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Neural tissue,
ectoderm
Indirect induction
of neural tissue
Direct induction
of neural tissue
Animal
A
Equatorial
Mesoderm
E
Animal
Animal
Animal
Animal
Vegetal
o
o
+ candidate
mesoderm
inducer
+ candidate
neural
inducer
Endoderm
Vegetal
Vegetal
Neural
tissue
Neural
tissue
Isolate
A
A
A
o
B
E
FIGURE 1.17 Indirect neural induction versus direct neural
induction. The organizer transplant experiments show that the invo-
luting mesoderm has the capacity to induce neural tissue in the cells
of the animal cap ectoderm. When assaying for the factor released
from mesoderm that is responsible for this activity, it was important
to distinguish between the direct and indirect induction of neural
tissue when animal caps were treated with a candidate factor. In the
first example (left), mesoderm (blue) is induced by the factor, and
then neural tissue (red) is induced by the mesoderm. Thus, both
mesoderm and neural genes are turned on in the animal caps.
However, in the case of a direct neural inducer (right), neural genes
are turned on (red), but mesoderm-specific genes are not expressed.
Mesoderm
fails to develop
V
A
A
C
A
o
V
V
Mesoderm
New
mesoderm
forms
FIGURE 1.16 Interactions between the animal and vegetal cells
of the amphibian embryo are necessary for induction of the meso-
derm. A. The regions of the amphibian embryo that give rise to these
different tissue types are shown. The animal pole gives rise to epi-
dermal cells and neural tissue, the vegetal pole gives rise to endo-
dermal derivatives, like the gut, while the mesoderm (blue) arises
from the equatorial zone. B. If the animal cap and vegetal hemi-
spheres are isolated from one another, mesoderm does not develop.
C. If the equatorial zone is removed from an embryo and the iso-
lated animal and vegetal caps are recombined, a mesoderm forms at
a new equatorial zone.
also restore a normal body axis. Harland's group took
advantage of this fact and used pools of cDNA isolated
from the organizer region to rescue the UV-treated
embryos. By dividing the pools into smaller and
smaller collections, they isolated a cDNA that coded
for a unique secreted protein, which they named Noggin.
When Noggin was purified and supplied to animal
caps, it was capable of specifically inducing neural
genes, without inducing mesodermal genes. Noggin
mRNA is expressed in gastrulating embryos, by the
cells of the dorsal lip of the blastopore, precisely
where the organizer activity is known to reside. Injec-
tion of Noggin mRNA into UV-treated embryos at the
four-cell stage can restore body axis and even hyper-
dorsalize the embryos to give bigger brains than
normal.
At the same time that the Noggin studies were
being done, other labs were using additional
approaches to identify other neural-inducing mole-
cules. DeRobertis was interested in identifying genes
that were expressed in the dorsal blastopore lip organ-
izer region. They isolated a molecule they named
Chordin. Like Noggin, this is a secreted protein that is
expressed in the organizer during the period when
neural induction occurs (Figure 1.19). Overexpression
of Chordin in the ventral part of the embryo causes a
secondary axis, similar to goosecoid. Thus, chordin
appears to be similar to noggin as a putative neural
inducer.
Athird candidate neural inducer was identified by
Melton and his colleagues, as a previously identified
reproductive hormone known as follistatin (Hemmati-
Brivanlou et al., 1994). In the reproductive system, fol-
mal tissue. However, since the organizer at the dorsal
lip of the blastopore is made from mesoderm, it was
not clear whether the neural tissue that developed in
animal caps was directly induced by the exogenous
factor, or, alternatively, whether the factor first induced
the organizer and subsequently induced neural tissue.
(Figure 1.17). Therefore, to refine the assay to look for
direct neural induction, studies concentrated on iden-
tifying factors that would increase the expression of
neural genes without the concomitant induction of
mesoderm-specific gene expression.
The animal cap assay was used for the isolation of
the first candidate neural inducer. Richard Harland
and his colleagues (Lamb et al., 1993; Smith et al., 1993)
used a clever expression cloning system to identify
a neural inducing factor (Figure 1.18). The cloning
was done by taking advantage of the fact that UV-
irradiated frog embryos fail to develop a dorsal axis,
including the nervous system, and instead develop
only ventral structures. Nevertheless, the transplanta-
tion of a dorsal blastopore lip from a different embryo
can restore a normal body axis to the UV-treated
embryo, indicating that the UV embryo can still
respond to the neural inducing factor(s). Furthermore,
injection of mRNA from a hyperdorsalized embryo can
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