Almost 1% of human infants are born with urinary tract abnormalities, many of which are linked to abnormal connections between the distal ureters and the bladder. These defects impact on urine storage, renal function and reproduction, however cellular and molecular mechanisms underlying distal ureter maturation and insertion into the bladder are not well understood. We have used transgenic and knockout mouse lines to study normal and abnormal ureter maturation. Our published and preliminary studies with Hoxb7-Gfp mice, a line expressing the Gfp fluorophore in the kidney, ureters and Wolffian ducts has enabled us to formulate the first model of ureter maturation. We show that this complex process can be subdivided into three stages: ureteric bud formation, vertical displacement, when ureters and Wolffian duct move caudally to meet the primitive bladder, and lateral displacement when ureters separate from the Wolffian duct and move to their permanent site of insertion in the dorsal bladder wall., and our studies demonstrate that all of these processes are dependent on formation of an epithelial outgrowth from the base of the Wolffian ducts that we call the wedge, and that wedge outgrowth depends on Vitamin A and the RTK Ret. We will use mouse models of Vitamin A deficiency and time-lapse photography in organ culture studies to investigate how vitamin A induces wedge formation, and how wedge formation controls vertical and lateral displacement of the distal ureter. To define the role of the primitive bladder in these events, we will generate a new transgenic model in which Wolffian duct derivatives and the primitive bladder are differentially labeled with non-overlapping chromophores, enabling us to visualize their respective interactions in living tissue. Finally, we will use a Cre/loxP mediated recombination system to mark cells wedge cells and their descendents to test the hypothesis that the wedge is a primordium of the trigone, a tissue integral to the bladder crucial for urine storage. The focus of this proposal is to elucidate normal mechanisms underlying ureter insertion into the bladder. Results of these experiments will ultimately provide insight into the cellular and molecular defects causing urogenital abnormalities.