Development of the Urinary Tract
Overview
Kidneys and ureters are derived from intermediate mesoderm, which is located between paraxial mesoderm medially and lateral plate mesoderm laterally.
The lining of the urinary bladder and urethra are derived from endoderm lining the cloaca and urogenital sinus.
Three Kidneys
By the 5th week of development, three kidneys have developed. These are:
- Pronephros - First kidney -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.
Ureteric Bud vs Metanephros
The functional kidney is the metanephric kidney, which is derived from two embryonic tissues:
- ureteric bud or ureteric diverticulum
- metanephric mass or metanephric blastema.
The ureteric bud is continuous with the urogenital sinus, initially by way of the mesonephric duct, but later in development by its own attachment.
The urogenital sinus is part of a larger, endoderm lined space in the caudal part of the embryo, the cloaca. Cloaca is latin for sewer, aptly named as it is the site of termination for both the gut tube and the ureter.
The cloaca is separated from the outside of the body by the proctodeal membrane.
Development of the kidneys begins in the pelvis, but later in development, they ascend to their final position on the posterior abdominal wall, behind the peritoneum.
On the diagram to the left, all parts of the kidney that are derived by the original ureteric bud are indicated in red. These include the linings of the ureter, renal pelvis, major calyces, minor calyces and collecting tubules.
The components that make up the nephron are derived from the metanephric blastema (pale pink in the diagrams above). These parts include the renal corpuscle, the proximal tubules and the distal tubules.
Uriniferous Tubule
The functional unit of the kidney is the uriniferous tubule, derived from both the ureteric bud (yellow parts) and metanephric blastema (blue parts).
The epithelium of the collecting tubules, derived from the ureteric bud, induces the mesenchymal tissue of the metanephric blastema to begin to condense and to form vesicles.
These vesicles merge to form the tubules of the nephron. These eventually establish continuity with one another.
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.
Urogenital Sinus
As the urorectal septum divides the cloaca, the anterior portion of the cloaca becomes the urogenital sinus. It 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
Urorectal Septum
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.
It is subdivided into rectum posteriorly and urogenital sinus anteriorly by 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 presence of enterovesical fistulas.
