Urinary System Development

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3 systems emerge

During approximately weeks 4 and 5, three sets of excretory structures emerge; two regress, and the third eventually matures to the adult form.

These systems develop and regress from cranial to caudal, and overlap chronologically.

Pronephros

Emerges early in week 4 in intermediate mesoderm, which is located along the posterior abdominal cavity.

— Both the urinary and genital systems arise from a common ridge in the intermediate mesoderm.

Primary nephritic ducts (aka, pronephritic ducts) arise in the cervical segments as rods of condensed mesoderm, which then undergo epithelialization.
Five to seven nephritic vesicles (aka, nephrotomes) form simultaneously in the mesoderm; these small, hollow balls of epithelia are nonfunctional, and completely regress within a week of emerging.
The pronephros is a vestigial structure in humans, but is responsible for excretion in early vertebrates (i.e., lampreys).

— As the pronephros regresses during week 4, the mesonephros emerges in the thoracic region.

Mesonephros

Primary nephritic ducts continue their growth caudally (as their cranial ends degenerate).

— Their elongation is driven by mesenchymal-epithelial cell conversions taking place at their caudal tips; they extend from the thoracic region to the middle lumbar region.

Mesonephric buds and tubules develop in the mesoderm, also in cranial to caudal direction (the cranial-most mesonephric buds regress as caudal buds emerge).
The tubules form connections to the ducts, which are now called the mesonephric (aka, Wolffian) ducts.

— Overall, more than 40 mesonephric buds will form, but, because older buds regress as new buds form, there are never this many present at once.

Eventually, the mesonephric ducts elongate and fuse with the urogenital sinus (be aware that some texts state that the connection occurs earlier, with the cloaca).
The mesonehpros degenerates in the female (for the most part; remnants can be found as the epoophoron, paraoophoron, and Garner cyst); in the male, it persists to contribute to the reproductive duct system.

Metanephros

Definitive kidney

Ureteric bud arises as an outgrowth of the mesonephric duct, near its connection with the urogenital sinus.

— This process is driven by the interaction between glial cell line-derived neurotrophic factor (GDNF), and receptor-tyrosine-protein kinase (Ret), which is released by the undifferentiated mesonephric mesenchyme.

The bud grows towards, and, eventually into, the metanephric mesoderm (aka, blastema).
Upon contact, cellular interactions between the ureteric bud and the metanephric mesoderm drive repeated rounds of bifurcation the ureteric bud and the generation of excretory units (the nephrons) in the mesoderm (these processes are discussed in detail, elsewhere).
Metanephros development, specifically nephrogenesis, continues post-natally.

Ascent and 90-degree rotation

After the ureteric buds join with the metanephric tissue, the immature kidney begins its ascent and 90-degree rotation from the pelvis to the upper abdominal cavity.

Kidney is originally located at the S1-S2 level, inferior to the terminal aortic bifurcation.

— The renal hilum which is where the ureter enters the kidney, faces anteriorly. As the ureters lengthen, the kidneys ascend through the pelvic cavity while rotating, so that the hila begin to face each other.

Final position is at the level of T12-L3. The right kidney lies a little lower than the left; this is due to the presence of the liver on the right side.

— The paired renal arteries, which branch directly from the abdominal aorta, enter with the ureter at the hilum; though not shown, here, the renal veins drain blood from the kidneys to the inferior vena cava.

mesonephric ducts & ureteric bud incorpoprate into posterior bladder wall

Initially, the mesonephric ducts join with the bladder (as we saw in our first diagram, this occurs during week 5, when the mesonephric ducts connect with the urogenital sinus).
The ureteric buds are outgrowths of the mesonephric ducts.
As the ends of the mesonephric ducts begin to move inferiorly, they are incorporated into the bladder wall; this also brings the ureteric buds in closer contact with the bladder.
Eventually, as the mesonephric ducts continue their downward migration, the ureteric buds establish direct connections with the urinary bladder and cease to connect with the mesonephric ducts.
Finally, when the ends of the mesonephric ducts are fully incorporated into the posterior bladder wall, they contribute to the trigone. This triangle-shaped region may serve to funnel urine towards the urethra (recent research shows the trigone has endothelial contributions, too; recall that the urinary bladder, as a derivative of the cloaca, is of endothelial origins).
The ureters reach their final destination, at the superior corners of the trigone, show where the ureters drain urine into the urinary bladder.

Kidney Illustrations & Photos

Urinary System Development

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