Transcript Developmental Biology 8/e

10.20 Summary of experiments by Nieuwkoop and by Nakamura and Takasaki,
showing mesodermal induction by vegetal endoderm (Part 1)
10.20 Summary of experiments by Nieuwkoop and by Nakamura and Takasaki,
showing mesodermal induction by vegetal endoderm (Part 2)
10.21 Experiments on 64-cell amphibian embryos demonstrated that the vegetal cells underlying
the prospective dorsal blastopore lip region are responsible for causing the initiation of gastrulation
10.22 The regional specificity of mesoderm induction can be demonstrated by
recombining blastomeres of 32-cell Xenopus embryos
10.23 The role of Wnt pathway proteins in dorsal-ventral axis specification
A) Early 2 cell stage; β-catenin in orange
B) Dorsal side of a presumptive blastula
and nuclear β-catenin
C) No β-catenin in the ventral side
D) β-catenin dorsal localization through
gastrula stage
10.24 Model of the mechanism by which the Disheveled protein stabilizes -catenin in
the dorsal portion of the amphibian egg (Part 1)
10.24 Model of the mechanism by which the Disheveled protein stabilizes -catenin in
the dorsal portion of the amphibian egg (Part 2)
10.25 Summary of events hypothesized to bring about the induction of the organizer in
the dorsal mesoderm
Goosecoid appears to be
essential for specifying the
dorsal mesoderm.
Goosecoid expression
occurs when there is a
synergism between these
proteins and TGF-β signal
secreted by vegetal cells.
10.26 Model for mesoderm induction and organizer formation by the interaction of catenin and TGF- proteins
Nodal
related gene
Ventral and lateral
mesoderm
Functions of the organizer (page 312)
- While the Nieuwkoop center cells remain endodermal, the
cells of the organizer become the dorsal mesoderm and
migrate underneath the dorsal ectoderm.
- Once the dorsal portion of the embryo is established, the
movement of the involuting mesoderm establishes the
anterior-posterior axis.
- The endomesoderm that migrates first over the dorsal
blastopore lip give rise to the anterior structures.
- The mesoderm migrating over the lateral and ventral lips
forms the posterior structures.
The organizer contribute to four cell types:
1) Pharyngeal endoderm
2) Head mesoderm (prechordal plate)
Induce the forebrain
and midbrain
3) Dorsal mesoderm (primarily the notochord)- induces the
hindbrain and trunk
4) Dorsal blastopore lip- forms the dorsal mesoderm and
eventually becomes the chordaneural hinge that induces
the tip of the tail
10.27 Ability of goosecoid mRNA to induce a new axis
The Nieuwkoop center activates the goosecoid gene in the
organizer tissues.
A) Gastrula, one blastopore lip
B) Gastrula, two blastopore lips,
goosecoid was injected
C) Goosecoid injected, 2 axes
and controls
D) Twinned embryo produced by
goosecoid injection
10.28 Neural structures induced in presumptive ectoderm by newt dorsal lip tissue, separated from
the ectoderm by a nucleopore filter with an average pore diameter of 0.05 mm
-The epidermis that is induced to
form, not the neural tissue.
-The ectoderm is induced to
become epidermal tissue by
binding bone BMPs.
-The nervous system forms from
that region of the ectoderm that is
“protected” from epidermal
induction.
-The “default fate” of the ectoderm is to become neural tissue;
- Certain parts of the embryo induce the ectoderm to become epidermal
tissue by secreting BMPs
10.29 Rescue of dorsal structures by Noggin protein
Injection of Noggin mRNA into
1-cell, UV-radiated embryos
completely rescues dorsal
development.
-Noggin induces dorsal ectoderm
to form neural tissue;
-Noggin dorsalizes mesoderm
cells that would otherwise
contribute to the ventral
mesoderm.
- The development of dorsal
structures is a dosage-dependent.
10.30 Localization of noggin mRNA in the organizer tissue, shown by in situ hybridization
Noggin mRNA is first localized in
the dorsal blastopore lip
region and then becomes
expressed in the notochord.
Noggin binds to BMP4 and BMP2
and inhibits their binding to
receptors.
