axis determination in frog embryos
DESCRIPTION
Axis determination in frog embryos. How are embryonic axes set up? What signaling events regulate embryo development?. Time-lapse videos of Xenopus gastrulation: By David Shook : http://faculty.virginia.edu/shook/ShowMovies/index.htm By Mike Danilchik: - PowerPoint PPT PresentationTRANSCRIPT
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Axis determination in frog embryos
How are embryonic axes set up?What signaling events regulate
embryo development?
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Time-lapse videos of Xenopus gastrulation:
By David Shook :http://faculty.virginia.edu/shook/ShowMovies/index.htm
By Mike Danilchik:http://worms.zoology.wisc.edu/frogs/gastxen/gastxen_sagview.html
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Overview of frog gastrulation
Future Dorsal side (back)Future Ventral Side (belly)
Sperm entry in animal hemisphere
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Vg1 mRNA(in situ hybridization)
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A 3’ UTR sequence element localizes Vg1 mRNA
(From Mowry and Melton (1992) Science 255: 991-993)
i) Inject RNA molecules into oocyte
ii) Isolate animal and vegetal RNAs
iii) Northern blot using Vg1 probe
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A 3’ UTR sequence element localizes Vg1 mRNA(depends on microtubules, mRNA-binding proteins)
(From Mowry and Melton (1992) Science 255: 991-993)
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Sperm entry provides the asymmetric cue to set up the dorsal-ventral axis of Xenopus embryos
Cortical rotation requires the microtubules – may be nucleated by the centriole delivered by sperm.
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UV irradiation on vegetal side prevents cortical rotation.
No dorsal axis forms, no gastrulation occurs. Embryo is ventralized.
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Rotate zygote in a new plane at time of first cleavage: two blastopores form
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Rotated zygotes after gastrulation and hatching
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Rotate zygote in a new plane at time of first cleavage: two blastopores form
Shows sufficiency.
Prevent normal cortical rotation by UV-irradiating vegetal side of zygote: No dorsal axis or gastrulation
Shows necessity.
Does cortical rotation induce the dorsal axis?
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Cortical rotation creates asymmetry of cytoplasm
Asymmetric cell divisions – a general way to create distinct cell fates in daughter cells.(Relies on asymmetry in mother cell = cell polarity)
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Are cell identities determined (in developmental sense) as shown in the fate map?
Fate map of late blastula
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Experimental test of specification.
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Specification map: shows what the cells have been instructed to become
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Explant early blastula tissues: don’t get mesoderm
early
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Transplanting dorsal vegetal cells restores axis formation in embryos from UV-irradiated embryos
(32-64 cell stage)
Figure 10.11
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Transplanting dorsal vegetal
cells to the ventral vegetal side causes a second axis to
form
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Transplant organizer just before gastrulation
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Transplanted organizer
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~64-cell stage late blastula
Mesoderm induction
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Signals 3 and 4 induce further specialization of mesoderm cells.
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Different parts of the dorsal mesoderm signal differentlySource of transplant: Anterior mesoderm
Posterior mesoderm
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Dorsal blastopore lip signals differently at different times(Actually different cells at different times)
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Can responding cells acquire any fate?Transplanting presumptive epidermis
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Can responding cells acquire any fate?Transplanting presumptive neural ectoderm
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Transplant later, get different result:
Cells have become determined (or, perhaps an epidermal signal is missing later).