Conditional deletion of murine fibroblast growth factor receptors (Fgfrs) 1 and 2 in metanephric mesenchyme leads to renal agenesis with unbranched ureteric buds; however, there are occasionally two buds per nephric duct. Our goal was to determine whether conditional deletion of Fgfr1 or Fgfr2 alone resulted in multiple ureteric bud induction sites. Although deletion of Fgfr1 alone results in no abnormalities, loss of Fgfr2 often leads to multiple ureteric buds and anomalies including renal aplasia, misshaped kidneys, partially duplicated kidneys, duplicated ureters, and obstructed hydroureter. Deletion of Fgfr2 did not change expression domains of glial cell line-derived neurotrophic factor (GDNF), Robo2, bone morphogenetic protein 4, or Sprouty1, all of which regulate ureteric bud induction. Cultured Fgfr2 mutant nephric ducts were also not more sensitive to exogenous GDNF than controls. Whole mount in situ hybridization revealed that in mutant embryos, Fgfr2 was deleted from stromal cells around the nephric duct and ureteric bud base, which correlates well with the ureteric bud induction abnormalities. Thus, Fgfr2 is critical in ensuring that there is a single ureteric bud from the nephric duct. The plethora of later stage defects in Fgfr2 conditional knockouts is reminiscent of many human cases of genetic urogenital anomalies. (Pediatr Res 64: 592-598, 2008) F ibroblast growth factor receptors (Fgfrs) are receptor tyrosine kinases with four known signaling members and 22 ligands in mammals (1). Fgfrs are expressed throughout embryogenesis in many tissues, including the kidney (1).The metanephric kidney arises from two embryonic tissues, the metanephric mesenchyme and ureteric bud. At embryonic day (E) 10.5 in the mouse and the 5th week of gestation in humans, the metanephric mesenchyme induces a single ureteric bud from the Wolffian (nephric) duct near the hind limb (2). Subsequently, the ureteric bud elongates and branches within the metanephric mesenchyme, giving rise to the collecting ducts, pelvis, and ureter (2). At its tips, the ureteric bud induces local metanephric mesenchyme to condense and differentiate into nephron epithelia. Regions of stromal mesenchyme also surround the developing nephrons.Many studies have documented diverse actions of Fgfrs in developing kidneys. Addition of Fgf2 to isolated rat metanephric mesenchymal tissues prevented apoptosis (3,4) and promoted condensation (4). Transgenic mice with a dominant negative Fgfr fragment developed renal agenesis/severe dysgenesis (5). Mice null for Fgf7, Fgf10, or Fgfr2-IIIb (the receptor isoform for Fgf7 and Fgf10), have small kidneys with normal-appearing nephrons (6 -8). Mice with conditional deletion of Fgf8 from the metanephric mesenchyme, however, have interrupted nephron development (9,10).Although Fgfr1 and Fgfr2 null mice are early embryonic lethal (11-14), conditional knockout approaches have revealed roles for these receptors in kidney development. We observed that deletion of Fgfr2 from the ureteric bud results in aberran...