METANEPHROS

The paired metanephroi are the precursors of the definitive adult kidneys. They begin to form around the twenty-eighth day of gestation, shortly after the mesonephric ducts have fused with the cloaca. The caudal portion of each mesonephric duct sprouts a small diverticulum known as a ureteric bud. Each bud then grows toward a nearby mass of mesoderm known as the metanephric mesenchyme, which is located at the sacral end of the ipsilateral nephrogenic cord.

Once each ureteric bud enters its associated metanephric mesenchyme, it begins a process of iterative bifurcation that gives rise to the urine collecting system. The first eight bifurcations of the ureteric bud give rise to the renal pelvis, major calices, and minor calices. These initial divisions later fuse to a considerable extent, resulting in the definitive appearance of the pelvicaliceal system. The next dozen bifurcations give rise to the collecting duct system.