Fig. 2. Kidney development in Six1-deficient mice. (A-D) Metanephros (mt) in Six1^+/- (A,C) and Six1^-/- (B,D) mice at E12.5. Kidneys are absent in Six1^-/- mice (^*), but ureter (ur) was observed in the left primordium (D). The genital tracts appeared to be normal both in males and females. (E) In Six1^+/- embryos of E11.5, ureteric bud (ub) grows out from Wolffian duct and invades the mesenchyme and the metanephric mesenchyme (mm) are condensed around the bulging ureteric bud. (F) In Six1^-/- embryos, although the ureteric buds grow out from Wolffian ducts, they fail to invade the mesenchyme (arrows) completely. (G-J) TUNEL analysis of metanephros in Six1^+/- and Six1^-/- embryos at E11.5. Note the apoptotic cells were markedly increased in Six1^-/- metanephric mesenchyme (arrows). g, gonad; wd, Wolffian duct.