lec 16- establishing axes in xenopus embryos Flashcards
what axes is set up during oogenesis under the control of the mother?
the animal-vegetal axis
what side are yolks concentrated on in the axes of an egg?
vegetal
what mRNAs are asymmetrically localized along the animal -vegetal axis during development of the egg?
maternal mRNA, many are localized specifically to the vegetal side
why is the vegetal cytoplasm important?
if some of it is removed from the vegetal pole of the zygote before the first cell division occurs, the embryo fails to develop structures associated with the anterior-posterior axis
how is fertilization done and how does it affect the dorsal-ventral axis?
-sperm entry triggers a set of developmental events that define the DV axis
-sperm can fertilize the egg anywhere on the animal hemisphere
-after fertilization, the cortical layer of cytoplasm just under the plasma membrane rotates away from the site of sperm entry
is cortical rotation seen in the zygote of amphibians because of how the pigment moves?
yes
what is the grey crescent?
a grey area that forms on the side of the zygote opposite the sperm entry site
what happens when cortical rotation occurs?
only the outer layer of cytoplasm rotates, in some species this outer layer is more pigmented than the other underlying cytoplasm, rotation of the darker cytoplasm exposes a patch of the underlying more lightly pigmented cytoplasm (grey crescent)
what occurs after cortical rotation?
microtubules, positioned in the cortex of the cell serve as tracks along which dorsalizing factors (maternally supplied mRNA and proteins) are carried to the future dorsal side of the embryo
where does the Spemann-Mangold Organizer form?
the region of the embryo associated with the grey crescent and the dorsalizing factor, it is also where gastrulation occurs
what happens when the first cleavage occurs at the DV axis?
-it bisects the zygote so that the gray crescent and dorsalizing factors are segregated equally into the two blastomeres
-if the two halves have the gray crescent (future dorsal region) equally, each half develops normally
what happens if a 4 cell embryo is split in half so one half gets all of the future dorsal region and the other has none?
-the ventral half develops into a ventralized embryo while the dorsal half develops into a complete embryo that lacks a gut that devrloped from ventral blastomeres
what are the dorsal and ventral blastomeres specified to form?
dorsla and ventral regions of the embryo at the 4 cell stage
if the xenopus embryo is treated with LiCl, what happens?
cortical rotation occurs, normally and a blastula forms but the resulting embryos are hyper-dorsalized
what does LiCl activate?
the Wnt/B-catenin pathway
what are the two maternal factors that are relocated from the vegetal pole to the future dorsal pole during cortical rotation?
XWnt-11 mRNA and Dishevelled protein
when the two maternal factors are present, is B-catenin degraded?
no, it enters the nuclei and turns on the expression of genes needed for dorsalization
how exactly do XWnt-11 mRNA and Dishevelled protein help with the Wnt pathway?
-the two maternal factors are localized at the vegetal pole
-the relocalize to the dorsal side during cortical rotation
-at the 8-cell stage, Xwnt-11 mRNAs are translated into proteins, this turns on the Wnt pathway
-B-catenin enters nuclei and activates transcription of cells of the future dorsal region
what are the 4 pieces of evidence that Wnt pathway is needed for Dorsal formation?
- B-catenin is found specifically in nuclei cells on the presumptive dorsal side and not the ventral
- activation of the Wnt pathway everywhere in the embryo results in hyper-dorsalized embryos
- when the expression of one of the xenopus frizzled receptors is blocked, the signalling pathway cannot be activated and the dorsal structures are lost
- injection of B-catenin into ventral vegetal cells can specify another dorsal side to the embryos, an embryo with two dorsal sides is produced
how is the Nieuwkoop center established?
-the action of B-catenin and other maternal factors on the dorsal side of the vegetal region lead to the formation of a signalling center which is also called the Nieuwkoop center
what is thought to be the early developmental stage of the Spemann-Mangold Organizer?
Nieuwkoop center because the Spemann-mangold organizer arises just above the Nieuwkoop center at a late blastula stage
why is the Nieuwkoop center required?
it is required for further development of both the antero-posterior and dorsal ventral axes as well as for induction of the nervous system from the ectoderm
what is the evidence behind why the Nieuwkoop center is an organizer?
-the experiment where half of the embryo has the factors needed and the other half doesn’t
-the half lacking the Nieuwkoop center develops abnormally lacking all dorsal and anterior structures
-when cells containing the Nieuwkoop center are grafted onto the ventral side of another embryo, two dorsal sides form, while grafted ventral sides have no effect
explain the establishing of the Nieuwkoop center with respect to VegT and Siamois transcription factors:
-cortical rotation moves the dorsal determinants towards the future dorsal side of the embryo, the transcription factor VegT is present throughout the vegetal region
-cortical rotation creates a region on the dorsal side where B-catenin is present in nuclei
-cells in the region where B-catenin is inside nuclei and where the transcription factor VegT is present express Siamois, this is where the Nieuwkoop center arises
