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Neural plate
Neurogenic region
Zone (IMZ)
Spinal cord
Neurogenic region
Neural crest
Neural tube
Yolk plug
Ventral mesoderm
FIGURE 1.9 The neural plate (light red) rolls up into a tube sep-
arating from the rest of the ectoderm. The involuting cells condense
to form a rod-shaped structure—the notochord—just underneath
the neural plate. At the same time, the neural plate begins to roll up
and fuse at the dorsal margins. A group of cells known as the neural
crest (bright red) arises at the point of fusion of the neural tube.
Neural plate
Neural plate
Spinal cord
The complex tissue rearrangements that occur
during gastrulation in the amphibian occur in other
vertebrates in fundamentally the same way. However,
the details of these movements can be quite different.
Much of the difference in cell movements lies in dif-
ferences in the amount of yolk in the egg. Fish and bird
embryos have a substantial amount of yolk; since the
cleavage divisions proceed more slowly through the
yolk, these animals have many more cleavage divi-
sions in the animal pole than in the vegetal pole. In
zebrafish embryos, the blastomeres are situated at the
top of the egg, and as development proceeds these
cells divide and spread downward over the surface of
the yolk cells in a process known as epiboly. At 50%
epiboly, when the spread reaches the equator, there is
a transient pause as the process of gastrulation begins
at the future dorsal margin of the embryo, which is at
this point called the shield (Figure 1.10). The shield
begins to thicken as gastrulation commences. Prospec-
tive mesodermal cells delaminate, move inside the
ectodermal layer, and begin migrating back toward the
animal pole. The rest of the ectoderm then continues
its migration to the vegetal pole until the yolk is com-
pletely enveloped at 100% epiboly. As the dorsal meso-
FIGURE 1.8 The development of the central nervous system,
brain, and spinal cord in a frog embryo is shown from the egg cell
to the adult. After a series of cleavage divisions produce a blastula,
a group of cells known as the involuting marginal zone, or IMZ,
grow into the interior of the embryo at a point known as the blasto-
pore. This process of gastrulation is shown in two cross sections. The
involuting cells go on to form mesodermal tissues (blue) and induce
the cells of the overlying ectoderm to develop into neural tissue,
labeled as the neurogenic region (red). After the process of neural
induction, the neurogenic region is known as the neural plate and
is now restricted to giving rise to neural tissue. A cross section of the
embryo at the neural plate stage shows the relationships between
the tissues at this stage of development. The neural plate goes on to
generate the neurons and glia in the adult brain and spinal cord.
of vertebrates. Another source of neurons and glia is the
neural crest, a group of cells that arises at the junction
between the tube and the ectoderm (Figure 1.9). The
neural crest is the source of most of the neurons and glia
of the peripheral nervous system, whose cell bodies lie
outside the brain and spinal cord. This tissue is unique
to vertebrates, and has the capacity to generate many
diverse cell types; we will have more to say about
neural crest in later chapters.
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