Renal System, Embryological Development
The kidneys, urinary tract, and the majority of the reproductive organs arise in the intermediate mesoderm. The kidney goes through three stages of development that mimic filogenesis (biological evolution) of the kidney; pronephros, mesonephros, and metanephros.
The pronephros arises at the C3-T1 vertebral levels by the dorsal proliferation of cords of cells. Such cords become pronephric tubules that grow caudally and get connections in order to form a common pronephritic duct that extends caudally toward the cloaca. Beyond the pronephros it is termed the Wolffian duct. The tubules of the pronephros are in continuity with the coelomic cavity. The pronephric kidney does not work in humans, but it seems to induce the normal formation of the further renal structures.
The mesonephros develops by the formation of menonephric tubules from the intermediate mesoderm of the C6-L3 vertebral levels. The tubules of the mesonephros do not communicate with the coelom, but the blood is filtrated directly into the mesonephric tubules through a capillary glomerulus structure from the aorta that is encapsulated by the proximal blind end of the tubule. The metanephros arises caudal to the mesonephros at five weeks of development. It derives from intermediate mesoderm (L4-S1 vertebral levels), the metanephrogenic blastema, lateral to the developing urogenital sinus and lateral to the mesonephric duct. The ureteric bud arises as a diverticulum from the mesonephric (Wolfian) duct close to the entrance to the cloaca. It grows towards and inside the metanephrogenic blastema, and invades the center of the metanephros. At this point, as the bud grows, it is named the metanephric bud. Its most caudal part has the name metanephric duct. This juxtaposition of the ureteric bud and the specialized mesoderm stimulates the metanephrogenic blastema to form glomeruli, and proximal and distal tubules. When the ureteric bud touches the metanephros, progressive branching of the ureteric bud occurs, creating the major and minor calyces, and the collecting tubules that will provide a conduit for urine drainage in the mature kidney. This process is known as the induction of the kidney.
The metanephrogenic blastema induces the bud to grow and to branch, so that it arborises to form a tree-like collecting duct system. The ureteric bud induces the metanephrogenic blastema to gain a stem- cell phenotype and to multiply. It then induces groups of stem cells to differentiate into nephrons, a complex developmental process. Such a reciprocal induction occurs as a result of sequential activation of a series of genes from different families. These encode growth factors, receptors, oncoproteins, transcription factors, enzymes, signal transducers, and extracellular matrix components.
There is a time gradient in the development of a fetal kidney. The most external cortex is composed of stem cells that are not yet committed to differentiation, the region just inside it contains cells undergoing the earliest phases of nephrogenic differentiation, and the most inside area that contains maturing nephrons and supporting stromal cells. During differentiation of the nephrons, condensation is the process by which disorganized mesenchymal cells (about 100) become a highly organized epithelial tubule to form a distinct mass. Then, during the transition to epithelium, condensed cells lose their mesenchymal characteristics and gain epithelial ones including the basement membrane, cell-cell junctions, and a cellular apico-basal polarity. Condensation of the developing nephron then occurs as invaginations form comma-shape bodies and then an S-shaped body. At about this time, blood vessel progenitors begin construction of the vascular component of the glomerulus. The tubules mature as the peculiar transporting segments of the nephron differentiate, and the morphogenesis of convoluted tubules takes place. Finally, fusion with the collecting ducts leads to a continuous tubule system.
The cloaca is a common sinus that forms in the caudal region of the fetus. At the caudal end of the cloaca, ectoderm lies directly over endoderm forming the thin cloacal membrane. As development progresses, a mesoderm derivate septum named the urorectal septum forms, dividing the hindgut, the ventral (urogenital) sinus. This urorectal septum extends in a caudal direction. The mesonephric (Wolfian) duct descends from the mesonephros to meet the urogenital sinus. Once this connection is made, fetal urine drains into the urogenital sinus.
The urogenital sinus and a small portion of allantoidis enlarge to for the urinary bladder. Portion of the mesonephric and metanephric (ureter) ducts are incorporated into the urogenital sinus. The ureter enters the bladder and the mesonephric ducts enter more caudally in the less dilated portion of the urogenital sinus. This distal portion will become the urethra for both the genders, and the vestibule and part of the vagina for the female. The cranial portion of the bladder tapers to become the vescico-allantoic canal that successively closes, leaving the median umbilical ligament. The caudal end of the developing bladder thickens with smooth muscle in a triangle between the two ureteric orifices and the urethra.
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