Fig. 1. Rat metanephric mesenchymes recapitulate differentiation of kidney epithelia in vivo under defined culture conditions. (A) Metanephric mesenchymes cultured in basal media undergo apoptosis. (B) Addition of Fgf2 and Tgf to the culture media induces survival of clusters of progenitors. (C) These aggregates express Wnt4 as detected by in situ hybridization. (D) After continued culture with Fgf2 and Tgf, mesenchymes degenerate without differentiating into epithelia. (E-J) NHBF, Lif or NGAL, when combined with Fgf2 and Tgf, each induced continued expansion of metanephric mesenchymes and their differentiation into organotypic epithelia within 7 days of organ culture. Tubules stain positive for Cdh1 (E-cadherin; F), whereas glomerular-like structures express podocalyxin-like (Podxl) (G). Histologically, these structures resemble kidney epithelia at and beyond the S-shaped body stage (H-J). (K) The sequence of metanephric mesenchymal differentiation in organ culture recapitulates epithelial differentiation in vivo (in this case in the presence of Lif). Arrows delineate a mesenchymal aggregate, after 3-4 days of differentiation in vitro, reminiscent of pretubular aggregates in vivo. UB, ureteric bud; CM, condensed mesenchyme; PA, pretubular aggregate; SB, S-shaped body; Tb, tubule; Gl, glomerular-like structure.