The Kidney and Retroperitoneum: Anatomy and Congenital Abnormalities

Embryology

In humans, mature kidneys develop as a result of evolution of three successive sets of primitive excretory structures ( Box 2-1 ). These excretory organs, in order of development, are the pronephros, the mesonephros, and the metanephros. These develop in a cranial-to-caudal progression. The pronephros develops in the segmented mesoderm of the upper thoracic and cervical regions of the fetus during the third week of fetal life. The pronephros is a transient structure with no adult correlate. However, its development is crucial for induction of the next major phase of kidney development: differentiation of the mesonephros into the mesonephric duct. The mesonephros, which arises caudal to the pronephros, forms in unsegmented nephrogenic cord during the fourth to eighth week of fetal development, just as the pronephros is regressing. While the mesonephric duct forms the first functioning excretory duct of the fetus, it degenerates rapidly after the ninth week of gestation. Some segments of the mesonephric duct, also known as the wolffian duct , persist and develop into segments of the genital system. In men, the wolffian duct forms the efferent ductules of the testes, the epididymis, and the vas deferens. In women, the wolffian duct develops into the epoophoron and the paroophoron. During the fifth week of gestation, the metanephros begins to develop into the definitive kidney.

The metanephros develops from two separate cell lines, each with different potentials. These cell lines are the ureteric bud and the metanephric blastema. The ureteric bud develops as an outgrowth of the mesonephric duct, proximal to the cloacal entry. The ureteric bud ultimately gives rise to the ureter, renal pelvis, calyces, and collecting tubules of the renal medulla. The metanephric blastema develops from the caudal portion of the nephrogenic cord and gives rise to the excretory part of the kidney. Development of the metanephric blastema into the excretory system must be induced by contact with the ureteric bud. Induction of the metanephric blastema occurs after growth of the ureteric bud and physical contact with the metanephric blastema. As the ureteric bud grows, the end approaching the metanephric blastema enlarges. This ampullary segment later gives rise to the renal pelvis, whereas the remainder of the ureteric bud will become the ureter. After the ampullary portion of the ureteric bud contacts the metanephric blastema, multiple divisions of this portion of the ureteric bud commence. Division of the ureteric bud is dichotomous but asynchronous, and this dividing process continues for 12 to 14 generations. Each generation of the ureteric bud invaginates deeper into the metanephric tissue. First- and second-generation divisions give rise to the major calyces, and the minor calyces arise from the third through fifth generations. All subsequent generations provide the basis for development of the collecting tubules. These tubules are radially arrayed around minor calyces, forming the renal pyramids. After final development, there should be 10 to 25 fully formed calyces. Divisions of the ureteric bud in the polar regions of the kidney temporarily lag behind development in the interpolar segment of the kidney. This divisional delay often results in the development of fewer and less completely divided compound calyces in the polar regions of each kidney.

Metanephric blastema tissue forms a cap overlying the terminal ampullary segments of the ureteric bud. The metanephric blastema tissue is carried with the dividing and growing ureteric bud. Maturation of the blastema is induced by physical contact with the ureteric bud. The metanephric blastema differentiates into the excretory system of the kidney. Differentiation of the metanephric blastema eventually leads to the development of Bowman capsule, the proximal and distal convoluted tubules, the loop of Henle, and supporting tissue of the renal parenchyma. Development of the glomerulus induces angiogenesis. The developing glomerulus is supplied by branches of the renal artery and is connected to the developing convoluted tubules and the loop of Henle. The tubules come to communicate with the ampullary segment of the ureteric bud, allowing for excretion of urine into the collecting tubules. The metanephric blastema surrounding this developing excretory system differentiates into the interstitial supporting tissue of the renal parenchyma. Because the ureteric bud development is dominant in the embryology of the kidney, its branching pattern and induction of metanephric blastema define the renal lobe. A single renal lobe consists of a calyx, collecting ducts, and its overlying renal cortex. As they develop, these renal lobes coalesce to form a normal kidney made up of approximately 14 renal lobes. In utero, renal lobar anatomy is evident as early as the fourth month of gestation. Renal maturation continues from birth to the age of 5. During maturation, cellular multiplication in the renal cortex continues and leads to loss of definition of gross lobar anatomy in most individuals. Persistent fetal lobation, an anomaly of kidney maturation, is seen in up to 5% of adult patients ( Fig. 2-1 ). After the age of 5, renal cellular multiplication is no longer possible. Renal hypertrophy can occur well into adulthood. Hypertrophy leads not to an increased number of glomeruli, but rather to enlargement and increased capacity of the existing glomeruli. This enlargement can result in global or focal enlargement of the kidney, as is commonly seen in patients after loss of substantial volumes of functioning renal tissue. The remaining kidney enlarges and increases its excretory capacity to compensate for the lost renal parenchyma. This is evident even in individuals in their 70s. In older individuals, however, the capacity for renal hypertrophy is less than in children. When focal, this can result in masslike areas of prominent, but normal renal parenchyma, known as a pseudotumor. This is sometimes seen in patients with focal areas of renal atrophy from reflux nephropathy, where the mass effect of the pseudotumor is enhanced by being adjacent to areas of scarred, atrophic kidney ( Fig. 2-2 ). The metanephros begins its development in the upper sacral region of the fetus. At birth, the kidneys lie in the upper lumbar region due to the differential migration of fetal tissues during gestation. This apparent ascent of the kidney is actually due to the rapid longitudinal growth of the embryo in the lumbar and sacral regions caudal to the developing kidney. This cephalic migration to the adult position occurs from the fourth to the eighth week of gestation. Concomitant with the cephalic migration is a 90-degree medial rotation of the kidney about its longitudinal axis, which brings the ureteropelvic junction (UPJ) to a medial position in relation to the kidney. During ascent from the true pelvis, vascular supply to the kidneys comes from progressively higher branches off the aorta, and the inferior branches regress. The primitive kidney is originally supplied by lateral sacral branches of the aorta, but as it ascends to the adult position, it is supplied by progressively higher lateral branches from the aorta until it reaches its adult position. The renal artery then arises laterally from the aorta at approximately the level of the second lumbar vertebra. Although progressive ascent usually leads to regression of the inferior blood vessels, anoma­lous vessels are commonly seen supplying the kidney. In addition, failure of complete ascent leading to anomalous renal position, as seen with pelvic and horseshoe kidneys, is almost always associated with coexistent anomalous blood supply to the affected kidney, which reflects persistence of these inferior branches.

