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:http://worms.zoology.wisc.edu/frogs/gastxen/gastxen_sagview.html

Overview of frog gastrulation

Future Dorsal side (back)Future Ventral Side (belly)

Sperm entry in animal hemisphere

Vg1 mRNA(in situ hybridization)

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

A 3’ UTR sequence element localizes Vg1 mRNA(depends on microtubules, mRNA-binding proteins)

(From Mowry and Melton (1992) Science 255: 991-993)

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.

UV irradiation on vegetal side prevents cortical rotation.

No dorsal axis forms, no gastrulation occurs. Embryo is ventralized.

Rotate zygote in a new plane at time of first cleavage: two blastopores form

Rotated zygotes after gastrulation and hatching

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

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)

Are cell identities determined (in developmental sense) as shown in the fate map?

Fate map of late blastula

Experimental test of specification.

Specification map: shows what the cells have been instructed to become

Explant early blastula tissues: don’t get mesoderm

early

Transplanting dorsal vegetal cells restores axis formation in embryos from UV-irradiated embryos

(32-64 cell stage)

Figure 10.11

Transplanting dorsal vegetal

cells to the ventral vegetal side causes a second axis to

form

Transplant organizer just before gastrulation

Transplanted organizer

~64-cell stage late blastula

Mesoderm induction

Signals 3 and 4 induce further specialization of mesoderm cells.

Additional signaling events act at later stages.

Different parts of the dorsal mesoderm signal differentlySource of transplant: Anterior mesoderm

Posterior mesoderm

Dorsal blastopore lip signals differently at different times(Actually different cells at different times)

Transplanting presumptive epidermis

Transplant presumptive neural ectoderm

Transplant later, get different result:

Cells have become determined (or, perhaps an epidermal signal is missing later).