Urinary Tract

The embryologic development of the kidneys encompasses three phases. These are the pronephros, meso-nephros, and metanephros. They go through overlapping cycles of growth and regression, resulting in the formation of the definitive kidneys and urogenital system from the latter two stages.

At the end of the third week, paired collections of primitive tubules form and empty into paired excretory ducts that lead to the cloaca.123 This pronephros never functions in humans and involutes by the early fifth week.124

While the pronephros is already involuting, a pair of structures forms just caudal to the pronephros. The tubules of this mesonephros actually function. They reach their full maturation by the end of the seventh week. Development, maturation, and involution all proceed in a craniocaudal progression, with considerable overlap-

ping.

The intermediate mesoderm from which the meso-nephros originates is divided by a longitudinal cleft, and a medially placed mesonephric duct is formed. A few days later, this duct communicates with the cloaca. Arteries that originate from the iliac arteries and the distal aorta supply the mesonephros itself. Drainage of the tubules is via the mesonephric duct (previously called the Wolffian duct). The latter partially involutes while play-

Urinary Tract Radiology

Fig. 2—62. Wandering spleen with torsion of vascular pedicle.

A 56-year-old man with chronic volvulus of an ectopic spleen.

(a) CT of the upper abdomen shows no evidence of the spleen in the left upper quadrant. There are prominent collateral veins (arrows) secondary to occlusion of the splenic vein. P = pancreas.

(b and c) The spleen (Sp) is located in the left lower abdomen. The vascular pedicle of the splenic hilum shows a whirled appearance (curved arrows).

(d) MRI (Tl-weighted coronal view) displays the spleen (Sp) in the left lower quadrant and numerous serpentine collateral veins (arrows). St = stomach.

Fig. 2—62. Wandering spleen with torsion of vascular pedicle.

A 56-year-old man with chronic volvulus of an ectopic spleen.

(a) CT of the upper abdomen shows no evidence of the spleen in the left upper quadrant. There are prominent collateral veins (arrows) secondary to occlusion of the splenic vein. P = pancreas.

(b and c) The spleen (Sp) is located in the left lower abdomen. The vascular pedicle of the splenic hilum shows a whirled appearance (curved arrows).

(d) MRI (Tl-weighted coronal view) displays the spleen (Sp) in the left lower quadrant and numerous serpentine collateral veins (arrows). St = stomach.

Femoral Hernia Mri

Fig. 2—63. Polysplenia syndrome in a 44-year-old man with abdominal pain.

(a) CT of the upper abdomen demonstrates a left-sided liver with absence of the hepatic segment of the IVC, a dilated hemiazygos vein (arrow) adjacent to the descending aorta, a right-sided heart (C), and two splenules on the right, lateral to a right-sided stomach (S).

(b) CT 4 cm lower shows a deep fissure divides the relatively normal-sized spleen.

(Reproduced from Gayer et al.122)

Fig. 2—63. Polysplenia syndrome in a 44-year-old man with abdominal pain.

(a) CT of the upper abdomen demonstrates a left-sided liver with absence of the hepatic segment of the IVC, a dilated hemiazygos vein (arrow) adjacent to the descending aorta, a right-sided heart (C), and two splenules on the right, lateral to a right-sided stomach (S).

(b) CT 4 cm lower shows a deep fissure divides the relatively normal-sized spleen.

(Reproduced from Gayer et al.122)

ing a major role in the development of the genital system in the male.

As the mesonephros and its duct regress, the meta-nephros starts to develop, culminating in the development of the definitive kidney and ureter. This process starts with the formation of a small bud from the mes-onephric duct, just proximal to its emptying into the cloaca (Fig. 2-65). As this bud elongates, it develops a bulbous end (ampulla) and a slender neck. The ampul-

Hemiazygos Vein

Fig. 2—64. Polysplenia syndrome in a 70-year-old woman.

(a) CT of the upper abdomen shows three rounded splenules in the left upper quadrant (thick arrow) and dilated azygos and hemiazygos veins in the retrocrural area (thin arrows).

(b) Coronal Tl-weighted MR image demonstrates the anomalous course of the interrupted IVC with its azygos continuation (arrows).

(Reproduced from Gayer et al.122)

lary end eventually becomes the renal pelvis, and the slender neck becomes the ureter proper.124 Specialized cells at the ampullary end interact with the surrounding mesoderm (metanephric blastema) to form the neph-rons and connective tissue of the kidney.123

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Fig. 2-65. Sagittal schematic of a 5-week embryo shows the allantois and hindgut emptying into the cloaca. The developing urorectal septum (URS) has started to partition the cloaca into separate GI and urinary compartments. The mesonephros (MES) and its duct are seen dorsally and superiorly. A short diverticulum from the mesonephric duct (developing ureter) is capped by the metanephric blastema (MET).

