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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

©Cebra-Thomas, 2001

Last Modified: 2 August 2001

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