Frogs Flashcards
Which frog species are model organisms for development?
Rana pipens and Xenopus
Why are frogs good model organisms?
Large broods, rapid development and good for transplantation studies
What types of studies are frogs not good for?
Genetic studies, polyploidy is a problem
What type of yolk distribution is found in frog eggs?
Mesolecithal, vast majority is in the vegetal pole
Where on the egg does fertilization occur?
Animal pole. The sperm can’t get through the yolk
What two things come from the sperm?
Nucleus and centrioles that set the first plane of division
What is a cortical rotation?
The cytoplasm next to the membrane spins within the egg. The spin is caused by microtubules from the centrioles
What part of the egg forms from the cortical rotation?
Grey crescent
What happens in the first cleavage?
Meridional, bisects grey crescent
What happens in the second cleavage?
Meridional, perpendicular to the first cleavage
What happens in the third cleavage?
Equatorial, displaced due to the yolk
What type of cleavage is seen in frogs?
Holoblastic, radial, unequal
Why is unequal cleavage seen in frogs?
The vegetal pole cells divide more slowly than the animal pole cells because the yolk resists cytokinesis
What happens in the 12th division?
Mid-blastula transition
What is the mid-blastula transition?
The transition from maternal signals to the zygotic genome
What changes in the embryo after the mid-blastula transition?
Transcription and translation get increased and the cell cycle is slowed down. Each cell division takes longer
Why is the blastocoel displaced in a frog blastula?
It can’t form in the vegetal pole because of the yolk and slow division, so it can only form in the animal pole
What is the fate of the epidermal ectoderm?
Skin
What is the fate of the neural plate ectoderm?
Brain and nervous tissue
What is the fate of the axial mesoderm?
Notochord
What is the fate of the paraxial mesoderm?
Somites
What are somites?
Regular mesoderm divisions on either side of the developing nervous system and notochord
What is the fate of lateral plate mesoderm?
Liver, spleen, body cavity
What is the fate of the endoderm?
Gut and lungs
What 5 morphogenetic events happen during gastrulation in frogs?
Epiboly, vegetal rotation, invagination, involution, intercalation
Which cells will undergo the epiboly during gastrulation?
Animal pole cells
What happens during the epiboly in gastrulation?
The animal pole cell layers intercalate and become narrower, which forces the cells to expand down over the vegetal pole
What happens during the vegetal rotation in gastrulation?
The vegetal cells are forced upwards and starting buckling in the blastocoel
Where is the invagination during gastrulation occurring?
Blastopore
What structure results from the invagination of the blastopore cells?
The dorsal blastopore lip
Which two structures are formed by the involution during gastrulation?
Archenteron and yolk plug
How is the archenteron created?
Involution of the cells over the blastopore lips causes the new cavity to be formed and spreads endoderm and mesoderm cells through the animal pole
What happens to the blastocoel during gastrulation?
It gets displaced by the archenteron and eventually replaced. The fluid inside squeezes in between the loosely packed cells between the blastocoel and archenteron
What drives the involution of cells and lets them recognize and follow a path?
Integrin-fibronectin communication
How do involuting cells use fibronectin to recognize and follow a path?
The lead cells reach out with lamellipodia and contact the fibronectin, then follow that signal while dragging other cells behind them
What are lamellipodia?
Sheet-like extension of plasma membrane for communication
Which cells undergo intercalation during gastrulation?
Mesoderm
What are the 3 mechanisms that drive the intercalation during gastrulation?
Polarized cell adhesion and the recognition of fibronectin, differential expression of protocadherins in the axial and paraxial mesoderm, actin contraction caused by calcium
Why is the grey crescent important?
Contains a maternal signal that specifies the organizer
Why does transplanting cells from the dorsal blastopore lip onto another embryo create a new body axis?
They signal and organize the surrounding cells to become specified
What did Barth and Holtfreter observe when they cultured amphibian tissue on glass?
It differentiated into ectodermal structures even though no inducing tissues were present
What did de Robertis show was occurring when amphibian tissue cultured on glass differentiated?
The glass was stimulating the MAPK/Erk pathway, which inhibits Smad1
How do the findings of de Robertis work with the idea of a chemical inducing agent?
The glass is stopping the signal that is normally there in the embryo, so the pathway inhibits Smad1 and causes differentiation into brain instead of skin
Which signal needs to be blocked in order for cells to become neural tissue?
BMP
Where is goosecoid expressed?
Brain cells
Where is brachyury expressed?
Notochord
What class of signals do chordin, follistatin, and noggin belong to?
Neural inducer genes and BMP inhibitors
What does chordin do?
Establishes the neural chord and competes with BMP for its receptor
What does noggin do?
Establishes the placement of the brain and competes with BMP for its receptor
Which side of the embryo expresses BMP?
Ventral
Which side of the embryo expresses chordin, follistatin and noggin?
Dorsal
How does BMP determine the fate of ectoderm cells?
An absence of BMP leads to specification of neural ectoderm, its presence leads to specification of epidermal ectoderm
What is the organizer in frogs?
Dorsal blastopore lip
What is the Nieuwkoop centre?
A set of maternal signals in the dorsal side of the vegetal cells of the embryo that specifies overlying cells to become the organizer
What are VegT and Vg1? Are they maternal or zygotic signals?
Maternal transcription factors that activate TGF-beta family paracrine signals by working through the Smad2 pathway
Where is beta-catenin in the vegetal cells of the embryo?
Dorsal side
How does the cortical rotation from fertilization affect the placement of beta-catenin?
The rotation carries maternal signals to the dorsal side of the vegetal cells. The signals brought include Wnt II, dishevelled, and GBP which inhibit GSK-3 and prevent beta-catenin breakdown
What is Tcf3?
A transcription factor that is suppressing transcription of siamois and twin
What happens to Tcf3 when beta-catenin is present?
Beta catenin binds to Tcf3 and it becomes an activator instead of a suppressor
What do siamois and twin do?
Activate transcription of neural inducer and organizer genes by working with Smad2 (i.e. noggin, chordin, goosecoid)
How are Nodal concentrations related to VegT and Vg1?
High VegT and Vg1 concentrations means low Nodal
How are Nodal concentrations related to beta-catenin?
High beta-catenin means high Nodal
High beta-catenin and high Nodal specifies what?
The organizer
High VegT and Vg1 and low Nodal specifies what?
Ventral mesoderm
What are the 4 functions of the organizer?
- self-differentiation of dorsal mesoderm
- dorsalizes surrounding mesoderm into paraxial mesoderm
- dorsalizes ectoderm into the neural tube
- initiates gastrulation
