Body plan (part 2) Flashcards
what is the life cycle of the xenopus, mouse and zebrafish to reach sexual maturity
xenopus = 1 year
zebrafish = 2-3 months
mouse = 2.5-3 months
Describe the xenopus egg before fertilisation
radial symmetry on all sides. darker patch on the top/animal pole as an environmental adaptation to hide from predation
describe what the animal and vegetal poles are on the xenopus
animal pole = upper part of the egg, where cytoplasm is most concentrated and where sperm enters in fertilisation
vegetal pole = bottom end of the egg, where the yolk is - contains nutritional value for development
Describe how the xenopus egg undergoes rapid cell division
takes around 5 hours and involves no active cell movement yet. cell undergoes S-M cell cycles with no G1/G2 meaning that DNA is replicated and the cell undergoes mitosis immediately
After the xenopus cell has rapidly undergone cell division, describe the stage that follows
gastrulation - mesoderm is induced. germ layers are moved so that the ectoderm is on the outside, endoderm inside and mesoderm between. this is a stage where cells can actively begin moving. notochord is formed from the dorsal mesoderm
How does the cycle of zebrafish differ to xenopus
cell division occurs on top of the large yolk cell. gastrulation is extremely similar to xenopus
when is the first asymmetry in mouse development established
at 6 days, when gastrulation begins. the asymmetry is the primitive streak that forms at the future posterior end
What events result in the mouse embryo having specialised organ primordia
late gastrulation and embryo turning
what is the phylotypic stage
when main properties of the phylum are apparent - notochord, tubular dorsal CNS, somites. stage where there are the greatest degree of similarities between the classes
what are the features of external development
numerous offspring with yolk, maternal contribution via the egg, rapid development before growth (swim away, muscles before cardiovascular system, autonomous feeding develops later after the GI tract develops)
what are the features of internal development
few offspring due to limited maternal food/gas supply. different growth and development rates to external -> cardiovascular system before muscle, metabolite needs in order for growth mean no need for functional muscle early on
what structure lies as a pair either side of the notochord
somites
How is the D/V axis established at fertilisation in xenopus
sperm entry point is on the future ventral side and breaks radial symmetry. entry of sperm triggers calcium signalling, cortical rotation occurs meaning that microtubules are moved so that dorsal determinants (Wnt signalling components) are moved to the dorsal side
How does the Wnt signalling pathway establish the dorsal side
stabilisation and nuclear translocation of b-catenin which activates target genes with the DNA binding protein TCF
How does UV irradiation of the vegetal pole prove the role of the Wnt pathway
results in ventralised embryo as microtubules are disrupted and pathway is blocked. by ‘tipping’ the cell, a normal embryo is formed, showing that UV misplaces dorsal factors and that the role of cortical rotation is to move Wnt components
What can activating the Wnt pathway too early in development cause
multiple heads and no tail/trunk structures. anteriorisation = more anterior characters formed at the expense of the posterior
How does the role of Wnt signalling change before and after gastrulation
before = establishes dorsal side of D/V axis
after = establishes posterior side of A/P axis
How can D/V axis development in zebrafish be compared to xenopus
involves the local accumulation of b-catenin on the future dorsal side- the same as xenopus
yet, does not involve sperm entry
What is the role of the maternal Wnt ?
to active direct target gene siamois. siamois encodes a TF that regulates genes expressed in the organiser
What is the equivalent of the organiser in xenopus in the 3 other model organisms?
zebrafish = the shield
chick = Hensen’s node
mouse = node
What does transplanting the organiser result in?
axis duplication
Why are quail-chick transplants used to demonstrate the organiser activity of avian Hensens node?
because they are similar avians but can be distinguished by simple histological procedures that do not need molecular markers
What is the role of vegT and how can it be enhanced?
role of vegT by itself is to activate nodal gene expression. can be enhanced by being expressed alongside beta catenin to enhance nodal expression
What is Vg1?
a maternal RNA localised to the vegetal pole. encodes a member of the TGF beta family and is well conserved in the chick (cVg1), with a similar gene in the mouse (GDF1)
What are nodal genes?
a member of the TGF beta superfamily. nodal signalling is the principle mesoderm inducer in vertebrates. works through activating SMAD proteins that act on target genes to induce biological responses
Which germ layers in Xenopus are maternally induced?
Ectoderm and endoderm. mesoderm is induced
How is mesoderm induced?
ectodermal interacting with the vegetal tissue (prospective endoderm). through cell signalling
How are different types of mesoderm induced?
the dorsal and ventral cells of the vegetal region will induce different properties in cells.
dorsal -> skeletal muscle
ventral -> blood, kidney cells
mesoderm-inducing signal is not uniform
How is the mesoderm induced?
from animal cap cells by signals from the vegetal pole
Aside from nodal, what other protein was found to be a powerful inducer of the mesoderm?
Activin
What is activin?
a member of the TGF beta family. induces all types of mesoderm and endoderm to form animal pole explants (the prospective ectodermal).
How does the concentration of activin impact development?
activin is concentration dependent.
lower dose = induces blood, higher muscle, higher notochord and endoderm
concentration of the inducing factor is highest near its source at the vegetal pole and lowest in the animal pole. creates a gradient
Explain the difference between just VegT/Vg1 expression and VegT/Vg1 expression alongside B-catenin?
just VegT = low nodal proteins produced, gives rise to ventral mesoderm = blood
VegT and B-catenin = high nodal proteins, gives rise to the organiser
What must occur in order for the head and CNS to develop?
level of Wnt signalling at the organisers must decrease. organisers secretes antagonists of the BMP and Wnt pathways
What occurs in mouse, zebrafish and xenopus that are deficient in nodal signalling?
mouse - gastrulation failure. no mesoderm or endoderm. lethal
zebrafish and xenopus - no or very little mesoderm and endoderm, results in posteriorisation
What germ layers do UV ventralised embryos have?
All 3, ectoderm, mesoderm, endoderm
What type of stem cells are Embryonic stem cells?
Pluripotent
What does the aggregation of 2 embryos produce?
Normal chimaeras embryo. If 2 aggregated embryos can be distinguished then their contribution to the resulting animal can be visualised and analysed. I.e black and which coat characteristics develop independently.
What is Wnt required for?
Gastrulation and mesoderm formation
What happens when wnt signalling increases ?
The extent of forebrain and mid brain is reduced
What is the Niewkoop center?
Determined by transformation of cell from one place to another - is a primary inducing region- has a central role in the induction of dorsal mesodermal cells to form the organiser.
What is the organiser?
Signals to direct the fate and organisation of nearby cells. SMALL PART OF EARLY GASTRULATION That patterns the entire body axis. Differentiate into notochord
What does the organiser differentiate into?
Notochord
When the dorsal-vegetal blastomere is transplanted to the opposite side (posterior-vegetal) of a recipient embryo what happens?
Induces a second axis
What does ventral expression of agonist of the wnt signal transduction pathway cause?
Axis duplication
What is the determined past of Gastrula?
The organiser
How does wnt signalling work?
Active Wnt signalling operates by interacting with Veg T, a maternal transcription factor, to induce expression of nodal genes
Where is VegT mRNA localised to?
The vegetal Vegetal half of an unfertilised egg
What is the principal mesoderm inducer in vertebrates?
Nodal signalling
