Axis Specification Flashcards
(17 cards)
When can an axis be defined and give some examples of when and how axis are defined?
Axis can defined before or after fertilisation or a bit of both.
In frogs all axis are defined before fertilisation. A-P is defined by maternal mRNA and D-V is defined by sperm entry point
In chick all axis are determined afterfertilisation. D-V is by the position of the blastoderm and A-P is due to gravity when rolling down the oviduct.
Drosophila axis are all determined by the mother, A-P from intracellular signals and D-V from extracellular signals.
What are nurse cells?
Oogenesis starts with one daughter cell. This divides by miotsis 4 times. The 2 starting cells will be connected to 4 other cells each. One of these two forms the egg. The other 15 undergo endo-replication - they swell and their DNA replicates. They dump all their cytoplasm into the egg cell via ring canals and then apoptose after fertilisation.
How is the A-P axis defined in drosophila?
AP due to position of nucleus polarising the surrounding follicle cells, these are maternal somatic cells that surround the oocytes and product its waxy exterior. The nurse cells produce bicoid and nanos which are transported and localise to the anterior and posterior sides of the oocytes respectively
What creates the polarisation that allows AP defining?
Oocyte nucelus is found in the posterior potion of the egg. It produces gurken signal which binds to its membrane. Gurken induces the nearby follicle cells to enter into a posterior cell fate. Gurken mutants form two anterior follicle cell populations as this is the default.
Gurken trigger Torpedo (fly EGFR) and this results in posterior differentiation of the nearby follicle cells.
How is DV axis defined?
Posterior follicle cells reorganise the micro-tubule of the occyte resulting in the nucelus being pulled anterior to the dorsal portion of the oocyte.
12 proteins shows a D-V mutant phenotype in screen. Some were mRNAs from nurse cells others were proteins from follicle cells. They were all however expressed uniformly.
Pipe is asymmetric and only seen in the ventral side of the oocyte as it is inhibited by gurken signals. Grk mutants have increased pipe and when pipe is null you can direct dorsal patterning using pip transgene.
How does pipe define DV axis?
Ventral follicle cells produce pipe which modified proteoglycan egg shell protein leading to a protein cascade. This cascade is made up of zymogens and are all changes into their active form by the presence of pipe. This cascade ends in spatzle. Spatzle acts on toll receptor which triggers the release of dorsal from cactus.
During embryogenesis the eggs cell endoreplicates and so all the nuceli close to the dorsal edge will be influence by a gradient of dorsal signal - TF. Different genes respond to different concentrations of dorsal.
Pathways is conserved in vertebrates but its end result is involved in the inflammatory response.
Why is cell adhesion important - give some examples?
Help define how cells can move, sort and creates structures. neural crest cells migrate cells migrate during development, skin cell migrate in adults as well. Also in cancer migration adhesion is important for metastasis and EMT.
What controls cell adhesion?
Cells either stick or repel each other. Different tissue types can sort themselves out after mixing for example frog mesoderm and ectoderm.
This is as a result of cadherin proteins. - 5 extracellular domains, trans-membrane domain and intracellular domain connected to cytoskeleton to aid movement. If same cadherins then cell stick together - evidence transfect 2 cell populations with 2 different cadherins and they will self sort into those populations. Ectoderm expresses e-cadherin and neural plate expresses n-cadherin.
How do we know cadherin is important for development?
Cadherin lacking extracellular domain results in disorganised cell layers.
How is cadherin involved in oogenesis in drosophila?
Both oocyte and posterior follicle cells express E-Cadherin and are important in getting the egg cells to the posterior side of the follicle. In drosophila E cadherin is called shotgun.
In shotgun mutants oocyte is miss placed and you get 2 x anterior follicle cells. Using mosaics we can determine where shotgun is needed.
Take heterozygotes for shotgun with FLP and FRT sites. Induce recombination to create some -/- cell populations. shotgun needed in germline and follicle cells for normal migration of egg cell.
What happens in different shotgun mosaics?
Oocytes with shotgun always prefer follicles with shotgun. If oocyte doesn’t have it then there is no preference. Oocytes prefers follicle with higher shotgun expression.
In WT shotgun is highest in posterior follicles but also in anterior follicles. If posterior are -/- then oocytes attaches to anterior follicle cells.
What are border cells?
Anterior follicle cell population that detatches and migrates to the egg cell throughthe nurse cells. An enhancer trap mutant was creates where this migration is much slower - slbo and marked them with GFP. Shotgun is also important for this - low expression - slow migration.
Why is studying adhesion and migration in oogeneiss in the fly useful?
1 - few cells involved 2 - easy to identify cells 3 - integrity not dependent on shg 4 - genetic manipulation is possible 5 - includes sorting, positioning and migration
What are polar cells and how are they involved in border cell migration?
Polar cells are found within the border cells. They induce the border cell fate. This results in SLBO and JING expression whihc stimulates shg. Ecdysone stimulates the migration event (systemic hormone) by binding to ECR/USP dimer and interacting with taimen - a transcriptional co activator.
Why might studying taimen be useful?
Also up regulated in ovarian and breast cancer as it is a transcriptional co activator for oestrogen. so studying may help our understanding of hormones and inhibitory drugs.
How are these border cells attracted to the oocyte?
This occurs in two stages
Stage one - movement posteriorly. This occurs via a receptor tyrosine kinase that is the fly homologue for PDGFr (again involved in tumour angiogenesis so useful to study). The ligand for this was found using miss-expression studies looking for abnormal BC cell migration (done using P element with UAS right at the end and cross to GAL4 line).
Stage 2 - movement dorsally. Gurken singalling responsible here and binds to EGF-R. Found by screening for mutant RTK. Easy to track BC mutants as the females are sterile because they are needed to drill a hole for the sperm in the waxy egg shell.
How is the location of the dorsal appendages defined?
These are used for gas exchange. Egg shell is secreted by the follicle cells. As nurse cell expand the anterior follicle cells are split into two group and end up dorsally and ventrally. Both produce dpp (decapentaplegic) but only the dorsal ones also have gurken
