Germ layer formation and gastrulation
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Why is the oocyte polarised even before fertilisation?
Due to gravity some cytoplasmic factors are found exclusively in the vegetal hemisphere
What leads to blastocoel formation?
Changes in osmolarity- results in an influx of water
Where are Vg1+ expressing cells and Vg1- expressing cells found?
Vg1+ found below the blastocoel Vg1- found above and by the sides of the blastocoel
What do Vg1 cells express?
VgT- a transcription factor
What does VgT bind to?
Promoter of the morphogen Nodal
Which levels of Nodal are found in which germ layer?
Highest level of nodal= Endoderm Mid level of nodal= Mesoderm No nodal= Ectoderm
What does the differing levels of Nodal explain?
• Why the vegetal hemisphere is so important- induces mesoderm and endoderm- otherwise the animal hemisphere would just be ectoderm • How the germ layers form
What happens when sperm fertilises an egg?
A rotation of the cortex of the zygote occurs- by 30 degrees- dorsalising factors in the cortex moved to the dorsal side of the zygote
What produces the ‘Nieukwoop centre?’ And what does this overlap cause?
An overlap of nodal (TGF- beta) and beta catenin- in the dorsal region of the vegetal hemisphere Overlap of these 2 regions causes ‘higher nodal activity’
Where is the Wnt signalling pathway activated?
The dorsal side of the embryo- marking the site where gastrulation will begin
What happens in regions of higher nodal activity and lower nodal activity?
Higher nodal activity- Organiser induced Lower nodal activity- Ventral mesoderm induced
What T. factors do organiser cells express?
Goosecoid Siamois
When is goosecoid expressed?
When both a beta catenin and TBF-beta downstream effector are bound
What do organiser cells differentiate into?
Axial mesoderm
Different levels of which genes cause the different types of axial mesoderm- notochord and prechordal mesoderm?
Siamois and goosecoid
What are the main roles of the organiser?
• Induce neural fate 2. Dorsalise the mesoderm