Healthcare and Medicine Reference
In-Depth Information
SPECIFICATION AND
DIFFERENTIATION THROUGH
CELLULAR INTERACTIONS
AND INTERACTIONS WITH THE
LOCAL ENVIRONMENT
A
Cranial
S2
Vagal
The vertebrate neural crest is a transient stem cell
population that arises along the lateral edges of the
neural plate induced by the convergence of secreted
signals (notably Wnts, BMPs, and FGFs), at the juxta-
position of neural plate, lateral epidermis, and subja-
cent paraxial mesoderm (see Chapters 1 and 2 for
details of crest induction). As described in Chapter 3,
these cells migrate from their site of origin at the
dorsal-most part of the neural tube, along stereotypic
pathways through the rest of the embryo. In this
section, we discuss how the neural crest cells become
specified toward different fates as they migrate
through different environments. The neural crest pro-
genitors continue to divide as they migrate until they
coalesce at their destinations. Crest cells generate a
variety of cell types. Not only does the crest produce
the entire peripheral nervous system, including the
autonomic and sensory ganglia, and the peripheral
glia (Schwann cells), but it also produces endocrine
chromaffin cells of the adrenal medulla, smooth
muscle cells of the aorta, melanocytes, cranial cartilage
and teeth, and a variety of other nonneural compo-
nents. Because of its variety of descendants, crest has
been a popular model for testing the mechanisms that
generate cell diversity (Le Douarin, 1982).
To test whether premigratory crest cells are com-
mitted to a particular fate, Le Douarin and colleagues
transplanted the crest between different anterior-
posterior positions (Figure 4.12). These experiments
took advantage of the chick-quail chimeric system
described in Chapter 2. The results show that crest
cells acquire instructions to differentiate during their
migration, as well as when they arrive at their final
destination. For example, crest cells from the trunk
normally give rise to adrenergic cells of the sympa-
thetic nervous system, whereas the more anterior crest
cells from the vagal region give rise to cholinergic
parasympathetic neurons that innervate the gut. When
vagal crest cells from quail embryos were transplanted
into the trunk region of chicken embryos, the trans-
planted vagal crest migrated along the trunk pathways
and differentiated into adrenergic neurons in sympa-
thetic ganglia. Similarly, trunk crest cells that were
transplanted to the vagal region gave rise to choliner-
gic neurons of the gut. Similar experiments have been
done to test the competence of crest cells to form a
variety of different cell types, and the general conclu-
Cholinergic neurons
(parasympathetic)
S7
Adrenergic neurons
(sympathetic)
S18
Trunk
Adrenal
medullary
region
S24
Lumbosacral
B
Cholinergic
neurons
Transplant
Adrenergic
neurons
Quail
Chick
FIGURE 4.12 The environment influences the fate of neural crest
cells. A. Crest cells from the trunk normally give rise to adrenergic
cells of the sympathetic nervous system, whereas the more anterior
crest cells give rise to cholinergic parasympathetic neurons that
innervate the gut. B. When anterior crest cells from quail embryos
are transplanted into the trunk region of chicken embryos, they dif-
ferentiate into adrenergic neurons. Similarly, trunk crest cells that are
transplanted anteriorly give rise to cholinergic neurons. (After Le
Douarin et al., 1975)
sion is that crest cells display great flexibility in
responding to local environmental cues. It could be
that each region of the crest contains a complement of
specified progenitors, only some of which survive in
each location, but it seems more likely, given the evi-
dence below, that commitment to a particular fate is a
multistep process of determination.
Migrating crest cells become exposed to a sequence
of instructive environments, each with a unique set of
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