Sources of Development and Primordia Differentiation
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 longitudinal axis of the embryo's body, this primordium behaves differently:
- In the upper part of the trunk, the mesoderm undergoes segmentation. This produces segmental plates, also known as nephrotomes.
- In the lower (caudal) part of the trunk, segmentation does not occur. Instead, it forms a continuous, unsegmented cord known as the nephrogenic cord.
The epithelial lining of the renal tubules, which forms from these structures, has a specific mesodermal origin and belongs to the coelonephroderma type.
First Stage: Pronephros
The earliest generation of kidneys is laid down in the embryo at the beginning of the fourth week of development. The pronephros forms from the anterior segmented plates of the mesoderm (approximately 8–10 cranial nephrotomes).
An essential feature of the pronephros is that it is practically non-functional in excretion. The nephrotomes comprising it undergo rapid regression. However, simultaneously with the pronephros, slightly more laterally, the mesonephric duct (Wolffian duct) forms, playing a key role in the subsequent development of the urinary tract.
Second Stage: Mesonephros
The pronephros is succeeded by the mesonephros (primary kidney), derived from 20–25 trunk nephrotomes (intermediate segmental plates). These exclusively give rise to epithelial tubules.
Mechanism of renal corpuscle formation: Corpuscles develop through the simultaneous growth of two independent systems:
- The blind ends of the epithelial tubules grow toward the blood vessels.
- Branches originating from the aorta branch into capillary networks, forming vascular glomeruli.
- The blind ends of the tubules envelop these glomeruli, forming a double-walled cup—the capsule.
The opposite ends of the tubules grow toward and open into the mesonephric duct. During this period, the mesonephric duct assumes the function of a ureter, emptying into the hindgut and contributing to the cloaca. The mesonephroi reach maximum development by the middle of the second month and function as the primary excretory organs during the first half of embryogenesis. Additionally, the epithelium on the medial side of the mesonephros forms the gonadal ridges—the future precursors of the gonad epithelium.
Third Stage: Metanephros (Permanent Kidney)
The permanent kidney develops from the unsegmented nephrogenic cord located in the caudal region of the embryo.
The morphogenesis of the metanephros is entirely analogous to that of the mesonephros: epithelial tubules interact with vascular glomeruli to form functional nephrons with renal corpuscles. The permanent kidneys take over the primary excretory function in the second half of the prenatal period.
The drainage system of the permanent kidney develops through the mesonephric duct, which gives off a specialized outgrowth (the ureteric bud). This bud induces the formation of the collecting duct system, renal calyces, renal pelvis, and ureter. Subsequently, the openings of the permanent ureters shift to open directly into the urinary bladder.
Duct Formation and Sexual Dimorphism
In addition to the mesonephric (Wolffian) duct—whose cranial part regresses while its caudal part elongates downward—the paramesonephric (Müllerian) duct forms laterally. Its fate depends directly on the genetic sex of the fetus:
- In male fetuses: the paramesonephric duct has no functional role and rapidly regresses.
- In female fetuses: this duct serves as the vital primordium for the reproductive system. It forms 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.