Osmosis video - Development of the renal system

The renal system starts to form at about week 4 of gestation from a portion of the urogenital ridge called the nephrogenic cord. The nephrogenic cord gives rise to three overlapping developmental stages: the pronephros, the mesonephros, and the metanephros. Pronephros consists of an early and nonfunctional system, which regresses by week 4. Next is the mesonephros, which functions as a primitive excretory system in the embryo. Most tubules regress by week eight and are replaced by the metanephros. Metanephros give rise to actual kidneys, which appear at around week five, and become mature enough to secrete urine around week ten.