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Notochord removed
Notochord transplanted
Roof plate
Roof plate
Roof plate
ectopic motor neurons
ectopic floorplate
No motor
No floorplate
FIGURE 2.23 Differentiation in the neural tube is dependent on factors derived from adjacent, nonneural
tissues. The diagrams at the top of the figure show that if the notochord, a mesodermally derived structure,
is removed prior to neural tube closure, the neural tube fails to display characteristics of ventral differentia-
tion, such as the development of the floorplate (blue) and the spinal motoneurons (red). This shows that the
notochord is necessary for the development of ventral neural tube fates. If an additional notochord is trans-
planted to the lateral part of the neural tube at this same time in embryogenesis, a new floorplate is induced
adjacent to the transplanted notochord. New motoneurons are also induced to form adjacent to the ectopic
floorplate. Thus, the notochord is sufficient to specify ventral cell fates. In the lower panels, the experiment
diagrammed at the top, the transplantation of an extra notochord, is shown next to a normal neural tube
labeled with a marker for motorneurons. The extra notochord is labeled as n ยข. In the lower right, the expres-
sion of sonic hedgehog in the notochord and floorplate (arrow) of the neural tube is shown. (B, C, and D cour-
tesy of Henk Roelink)
of the neural tube. First, antibodies raised against Shh
will block the differentiation of floorplate and motor
neurons when added to neural tube explants. Second,
targeted deletion of the Shh gene in mice results in the
failure of the development of the ventral cell types in
the spinal cord (see Chapter 4).
In addition to its role in the ventralization of the
neural tube, Shh is also expressed in the more anterior
regions of the body axis immediately subjacent to the
neural tube, in what is known as the prechordal meso-
derm. Here the function of Shh is similar to that of the
notochord and floorplate: it serves to induce ventral
differentiation in the forebrain. In the forebrain, the
growth of the different brain vesicles gives rise to
complex anatomy, and so the induction of ventral fore-
brain is critical for a number of subsequent morpho-
genetic events. Consequently, the loss of Shh signaling
in the prechordal mesoderm produces dramatic phe-
notypic changes in embryos and the resulting animals.
One particularly striking phenotype that arises from
the disruption of Shh in embryogenesis is cyclopia
(Roessler et al., 1997). The eyes normally form from
paired evaginations of the ventral diencephalon (see
above). However, in the neural plate, the eye field is
initially continuous across the midline and is split into
two by the inhibition of eye-forming potential by Shh
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