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Fig. 8. Model of the molecular pathways and their potential interactions involved
during the development of the pyloric sphincter. Schematic representations of
avian stomach (left panel) and the molecular pathways involved (right panel).
The avian stomach can be divided in proventriculus (glandular stomach) and
gizzard (muscular stomach). The pyloric sphincter is a highly conserved
structure present in all vertebrates, which (in avians) anatomically separates
the gizzard from the duodenum. Shh from epithelium induces
Bmp4 expression in the adjacent mesenchyme, except in the gizzard
where Bapx1 prevents Bmp4 expression. In the small
intestine, Bmp4 activates the BMP signaling pathway in the mesoderm
and endoderm (S.F. and P.d.S.B., unpublished). In the pyloric sphincter,
Bmp4 is able to activate the expressions of Nkx2.5 and
Sox9, which are both sufficient to induce pyloric epithelial
phenotype through mesenchymal-epithelial signal modulation. Importantly, our
data show that there is no cross-regulation between Sox9 and
Nkx2.5 at the transcriptional level. SOX9 is able to control
Gremlin expression in the pyloric sphincter mesenchyme. Gremlin, a
diffusible factor, could modulate endodermal BMP pathway activation, in order
to induce specific pyloric epithelium differentiation.
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