Lecture 2

Question 1

What are the 2 fates of Ectodermal Cells & when do they differentiate:

Answer 1

Neural stem cells
Epidermal cells

Week 4.

Question 2

Pathway of Signal Transduction behind Ectodermal differentiation

Answer 2

Receptor-Ligand binding between ectodermal cell & signal

Signal transduction: via secondary messengers

Cellular Response –> TF enters nucleus

Changes in gene expression.

Question 3

Neural Induction: BMP signalling Pathway is key.

Answer 3

• Bone Morphogenetic Proteins

BMP bind to their receptors (BMPR)
This triggers the signal transduction cascade

‘Smad’ proteins in the cytoplasm are phosphorylated - allowing them to act as transduction factors when entering the nucleus…
which alter the gene expression

Chordin & Noggin then mop up the remaining BMP outside the cell: they’re BMP antagonists.

Question 4

(Organism used to study Neural Induction)

Answer 4

(Amphibians)

Question 5

Organiser Cells

Answer 5

Develop dorsally in the Mesoderm

Neurally induce the Ectoderm cells above them (dorsal ectoderm).

Question 6

Organiser cells Function:

Answer 6

Secrete antagonists of BMP pathway:
Chordin, Noggin, BMP.

These ectodermal cells with a blocked BMP pathway alter their transcriptional profile…
and start transcribing neural stem cell genes.

Question 7

Why Do Inhibitory Molecules elicit a positive transcriptional response

Question 8

Molecular Pathway of Neural Development:

Answer 8

BMP inhibition prevents Smad proteins being phosphorylated

Triggers expression of Sox transcription factors

Sox TF’s induce neuronal fate.

Vs

BMP molecules present –> Smad proteins phosphorylated –> epidermal TF’s expressed.

Question 9

Mesodermal Patterning

Answer 9

Nodal Gradient:
High levels dorsally = organiser cells
Low levels ventrally (leads to muscles, gonads etc).

(Organiser cell signals also inhibit BMP action in ectoderm triggering neural cell fate).

Question 10

Xenopus Gastrula Fate Map

Answer 10

Dyes used to show the variation in cell fate.
(shows ectodermal & mesodermal gradients)

Question 11

Mangold ‘organiser graft’ experiment in Newts

Answer 11

• removed organiser cells from one newt embryo
• ectopically transplanted into ventral mesoderm of second newt
• the recipient newt now has 2 sets of organiser cells= a gain of function study.

Question 12

2 main Conclusions of Newt organiser graft experiment:

Answer 12

1) Formation of 2 neural tubes:
induces a secondary neural axis… had the normal neural cells but also a secondary set derived from the host’s transplanted organiser cells.

2) Part of the mesoderm was donor-derived:
shows organiser cells triggered self-differentiation

(represents the impact of ectopically transplanting these organiser cells in the ventral mesoderm).

Question 13

Identifying how BMP antagonists were responsible for Neural cell Fate.

Answer 13

Extracted all mRNA for organiser cells, reverse transcribed the eDNA…

Each was then ectopically introduced to identify the specific gene/protein responsible for inducing neural fate
(found to all be BMP antagonists/inhibitors.)

Question 14

Neural by Default model?

Answer 14

A model with BMP removed –> leads to automatic neural identify.

Question 15

Identifying this process (first step is to look)

Answer 15

In situ Hybridisation to characterise mRNA & protein signatures…

Chordin dorsally expressed in mesoderm

Expression of Sox2 cells is visible
Overtime can see the shape of cells changing to resemble more of the nervous system (especially post-gastrulation).

(As its a top-down view its difficult to see the dorsal-ventral patterning of Sox2… will still be dorsal though & within the neural plate.)

Question 16

Other Names for Organiser cells

Answer 16

The Node
Hensen’s node
the Shield

(not too important)