Axis determination and early
development in amphibians
We are standing and walking with parts of our body
which could have been used for thinking had they
developed in another part of the body
(Hans Spemann 1943)
Spemann’s initial experiments
Spemann’s organiser
• The term "Organizer" was coined to emphasize the ability of this
dorsal blastopore lip tissue to direct the development of the host
tissue and to give these redirected cells a coherent, unified,
organization. "A piece of the upper blastoporal lip of an
amphibian embryo undergoing gastrulation exerts an organizing
effect on its environment in such a way that, if transplanted to an
indifferent region of another embryo, it causes there the
formation of a secondary embryonic anlage. Such a piece can
therefore be designated as an organizer." This tissue had the
ability to invaginate and differentiate autonomously, to induce
the neural plate and, by assimilative induction, to organize
somites from the lateral plate mesoderm of the host.
The functions of the organiser
• The ability to self-differentiate dorsal
mesoderm
• The ability to dorsalise the surrounding
mesoderm into paraxial (somite-forming)
mesoderm (when it would otherwise form
ventral mesoderm)
• The ability to dorsalise the ectoderm,
inducing the formation of the neural tube
• The ability to initiate the movements of
gastrulation
Molecular events forming the Organiser
sperm
Dsh
Dsh
V
D
No -catenin in nuclei
Cortical
rotation
V
GSK Dsh
D
GSK
-catenin degraded
-catenin stable
-catenin in nuclei
WNT pathway
Transcriptional events
V
D
-catenin degraded
Tcf3 proteins
-catenin stabilised (nuclear)
Siamois gene
Repressed
Input from TGF- (Vg1, Nodal)
signalling pathway
Tcf3 proteins+
-catenin
Activated
Siamois protein
goosecoid gene
Activated
Dorsal axis
Ability of goosecoid mRNA to induce a
new D-V axis
• An embryo (16-cell stage) whose
ventral vegital pole was injected with
goosecoid mRNA induced a second
axis (with two complete sets of head
structures precursors). This did not
happen when embryos were injected
with a mutant form of the DNA-binding
homeodomain (Niehrs et al, 1993 Cell
72: 491-503)
Model for organiser induction and
mesoderm formation
Stage 8
V
Stage 9
D
VegT, Veg1
V
D
-catenin
Xnr
-catenin
VegT, Vg1
Stage 10
Nodal related high
Nodal related low
V
D
BMP4 gradient
Organiser
(ventral and lateral mesoderm)
Organiser
Ventral mesoderm
The Diffusible proteins of the Organiser I:
BMP inhibitors
•
Noggin: diffusible protein accomplishing two of the major functions of the
organiser: induction of dorsal ectoderm to form neural tube and formation of
dorsal mesoderm. Binds to BMP4 and BMP2 and inhibits their binding to
receptors (Zimmerman et al 1996 Cell 86, 599-606).
•
•
•
Chordin: diffusible protein directing central nervous system development. Binds
to and inhibits BMP4 (Sasai et al 1994 Cell 79, 779-790).
Nodal-related protein 3 (Xnr-3): diffusible protein synthesized by the superficial
cells of the organiser and is also able to block BMP4 (Smith et al 1995 Cell 82,
37-46).
Follistatin: secreted glycoprotein that induces epidermal cell fate (skin)
overriding the default neural fate (Hemmati-Brinvalou and Melton 1994 Cell 88,
273-281).
Rescue of dorsalised structures by Noggin
Noggin expression
Chordin expression
Bone Morphogenetic Factor 4 (BMP4): an
Organiser antagonist
• The Xenopus BMP4 is a member of the TGF family of ligands. It induces ventral
mesoderm and (ventral) epidermal fates and
suppresses neural ones.
Blood cells
kidneys
muscle
BMP4 relative concentration
The Diffusible proteins of the Organiser II:
Wnt inhibitors
•
•
Cerberus responsible for the anterior-most head structures; cytokine of the
cysteine knot superfamily binds Xwnt8 (member of the Wnt family) as well as
BMPs and Nodal-related (Bouwmeester et al 1996 Nature 382, 595-601).
FRZB and Dickkopf Wnt receptor antagonists prevent signalling from Wnt
receptors. Frzb is able to bind Wnt agonists and by binding to Frizzled proteins
prevent the interaction between ligand and receptor (Wang et al, 1997 Cell 88,
757-766; Glinka et al, 1998 Nature 391, 357-362).
Cerberus induces head structures
Frzb expression and function
Frzb binds to Xwnt8
Dorsal
Pharyngeal
endoderm Prechordal plate mesoderm Notochordal mesoderm
Cerberus
Dickkopf
Frzb
Chordin
Noggin
Anterior
Follistatin
Posterior
Xwnt8
Nodal-related
Ventral
BMP4
Posterior transforming proteins: Wnt signals
and retinoic acid
In Xenopus embryos a gradient of Wnt
signalling (Xwnt8) and -catenin is highest in
the posterior and absent in the anterior. There
is also a high concentration of retinoic acid
(RA) at the posterior end of the neural tube.
RA is especially important in patterning the
hindbrain but not the forebrain (Dupé and
Lumsden 2001 Development 128, 2199-2208)
The anterior transforming proteins: Insulin-like growth
factors
In addition to proteins that block BMP4 and
Wnt signalling in the head there is a positive
signal that promotes anterior head
development. Insulin-like growth factors
(IGFs) are required to form the anterior-neural
tube with its brain and sensory placodes.
(Pera et al Dev Cell 1, 655-665)
Summary
Anterior
O
R
G
A Dorsal
N
I
S
E
R
IGFs
Ventral
BMP4
RA
Wnt
Posterior
Dorso-ventral patterning is controlled by
the same genes in flies and frogs
Flies and frogs
• Dorsal neural tube of the vertebrate and
ventral nerve cord of the fly appear to be
fly
generated by the same set of instructions.
frog
Tolloid Xolloid
Sog
Chordin
Dpp
BMP-4
Can the fly sog gene rescue
ventralised Xenopus embryos?
Embryos
completely
ventralised by UV
irradiation
Ventralised
embryos partially
rescued by injection
of sog mRNA
Ventralised
embryos completely
rescued by injection
of sog mRNA
• sog and chordin can substitute for each
other!