Development of the Urinary System

Sources of development and differentiation of primordia

The primary material for the formation of the urinary system is the intermediate mesoderm, located between the somites and the splanchnotome. Depending on its anatomical position along the axis of the embryo's trunk, this primordium behaves differently:

• In the upper part of the trunk, the mesoderm undergoes segmentation, resulting in the formation of segmental stalks, also called nephrotomes.
• In the lower (caudal) part of the trunk, segmentation does not occur; instead, a single unsegmented cord forms—the nephrogenic cord.

The epithelial lining of the renal tubules, which forms from these structures, has a specific mesodermal origin and belongs to the coelonephrodermal type.

Stage 1: Pronephros

The very first generation of kidneys develops in the embryo at the beginning of the fourth week of development. The pronephros forms from the anterior segmented stalks of the mesoderm (approximately from the first 8–10 nephrotomes).

A crucial feature of the pronephros is that it practically performs no excretory functions. Its constituent nephrotomes undergo very rapid regression. However, simultaneously with the pronephros, slightly more laterally, the mesonephric duct (Wolffian duct) develops, playing a key role in the subsequent formation of the urinary tract.

Stage 2: Mesonephros

The pronephros is succeeded by the mesonephros (primary kidney), derived from 20–25 trunk nephrotomes (middle segmental stalks), which form exclusively epithelial tubules.

Mechanism of renal corpuscle formation: The development of the corpuscles occurs through the reciprocal growth of two independent systems:

1. The blind ends of the epithelial tubules grow toward the blood vessels.
2. Branches arise from the aorta and break up into a capillary network, forming vascular glomeruli.
3. The blind ends of the tubules grow around these glomeruli, forming a double-walled cup—the capsule.

The opposite end of the tubules grows toward the mesonephric duct and opens into it. During this period, the mesonephric duct takes on the function of the ureter, emptying into the hindgut and forming the cloaca. The mesonephroi reach their maximum development by the middle of the second month and function as the main excretory organs during the first half of embryogenesis. In addition, the epithelium on the medial side of the mesonephros forms the genital ridges—the future epithelial primordia of the gonads.

Stage 3: Metanephros

The definitive kidney (metanephros) develops from the unsegmented nephrogenic cord located in the caudal part of the embryo's body.

The principle of metanephros morphogenesis is entirely analogous to the development of the mesonephros: epithelial tubules interact with vascular glomeruli in the exact same way, forming functional nephrons with renal corpuscles. The definitive kidneys take over the primary excretory function in the second half of the prenatal period.

The excretory ducts for the definitive kidney are formed by the mesonephric duct, which gives rise to a special outgrowth that induces the formation of the collecting duct system, renal calyces, renal pelvis, and ureter. Subsequently, the orifices of the definitive ureters open directly into the urinary bladder.

Formation of ducts and sexual dimorphism

In addition to the mesonephric (Wolffian) duct—whose cranial part obliterates while its caudal part grows downward—the paramesonephric (Müllerian) duct develops more laterally. Its fate depends directly on the genetic sex of the fetus:

• In male fetuses: the paramesonephric duct finds no use and rapidly regresses.
• In female fetuses: this duct serves as the vital source for the development of the reproductive organs, forming the uterine tubes, uterus, and vagina. Specifically, to form the uterus and vagina (unpaired organs), the lower segments of the two Müllerian ducts fuse together.

Briefly: answers to topical questions

Class and exam questions

What else to study on this topic

• Mechanisms of renal corpuscle capsule formation and capillary ingrowth
• The connection of the mesonephros with the formation of the genital ridges
• The process of collecting duct induction by the mesonephric outgrowth
• Differences in the fate of the Wolffian and Müllerian ducts depending on sex
• Histogenesis of the coelonephrodermal type of epithelium

Other topics in the "Urinary System" section

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