zebrafish development Flashcards
describe what the initial cleavage of the embryo is like
it is discoidal and meroblastic. the yolk- free cytoplasm forms a blastodisc and the yolk- rich forms a vegetal yolk
describe the stages of blastomere joining and the impact of this
initially the blastodiscs are joined to each other and to the yolk by cytoplasmic bridges. after the 16 cell stage only the inner cells are now joined to the yolk. these joins allow the diffusion of molecules larger than would be allowed by normal gap junctions
how does cleavage change at different cell stages
up to 32 cells the cleavage is radial and forms a sheet of cells. from the 64 to 512 cell stage the cleavage becomes irregular but is still synchronous
describe the mid blastula transition MBT
it happens at the 1000 cell stage (division 10) and marks the end of synchronous division. cells begin to differentiate and the zygotic genome is activated
what layers form at the MBT and what will these layers become
superficial animal cells form the enveloping layer EVL which will form the periplasm. deep cells lose their connection, gaps form and they will develop into the embryo. nuclei of around 20 marginal cells migrate into the yolk, divide and form the yolk syncytial layer YSL which is split into internal and external
what is epiboly and what drives it
the first morphogenetic movement at which the yolk domes and the blastoderm engulfs it. it is driven by the eYSL which moves downwards, pulling the EVL as it is joined by adherins and tight junctions
when does gastrulation happen and what takes place
it happens at 50% epiboly, halting it. some cells begin to internalise which forms the hypoblast (involuting cells which will become the endoderm and mesoderm) and the epiblast (external cells which will form the ectoderm)
what is the shield and what does it result in
it is the dorsal thickening of the germ ring due to the dorsal migration of surrounding cells. after its formation epiboly resumes
where does the AP axis form and why
the AP axis forms dorsally as dorsal hypoblast cells form the axial mesendoderm (PCP and notochord). lateral hypoblast cells move dorsally due to convergent extension and they form the paraxial and lateral mesoderm
what is special about somitogenesis and neurogenesis
they happen simultaneously at around 10.5 hours
describe the hatching process
from day 2 the embryos can hatch from the chorion. by day 5 they have used up most of the yolk so they develop swim bladders in order to feed
how does EVL division change and why does this alter cellular composition of the layers
initially the EVL undergoes both parallel and delamination division, hence some EVL cells contribute to the deep cell layer. after the late blastula stage they only divide parallel and form the periderm. deep cells cant enter the superficial EVL
why can’t a fate map be made at the gastrula stage
there is deep cell mixing so different and cells can form any germ layer
when does deep cell tissue restriction begin and what is the broad rule for determining fates
restriction begins at the shield stage where cell latitude shows some correlation with the germ layer of the progeny
what tissues arise from the ectoderm
neural tissue is the default but epidermal tissue can arise if BMP signals are received
what forms the mesendoderm and what tissues does it give rise to
it forms from the dorsal and lateral hypoderm and forms the PCP (gsc marker) and the notochord (tbxta/ntl/bra markers)
how does the endoderm for the gut tube
it arises form marginal cells which undergo EMT to become mesenchymal stem cells. these converge at the midline at somitogenesis to form the gut tube
how is germ cell fate decided
by vasa mRNA which encodes a specific germ cell protein. it localises at part of the 1st/2nd cleavage planes and then in four cells. these divide to form tissues which migrate to the gonads
how has it been shown that maternal molecules can act as morphogens
by the removal of the yolk which contains the maternal molecules. this caused mis development of dorsal/ anterior structures eg no head, no notochord etc
describe the movement of Wnt8a mRNA
it is a maternal molecule which is localised at the vegetal pole but is transported dorsally by microtubules
describe the effects of Wnt8a protein
it is secreted out of the yolk into dorsal cells in which is activates the Wnt pathway. some nuclei migrate to form the dorsal YSL (Nieuwkoop equivalent) and others form the shield (Spemann’s organiser equivalent). the Wnt pathway also gives rise to BMP which promotes default neural ectoderm
what is beta catenin and describe its mutants
it is a maternal mRNA, therefore mutants are maternal as opposed to zygotic. eg mich mutants which lose dorsoanterial structures and lack PCP/notochord markers
how has it been shown that the shield is the equivalent to the amphibian Spemann’s organiser
ventral transplantation saw the generation of a second axis on the ventral side
how is the zygotic genome activated
before the MBT the genome is blocked by histone pools which prevent transcription factors binding to DNA. after the MBT there is more DNA being synthesised but the number of histones is constant so they can no longer compete
what are the two methods of degrading maternal molecules
either with miR430 which works in association with RISC factors or by tagging them with m6A which is targeted and degraded by Ythdf2 proteins
how does are YSL genes activated and what do they do
they are activated by TF mxtx2 which is induced by TF nanog. YSL genes induce the mesoderm and endoderm
how are mesoderm and endoderm genes activated
they are activated by nodal related mRNA ndr1 and ndr2. these activate mesoderm at low concentration and endoderm at high concentration. when the concentration is the same, brief exposure induces bra/ntl and long exposure induces bra/ntl and gsc
what happens when ndr proteins are removed and how do phenotypes vary
their removal causes squint and cyclops mutants. single mutants are mild but double have no ntl/bra/tbxta/gsc in the shield and therefore no endoderm or axial mesoderm. there is some tail cells that still form due to the presence of ventroleteral ntl
what is one eyed pinhead oep and how do mutants show the role of genomes in nodal signalling
oep is a nodal co-receptor, where the Zoep mutation is embryonic lethal but can be rescued by injecting the embryo with oep mRNA. they discovered by the maternal and zygotic genomes are involved in nodal sequencing due to the role of compensation. in Moep the zygotic compensated, in Zoep the maternal mostly compensates but in MZoep there is almost no nodal signalling so there is no mesoderm or endoderm
what are the regulators of ntl and how are they regulated
ntl has two regulators E1 and E2. without E2 there is a loss of dorsal ntl expression, the same as when there is no nodal signalling, indicating that E2 is nodal-regulated. without E1 there is a loss of ventral ntl expression
what is a morpholino and how are they different to nucleotides
they are modified oligonucleotides. they have morpholine rings instead of pentose sugars and they have phosphorodiamidate bonds instead of phosphodiester
what are the two types of morpholino and how do they differ
there is 5’UTR/ATG which bind at the 5’UTR and prevent the initiation of translation by ribosomes. there is also splice which bind to the splice acceptor/donor, alter the transcript and produce abnormal or truncated proteins
what phenotypes do the morpholino types give
5’UTR/ATG gives MZoep and splice gives Zoep because the maternal mRNA is already spliced
what does TF MyoD do
it is expressed by skeletal cells in the paraxial mesoderm
what does TF Gata1 do
it is expressed by erythrocytes in the lateral mesoderm
what does vasa mRNA do
it encodes germ cell specific proteins which localise in parts of the cleavage planes
what does Wnt8a mRNA do
it is localised in the vegetal pole but is transported dorsally by microtubules. then the Wnt8a protein is secreted from the yolk cell and activates the Wnt pathway in certain cells
what does chordin do
its a BMP antagonist which promotes the development of default neural ectoderm
what does BMP do
its a bone morphogenic protein which causes the ectoderm to give rise to epidermis as opposed to default neural tissue
what does beta catenin do
it is an mRNA needed to induce dorsoanterial structures
what does nodal do
it is a protein which regulates mesendoderm formation
what does gsc do
it marks the prechordal plate in the axial mesendoderm
what does tbxta/ntl/bra do
it marks the notochord in the axial mesendoderm
