germ layer induction

Question 1

why are some cytoplasmic determinants specifically found in the vegetal hemisphere in mammals?

Answer 1

interaction with placenta

Question 2

describe process of germ layer induction in xenopus

Answer 2

1. cytoplasmic determinants are localised at bottom of egg
2. 3rd division separates top 4 cells from bottom 4 cells. top 4 cells = animal cells. bottom 4 cells = vegetal cells.
3. cytoplasmic determinants in vegetal cells transcriptionally activate VgT
4. VgT autonomously activates transcription of Nodal
5. Nodal is secreted from vegetal cells and diffuses to animal cells = morphogen
6. cells receiving highest levels of Nodal = endoderm
   cells receiving intermediate levels of Nodal = mesoderm
   cells receiving lowest/no levels of Nodal = ectoderm
7. activation of nodal signalling pathway causes change in transcriptional profile of cell and change in cell behaviour

Question 3

how is the organiser induced?

Answer 3

1. dorsalising factors are located at bottom of vegetal hemisphere of egg
2. sperm penetrates egg
3. 30 degree cortical rotation causes dorsalising factors to shift to one side of the embryo = future dorsal side
4. wnt signalling pathway activated in future dorsal side. wnt target genes activated. e.g. β-catenin upregulates nodal
5. high levels of nodal signalling through VgT and β-catenin AND high levels of wnt signalling induces organiser + turns on goosecoid + siamois
6. cells in the organiser undergo convergent extension to form a rod of axial mesoderm

Question 4

which gene is expressed in mesoderm in response to low levels of Nodal?

Answer 4

brachyury

Question 5

what genes are expressed in mesoderm in response to high levels of Nodal and Wnt signalling

Answer 5

goosecoid
chordin
xnot
xlim1