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Polarity and Segmentation
cephalon remains as a single vesicle and does not
expand to the same extent as the other regions of
the brain. The rhombencephalon divides into the
metencephalon and the myelencephalon. These two
vesicles will form the cerebellum and the medulla,
The most caudal brain region is the rhomben-
cephalon, the region that will develop into the hind-
brain. At a particular time in the development of this
part of the brain, the rhombencephalon becomes
divided into segments, known as rhombomeres (see
below). The rhombomeres are regularly spaced repeat-
ing units of hindbrain cells and are separated by dis-
tinct boundaries. Since this is one of the clearest areas
of segmentation in the vertebrate brain, study of the
genes that control segmentation in rhombomeres
has received a lot of attention and will be discussed
in detail in the next section as a model of how the
anterior-posterior patterning of the nervous system
takes place in vertebrates.
The insect nervous system is made up of a series of
connected ganglia known as the ventral nerve cord. In
many insects, the ganglia fuse at the midline. The
segmental ganglia of the ventral nerve cord are not all
identical, but rather vary from anterior posterior in the
number and types of neurons they contain. The insect
brain is composed of three regions, known as the pro-
tocerebrum, the deutocerebrum, and the tritocerebrum
(Figure 2.2). The compound eyes connect through the
optic lobes to the rest of the brain. Thus, as in the ver-
tebrate, there are quite distinct regional differences
along the anterior-posterior axis of the insect nervous
system, and so there must be mechanisms that make
one part of the nervous system different from another
Like the rest of the body in most metazoans, the
nervous system is regionally specialized. The head
looks different from the tail, and the brain looks dif-
ferent from the spinal cord. There are a number of basic
body plans for animals with neurons, and in this
section, we will consider how the regional specializa-
tion of the nervous system arises during the develop-
ment of some of these animals. At least some of the
mechanisms that pattern the nervous system of
animals are the same as those that pattern the rest of
the animal's body. Similarly, many different types
of tissues play key roles in regulating the development
of the nervous system.
In the vertebrate embryo, most of the neural tube
will give rise to the spinal cord, while the rostral end
enlarges to form the three primary brain vesicles: the
prosencephalon (or forebrain), the mesencephalon
(or midbrain), and the rhombencephalon (or hind-
brain) (Figure 2.1). The prosencephalon will give
rise to the large paired cerebral hemispheres, the
mesencephalon will give rise to the midbrain, and
the rhombencephalon will give rise to the more
caudal regions of the brainstem. The three primary
brain vesicles become further subdivided into five
vesicles. The prosencephalon gives rise to both the
telencephalon and the diencephalon. In addition
to generating the thalamus and hypothalamus in
the mature brain, important features of the dien-
cephalon are the paired evaginations of the optic
vesicles. These develop into the retina and the
pigmented epithelial layers of the eyes. The mesen-
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