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 Ventral Side (belly)
Sperm entry in
animal hemisphere
Future Dorsal side (back)
Vg1 mRNA
(in situ
hybridization)
A 3’ UTR sequence element localizes Vg1 mRNA
i) Inject RNA molecules
into oocyte
ii) Isolate animal and
vegetal RNAs
iii) Northern blot using
Vg1 probe
(From Mowry and
Melton (1992) Science
255: 991-993)
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)
Fate map of late blastula
Are cell identities determined (in developmental sense) as shown in the
fate map?
Experimental test of specification.
Specification map:
shows what the cells have been instructed to become
early
Explant early blastula tissues: don’t get mesoderm
Transplanting
dorsal vegetal cells
restores axis
formation in
embryos from UVirradiated 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
Mesoderm induction
~64-cell stage
late blastula
Signals 3 and 4 induce further specialization of mesoderm cells.
Additional signaling
events act at later
stages.
Different parts of the dorsal mesoderm signal differently
Source 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).