axis determination in frog embryos
DESCRIPTION
Axis determination in frog embryos. How are embryonic axes set up? What signaling events regulate embryo development?. Overview of frog gastrulation. Future Ventral Side (belly). Future Dorsal side (back). Sperm entry in animal hemisphere. Vg1 mRNA ( in situ hybridization). - 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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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
Prevent normal cortical rotation by UV-irradiating vegetal side of zygote: No dorsal axis or gastrulation
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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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Additional signaling events act at later stages.
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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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Transplanting presumptive epidermis
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Transplant presumptive neural ectoderm
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Transplant later, get different result:
Cells have become determined (or, perhaps an epidermal signal is missing later).