Congenital Malformations of the Kidney

Although transient vestigial excretory organs (the pro- and mesonephros) are recognizable in the human embryo, the definitive kidney is the meta- nephros. This is formed in two parts: the nephrons from the nephrogenic cord and the excretory ducts (collecting tubules, calyces, pelvis and ureter) from the ureteric bud which grows as a branch from the caudal portion of the mesonephric (Wolffian) duct. During early development the ureteric bud grows cranially and impinges on the caudal end of the nephrogenic cord (called the metanephric blastema) where it begins a process of rapid dichotomous branching. The first few generations of branches coalesce to form the renal pelvis and calyces. As further branching occurs, condensations of meta-nephric blastema, from which the nephrons develop, become related to the dilated tip, or ampulla, of each branch of the ureteric bud (Figure 2.1). As the nephrons form they become attached to the ampullae which in turn develop into collecting ducts. Attachment to the growing tip of the ureteric bud branches ensures that the nephrons are carried outwards as they develop.