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Iseki 1999). It is also known that
FGF8 signaling is necessary for the proper separation of the
midbrain and hindbrain
(Shigetani2000). Embryos
treated with SU5402 were morphologically different from the
control embryos primarily in the craniofacial region. The
size of the experimental embryo's brain sections were
smaller and narrower in shape than in the controls. We
attribute the observed malformations to the interference of
FGF signaling and conclude FGF signaling is necessary for
proper craniofacial development.
In the craniofacial region, FGF is known to signal
Bmp4which in turn
downregulates shh (Cebra-Thomas, personal
communication). According to
our results for the in situ hybridization, we propose
that FGF signaling is reduced by SU5402, then
Bmp4is also reduced
which leaves a minimal amount of signaling factors to
downregulate the shh. The control embryos which were
processed for in
situhybridization with a
shhantisense probe
have blue outlines of the the craniofacial region indicating
shh expression. The SU5402 treated experimental
embryos, which underwent the same procedure and staining,
had an excess of blue staining in the craniofacial
region,particularly the anterior midbrain. This supports the
conclusion that the feedback loop of fgf and shh was broken,
and that there was severely reduced ability to downregulate
shh. If the loop was still functioning, then staining in the
craniofacial region of the experimental embryos would have
been present but in a reduced and controlled manner. Results
show the craniofacial region and limb bud region of chicken
embryos as
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