Schematic presentation of three stages of kidney development: pronephros, mesonephros, and metanephros.
Pronephros is the earliest nephric stage in humans, and constitutes the mature kidney in most primitive vertebrates. It extends from the 4th to the 14th somites and consists of 6-10 pairs of tubules. These spill into a pair of primary ducts that are formed at the same level, extend caudally, and eventually reach and spill into the cloaca. The pronephros is a vestigial structure that disappears completely by the 4th week of human embryonic life.
Mesonephros develops by the formation of mesonephric tubules from the intermediate mesoderm, it is the principal excretory organ during early embryonic life (4—8 weeks). It gradually degenerates, although parts of its duct system become associated with the male reproductive organs.
Metanephros arises caudal to the mesonephros at five weeks of development; it is the permanent and functional kidney in higher vertebrates. It is derived from the intermediate mesoderm. The ureteric bud arises as a diverticulum from the Wollfian duct, close to the entrance to the cloaca and grows towards and inside the metanephric mesenchyme.