Brain Development
107
are known to delineate prosomeres in the
mouse brain, and dissociated cells have
been demonstrated to sort each other
out
in vitro
, dependent on the cadherin
subclass expressed. In the E11.5 mouse
telencephalon, cadherin-6 demarcates the
lge (p5), while R-cadherin complementary
delineates the ctx (p4). As the overexpres-
sion of cadherins at the ctx/lge boundary
using the
in vivo
EP method (see Sect. 7
for this methodology) results in the sort-
ing of cells with ectopic cadherin into
the territory expressing the same cad-
herin at the boundary, this indicates that
the differential expression of cadherins
plays a role in maintaining the com-
partment boundary. Notably, R-cadherin
expression in the ctx has been shown to
be downstream of Pax6. Transcription fac-
tors might therefore regulate cell adhesion
molecules in order to maintain their ex-
pression pattern by sorting mechanisms
in the telencephalon.
Most recently, an important genetic cas-
cade that patterns the rostro-caudal axis
within the mouse cerebral cortex at later
stages of development has been identiFed.
As summarized in ±ig. 8(c), the expres-
sion level of transcription factors Pax6 and
Emx2 was found to make the countergra-
dients along the mediolateral axis in the
neocortex primordium at E12.5, and if one
of the gradients was deprived by means of
genetic mutation, regional identities were
revealed to be shifted accordingly. Up-
stream of Pax6/Emx2 countergradients,
±gf8 expressed at the anterior neural ridge
has been suggested as playing a pivotal
role. Indeed, overexpression of ±gf8 in the
posterior cerebral neocortex caused the du-
plication of the cortical area with its mirror
image, indicating the organizer activity of
±G±8 in the cerebral cortex primordium
(±ig. 8d). As the mammalian neocortex
has been thought to evolve via multiple
duplication of identities, local changes in
organizer activities such as ectopic sources
of ±G±8 expression might actually con-
tribute to emerge the diversity of brain
regions.
3
Cellular Aspects to Generate Distinct Types
of Cells and/or Cellular Organization (e.g.
Nuclei, Layers, and so on) in Patterned
Tissue
While the neural tissue at earlier stages
has just a single-cell layer organization
with the epithelial character, differentiated
tissues contain multiple types of cells
with complex cellular organizations. There
are three major classes of cells in the
developing and adult CNS:
nerve, glial,
and
ependymal cells
.
Nerve cells, also called
neurons
,p
l
a
y
a central role in constituting functional
neuronal networks of the nervous system.
Most nerve cells have large cell bodies
called
soma
, which are accompanied by
relatively large cell nuclei as well as
multiple processes called either
axons
or
dendrites
: the former are deFned by
their ability to gather the action potentials
ands
ende
l
e
c
t
r
i
cs
ign
a
l
stoo
th
e
rn
e
r
v
e
cells, and the latter are the sites for
receiving these electric signals from axons.
One nerve cell normally has one axon
and multiple dendrites. In an
in vitro
situation, the processes of nerve cells
are also called neurites, and the longest
neurite has been shown to acquire axon
characters. In the CNS, axon terminals of
nerve cells make connections with other
nerve cells and the contact sites between
nerve cells are called
synapses
. Nerve cells
also innervate muscles and the contact
site is speciFcally called the
neuromuscular
junction
(NMJ).
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