Vertebrate Embryonic Patterning 2
Molecular Basis of Nieuwkoop’s
Center
Properties of Dorsal Lip Region
• Autonomously specified
• Dorsal lip never respecified by tissue at
location of transplantation
• Donor cells always contribute to notochord
(mesoderm)
• Secondary neural tube forms from recruited
host cells
• Donor lip cells organize host cells
Mesoderm Induced by Vegetal Cells
Nieuwkoop’s Experiment
Nieuwkoop’s Center Rescues Dorsal Axis
& Duplicates Axis in Wild-type
Inducing Centers in the Blastula
“Spemann”
(General)
Fate Map of Organizer and
Nieuwkoop’s Center
Molecular Mechanism of Dorsal
Specification
VegT, Vg1, Xnr’s
mRNAs
VegT, Vg1
VegT, Vg1
Accumulation of b-catenin in Dorsal
Nuclei
V
D
Dorsal view
b-catenin
Ventral view
Stimulating b-catenin Expression
Duplicates Axis
Injection of wnt 1
into ventral
vegetal (C4)
duplicates axes
Ventral expression
of dominant
negative GSK-3
duplicates axes
siamois
Mis-Expression of Int1 Oncogene
Duplicates Axes
Int 1 is gene at
site of MMLV
integration.
Identified by H.
Varmus
McMahon & Moon, 1989
Neural tubes
Dominant Negative GSK3
Duplicates Axes
Wnt Proteins Not Expressed on
Dorsal Side!!! What The…
Wnt 8
Some Wnt proteins
expressed in the ventral
mesoderm and ectoderm
Dorsal
Blastopore
Lip
VegT Necessary for Endoderm
Depletion of Maternal mRNAs
Injection of a-sense morpholino
oligonucleotide causes
degradation of complementary
mRNA thereby depleting the
oocyte/embryo of that message
and the encoded protein.
Some maternal proteins may still
be present depending on the
actual stability of the protein.
Technique works best on
depleting stored mRNAs which
are not translated until after
fertilization.
Cooperation of Vg1, VegT & b-catenin
siamois
Xbra & Gsc
Xbra
General Mesoderm Markers
D
Xenopus brachyury
(Xbra)
vegetal view
The initial activation of
VegT & Vg1 induces
mesoderm. Xbra is
expressed throughout the
equatorial region prior to
gastrulation.
Xbra is the founding
member of the T-box
transcription factor family.
Smads and Siamois
Specify Nieuwkoop
Center
Vg1 ligand