Diagram of normal and abnormal kidney development

Uppermost are the precursor structures of a single renal tract, as would be present in a human fetus of 4 to 5 weeks' gestation; note the mesonephric duct, metanephric mesenchyme (which will form nephrons), and ureteric bud. Solid arrows indicate normal developmental pathways and dotted arrows indicate abnormal pathways. In normal development, the ureteric bud grows into the metanephric mesenchyme and is followed by bud branching and nephron induction and, finally, development of a fully functional, normal kidney containing multiple layers of nephrons. Hypoplastic kidneys contain fewer layers of nephrons than normal but retain a moderated degree of excretory function. Cystic dysplastic kidneys contain malformed tubules and some small cysts but generally retain some excretory function. A discrete renal pelvis (white) is present in normal, hypoplastic and cystic dysplastic kidneys; this feature would appear distended (hydronephrotic) if the kidney was attached to an obstructed lower tract (not shown). The multicystic dysplastic kidney has no useful excretory function, and its pelvis is absent or severely disorganized. Aplastic kidneys undergo no primary growth or development; a similar phenotype is seen in the organ remnant left after the spontaneous involution of a multicystic dysplastic kidney. In the most profound type of malformation, agenesis, the first steps of kidney development do not occur, and the kidney (and often the ureter) is absent.