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FIGURE 1.31 Overexpression of the proneural bHLH gene NeuroD in frog embryos leads to the forma-
tion of a large number of ectopic neurons (right) all over the side of the animal. These neurons are never
present in the normal animal (left). Neural tissue expresses the neural cell adhesion molecule (NCAM), and
both embryos have been strained for this protein. (From Lee et al., 1995)
FIGURE 1.32 Notch ICD expression in frog embryos blocks neural differentiation. In this experiment,
the intracellular domain of the Notch receptor was overexpressed in the frog embryo. A. A normal neural
plate stage frog embryo labeled with a neuron marker, Neuron-Specific-Tubulin. There are three purple
stripes of neurons on either side of the midline, and these are the primary neurons of the spinal cord.
They are already beginning their differentiation at neural plate stages, before the neural tube has rolled
up. B. A neural plate stage embryo that has been injected at the two-cell stage in the left blastomere
with the Notch intracellular domain. All of the neurons on the left side of the embryo have failed to
develop, although the right side develops the three normal stripes of primary neurons. (From Wettstein et
al., 1997; Chitnis et al., 1992)
of the developing nervous system. In addition, verte-
brates also have several ligands for Notch receptors,
including two Delta ligands and two Serrate-like
ligands, also known as Jagged in mammals (Kintner
and Weinmaster, 2003). The Notch signaling system is
active during many of the cell fate decisions made in
the developing embryo, not just those in the nervous
system. Therefore, experimental manipulations of this
pathway often have widespread consequences during
development. Nevertheless, the findings have been
consistent with a model of Notch/Delta function
that is similar to that discovered in Drosophila . For ex-
ample, overexpression of activated Notch1 in Xenopus
embryos prevents neural differentiation; the progeni-
tor cells remain undifferentiated and do not form
neurons (Figure 1.32). The various Notch receptors
have been eliminated in mice and cause significant dis-
ruption in development. Examination of the nervous
system in Notch1 -/- mouse embryos shows that
neurons prematurely differentiate in the neural tube.
This result further supports the model that
Notch/Delta interactions inhibit neuron formation
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