Secreted molecules in metanephric induction.

Nearly 50 yr. ago, Clifford Grobstein made the observation that the ureteric bud induced the nephrogenic mesenchyme to undergo tubulogenesis. Since that discovery, scientists have attempted to characterize the molecular nature of the inducer. To date, no single molecule that is both necessary and sufficient for nephric induction has been identified. Because of recent insights regarding the role of several secreted molecules in tubulogenesis, it has become necessary to revise the classic model of metanephric induction. The studies of the classic ureteric inducer performed to date have most likely been characterizations of a mesenchyme-specific inducer, Wnt-4, and its role in tubulogenesis. Ureteric induction most likely involves a series of distinct events that provide proliferative, survival, and condensation signals to the mesenchyme, integrating the growth of the ureteric system with tubulogenesis.