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Fig. 7. Schematic model of some mediators of inductive tissue interactions in the
embryonic kidney. Ret, a tyrosine kinase receptor, and its ligand Gdnf are
essential for kidney development
(Airaksinen and Saarma, 2002).
Wnt11 is necessary for Gdnf expression, while Ret
signalling regulates Wnt11 expression
(Majumdar et al., 2003).
Stromal components in the retinoic acid (RA) signalling system also contribute
to Ret expression (Batourina et
al., 2001), perhaps indirectly. Sprouty signalling may contribute
to the coordination of Wnt11/Gdnf/Ret pathways
(Majumdar et al., 2003). As a
RTK receptor effector, Sprouty may either directly antagonize Ret
function or have an indirect effect via changes in reciprocal stromal
signalling, such as Fgf7 or Foxd1-regulated signals
(Hatini et al., 1996).
Fgf7 signalling regulates Spry2 expression and is also
influenced by human SPRY2 expression in the ureteric bud (red arrow),
suggesting a link in inductive signalling between epithelial ureteric bud (U)
and stromal cells (S). W, Wolffian duct; N, nephrogenic mesenchyme.
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