Axis Determination, Gastrulation Pt I Flashcards
what has been determined to be important for creating D-V axis?
what is one method to prevent D-V axis creation?
cortical rotation (occurs upon fertilization) is key in frogs for establishing D-V axis.
superficial UV-irradiation prevents rotation, leads to grossly abnormal embryos that are radially symmetrical, contain only ventral mesoderm structures.
describe the UV experiments proving cortical rotation is necessary for establishing D-V axis
when a bcr (before cortical rotation) fertilized egg is irradiated with UV and left over developmental time st it is the same age as an acr (after cortical rotation) embryo, and V cytoplasm is transferred into a 16-cell recipient, 2 embryonic axes form, rather than the normal 1.
describe V cytoplasmic transfer experiments (involving cortical rotation)
transferring V (vegetal) cytoplasm before cortical rotation to a 16-cell recipient induced extra embryonic axis formation, whereas transfer of vegetal cytoplasm from an acr egg to a 16-cell recipient resulted in normal axis formation (1 axis).
microinjection of dorsal cytoplasm from an acr embryo to a 16-cell recipient also induced extra embryonic axis formation.
describe cytoplasmic removal experiments. what do all of these experiments suggest?
removing vegetal cytoplasm before cortical rotation yields no dorsal organs in the developed embryo
removing vegetal cytoplasm after cortical rotation will still yield dorsal organs.
thus suggests that determinants move from vegetal pole to dorsal side of embryo
what is localized during dorsal specification?
B-catenin activity is localized during dorsal specification
what is one model for B-catenin activity localization?
1) B-catenin proteins are transported by MTs during cortical rotation to dorsal region
2) meanwhile, any remaining B-catenin is downregulated by GSK-3 activity in ventral region
what is another model to explain B-catenin localization?
1) as B-catenin is being translated from maternal mRNA, it is evenly distributed bcr, but so is GSK-3 activity - B-catenin activity thus uniformaly suppressed
2) a GSK-3 inhibitor (in form of Disheveled [Dsh], + GBP [GSK-3 binding protein]) that is transported from ventral pole to dorsal pole
3) Wnt 11 RNA is transported, translated in future dorsal region. stabilizes GBP and GSH, protect B-catenin
4) bcr, ven. pole cyto. can impart dorsalizing activity to ventral blastomeres bc its injection introduces GSK-3 inhibitor + Wnt 11 mRNA, allows for B-catenin activity
5) acr, B-catenin is downregulated in ventral pole due to absence of GSK-3 inhibitor
6) Dsh + GBP are carried on MTs in kinesin-dependent fashion
why was this more complex model proposed?
evoked to explain that B-catenin is homogeneously distributed in oocytes bcr initially
if this is the case, in experiments involving transplantation of cyto. from future dorsal pole from eggs bcr into ventral blastomeres of 16-cell embryos, you would see secondary embryonic axis formation
thus, new model attempts to explain inconsistency
what is one gene that has been identified in playing a role in left-right axis formation?
what do mutations in this gene cause?
situs inversus viscerum+ (or iv+ for short) was identified in playing role in L-R axis formation
mutations in this gene can cause “situs inversus” - internal organs are incorrectly oriented w/ respect to other body axes (L-R orientations are randomized)
why is situs inversus more dangerous than situs inversus totalis?
inversus totalis is the full switch of L-R orientation (organs on left side will be on right side).
inversus totalis means the relative positioning and fit of the organs will remain the same. thus, the body can still function properly. in situs inversus, the body plan is messed up and organs may not fit or function properly.
what does iv encode?
a subunit of dynein
what is one result of iv-/iv- in mice?
certain cilia (called monocilia, have 9+0 arrangement of MTs and swing in a circular motion) in an area called the primitive node (think mammalian version of Spemann’s organizer) in iv-/iv- mice are immobile.
important for L-R axis determination, will direct determinants to their respective sides of the cell (mutated monocilia obviously cannot do this).
what is another gene that has been implicated?
what is the role of kinesin in cilia?
kif3B - encodes a kinesin - cilia are altogether missing in Henson’s node
kinesin is thought to be necessary for cilium assembly (perhaps by mediating transport of necessary components) while the above-mentioned dynein is required for movement of cilia.
how has monocilia movement been monitored?
fluorescent beads placed in fluid around node have shown that fluid is moved in a net leftward direction, but in absence of functional monocilia (i.e. iv-/iv-) this does not occur.
what is the theory then behind the importance of monocilia?
what is the “swept” molecule?
theory is that signaling molecules are swept to left side of embryo, results in differential gene expression across left and right sides
swept molecule = member of TGF-B family (secreted peptide) called Nodal
