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A
B
mb
di
hb
mx
ma
tb
sc
ht
2
E9.0
C
D ch
ch
mb
di
di
cb
cp
cp
my
my
6
FIGURE 2.15 Several important signaling molecules have been localized to the midbrain-hindbrain
boundary, a key signaling center in the brain. wnt1 (A) engrailed-1 and fgf8 (B) form an interconnected network
that specifies this boundary and is necessary for the growth of the midbrain and the cerebellum. Deletion of
any of these molecules in mice results in a loss of the midbrain and reduction in cerebellar size. A section
through the brain of a wild-type embryo is shown in C, while a wnt1 knockout mouse brain is shown in D.
Note the loss of the midbrain (mb) and cerebellum (cb) in the mutant brain. Other structures are normal (ch
= cerebral cortex; cp = choroids plexus; di = diencephalon; my = myelencephalon). (A, B, D modified from
Danielson et al., 1997; B modified from Crossley and Martin, 1995)
distance over which these molecules can signal. Once
the number of cells exceeds the range over which the
signal can act, the brain just makes a new signaling
source. The same principle seems to be driving the
construction of cell phone towers.
of homeodomain proteins are expressed in these more
anterior regions of both vertebrate and invertebrate
embryos, and they perform a role similar to that of
Hox gene clusters in more caudal segments. Below, we
explore the evidence that homeodomain proteins
specify the structures that comprise the head and brain.
The most widely held view is that different parts of
the brain are generated through the progressive sub-
division of initially similar domains. The neural plate
begins to show regional differences in the anterior-
posterior direction at its formation. Embryologists at
the beginning of the last century applied small amounts
of dyes to specific parts of developing embryos and
found that particular regions of the neural plate are
already constrained to produce a particular part of the
nervous system. Many embryologists have also used
transplantation between species to define the contri-
FOREBRAIN DEVELOPMENT,
PROSOMERES, AND PAX GENES
To this point, we have explored how Hox genes
control the specification of anterior-posterior position
in the nervous system. However, Hox gene expression
stops at the anterior boundary of the metencephalon.
Are there similar transcription factors that control posi-
tional identity in the rest of the brain? Many other types
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