A) At gastrulation at dorsal marginal zone
B) When cells involute in the dorsal blastopore lip
C) During convergent estension in the precursors of notochord
D) Extend beneath the ectoderm in the center of the embryo
10.31 Localization of chordin mRNA
Chordin was found to be localized in the dorsal blastopore lip and later
in the notochord.
Of all organizer genes, chordin is the one most acutely activated by βcatenin.
Chordin binds to BMP2 and BMP4 and prevents their complexing
with their receptors.
Follistatin and BMPs
- Follistatin is also transcribed in the dorsal blastopore lip and
notochord.
- Follistatin is an inhibitor of both activin and BMPs, causing
ectoderm to become neural tissue.
- In Xenopus, the epidermal inducers are BMPs (BMP4,
BMP2, BMP7) and some relatives such as ADMP (antidorsalizing morphogenic protein).
- BMP4 induced ectodermal cells to become epidermal.
10.32 Model for the action of the organizer
Thus, the epidermis is instructed by
BMP signaling, and the organizer
works by blocking that BMP signal
from reaching the ectoderm above it.
10.34 Regional specificity of induction can be demonstrated by implanting different
regions (color) of the archenteron roof into early Triturus gastrulae
10.35 Regionally specific inducing action of the dorsal blastopore lip
10.36 Paracrine factor antagonists from the organizer are able to block specific
paracrine factors to distinguish head from tail (Part 1)
10.36 Paracrine factor antagonists from the organizer are able to block specific
paracrine factors to distinguish head from tail (Part 2)
10.37 Cerberus mRNA injected into a single D4 blastomere of a 32-cell Xenopus
embryo induces head structures as well as a duplicated heart and liver
Induction of the most anterior head structures could be accomplished
by a secreted protein called Cerberus which promotes the formation of
cement glands (the most anterior region of the tadpole ectoderm),
eyes, and olfactory placodes.
Cerberus mRNA
injected in a vegetal
ventral Xenopus
blastomere originated
head structures.
10.38 Xwnt8 is capable of ventralizing the mesoderm and preventing anterior
head formation in the ectoderm
-Frzb and Dickkopf are synthesized
in the involuting endomesoderm
(the most anterior portion of the
organizer).
-Frzd is a small, soluble form of
Frizzled that can bind to Wnt.
- Dickkopf also appears to interact
directly with the Wnt receptors,
preventing Wnt signaling.
10.39 Insulin-like growth factors enhance anterior neural development
IGFs are required for the formation of the anterior neural tube with its
brain and sensory placodes.
IGFs accumulate in the dorsal midline and are specially prominent in
the anterior neural tube (A).
B) Igf2 injected into
ventral marginal zone
blastomeres showing
ectopic headlike structure.
C) Control and IGF inhibitor
10.40 The Wnt signaling pathway and posteriorization of the neural tube (Part 1)
The primary protein involved in
posteriorizing the neural tube is
thought to be a member of the Wnt
family of paracrine factors, most likely
Xwnt8.
-It appears that a gradient of Wnt proteins is necessary for specifying
the posterior region of the neural plate.
- In Xenopus, a gradient of Wnt signaling and β-catenin is higher in the
posterior and absent in the anterior.
10.40 The Wnt signaling pathway and posteriorization of the neural tube (Part 2)
There appear to be two major gradients in the amphibian
gastrula:
- BMP gradient that specifies the dorsal-ventral axis and
- Wnt gradient that specify the anterior-posterior axis
10.41 Model of organizer function and axis specification in the Xenopus gastrula
10.42 Pitx2 determines the direction of heart looping and gut coiling
The embryo has a right-left axis.
The crucial event in this axis formation is the expression of a nodal gene
in the lateral plate mesoderm on the left side of the embryo.
In Xenopus, this gene is Xnr1.
It seems that the microtubules are involved in the establishment of the
Xnr1 expression pattern.
Vg1 seems to processed into its active form predominantly on the left
hand side of the embryo.
Vg1- Xnr1- activates pitx2
Pitx2 persists on the embryo’s left side as the head and gut develop,
controlling their respective positions.