Initial differentiation of the metanephric mesenchyme is independent of WT1 and the ureteric bud

The early development of the metanephric kidney is characterized by the ind uced differentiation of mesenchymal cells into a stem cell population that undergoes a mesenchymal to epithelial transformation in response to stimuli from the ureteric bud. The Wilms' tumor suppressor gene, Wt1, is required for mesenchymal cells to complete this developmental program. in the absenc e of WT1, a prospective metanephric mesenchyme appears, but becomes apoptot ic, and outgrowth of the ureteric bud from the Wolffian duct does not occur . Therefore, the examination of Wt1 -/- embryos allows the determination of those markers of early metanephric differentiation that do not require the ureteric bud or WT1 for their expression. Here, we demonstrate that severa l markers, including Pax-2, Six-2, and GDNF, were present as RNAs in the me tanephric mesenchyme of Wt1 -/- embryos. These findings demonstrate that th e metanephric mesenchyme in mutant embryos has begun to differentiate towar ds the nephrogenic lineage, and that this early differentiation does not re quire either WT1 or the presence of the ureteric bud. To determine whether WT1 functions other than to induce expression of factors that stimulate ure teric bud outgrowth, Wt1 -/- metanephric mesenchymes were recombined with w ild-type ureteric buds in organ culture, but this failed to rescue tubuloge nesis. However, the Wolffian duct from Wt1 -/- embryos was a competent indu cer of wild-type metanephric mesenchyme. (C) 1999Wiley-Liss, Inc.