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A
Epidermis
vs neural
O
O
O
O
O
O
Proneural
genes
hes
hes
Notch
Notch
B
Neural
vs glial
Neuron
Proneural
gene
hes
Astrocyte
FIGURE 3.13 The proneural genes regulate neurogenesis at
early and late stages of development. A. As described in Chapter 1,
the proneural genes are important in the initial segregation of neural
tissue from the epidermis in both Drosophila and vertebrates. B.
Proneural genes are also important in the decision of a progenitor
in the neural tube to generate either a neuron or a glial cell. The pro-
genitors express one of several proneural genes, and this allows
them to generate neurons. If the Notch pathway is activated in the
progenitor cell, the proneural gene expression in the progenitor is
inhibited by the antagonist Hairy/enhancer of split ( hes ) gene expres-
sion and the progenitors primarily generate astrocytes.
FIGURE 3.14 The proneural gene Cash1 is expressed in progen-
itor cells. Combined in situ hybridization for the proneural gene
chicken achaete scute homolog 1 ( Cash1 ) and immunohistochemistry
for BrdU , a marker for mitotically active progenitor cells shows that
many of the BrdU -labeled cells also express the proneural gene Cash1
in this section through the neuroepithelium of the embryonic chick
retina. There are many other BrdU- labeled cells that do not express
the Cash1 gene, and presumably these express a different proneural
gene. (Image thanks to Dr. Branden Nelson)
tion of the nervous system and in the control of the
progenitor cells later in development to direct them to
a neural fate.
tinct cell types: a more superficial marginal zone, con-
taining a group of large, stellate-shaped cells, known
as Cajal-Retzius cells, and a deeper zone of cells called
the subplate cells (Marin-Padilla, 1988; Allendoerfer
and Shatz, 1994). The next stage of cortical develop-
ment is characterized by a large accumulation of newly
postmitotic neurons within the preplate (Marin-
Padilla, 1988). These new neurons form the cortical
plate. The cortical plate divides the preplate into the
superficial marginal zone, composed primarily of the
Cajal-Retzius cells, and the intermediate zone, com-
posed of the subplate cells and increasing numbers
of incoming axons. The developing cortex is thus
described as having four layers: the ventricular zone,
the intermediate zone, the cortical plate, and the mar-
ginal zone.
At the very earliest stages of cortical development,
the processes of the progenitor cells span the entire
thickness of the cortex. The first cortical neurons that
are generated use the predominantly radial orientation
of their neighboring progenitor cells to guide their
C EREBRAL CORTEX HISTOGENES IS
In the next section, the histogenesis of two specific
regions of the CNS—the cerebral cortex and the cere-
bellum—will be highlighted; histogenesis of these
structures has been the subject of intense study for
many years. As noted in the previous chapter, the cere-
bral hemispheres develop from the wall of the telen-
cephalic vesicle. The neuroepithelial cells initially
span the thickness of the wall, and as they continue
to undergo cell division, the area of the hemispheres
expands. At this early stage of development, the pro-
genitor cells are thought to undergo primarily sym-
metric cell divisions, and their progeny both remain in
the cell cycle. Soon, however, a few cells withdraw
from the cycle to develop as the first cortical neurons.
These neurons migrate a short distance to form a dis-
tinct layer, just beneath the pial surface, known as the
preplate (Figure 3.15). The preplate consists of two dis-
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