Embryonic renal epithelia: Induction, nephrogenesis, and cell differentiation

Embryonic metanephroi, differentiating into the adult kidney, have come to be a generally accepted model system for organogenesis. Nephrogenesis impli es a highly controlled series of morphogenetic and differentiation events t hat starts with reciprocal inductive interactions between two different pri mordial tissues and leads, in one of two mainstream processes, to the forma tion of mesenchymal condensations and aggregates. These go through the intr icate process of mesenchyme-to-epithelium transition by which epithelial ce ll polarization is initiated, and they continue to differentiate into the h ighly specialized epithelial cell populations of the nephron. Each step alo ng the developmental metanephrogenic pathway is initiated and organized by signaling molecules that are locally secreted polypeptides encoded by diffe rent gene families and regulated by transcription factors. Nephrogenesis pr oceeds from the deep to the outer cortex, and it is directed by a second, e ntirely different developmental process, the ductal branching of the ureter ic bud-derived collecting tubule. Both systems, the nephrogenic (mesenchyma l) and the ductogenic (urcteric), undergo a repeat series of inductive sign aling that serves to organize the architecture and differentiated cell func tions in a cascade of developmental gene programs. The aim of this review i s to present a coherent picture of principles and mechanisms in embryonic r enal epithelia.