The kidney develops by cycles of ureteric bud branching and nephron formation. The cycles begin and are sustained by reciprocal inductive interactions and feedback between ureteric bud tips and the surrounding mesenchyme. Understanding how the cycles end is important because it controls nephron number. During the period when nephrogenesis ends in mice, we examined the morphology, gene expression, and function of the domains that control branching and nephrogenesis. We found that the nephrogenic mesenchyme, which is required for continued branching, was gone by the third postnatal day. This was associated with an accelerated rate of new nephron formation in the absence of apoptosis. At the same time, the tips of the ureteric bud branches lost the typical appearance of an ampulla and lost Wnt11 expression, consistent with the absence of the capping mesenchyme. Surprisingly, expression of Wnt9b, a gene necessary for mesenchyme induction, continued. We then tested the postnatal day three bud branch tip and showed that it maintained its ability both to promote survival of metanephric mesenchyme and to induce nephrogenesis in culture. These results suggest that the sequence of events leading to disruption of the cycle of branching morphogenesis and nephrogenesis began with the loss of mesenchyme that resulted from its conversion into nephrons.

No abstract

BackgroundThe production of nephrons suddenly ends in mice shortly after birth when the remaining cells of the multi-potent progenitor mesenchyme begin to differentiate into nephrons. We exploited this terminal wave of nephron production using both microarrays and RNA-Seq to serially evaluate gene transcript levels in the progenitors. This strategy allowed us to define the changing gene expression states following induction and the onset of differentiation after birth.ResultsMicroarray and RNA-Seq studies of the progenitors detected a change in the expression profiles of several classes of genes early after birth. One functional class, a class of genes associated with cellular proliferation, was activated. Analysis of proliferation with a nucleotide analog demonstrated in vivo that entry into the S-phase of the cell cycle preceded increases in transcript levels of genetic markers of differentiation. Microarrays and RNA-Seq also detected the onset of expression of markers of differentiation within the population of progenitors prior to detectable Six2 repression. Validation by in situ hybridization demonstrated that the markers were expressed in a subset of Six2 expressing progenitors. Finally, the studies identified a third set of genes that provide indirect evidence of an altered cellular microenvironment of the multi-potential progenitors after birth.ConclusionsThese results demonstrate that Six2 expression is not sufficient to suppress activation of genes associated with growth and differentiation of nephrons. They also better define the sequence of events after induction and suggest mechanisms contributing to the rapid end of nephron production after birth in mice.