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Dorsal lip
Animal cap
Animal cap
animal cap
animal cap
Neural tissue
FIGURE 1.13 Isolation of fragments of embryos at different
stages of development demonstrates when tissue becomes commit-
ted to the neural lineage. If the animal cap is isolated from the rest
of the embryo (left), the cells develop as epidermis, or skin. If the
same region of the embryo is isolated a few hours later, during gas-
trulation (right), it will develop into neural tissue (shown in the
figure as red neurons). Experiments like these led to the idea that
the neural lineage arises during gastrulation.
FIGURE 1.14 Spemann and Mangold transplanted the dorsal lip
of the blastopore from a pigmented embryo (shown as red) to a non-
pigmented host embryo. A second axis, including the neural tube,
was induced by the transplanted tissue. The transplanted dorsal
blastopore lip cells gave rise to some of the tissue in the secondary
axis, but some of the host cells also contributed to the new body axis.
They concluded that the dorsal lip cells could “organize” the host
cells to form a new body axis, and they named this special region of
the embryo the organizer.
first realizations that came from these additional
studies was that “the organizer” has subdivisions,
each capable of inducing specific types of differentia-
tion. Holtfreter subdivided the organizer region into
pieces, and, using the same transplantation strategy,
he found that when more lateral aspects of the dorsal
lip were used, tails were induced, whereas when
more medial regions of the organizer region were
transplanted, heads were induced. In an attempt to
more precisely define the heterogeneity of the region,
Holtfreter also cultured small bits of the dorsal lip and
found that that these develop into more or less well-
defined structures, such as single eyes or ears! As
Holtfreter succinctly summarizes: “even at the gas-
trula stage the head organizer is not actually an
equipotential entity, but is subdivided into specialized
inductors although distinct boundaries between them
do not seem to exist.”
Neural induction does not appear to act solely
through a vertical signal passed from the involuting
mesoderm; there is also evidence that a neural induc-
ing signal can be passed through the plane of the ecto-
derm. As noted above, when blastulas are placed in
hypotonic solutions just prior to gastrulation, the IMZ
cells fail to involute, and instead evaginate to produce
an exogastrula. Under these conditions, the process of
signaling between the mesoderm and the ectoderm
should be blocked, since involuting mesoderm is no
longer underneath the ectoderm. Surprisingly, how-
ever, some neural induction does appear to take place.
Although this was difficult to determine in Holtfreter's
time, the use of antibodies and probes for neural-
specific proteins and gene expression clearly shows
that organized neural tissue forms from such exogas-
trulated ectoderm (Holtfreter, 1939; Ruiz i Altaba,
1992). This so-called planar induction can also be
demonstrated in a unique tissue combination invented
by Ray Keller that bears his name, the Keller sandwich.
In this preparation, the presumptive neural ectoderm
and the dorsal lip are dissected from two embryos and
sandwiched together. In these, the mesoderm does not
move inside of the sandwich, but rather extends away
from the neurectoderm like an exogastrula (Keller et
al., 1992; Figure 1.15). Only a thin bridge of tissue con-
nects the mesoderm with the neural ectoderm, but
nevertheless, extensive and patterned neural develop-
ment occurs in these cultures (Figure 1.15).
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