Anatomic relationship between the genital and urinary systems

The development of the genital apparatus accompanies that of the urinary system (formation of the urinary apparatus). It also has its origin in the intermediate mesoderm (stage 9) and urogenital sinus (stage 15). The primordial germ cells (stage 7) share in the formation of the gonads, but have an ectodermal origin (stage 3). In males, the development of the testes is closely tied with that of the mesonephros. In females, on the other hand, the mesonephros plays no role at all.

The intermediate mesoblast is the origin of an elongated structure, the urogenital ridge (crista urogenitalis), which lies on both sides of the midline between the lateral mesoderm and the root of the dorsal mesenterium of the embryo. It consists of two main components, the nephrogenic cord, out of which the urinary apparatus arises and the genital ridge (stage 14) as the origin of the gonads. The genital ridge extends from the upper thorax region to the level of the cloaca. The true gonad anlage develops, though, out of only the middle area.
The cranial and caudal parts of the urogenital ridge form the upper and lower gonadal bands, respectively, that as embryonic structures secure the gonads cranially and caudally (more info: the development of the ligaments).