Lecture 8: vertebrate early development Flashcards
Explain the process after sperm entry and the role of maternal genes in the Xenopus oocyte and the formation of the D-V axis. Draw diagram.
The unfertilised xenopus egg has a animal hemisphere (top) where the sperm can enter, and a vegetal hemisphere (bottom)
Sperm entry triggers a 30-degree cortical rotation so that the positive ends of sub-cortical microtubules end up facing away from the entry point, redistributing and activating maternal dorsalising factors on the opposite side of the embryo, including members of the Wnt signalling pathway, leading to an increase in B-catenin dorsally.
One of the cytoplasmic determinants in the vegetal layer of an unfertilised egg is an mRNA that codes a TF VegT.
Activated B-catenin, as a result of Wnt signalling accumulates in cells on the dorsal side of the embryo causing transcription of Wnt target pathway genes. This leads to the formation of the Nieuwkoop centre which then produces signals to induce the organiser.
One target of B-catenin is Xnr (xenopus nodal related) genes. The dorsal side expresses more Xnr genes => gradient of Xnr activity from dorsal to ventral side (higher on dorsal side where the Nieuwkwoop centre is). .
What is the Nieuwkoop centre? Draw diagram.
The Nieuwkoop centre is an area of the early Xenopus embryo located opposite the point of sperm entry. It contains both VegT and B-catenin which are both TFs for Xnr genes => it is an area rich in Xnr signalling leaving a gradient of Xnr gene transcription across the vegetal tissue.
The Nieukwoop centre induces the Organiser in the mesoderm through high levels of SMAD2 as a result of Xnr signalling.
Explain how the three germ layers form.
- Unfertilised egg vegetal layer contains cytoplasmic determinant mRNA that encodes VegT TFs which are encoded after fertilization.
- VegT TF transcribe Xnr genes which encode nodal.
- Nodal is a morphogen, secreted from the vegetal pole towards the equatorial cells.
- Cells closest to nodal source become pharyngeal endoderm
- Cells at intermediate distance become mesoderm
- Cells furthest become ectoderm (fate isn’t changed because too far away => default domain)
Top = ectoderm = embryonic tissue that gives rise to skin and NS. Comes from the vegetal hemisphere.
Middle = mesoderm = muscle blood bone kidney etc. Comes from animal hemisphere in the marginal zone (equatorial cells).
Bottom = endoderm = gut and associated organs, and lungs. Comes from the vegetal hemisphere
Draw a diagram of the blastula fate map.
Explain organiser induction and its fate in gastrulation.
High levels of SMAD2 induced by high levels of Xnr and beta-catenin activity, create the Spemann-mangold organiser visible as the dorsal lip
Defining features of Spemann-organiser produced by signalling from the Nieuwkoop centre:
- specifically activated by B-catenin which positively activates Siamois
- High levels of Xnr proteins lead to high levels of SMAD2 protein activation.
The two TF collaborate to activate a set of genes that pattern the antero-posterior and the dorso-ventral axes as they begin to get built in gastrulation.
Goosecoid is a TF that regulates cell behaviors that the organiser cells perform as it does its job of leading gastrulation by creating dorsal axial mesoderm with Brachyury targets.
During gastrulation, the tissue at the dorsal lip undergoes convergent extension – the cells extend forward into the interior of the embryo, leading to a long lip of dorsal mesoderm.
At the same time cells at different positions acquire distinct cell fates creating and patterning the axis.
The Organizer is a dynamic mix of axial mesodermal progenitor cells that gives rise to three distinct embryonic tissues:
- pharyngeal endoderm
- prechordal mesoderm
- notochord
and determines the proportions of paraxial, intermediate, lateral plate and ventral mesoderm in the rest of the mesoderm.
Organizer cells undergo convergent extension, involving Involution, intercalation and migration of axial mesoderm as an extending rod of tissue across the blastocoel roof, thus defining the future dorsal midline and the Anterior-Posterior Axis of the embryo.
Outline what gastrulation achieves in the early vertebrate embryo.
- Germ layer formation and positioning
- Development of A-P and D-V axes
- Establishment of body plan, midline signalling centres, neural induction and mesoderm dorsalisation
