What is the first tissue where shh is expressed?
Tissue that lies underneath the neural tube and is ventral in character
Rod like group of axial mesoderm with the notochord posteriorly and a fan like group of cells anteriorly (PM)
When does the PM express shh?
Soon after the neural plate has become the neural tube
Where does the PM lie?
Underneath the anterior ventral midline region of the neural tube
What is the tissue most likely to be inducing ventral midline of the forebrain and separating the eye fields? Why?
The PM - expresses shh early on in development and lies anteriorly and ventrally underneath the neural tube
What experiments showed that shh deriving from the PM (fan shaped) induces a fan-shaped set of cells in the notochord above it?
2000-2005
1) Surgically ABLATE the PM –> no shh
2) GRAFT in extra PM –> ectopic shh
3) Culture PM and NIAVE notochord –> induce shh
4) Same as 3 but PRE-TREAT with shh blocker –> prevent effects
5) Conditional KNOCK-DOWN of shh in the PM in vivo
What can be seen if cut a midline section through the embyro?
Shh expressing PM site underneath the cells induced to express shh
At what levels can GOF and LOF studies occur?
1) Tissue level (graft in/ablate)
2) Genetic level (KO/over-express)
3) Protein level
What was the first theory of the pattern of hypothalamic development?
- Formation of arcs of morphogen gradient released from the end of the PM around the end of the the floor plate (PM doesn’t go all the way to the end of the neural tube)
- Formation of ARCS of progenitors around the end of the FP (see different concentrations of the morphogen
What is seen in the chick 0.5 days?
See such patterns as explained by the ‘arc theory’:
- FP marker (Foxa2) - high levels shh
- nkx 2.2 - intermediate levels of shh
- pax6 - gene repressed by shh (low/no shh)
What marker is in the FP (high shh levels)?
Foxa2
What marker in response to intermediate shh levels?
nkx2.2
What is repressed by shh (marker of no shh)?
Pax6
Why did the ‘arc model’ fall apart?
Shortly after 0.5 days in the chick - progenitor domains are rapidly obscured (no neat arcs)
Antisense ISH of KNOWN progenitors –> chaotic clustering around the edge of the floor plate basal plate territory
That could NOT be explained by a simple shh morphogen gradient/arcs of progenitors
What does the PM initially express?
What happens to the PM after shh ventral midline cells are induced in the forebrain?
What does this cause?
Initially expresses shh
PM rapidly changes signalling properties –> up regulates BMPs
Causes:
1) BMPS act on shh+ cells that have just been induced - causing them to UPREGULATE pSmad157 and BMP
2) BMP from the PM and the ventral midline cells act on THEMSELVES to upregulate the expression of Fgf10 and components that react to Fgf10 signalling
When are shh ventral midline cells induced in the forebrain of the chick?
0.5 days
What is pSmad157?
A TRANSDUCER of BMP signalling
What does Fgf cause?
What does this explain?
The PROLIFERATION of cells
Explains the difference in proliferation between the forebrain and the spinal cord (not proliferative - glial-like population)
When does this BMP and Fgf10 up regulation occur in the fan shaped region above the PM in the chick?
1.5 dpf
When does this BMP and Fgf10 up regulation occur in the fan shaped region above the PM in the human?
26-32 dpf
Describe the differential expression of fgf10 and shh in the ventral midline
Shh is expressed in the ventral midline all throughout the axis (in forebrain and neural tube)
Only the cells in the FOREBRAIN (fan shaped region above the PM) express fgf10 (these cells express shh AND Ffg10)
What does double ISH of the fan shaped cell population above the PM show?
Why is this unusual?
Co-expression of:
- Shh
- BMP7
- Fgf
- Components of these signalling pathways
Unusual because Shh and BMP7 normally REPRESS each others transcription - likely to have very different properties in the brain
What does fate-mapping studies of the Fgf10+ cells show?
Describe this experiment
What does this tell us?
Population grows and gives rise to an extensive part of the ventral hypothalamus
Experiment:
- Inject dye underneath the developing chick embryo
- Shine light onto the egg
- Slit along dorsal midline
- Can see PM due to appearance
- Target neural tissue the lies above the PM with a DYE
- Close egg and develop embyro in the egg until distinct forebrain regions identified
- See the dye FILLS the WHOLE part of the VENTRAL FOREBRAIN
Tells us:
- Cells above the PM –> give rise to the VENTRAL FOREBRAIN
How does the region of cells above the PM give rise to the ventral forebrain? (model)
Sequential anisotropic growth model:
Anterior and mammillary regions grow sequentially in opposite directions (tangentially) from the Fgf10+ pre hypothalamus cells
Whilst MAINTAINING a CONSTANT population of Fgf10+ expressing cells
1) As the Fgf10+ cells (above PM - in the middle of the anterior and mammillary regions) proliferate:
- Give rise to 2 daughters
- 1 daughter displaced ANTEIRORLY and DOWNREGULATES Fgf10 (becomes a different sort of progenitor)
- Happens over and over again
–> Get anterior hypothalamus forming
2) Then, a SWITCH and the same thing happens
- Daughter cells are displaced POSTERIORLY
–> Give rise to the posterior (mammillary) hypothalamus
What is the early patterning of shh obscured by?
The growth of cells from the Fgf10+ progenitor cells
Where is the PVN likely to develop from?
Pax6 cells that were REPRESSED at very early stages
Where are the the Arc, VMN and LH likely to develop from?
From Fgf10+ progenitors (overlapping shh expression)
Where are the ME/post pituitary likely to develop from?
The Fgf10+ cells
So, what do the Fgf10+ cells give rise to?
Anterior and mammillary progenitors that give rise to neurons that regulate key aspects of homeostasis:
SCN: circadian cycle
Arc: eating, reproduction
TMN/LH: sleep-wake
What is the hypothesis about the Fgf10+ cell population? Why?
Is this hypothesis proved?
Harbours a HYPOTHALMIC STEM CELL - through development, maintain a CONSTANT number of Fgf10+ cells
- Stem cell: cell that can SELF-RENEW to get rise to more of itself at the SAME TIME as giving rise to many other cell types
Not yet proved
