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

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?

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.

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)

Can responding cells acquire any fate?Transplanting presumptive epidermis

Can responding cells acquire any fate?Transplanting presumptive neural ectoderm

Transplant later, get different result:

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