The ureteric bud undergoes simultaneous lengthening and division. The developing metanephros grows into the space vacated by the regressing mesonephros.124 The branching of the ureteric bud continues through many generations, leading to the development of the renal pelvis, calyces, papillae, cribriform plates, and even the collecting tubules. This branching also leads to the lobular contours of the fetal kidney that may persist into adulthood. The fetal kidney also undergoes a gradual fusion of calyces, but not papillae, that continues into the postpartum period, resulting in compound calyces.123

The development of the metanephros is accompanied by a change in its position and orientation. When the ureteral bud first contacts the metanephric blastema, it is at the level of the lower lumbar spine or upper sacrum. Originally, the paired metanephros almost touch each other in the midline. During the rapid growth of the caudal end of the fetus, they grow into the space occupied by the involuting mesonephros. Straightening of the fetal spinal curvature adds to the apparent migration of the kidney.125 By the end of the third month, the kidney is located at the level of the second or third lumbar vertebra, and at birth, it has ascended to the T12-L1 level. Rotation of the kidney along its longitudinal axis accompanies this ascent. The original renal pelvis lies along the anterior aspect of the developing kidney, eventually reaching an anteromedial location by the ninth week. Rarely, the kidney overrotates, leading the blood vessels to wrap around its posterior surface.

The blood supply to the developing kidney also undergoes change as it migrates cephalad. Lateral sacral branches from the distal aorta supply the metanephros. As the kidney migrates superiorly, lateral stem (end) arteries from more rostral levels of the aorta progressively supply it until the definitive renal arteries develop. Persistence of the more caudal arteries results in multiple and/or aberrant renal arteries.

Renal Agenesis

Agenesis of the kidney may result from failure of the ureteral bud to form, failure of the bud to encounter and induce the metanephric blastema, or absence of the blastema itself. When agenesis occurs, it is associated with abnormalities of the ipsilateral genital structures, including seminal vesicle cysts.126 A hemitrigone may be formed in the urinary bladder (Fig. 2-66). Agenesis of the kidney is discussed and illustrated in Chapter 9.

Crossed Fused Renal Ectopia, Horseshoe Kidney, and Blind-Ending Bifid Ureter

If there is asymmetric inhibition of ureteral bud growth prior to the separation of the metanephric blastemae, a crossed fused ectopia will occur127 (Fig. 2-67). If the inhibition is symmetric, then a horseshoe kidney results (Fig. 2-68). The two portions of the kidney may be united at their lower pole by either a band of fibrous tissue or, more commonly, functioning parenchyma.124 The renal pelves are still directed anteriorly, and the ureter must pass over and anterior to the resultant mass effect. This accounts for the common ureteropelvic (UPJ) obstruction. A crossing renal vessel may exacerbate this as well.124 The fused lower poles of a horseshoe kidney may also press on the underlying IVC, causing obstruction of that vessel and resultant bilateral leg edema.128 If inhibition occurs after the separation, either uni- or bilateral ectopia will occur.127 Early division of the ureteral bud that does not contact a metanephric blastema gives rise to a blind-ending bifid ureter. Although clearly demonstrated on excretory urography (Fig. 2-69) or a retrograde pyelogram, a blind-ending bifid ureter may be very difficult to diagnose on cross-sectional imaging.129 Early division that does contact the blastema leads to a partially or completely duplicated ureter and collecting system.127

Urinary Bladder

The bladder and portions of both the male and female genital tract are partially derived from the distal hindgut.

Partially Duplicated Collecting SystemHemitrigone Urinary Bladder

Fig. 2-66. Bladder hemitrigone associated with renal agenesis.

Supine film from an intravenous urogram reveals a large solitary kidney on the right. The distal ureter is noted to cross the midline (arrows) emptying onto a hemitrigone in this patient with left renal agenesis. (Courtesy of Marc Z. Simmons, M.D.)

Fig. 2-66. Bladder hemitrigone associated with renal agenesis.

Supine film from an intravenous urogram reveals a large solitary kidney on the right. The distal ureter is noted to cross the midline (arrows) emptying onto a hemitrigone in this patient with left renal agenesis. (Courtesy of Marc Z. Simmons, M.D.)

Crossed Fused Renal Ectopia Radiology

Fig. 2-67. Crossed-fused renal ectopia.

Left retrograde ureterogram shows a cross-fused ectopia. The left kidney lies inferior and medial to the previously opacified right kidney (curved arrow). The distal right ureter (straight arrow) can be seen in its normal location. (Courtesy of Marc Z. Simmons, M.D.)

Fig. 2-67. Crossed-fused renal ectopia.

Left retrograde ureterogram shows a cross-fused ectopia. The left kidney lies inferior and medial to the previously opacified right kidney (curved arrow). The distal right ureter (straight arrow) can be seen in its normal location. (Courtesy of Marc Z. Simmons, M.D.)

Fig. 2-68. Horseshoe kidney.

Contrast-enhanced CT of the pelvis demonstrates a horseshoe kidney. Both renal pelves can be seen along the anterior border (curved arrows). The two kidneys are connected by functioning renal parenchyma. (Courtesy of Marc Z. Simmons, M.D.)

Two Ureteric Buds

Fig. 2-69. Blind-ending ureteral bud.

Supine view of the pelvis from an intravenous urogram shows a short, blind-ending segment of ureter that lies just medial to the normal ureter draining the right kidney. No collecting system or functioning renal parenchyma is visualized.

Fig. 2-69. Blind-ending ureteral bud.

Supine view of the pelvis from an intravenous urogram shows a short, blind-ending segment of ureter that lies just medial to the normal ureter draining the right kidney. No collecting system or functioning renal parenchyma is visualized.

Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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Responses

  • klaus
    Can renal agenesis be discovered at birth?
    7 years ago

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