Lecture 2

Question 1

Which cells in the ventral midline is sonic hedgehog expressed in during development

Answer 1

Axial mesoderm and floor plate

Question 2

What is the effects of the gradient of sonic hedgehog morphogen in the formation of the neural tube

Answer 2

The gradient of the morphogen Shh results in distinct progenitor cell fields along the dorso-ventral axis that act as a readout of the level of sonic hedgehog signal they received. These will ultimately differentiate into defined neurons based on levels of sonic hedgehog signalling

Question 3

Explain the observation that sonic hedgehog expression tends to initially spread laterally of the midline prior to neural tube patterning

Answer 3

The spread of sonic hedgehog expression is seen and increase in the size of the floor plate either side to the initial cells induced by the axial mesoderm and sonic hedgehog upregulating its own expression

Question 4

The developing ventral brain is underlain by the axial mesoderm T or F

Answer 4

F – it is only underlain by the PM and not the notochord

Question 5

Why is it that by the time regions of the forebrain have become distinguishable the prechordal mesoderm doesn’t appear visible

Answer 5

By the time different regions of the forebrain are obvious the prechordal mesoderm has undergone EMT-like transitions and its cells have dispersed and migrated away

Question 6

Describe the experiments that lead to the identification that the cells that lie above the prechordal mesoderm give rise to the hypothalamus

Answer 6

Lineage tracing experiments where a tracer known as Dil was injected into the cells of the PM. The chick embryo was then able to develop until distinct forebrain regions were observable. Here the dye had become localised to a region that was the prospective hypothalamus

Question 7

How was it that the markers involved in determining the hypothalamus were identified

Answer 7

The expression of known signals and transcription factors was investigated to determine which ones correlated with the underlying prechordal mesoderm

Question 8

Which factors other than FGF10 were found to be expressed by all hypothalamic precursor cells

Answer 8

BMP7 and the transcription factors Tbx2 and Nkx2.1

Question 9

What is the relation between these factors found to be expressed in hypothalamic precursors

Answer 9

pSmad1/5/8 indicates the activity of BMP signalling and Tbx2 and Nkx2.1 are targets of pSmads1/5/8

Question 10

What is seen as a result of physical ablation of the prechordal mesoderm in the ventral forebrain

Answer 10

A failure of induction of hypothalamic markers

Question 11

What can be said about the role of the prechordal mesoderm in triggering the expression of signalling molecules in the ventral brain

Answer 11

A signal from the prechordal mesoderm induces the expression of BMP7 by the prospective hypothalamic cells

Question 12

What is the main difference between the development of the ventral forebrain and the spinal cord in terms of the expression of various markers

Answer 12

The main difference is that in the ventral forebrain signals from the prechordal mesoderm induces the expression of BMPs from those ventral cells. In contrast in the spinal cord BMP expression is localised to the dorsal neural tube.

Question 13

Sonic hedgehog floor plate cells in the ventral forebrain remain in that state for the entirety of embryogenesis T or F

Answer 13

F – this is another difference to the same cells in the spinal cord

Question 14

What structure is the prechordal mesoderm responsible for inducing

Answer 14

Hypothalamus

Question 15

Once it was determined that the prechordal mesoderm is necessary for the induction of the hypothalamus describe a way in which you could test its sufficiency in determining hypothalamic progenitors

Answer 15

Culturing a piece of neural tissue alone or with added prechordal mesoderm to see if the prechordal mesoderm can induce this tissue to become hypothalamic

Question 16

The prechordal mesoderm is sufficient to induce the expression of hypothalamic markers T or F

Answer 16

T

Question 17

What is the significance of invertebrates not possessing hypothalamic progenitors

Answer 17

This indicates that a key point in evolution was the emergence of the axial mesoderm

Question 18

The hypothalamus maintains homeostasis integrates autonomic response and endocrine function with behaviour. Explain its direct and indirect actions

Answer 18

Direct regulation of the pituitary gland(physical homeostasis: temperature sleep metabolism stress reproduction). Indirectly through interactions with other components of limbic system (mental homeostasis mood motivation)

Question 19

How was it determined that signals from the prechordal mesoderm were responsible for inducing the hypothalamic progenitors

Answer 19

Factors were looked that were expressed specifically at the right time

Question 20

What markers could you look for to verify that you had prechordal mesoderm tissue

Answer 20

Stain for goosecoid whose expression remains in the prechordal mesoderm

Question 21

What other factors are expressed in and play a part in patterning the prechordal mesoderm hypothalamus and ventral forebrain

Answer 21

Sonic hedgehog nodal (and BMP7)

Question 22

What is the specific phenotype seen in a sonic hedgehog knockout that indicates its role in the development of the ventral forebrain and hypothalamus and why is this

Answer 22

Knockout of sonic hedgehog leads to a failure of the hypothalamus to develop. Shh is in fact required to maintain the prechordal mesoderm cells