Title: References
1References Gilbert (7th edition) Chapter 10,
320-330 Gilbert (6th edition) Chapter 10,
316-329 Wolpert (2nd edition) Chapter 3, 89-104
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3Gastrulation in Xenopus (Gilbert fig 10.7)
4Xenopus animal/vegetal axis Nieuwkoop Centre
- In Xenopus, before fertilization, the egg has a
radial symmetry. - Sperm entry sets up the D/V axis with the dorsal
side of the embryo at a place opposite of the
sperm entry point. - Within the first 90 minutes after fertilization,
the cortex rotates approximately 30 degrees. - The cortex is a gel-like layer of actin
filaments about 5 micrometres thick just under
the membrane. - The vegetal cortex opposite the sperm entry point
moves towards the animal pole. - This leads to the formation of a 'signaling
centre' opposite the site of sperm entry known as
the Nieuwkoop Centre.
5Wnt signalling
6Nieuwkoop Centre specifies the Spemann Organizer
- The Nieuwkoop Centre's main role is to specify
another signaling centre - the Spemann Organizer which in turn leads to the
induction and patterning of the mesoderm and
patterning of the ectoderm
7Spemann Organiser
Organization of a secondary axis by dorsal
blastopore lip tissue. (A) Dorsal lip tissue
from an early gastrula is transplanted into
another early gastrula in the region that
normally becomes ventral epidermis. (B) The donor
tissue invaginates and forms a second
archenteron, and then a second embryonic axis.
Both donor and host tissues are seen in the new
neural tube, notochord, and somites. (C)
Eventually, a second embryo forms that is joined
to the host. (D) Structure of the dorsal
blastopore lip region in an early Xenopus
gastrula.
8- Nieuwkoop centre formed in the blastula.
- Cells of the Nieuwkoop Centre contribute to
endoderm - The Spemanns Organiser forms just above the
Nieuwkoop Centre during late blasula, early
gastrula - Cells of the Organiser contribute to dorsal
mesodermal derivatives - Blastula transplantation experiments explained
by induction of Organiser
9Induction of mesoderm in Xenopus
- Animal pole explants form ectoderm.
- Vegetal pole explants form endoderm.
- Animal pole explants cultured in contact with
vegetal tissue produces some mesodermal tissue. - Therefore, mesoderm depends upon signals from the
vegetal region to turn animals pole cells from
ectodermal fate to a mesodermal fate. - If animal and vegetal tissues are separated by a
filter, then mesoderm develops.
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14The four signal model of mesoderm induction
- Class 1) General mesoderm inducer from the
vegetal region makes default ventral mesoderm. - Class 2) Dorsal mesoderm inducer from the vegetal
region generates Spemann Organizer and
notochord. - Class 3) Spemann Organizer signal from dorsal
region modifies the patterning of ventral
mesoderm. - Class 4) Ventral mesoderm patterning signal from
the ventral region subdivides muscle, kidney and
blood.
15Induction of mesoderm by endoderm
- At least 2 signals must come from the endoderm
- ventral mesoderm
- dorsal mesoderm (including the Organiser)
- The dorsal mesoderm signal comes from the
Nieuwkoop Centre - 1 component of the Nieuwkoop Centre is
beta-catenin (but this is not the signal!) - What is the ventralising signal?
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17From Wolpert, chapter 3
18An inhibitor of Nodal-related (Xnr) proteins
prevents the formation of the dorsal blastopore
lip
It also inhibits the production of Goosecoid, a
protein that is synthesised just in the Spemanns
Organiser.
19Nodal-related (Xnr proteins)
- Are members of the TGF-b superfamily
- Produced in the vegetal hemisphere
- Diffuse and induce mesoderm in overlying animal
cells - Regions with little Xnr proteins develop as
ventral mesoderm - Regions with medium Xnr proteins develop as
lateral mesoderm - Regions with high Xnr develop as dorsal mesoderm
(Organiser)
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22Spemann Organiser
Organization of a secondary axis by dorsal
blastopore lip tissue. (A) Dorsal lip tissue
from an early gastrula is transplanted into
another early gastrula in the region that
normally becomes ventral epidermis. (B) The donor
tissue invaginates and forms a second
archenteron, and then a second embryonic axis.
Both donor and host tissues are seen in the new
neural tube, notochord, and somites. (C)
Eventually, a second embryo forms that is joined
to the host. (D) Structure of the dorsal
blastopore lip region in an early Xenopus
gastrula.
23Mesoderm induction
- Several secreted proteins are synthesised in the
Organiser - These include Noggin, chordin and frizbee
- They are good candidates because
- they cant induce mesoderm in animal cap
experiments - But
- they can dorsalise ventral mesoderm
24Rescue of dorsal structures by Noggin protein.
When Xenopus eggs are exposed to ultraviolet
radiation, cortical rotation fails to occur, and
the embryos lack dorsal structures (top). If such
an embryo is injected with noggin mRNA, it
develops dorsal structures in a dosage-related
fashion (top to bottom). If too much noggin
message is injected, the embryo produces dorsal
anterior tissue at the expense of ventral and
posterior tissue, becoming little more than a
head (bottom).
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26Ventralising signals
- Candidates are
- BMP4 (TGF-b family). Knock out BMP4 function,
get dorsalisation of mesoderm. - Xwnt8. Xwnt8 ventralises mesodermal explants
- The big surprise was that the signals from the
Organiser (Noggin, Frizbee etc.) interact with
the ventralising signals - Noggin interacts with BMP4 and prevent it
binding to its receptor - Frizbee interacts with Xwnt8 and prevents it
binding to its receptor
27 Xwnt8 is capable of ventralizing the mesoderm
and preventing anterior head formation in the
ectoderm. (A) Frzb is a protein secreted by the
anterior region of the organizer. It binds to
Xwnt8 before that inducer can bind to its
receptor. Frzb resembles the Wnt-binding domain
of the Wnt receptor (Frizzled protein), but is a
soluble molecule. (B) Xwnt8 is made throughout
the marginal zone.
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30The three main stages of vertebrate development
- 1) setting up the main body axes (A/P and D/V)
- 2) specification of three germ layers (endoderm,
mesoderm and ectoderm) - 3) germ layer patterning (mesoderm and early
nervous system) - Maternal genes provide factor (RNA and proteins)
to the egg during oogenesis (including
sub-cellular localization to specific regions. - Zygotic genes are expressed by the embryo's
genes. - Both maternal and zygotic genes may have long
term effects upon the embryo's development.