The organs of the vertebrate excretory system, or urinary system, include the kidney, ureter, bladder, and urethra. The excretory system is responsible for maintaining organismal homeostasis by removing waste produced by metabolic processes and regulating osmolality. The kidneys arise from the intermediate mesoderm through three embryonic stages: pronephos, mesonephros, and metanephros. During nephrogenesis, cross-signaling between the ureteric bud epithelium and nephrogenic mesenchyme gives rise to the collecting ducts and nephrons, respectively. GDNF is produced by nephrogenic mesenchymal cells and induces branching morphogenesis in the ureteric bud epithelium. The ureteric bud induces the mesenchymal cells to condense and undergo a mesenchymal to epithelial transition to produce the absorptive epithelial cells of the nephron. The ureter develops from the nephric duct and transports urine from the kidneys to the endodermally-derived bladder for excretion through the urethra.