Lecture 2 Flashcards
What are the 2 fates of Ectodermal Cells & when do they differentiate:
Neural stem cells
Epidermal cells
Week 4.
Pathway of Signal Transduction behind Ectodermal differentiation
Receptor-Ligand binding between ectodermal cell & signal
Signal transduction: via secondary messengers
Cellular Response –> TF enters nucleus
Changes in gene expression.
Neural Induction: BMP signalling Pathway is key.
- 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.
(Organism used to study Neural Induction)
(Amphibians)
Organiser Cells
Develop dorsally in the Mesoderm
Neurally induce the Ectoderm cells above them (dorsal ectoderm).
Organiser cells Function:
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.
Why Do Inhibitory Molecules elicit a positive transcriptional response
Molecular Pathway of Neural Development:
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.
Mesodermal Patterning
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).
Xenopus Gastrula Fate Map
Dyes used to show the variation in cell fate.
(shows ectodermal & mesodermal gradients)
Mangold ‘organiser graft’ experiment in Newts
- 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.
2 main Conclusions of Newt organiser graft experiment:
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).
Identifying how BMP antagonists were responsible for Neural cell Fate.
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.)
Neural by Default model?
A model with BMP removed –> leads to automatic neural identify.
Identifying this process (first step is to look)
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.)