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The interactions between soluble morphogens, metanephric mesenchyme and
matrix molecules during the branching process of the ureteric tree. (A) Local
and possibly global gradients of soluble branch promoting growth factors, such
as pleiotrophin and fibroblast growth factors, may promote vectorial branching
and elongation of the ureteric bud (UB) stalks
(Qiao et al., 2001;
Sakurai et al., 2001). (B)
Inhibitory growth factors such as transforming growth factor ß and the
bone morphogenetic proteins may negatively feed back to regulate growth and
branching and may also function to maintain luminal caliber
(Santos and Nigam, 1993;
Dunn et al., 1997;
Bush et al., 2004a). These
stimulatory and inhibitory branching morphogens may govern the process of
vectorial branching by altering expression of distal effector molecules at
tips, branches and stalks, which are known to be important in branching
morphogenesis, such as (C) proteinases [e.g. matrix metalloproteinases
(Lelongt et al., 1997;
Pohl et al., 2000a)], (D)
inhibitors of proteinases (Sakurai and
Nigam, 1997), (E) extracellular matrix proteins (e.g.
proteoglycans) (Pohl et al.,
2000b), and cell-adhesion molecules (e.g. integrins)
(Qiao et al., 2001;
Zent et al., 2001). Cell-cell
contact with the metanephric mesenchyme may also be necessary for proper
patterning (vectorial branching and stalk elongation) of the ureteric tree
(Qiao et al., 1999a) (G).
Adapted, with permission, from Stuart and Nigam
(Stuart and Nigam, 1995).
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