The intermediate mesoderm is a germ layer aptly named for its intermediate position between the paraxial and lateral plate mesoderm. The paired cylindrical masses of the intermediate mesoderm are arranged along the posterior aspect of the embryo, laterally to the paraxial mesoderm.
The intermediate mesoderm develops into the urogenital system, which includes the kidneys and gonads, and their respective duct systems, as well as the adrenal cortex. The intermediate mesoderm forms paired elevations called urogenital ridges. The nephric duct arises from intermediate mesoderm and is essential for all further urogenital development. In the third week of fetal development, the intermediate mesoderm differentiates into the kidneys. Three pairs of kidneys successively form within the intermediate mesoderm: the pronephros, the mesonephros, and the metanephros, where the last pair persists as the functional kidneys of the newborn.
The male and female gonads develop from gonadal ridges that arise in the fifth week of development from growth of the intermediate mesoderm just medial to the mesonephros (intermediate kidney). The nephric ducts, or Wolffian ducts lie adjacent to the gonads, and serves as the anlage for certain structures of the male reproductive system. A second pair of ducts, the paramesonephric ducts or Mullerian ducts develop lateral to the mesonephric ducts and eventually form structures of the female reproductive system. Both sets of ducts empty into the urogenital sinus. An early embryo has the potential to follow either the male or the female pattern of development, as it contains both sets of ducts and genital ridges that can differentiate into either testes or ovaries.