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Flashcards in Lecture 5 Deck (24):

Why is the african claw toed-frog used as a model organism for developmental biologists?

The eggs it produces are relatively large and are polarised with a pigmented animal hemisphere and a yolky vegetable hemisphere
They are easy to obtain after injection with chorionic gonadotropin
There is high number of eggs produced per treatment
Easy to fertilize artificially


How does gastrulation proceed in the xenopus laevis?

Prospective mesodermal cells on the dorsal side undergo a change in cell shape to become bottle cells forming a groove and the bastopore
Cells of the marginal zone and future endoderm then involute rolling under the blastopore s a coherent sheet
The cells then converge into the midline and extend along the antero-posterior axis beneath the dorsal ectoderm and there is convergent extension of the mesoderm


What occurs during the convergent extension of the mesoderm in gastrulation in the frog?

Initially the presumptive mesoderm formed an equatorial ring however during gastrulation it converges and extends along the antero-posterior axis
At the same time the ectoderm spreads downward to cover the whole embryo and the archenteron forms with loss of the blastocoel


What process succeeds gastrulation in xenopus lavis?

Neurolation which is formation of the neural tube marking the beginning of organogenesis


How does neurolation proceed in xenopus lavis?

Neural folds form at the edges of the neural plate
These folds then rise and fuse forming the neural tube which then sinks below the epidermis
Neural crest cells then detach from the tube and migrate to form various structures


What is the neural plate?

A thickening of the ectoderm overlying the notochord


How are the body axis established in xenopus lavis?

The animal vegetable axis is maternally derived with the egg being polarized into two hemispheres
While unfertilised there is radial symmetry around this axis, but it is broken upon fertilization


Where can sperm entry occur in the xenopus lavis egg?

Sperm entry can only occur in the animal hemisphere


What occurs upon fertilization of a xenopus lavis egg?

After sperm entry the cortex rotates about 30 degrees over the underlying cytoplasm to establish the dorsal crescent


What is the dorsal crescent in the fertilized xenopus lavis egg?

A poorly pigemented zone on the dorsal side away from the site of sperm entry


What is the importance of the cortical rotation triggered by sperm entry in the egg of xenopus lavis?

It polarizes sub-cotrical microtubules to have there positive ends directed away from the site of sperm entry
This allows the aligned microtubules to act as tracks for the directed movement of maternal proteins and mRNAs


What maternal factors are moved after sperm entry in the fertilized egg of xenopus lavis?

XWnt-11 mRNA and dishevelled protein relocate from the vegetable pole to the site opposite sperm entry establishing the nieuwkoop centre


What is the importance of establishing the nieuwkoop centre in xenopus lavis?

It specifies the dorsal side side allowing the dorsal ventral axis to be established


What is the cannoncal wnt/beta catenin signalling pathway in the frog?

Wnt binds to its receptor frizzled causing dishevelled to be activated
Dishevelled inhibits the axin/GSK-3/APC complex allowing beta catenin to reach the nucleus
Beta catenin will interact with the TCF/LEF Hign mobility group domain of transcriptional regulators
In the absence of Wnt GSK will phosphorylate beta-catenin marking it for degradation preventing it from influencing transcription


What is the spemanns organizer?

A centre that arises in the dorsal lip of the blastopore, dorsal to the nieuwkoop centre in the future dorsal mesoderm


What is the function of the spemmans organizer?

It is essential of further development of the antero-posterior axis and dorsal ventral axis and induction of the nervous system from ectoderm


When does determination of the fate of cells occur in the xenopus embryo?

Fate maps can be drawn at the blastula stage however experiments with transplantation show that the cells are not yet determined at this stage


What specifies the endo and ectoderm in xenopus lavis?

Maternal factors probably Veg T for endoderm and Ectodermin for ectoderm


What specifies the mesoderm in xenopus lavis?

It is induced from ectoderm by signals from the vegetal region


Why does there at first appear to be no source for mesoderm in the development of xenopus?

Explants from the animal pole form balls of epidermal cells and explants from the vegetal pole form endodermal structures leaving no apparent mesoderm


How is mesoderm inducted in the xenopus lavis?

The vegetal region secretes a signal that causes some of the animal region cells to become mesoderm


What is the signal that the vegetal region of xenopus’ secretes to induce the formation of mesoderm?

VegT mRNA becomes trapped in the vegetal hemisphere and the protein it produces causes transcription of xenopus-nodal-related genes which initiate mesoderm formation


What is the dorsalizing signal in xenopus?

During development the notochord becomes the most dorsal while the blood becomes the most ventral
Beta catenin accumulates on the future dorsal side acting as a transcription factor to activate xenopus nodal-related genes in a dorsal-ventral gradient


How does spemann’s organizer play a role in neural induction?

It inhibits bone morphogenetic protein signalling by BMP antagonists such as noggin and chordin which it secretes
This absence of bone morphogenetic protein causes ectoderm to become neural tissue