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parts which elaborate it in smaller quantities, would
induce neural crest.” Since the cells that will ultimately
develop into dorsal neural tube are initially immedi-
ately adjacent to the nonneural ectodermal cells, these
could provide a signal for dorsal differentiation similar
to the notochord-derived Shh for ventralization of
the neural tube. This idea has been postulated for a
number of years in various forms but has only recently
been tested with perturbations of specific candidate-
inducing molecules.
Several lines of evidence now support the hypoth-
esis that the ectoderm provides the molecular signals
to promote dorsal differentiation in the lateral regions
of the spinal cord, and likely in the more anterior
regions of the neuraxis. Moury and Jacobson (1990)
first tested whether interactions between the neural
plate and the surrounding ectoderm were responsible
for the induction of neural crest by transplanting a
small piece of the neural plate from a pigmented
animal to the ventral surface of the embryo. When the
embryo was allowed to develop further, the transplant
rolled into a small tube and at the margins gave rise to
neural crest cells, as evidenced by the pigmented
melanocytes that migrated from the ectopic neural
tissue. These results were extended by the similar
experiments of Selleck and Bronner-Fraser (1995) in
the chick embryo, and in addition, they used an
explant culture system, in which neural plate and epi-
dermis were co-cultured and analyzed for proteins
and genes normally expressed by neural crest. They
found that the neural crest was induced to form from
the neural tube when placed adjacent to the epidermis.
The initial steps toward identifying the crest inducer
were made by Liem et al. (1995). BMPs, discussed in
the previous chapter for their role in neural induction,
also play important functions in specifying dorsal
regional identity in the developing spinal cord. Liem
et al. (1995) used a similar explant culture system as
that used for the analysis of Shh effects on ventraliza-
tion of the neural tube. The neural tube was dissected
into a ventral piece, a dorsal piece, and an intermedi-
ate piece (Figure 2.27). They then analyzed the expres-
sion of genes normally restricted to either the dorsal
neural tube or the ventral neural tube to determine
whether these genes were specifically induced by co-
culture with the ectoderm. They found that certain
dorsally localized genes, such as pax3 and msx1 , are
initially expressed throughout the neural tube and are
progressively restricted from the ventral neural tube
by Shh from the notochord and floorplate. However,
co-culture with the ectoderm was necessary to induce
the expression of other, more definitive, dorsal
markers, such as HNK1 and slug . BMPs were found to
effectively replace the ectodermally derived signal,
A
B
Prosencephalon
Mesencephalon
s1
Rhombencephalon
s1
s4
s4
s7
s7
Cervical
Spinal Cord
s18
Mesectoderm
Pigment cells
Parasympathetic ganglia
Enteric ganglia
Sensory ganglia
Sympathetic ganglia
Endocrine cells
Thoracic
Spinal Cord
s24
s28
Lumbosacral
Spinal Cord
FIGURE 2.26 The fate map of neural crest in the chick embryo.
Various types of tissues, including pigment cells, sensory ganglia,
and endocrine cells, are derived from the neural crest. The cells
migrating from the various positions along the neural tube give rise
to different tissues; for example, the sympathetic ganglia arise from
the neural crest of the trunk, but not from the head. Similarly, the
parasympathetic ganglia arise from the neural crest of the head but
not from the crest that migrates from most trunk regions. (Repro-
duced from Le Douarin et al., 2004)
forms first as a thickening of the neurectoderm, known
as the neural keel, and tube formation occurs later by
a process of cavitation, but the neural crest still forms
from the lateral edges of the plate. Additional recent
studies have also shown that although most of the
neural crest normally arises from the lateral edges of
the neural plate, there is a late-migrating population of
crest cells that are derived from the neural tube.
The first experimental studies to indicate that the
induction of the neural crest may involve some of the
same factors as those responsible for neural induction
were those of Raven and Kloos (1945). They found that
neural crest was induced from ectoderm by lateral
pieces of the archenteron roof, whereas neural tube
was induced by medial pieces, such as the presump-
tive notochord. Similar results led Dalq (1941) to
propose that a concentration gradient of a particular
organizing substance originating in the midline tissue
of the archenteron roof could set up medial-lateral dis-
tinctions across the neural plate—“the median strip of
the archenteron roof, supposedly rich in organisine,
would induce neural structures, while the more lateral
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