Embryonic Development

The kidney is derived from interaction between the ureteral bud and the metanephric blastema. During the 5th wk of gestation, the ureteral bud arises from the mesonephric (wolffian) duct and penetrates the metanephric blastema, which is an area of undifferentiated mesenchyme on the nephrogenic ridge. The ureteral bud undergoes a series of approximately 15 generations of divisions and by the 20th wk of gestation forms the entire collecting system: the ureter, renal pelvis, calyces, papillary ducts, and collecting tubules. Signals from the mesenchymal cells induce ureteric bud formation from the wolffian duct as well as ureteric bud branching. Reciprocal signals from the ureteric bud and, later, from its branching tips induce mesenchymal cells to condense, proliferate, and convert into epithelial cells. Under the inductive influence of the ureteral bud, nephron differentiation begins during the 7th wk of gestation. By the 20th wk of gestation, when the collecting system is developed, approximately 30% of the nephrons are present. Nephrogenesis continues at a nearly exponential rate and is complete by the 36th wk of gestation. During nephrogenesis, the kidneys ascend to a lumbar site just below the adrenal glands. At least 16 signaling agents have been identified that regulate renal development. Defects in any of the signaling activities could cause a kidney not to form (renal agenesis), to differentiate abnormally (renal dysgenesis). Dysgenesis of the kidney includes aplasia, dysplasia, hypoplasia, and cystic disease.

The fetal kidneys play a minor role in the maintenance of fetal salt and water homeostasis. The rate of urine production increases throughout gestation and, at term, volumes have been reported to be 50 mL/hr. The glomerular filtration rate is 25 mL/min/1.73 m² at term and triples by 3 mo post-term. The increase in glomerular filtration rate is caused by a reduction in intrarenal vascular resistance and redistribution of intrarenal blood flow to the cortex, where more nephrons are located.

Renal Agenesis

Renal agenesis, or absent kidney development, can occur secondary to a defect of the wolffian duct, ureteric bud, or metanephric blastema. Unilateral renal agenesis has an incidence of 1/450-1,000 births. Unilateral renal agenesis often is discovered during the course of an evaluation for other congenital anomalies (VATER syndrome; e.g., Chapter 311). Its incidence is increased in newborns with a single umbilical artery. In true agenesis, the ureter and the ipsilateral bladder hemitrigone are absent. The contralateral kidney undergoes compensatory hypertrophy, to some degree prenatally but primarily after birth. Approximately 15% of these children have contralateral vesicoureteral reflux, and most males have an ipsilateral absent vas deferens because the wolffian duct is absent. Because the wolffian and müllerian ducts are contiguous, müllerian abnormalities in girls also are common. The Mayer-Rokitansky-Kuster-Hauser syndrome is a group of associated findings that includes unilateral renal agenesis or ectopia, ipsilateral müllerian defects, and vaginal agenesis (Chapter 548).

Renal agenesis is distinguished from aplasia, in which a nubbin of nonfunctioning tissue is seen capping a normal or abnormal ureter. This distinction may be difficult but usually is clinically insignificant. Unilateral renal agenesis is diagnosed in some patients based on the finding of an absent kidney on ultrasonography or excretory urography. Some of these patients actually were born with a hypoplastic kidney or a multicystic dysplastic kidney that underwent complete cyst regression. Although the specific diagnosis is not critical, if the finding of an absent kidney is based on an ultrasonogram, a functional imaging study such as an excretory urogram or renal scan should be performed because some of these patients have an ectopic kidney in the pelvis. If there is a normal contralateral kidney, long-term renal function should remain normal.

Bilateral renal agenesis is incompatible with extrauterine life and produces the Potter syndrome. Death occurs shortly after birth from pulmonary hypoplasia. The newborn has a characteristic facial appearance, termed Potter facies (Fig. 531-1). The eyes are widely separated with epicanthic folds, the ears are low set, the nose is broad and compressed flat, the chin is receding, and there are limb anomalies. Bilateral renal agenesis should be suspected when maternal ultrasonography demonstrates oligohydramnios, nonvisualization of the bladder, and absent kidneys. The incidence of this disorder is 1/3,000 births, with a male predominance, and represents 20% of newborns with the Potter phenotype. Other common causes of neonatal renal failure associated with the Potter phenotype include cystic renal dysplasia and obstructive uropathy. Less-common causes are autosomal recessive polycystic kidney disease (infantile), renal hypoplasia, and medullary dysplasia. Neonates with bilateral renal agenesis die of pulmonary insufficiency from pulmonary hypoplasia rather than renal failure (Chapter 95).

Figure 531-1 Stillborn infant with renal agenesis exhibiting characteristic Potter facies.

The term familial renal adysplasia