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in shape and use the neurotransmitter glutamate.
However, there are other populations of neurons in
the cerebral cortex, such as a class of stellate-shaped,
GABA-containing inhibitory interneurons. In a study
of chimeric mice, where single embryonic stem cells
with lacZ reporter genes were injected into wild-type
host embryos, clones of labeled progenitor cells could
be clearly identified in the cerebral cortex from very
early stages of development. Two patterns of clones
were identified: those that were made up primarily of
pyramidal neurons, arranged in radial columns or
clusters (Figure 3.20), and those that were more widely
dispersed and were GABA-containing stellate cells
(Tan et al., 1998). Anderson et al. (1997) found that
when cortical slices were cultured with the lateral
ganglion eminences attached, these GABA neurons
developed in the cortex. However, when the cortex was
isolated from this subcortical region, the number of
GABA-containing neurons was greatly reduced
(Figure 3.20). Thus, it seems clear that the precursors of
most GABA-containing interneurons in the cerebral
cortex migrate all the way from the subcortical germi-
nal ridges. Moreover, they could directly visualize this
migration by labeling the premigratory population in
the ganglionic eminence with the dye DiI , and track the
migration of the GABA-containing neurons to the cere-
bral cortex. Clearly, the tangential migrating popula-
tions of young neurons within the cortex do not interact
with radial glial cells in the same way as that described
for radially migrating cortical neurons. However, at
this time, much more is known about the factors that
guide neurons along radial glia (see below) than for
these tangentially migrating populations.
A
THE SUBVENTRICULAR
ZONE: A SECONDARY ZONE
OF NEUROGENESIS
Remove LGE
Cult ure co rtex
Cerebral
cortex
Reduced #
GABA +
neurons
As we have seen, neurons can at times migrate con-
siderable distances from their point of generation in
the ventricular zone. Although the cases described so
far involve the migration of postmitotic neurons, there
are also regions of the brain where the progenitor cells
themselves migrate from the ventricular zone and
continue to generate neurons in what are known as
secondary zones of neurogenesis. There are three
well-defined secondary zones of neurogenesis in the
mammalian brain: the external granule layer (to be
described in the section on cerebellar histogenesis), the
subventricular zone, and the hippocampal granule cell
precursors. The subventricular zone is a specialized
region of the anterior lateral wall of the lateral ventri-
cle (Figure 3.21). The SVZ forms as a secondary neu-
rogenic zone in the late embryonic period in rodents,
and, although most thoroughly studied in mice and
rats, is present in all mammals examined to date
(Jacobson, 1977). In rats, from E11 to E14 , mitoses occur
exclusively at the ventricular surface; however, at E16 ,
the SVZ forms, subadjacent to the VZ, and the SVZ
continues to generate neurons and glia long after the
VZ has ceased cell division at E19 . The majority of the
glia of the forebrain are thought to be derived from
the SVZ. In recent years, the SVZ has become the sub-
ject of intense investigation owing to its maintenance
in the adult brain, and more will be said about this
later in this chapter. The progenitor cells of the granule
LGE
Clones of
intrinsic cortical
progenitors
GABA +
neurons
B
FIGURE 3.20 The neurons of the cerebral cortex derive from
both intrinsic and extrinsic sources. Most of the neurons in the cortex
are derived from the ventricular zone cells immediately below their
adult location. Clones of cells in the cortex show only a limited
amount of radial dispersion when labeled from very early stages of
development using chimeric mice. However, the GABA containing
stellate cells in the cortex arise from the lateral ganglionic eminence,
a subcortical structure long thought to generate the neurons of
the basal ganglia. If the cortex is isolated from the LGE early in its
development and the cortex is allowed to develop further in vitro,
the number of GABA containing neurons in the cortex is greatly
reduced.
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