The urogenital system arises during the fourth week of development from urogenital ridges in the intermediate mesoderm on each side of the primitive aorta. The nephrogenic ridge is the part of the urogenital ridge that forms the urinary system. Three sets of kidneys develop sequentially in the embryo: The pronephros is rudimentary and nonfunctional, and regresses completely. The mesonephros is functional for only a short period of time, and remains as the mesonephric (Wolffian) duct. The metanephros remains as the permanent adult kidney. It develops from the uteric bud, an outgrowth of the mesonephric duct, and the metanephric mesoderm, derived from the caudal part of the nephrogenic ridge.
Urine excreted into the amniotic cavity by the fetus forms a major component of the amniotic fluid. Urine formation begins towards the end of the first trimester (weeks 11 to 12) and continues throughout fetal life.
The kidneys develop in the pelvis and ascend during development to their adult anatomical location at T12-L3. This normally happens by the ninth week.
Table 12 - Adult Derivatives of Embryonic Kidney Structures
The adrenal medulla forms from neural crest cells that migrate into the fetal cortex and differentiate into chromaffin cells.
The urinary bladder develops from the upper end of the urogenital sinus, which is continuous with the allantois. It is lined with endoderm. The lower ends of the metanephric ducts are incorporated into the wall of the urogenital sinus and form the trigone of the bladder. The connective tissue and smooth muscle surrounding the bladder are derived from adjacent splanchnic mesoderm.
The allantois degenerates and remains in the adult as a fibrous cord called the urachus (median umbilical ligament).
Figure 11 - Development of the kidneys
Absence of a kidney results when the ureteric bud fails to develop or regresses after development. If both kidneys are absent (bilateral renal agenesis) the fetus cannot urinate and amniotic fluid is deficient (< 400ml) resulting in oligohydramnios and characteristic physical deformations known as Potter facies (flattened nose, low-set ears, thickened, tapering fingers).
Congenital Polycystic Disease of the Kidneys
An autosomal recessive condition manifest by the presence of many heterogeneous cysts within the parenchyma of the kidney. The cause and pathogenesis is unknown.
Horseshoe kidney occurs when the inferior poles of the kidneys fuse together. The combined kidney is not able to ascend to its adult physiological location because it gets “hung up” on the inferior mesenteric artery.
A pelvic kidney is one that has failed to migrate to its adult anatomical location. In crossed ectopia one kidney and its associated ureter migrate to the opposite side of the body.
If the lumen of the allantois persists as the urachus forms it may give rise to an abnormal communication between the urinary bladder and the umbilicus known as an urachal fistula. Often with this condition urine will dribble from the umbilicus when the baby cries. A blind-ending communication that will not drain urine is known as an urachal sinus.