Development of the hypothalamus Flashcards by Karla Littleford

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

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How are the layers of neurons developed in the spinal cord?

Motor neurons differentiate in a STEREOTYPED pattern along the DV axis

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How are the layers of neurons developed in the cortex and retina?

Neurons are born in a specific ORDER –> layers

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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

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Why do we need to know how the hypothalamus develops?

Important if we are to understand hypothalamus FUNCTIONS

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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

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What are the first clues as to how the hypothalamus develops?

Shh studies in the MOUSE:

- DELETION of Shh –> failure of hypothalamic development

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What are the symptoms of failed hypothalamic development? (3)

1) Cyclopia - sinfle eye
2) Holoprocencephaly - no bilateralisation of the forebrain
3) Head - abnormal

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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

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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

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Why need to determine the first tissue where shh is expressed?

Probably important in ventral forebrain development

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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)

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When does the PM express shh?

Soon after the neural plate has become the neural tube

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Where does the PM lie?

Underneath the anterior ventral midline region of the neural tube

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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

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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

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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