Nephrogenesis has been a long standing model of studies on epithelial-mesenchymal interactions and differentiation. During nephrogenesis [reviewed in 1], the ingrowing ureter bud induces the differentiation of an apparently homogenous cell mass, the nephrogenic mesenchyme, to form intricately structured nephrons. The nature of the inducing signals is still obscure. First, the mesenchymal cells condense near the tip of the ureter bud, followed by the polarization of the condensate to form the primary vesicle. The vesicle then first invaginates distally, forming the comma-shaped body, and the second, proximal invagination results in the formation of the S-shaped body, which then develops into the mature nephron.