INTRODUCTION TO THE ANATOMY OF THE UROGENITAL STRUCTURES

The urinary system and reproductive systems are studied together in a comparative anatomy course because they both originate in the intermediate mesoderm, and commonly share parts which have roles in both the reproductive system and urinary system. The intermediate mesoderm of the embryo consists of two longitudinal bands that lie dorsal to the coelom and to either side of the dorsal aorta.

The initial kidney tissue of all vertebrates is positioned within this intermediate mesoderm. Nephric tissues begin to form at the anterior end of the intermediate mesoderm. In the early stages of many vertebrate kidneys these first nephric tubules function for a brief period as the pronephric kidneys. Later, a long portion of the intermediate mesoderm posterior to the pronephric tissue develops to form the more advanced mesonephric kidney. The mesonephric kidney is the kidney of fish and amphibians. The nephric tubules of the mesonephric region connect to a mesonephric duct. This mesonephric duct also develops from within the intermediate mesoderm.

The gonads, ovary or testis, also develop in the intermediate mesoderm. They originally form as swellings that lie just ventral to the anterior mesonephric kidney. A mullarian duct also develops in the intermediate mesoderm near the mesonephric duct.

Reptiles, birds, and mammals develop a metanephric kidney and lose the mesonephric kidney in the process of early development. The metanephric kidney begins as a small outgrowth from the more posterior end of the mesonephric duct. This is initially called a metanephric bud. As nephric tissue develops around this new duct a new kidney structure forms. The metanephric duct becomes the ureter. In both males and females the ureter eventually becomes separated from the mesonephric duct.

As males develop the mesonephric duct makes connection with the testis as the primary sperm conducting duct, and the mullerian duct is lost. In some fish species the testis secondarily grows a separate sperm duct as the embryo develops.

In females the mullerian duct becomes the passage for eggs. The females of fish and amphibians retain the mesonephric duct as a urinary duct. In reptiles, birds, and mammals (amniotes) the metanephric kidney replaces the mesonephric kidney. Subsequently, the mesonephric kidney and duct become small vestigial remnants.