108
Brain Development
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postmitotic and never generate siblings.
In the process of nerve cell production,
progenitor cells are known to get into
the elevator movement, in which the cell
body of the progenitor cells migrates from
the ventricular to the marginal zone in
a cell cycle dependent manner. During
the movement, the asymmetrical divisions
appear to produce neuroblasts and even-
tually generate various types of nerve
cells.
Nerve cells can be distributed in differ-
ent patterns in the CNS as the postmitotic
nerve cells usually migrate toward their
destinations after Fnal cell division. ±or
example, in the spinal cord, motor neu-
rons born in the ventricular zone migrate
radially and make clusters called motor
columns in the ventral horn. Within the
spinal motor column, neurons are fur-
ther segregated into groups by means
of their birthdays and targets: While the
earlier generated (i.e. older) motor neu-
rons constitute the lateral group in the
column and innervate the limb mus-
cles, the younger ones home in on the
medial column and innervate the axial
muscles. In the mammalian cerebral neo-
cortex, nerve cells born in the ventricular
zone travel radially as well as tangentially
and settle in the mantle zone depending
on their birthdays, forming the charac-
teristic layer organization. The migration
pattern of neurons born in the subventric-
ular zone of the telencephalon is another
interesting example, in which the post-
mitotic neurons adhere to each other to
make a chain migratory pathway into the
olfactory bulb. In the mature brain, neu-
rons with distinct functions often gather
and are anatomically distinguishable in
the gray matter as the group of cells
called the
nucleus
. At the dorsal surface of
the CNS, neurons with similar functions
tend to be assembled in the laminated
structure called the
cortex
.Th
en
eu
ron
s
with same functions may further exist
in a sparse manner, and such a dis-
tribution pattern is termed the
reticular
formation
, which plays a crucial role in
modulating both sensory and motor in-
puts.
Glial cells, or simply
glia
, are supporting
cells in the nervous system, and there are
normally between 10 and 50 times more
glial cells than neurons. Various types
of glial cells with distinct functions are
generated in both the CNS and PNS. ±or
example, the oligodendrocyte attaches to
the nerve axon and surrounds it to make
the myelin sheath, enhancing the speed
of electrical signal transmission along the
axon. The Schwann cells in the PNS have
a role similar to the oligodendrocyte. The
astrocyte is thought to relay nutrients
from blood vessels to neurons, and the
microglia plays a role in maintaining
and protecting the cellular organization
of the CNS.
±inally,
the
ependymal
cells
are
a
unique population in the CNS with stem
cell characteristics. They are homed in
the ventricular zone and are shown to
produce
nerve
cells
as
well
as
glial
cells (astrocytes). The ependymal cells
are sometimes called
radial glial cells
as
they extend long radial Fbers from the
ventricular to the pial surface, which are
thought to support the radial migration
of nerve cells during development. In the
adult brain, the ependymal cells continue
to produce siblings and this might help
generate novel neuronal circuits during
the process of learning and/or memory
formation. Several researchers have now
begun to apply this unique character
of the ependymal cells to regenerate a
variety of cells, including nerve cells in
the CNS.
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