The arched collecting tubules induce growth of the surrounding mesenchyme, which forms a tubule in the metanephric mass of mesoderm. These tubules form the parts of the nephron: glomerulus, Bowman's capsule, convoluted tubule, loop of Henle.
Formation of the nephrons of the permanent kidney begins at week 8. The metanephric vesicles elongate to form tubules which directly contact the arched collecting tubules. The metanephric tubules are lobulated and remain this way until after birth. They resemble tubes made up of clusters of cells with an interior lumen. Contact with the collecting tubule is essential to production of proteases which break down the extracellular matrix between the metanephric tubule and the collecting tubule. These tubules then become continuous to form the early nephron. A failure in the connection between the nephron and the collecting system is one of the theories suggested as the cause of a polycystic kidney.
