Development of the Urogenital Tract

In the cross section of the embryo to the right, note the presence of three main masses of mesoderm. Paraxial mesoderm that forms the vertebrae and associated muscles, Lateral plate mesoderm that forms the linings of the body cavities and mesothelial coverings of the visceral organs. The intermediate mesoderm, which is located between the two, gives rise to genitourinary structures.

By the 5th week of development, three kidneys have developed. These are:

• Pronephros - Transient during the early embryonic period. It rapidly degenerates and has no known function.
• Mesonephros - Middle kidney. For a short period maintains some functional activity, but largely regresses, except in male embryos where it contributes to development of male genital ducts.
• Metanephros - Definitive kidney that forms from two primordia, an outgrowth of the urogenital sinus, the ureteric bud and a mass of intermediate mesoderm, the metanephric blastema.

On the diagram to the left, all parts of the kidney that are derived by the original ureteric bud are indicated in yellow. These include the ureter, renal pelvis, major calyces, minor calyces and collecting tubules.
The components that make up the nephron are derived from the metanephric blastema (gray area in the diagrams above). These parts include the renal corpuscle, the proximal tubules and the distal tubules. 
 

Note that the epithelium of the collecting tubules, derived from the ureteric bud, induce the mesenchymal tissue of the metanephric blastema to begin to condense and to form vesicles. These vesicles will merge with one another to form the tubules of the nephron. Note that the nephron (brown) is continuous with the collecting tubule (yellow). The combination of the nephron and collecting tubule constitutes the uriniferous tubule or functional unit of the kidney. Knots of capillaries embed in the Bowman's capsule to form the glomeruli. The combination of Bowman's capsule and glomeruli constitutes the renal corpuscle.

The functional unit of the kidney is the uriniferous tubule, derived from both the ureteric bud (yellow parts) and metanephric blastema (blue parts).

At the caudal end of the embryo, the cloaca is a common area for the termination of the gut tube and urogenital system. The cloaca is lined by endoderm, which subsequently forms the lining of the rectum, urinary bladder and urethra. The cloaca is partitioned into the rectum caudally and the urogenital sinus anteriorly by the downward growth of the urorectal septum. The urogenital sinus goes on to form the bladder and urethra.
Roll over the image to see this. Failure of the urorectal septum to develop can result in the development of enterovesical fistulas

As the urorectal septum divides the cloaca, the portion of the cloaca that becomes the urogenital sinus is subdivided into three parts.

In the male these parts are:

• vesicle part becomes the urinary bladder
• pelvic part becomes the prostatic urethra and membranous urethra
• phallic part becomes the penile urethra

In the female these parts are:

• vesicle part becomes the urinary bladder
• pelvic part becomes the entire urethra
• phallic part becomes the vaginal vestibule