Obviously, kidney development involves a complex sequence of developmental processes during gestation. It is interesting to note that successive development and maturation of the primitive excretory organs—the pronephros, mesonephros, and metanephros—recapitulate the complex evolution of excretory organs in species of varying levels of sophistication. A primitive pronephros is the excretory organ of primitive fish. The mesonephros is the excretory organ of more advanced fish and amphibians. The metanephros is the excretory organ of reptiles, birds, and mammals. Ontogeny may indeed recapitulate phylogeny.

Classification of Congenital Renal Abnormalities

It is helpful to classify congenital renal abnormalities as anomalies of (1) number, (2) position, (3) fusion, (4) vasculature, (5) structure, and (6) UPJ obstruction. Abnormalities in each of these categories are outlined in Box 2-2 .

Abnormalities of Number

Renal Agenesis

Renal agenesis results from failure of the ureteric bud to reach the metanephric blastema because the ureteric bud fails to form or degenerates prematurely. In either case, the induction of differentiation in the metanephric blastema does not occur. Associated ureteral abnormalities are universally present ( Box 2-3 ). These include absence of the ipsilateral ureter and its associated hemitrigone, or presence of a blind-ending ureteral stump, a remnant of the incompletely developed ureteral bud. In 20% of the male individuals with renal agenesis, there is absence of the ipsilateral epididymis, vas deferens, or seminal vesicle, or presence of an associated ipsilateral seminal vesicle cyst ( Fig. 2-3 ). In 70% of women with unilateral renal agenesis, associated genital anomalies are present ( Fig. 2-4 ). These include absence or atresia of the uterus or vagina, a unicornuate uterus with absence or atresia of the vagina and ovary, or duplication anomalies of the genital tract. These complex müllerian duct anomalies are considered part of the Mayer-Rokitansky-Küster-Hauser syndrome. Absence of the ipsilateral adrenal gland is associated with renal agenesis in 10% of patients. In the remaining patients, the adrenal gland is present, but it often has a disklike shape ( Fig. 2-5 ). Whereas renal agenesis can be definitively diagnosed with ultrasound (US), computed tomography (CT), magnetic resonance imaging (MRI), or radionuclide renography, congenital absence of the kidney from the normal renal fossa may be suggested on plain radiographs of the abdomen. With absence of one kidney, bowel (duodenum or colon on the right and colon on the left) may fall into the empty renal fossa. With absence of the left kidney, the descending colon may course medially with respect to the distal transverse colon resulting in a looped configuration of the splenic flexure ( Box 2-4 ). These findings may also be seen on cross-sectional imaging ( Fig. 2-6 ).

Box 2-3

Abnormalities Commonly Present with Renal Agenesis

• Absence of ipsilateral ureter

• Absence of ipsilateral hemitrigone

• Absence of ipsilateral vas deferens

• Absence of ipsilateral seminal vesicle or cyst

• Uterine anomalies

• Abnormal bowel gas pattern

• Disk-shaped adrenal gland (adrenal gland absent in 10% of patients)

Box 2-4

Causes of Unilateral Absent Kidney Opacification in Renal Fossa

Common

• Previous nephrectomy

• Renal agenesis

• Renal ectopia

Uncommon

• Renal artery occlusion

• Renal vein occlusion

• Ureteral obstruction

• Pyonephrosis

• Pyelonephritis

• Xanthogranulomatous pyelonephritis

Rare

• Multicystic dysplastic kidney

• Tumor infiltration of kidney

Renal agenesis occurs in one in 1000 live births. Seventy-five percent of patients with renal agenesis are male. Bilateral involvement is rare, occurring approximately once in every 3000 live births. Bilateral renal agenesis is incompatible with life. With bilateral renal agenesis, intrauterine growth does occur because the placenta serves as the excretory organ for the fetus. As no urine is excreted, there is associated oligohy­dramnios. This causes pulmonary hypoplasia and facial abnormalities, which are features of the well-known Potter syndrome ( Fig. 2-7 ). Potter syndrome includes a typical facial pattern with low-set ears, a broad flat nose, and prominent skin folds below the lower eyelids, coupled with pulmonary hypoplasia and development of pneumothoraces at birth. Unilateral renal agenesis may remain asymptomatic as long as the contralateral kidney functions normally. Usually the contralateral kidney becomes hypertrophied and appears enlarged (see Fig. 2-4 ), an expected development. With unilateral renal agenesis, congenital malformations in the remaining solitary kidney are common. If these abnormalities impair renal function, then symptomatic renal insufficiency can develop.

Supernumerary Kidney

Very rarely, more than two discrete kidneys are present, probably due to the formation of two ureteral buds on one side. Usually, the supernumerary kidney occurs on the left side caudal to the normal kidney and is hypoplastic. Supernumerary kidneys are of two basic types. In the first type, a bifid ureter also drains the second kidney on the ipsilateral side. In the second type, a separate ureter drains the supernumerary kidney and another ureter drains the second kidney on the ipsilateral side ( Fig. 2-8 ).