Renal Pelvis and Ureter - Clinical Tree

The renal pelvis and ureter are muscular conduits lined by urothelium that function to propel urine from the renal calyceal system to the urinary bladder. The ureter and renal pelvis are affected by developmental, reactive, and neoplastic disorders. The developmental disorders are a group of closely related entities that include abnormalities in ureteral number, ureteral location, and structure and function of pelvic and ureteral muscularis propria. The mucosa is the site of major reactive and neoplastic disorders.

Development

The ureter and renal pelvis develop from the ampullary bud, which arises from the distal mesonephric duct during the fourth week of development. Contact of the ampullary bud with metanephric blastema induces nephrogenesis. During the months that follow, the ampullary bud elongates and branches dichotomously in parallel with development of the nephrons to create the adult metanephric kidney with its renal pelvis and ureter. As the ureter elongates, there is a period of luminal obliteration followed by recanalization in the fifth week. Recanalization begins in the middle of the ureter and extends proximally and distally with the ureteropelvic and ureterovesical junctions, which are the last segments to recanalize.

The mesonephric duct distal to the ampullary bud (the common nephric duct) is incorporated into the developing urogenital sinus, whereas the ureteral orifice migrates to the trigone of the urinary bladder. The common nephric duct forms the trigone and contributes to the prostatic urethra in the male. Concomitant development of the male and female reproductive tracts from the mesonephric (wolffian) and Müllerian ducts, respectively, and division of the cloaca into bladder and hindgut occur nearby as the ureter and kidney develop. Thus multiple malformations in these areas often occur together.

Anatomy

The lumen of the renal pelvis and ureter is lined by the urothelium, which rests on a basement membrane over a lamina propria composed of highly vascular loose connective tissue ( Fig. 3.1 A ). The urothelium is composed of three to five layers of cells in the pelvis and four to seven layers of cells in the ureter ( Fig. 3.1 B). The pelvis and ureter have a continuous muscular wall that originates in the fornices of the minor calyces as small interlacing fascicles of smooth muscle cells. These take on a spiral architecture in the pelvis and ureter that is necessary for effective peristalsis. The muscularis propria is not divided into distinct layers. Near the bladder, the ureter acquires an external sheath from the detrusor muscle, and the muscle fascicles become oriented longitudinally. The longitudinal fibers continue through the wall of the bladder and into the submucosa, where they spread about the ureteral orifice to contribute to the trigone muscle. Ultimately they terminate near the bladder neck in the female and at the verumontanum in the male.

Peristalsis is initiated by “pacemaker” cells in the renal pelvic muscle near the calyces. These generate electrical impulses that propagate from cell to cell through gap junctions. Effective peristalsis requires both continuity of gap junctions and appropriate quantity and organization of muscle fascicles. As discussed later, disruption of this pattern, even focally, may cause ureteral incompetence or functional obstruction.

Congenital Malformations

Genitourinary tract malformations occur in 10% of the population and are the most common group of congenital anomalies ( Fig. 3.2 ). Some, such as bifid ureters, are clinically insignificant. Others are associated with ureteral incompetence or obstruction, with increased risk for renal damage or renal dysplasia. Some are components of multiple malformation syndromes (e.g., VATER [vertebrae, anus, trachea, esophagus, renal] association or prune belly syndrome), are associated with chromosomal abnormalities (e.g., trisomy syndromes), or have a familial predisposition. Congenital malformations of the ureters frequently occur together ( Table 3.1 ).

Fig. 3.2, Bifid ureter joining to form a single ureter above the bladder.

Table 3.1

Common Associations Among Ureteral Anomalies

Ureteral Anomaly	Bifid/Duplex	Ectopia	Reflux	Obstruction of Ureteropelvic Junction	Ureterocele	Dysplasia
Bifid/duplex		+	+	+	+	+
Ectopia	+		+	+	+	+
Reflux	+	+		+	+	+
Obstruction of ureteropelvic junction	+	+	+		+	+
Ureterocele	+	+	+	+		+
Diverticulum	-	+	+	-	-	+
Primary megaureter	-	-	-	-	-	+

Patients with ureteropelvic anomalies usually present with symptoms of ureteral or pelvic distention, such as flank pain or mass, or with complications such as infection, calculi, or renal insufficiency. Magnetic resonance imaging is the examination of reference. Most such lesions encountered in surgical pathology consist of intrinsic structural defects of the muscularis, usually involving the ends of the ureters, the last segments to recanalize during embryogenesis. These malformations are congenital and of developmental origin, but patients may present at any age from newborn to adulthood. Surgical therapy usually consists of excision of the abnormal segment to preserve renal function. The pathologist should define the anatomic basis of the functional deficit, which usually consists of a distinct but localized defect in smooth muscle quantity or organization ( Table 3.2 ). Recognition of these lesions requires an appreciation of the normal muscle pattern, and their histologic demonstration requires well-oriented sections in which the pattern of the muscle fascicles is highlighted by a trichrome stain. For most lesions, longitudinal orientation of the specimen best shows the deviation from the normal muscle pattern. Primary megaureter (discussed later) is an exception in which cross sections optimally display the predominance of circular fibers and thickening of the periureteral sheath.

Table 3.2

Ureteral Muscle Findings in Ureteral Anomalies

Ureteral Anomalies	Muscle Normal	Muscle Deficient	Muscle Dysplastic	Longitudinal Fiber Predominance	Circular Fiber Predominance	Sheath Thick
Refluxing megaureter	+	+	+	-	-	-
Obstruction of ureteropelvic junction	+	+	+	+	-	-
Primary megaureter	-	-	+	-	+	+
Ureterocele	+	+	+	-	-	-
Paraureteral diverticulum	+	+	+	-	-	-

Abnormalities in Number or Location of Ureters

Ureteral agenesis, ureteral duplication, and ureteral ectopia are a group of related malformations resulting from defective formation of the ampullary bud. Isolated failure of bud formation causes ureteral agenesis with absence of the ipsilateral hemitrigone and kidney. Another cause of agenesis of the ureter and kidney is wolffian duct failure, which is often associated with genital tract malformations (e.g., absent testis or unicornuate uterus). Unilateral agenesis of the kidney and ureter is associated with additional urologic malformations in 20% to 40% of patients. Bilateral renal agenesis (Potter syndrome) is lethal because of associated pulmonary hypoplasia; treatment possibilities exist but introduce ethical problems.

Bifid ureter and duplex ureter, the most common (0.8% of all autopsies) ureteral anomalies, result from premature branching of the ampullary bud or development of two separate ampullary buds. Premature branching results in two separate renal pelves and proximal ureters that join to form a single ureter at some point above the bladder ( Fig. 3.3 ). Duplex ureters ( Fig. 3.3 ) have two separate ureteral orifices in the bladder. The ureter from the lower pole usually has its orifice normally situated on the trigone or displaced laterally. The orifice of the ureter from the upper pole can be normally placed, but displacement toward the bladder neck or to an extravesical location is more common. Ureteral ectopia results from abnormally high or low origin of the ampullary bud from the mesonephric duct. Eighty percent of ureteral orifice ectopia is associated with the ureter from the upper pole of a duplicated system. The ectopic ureteral orifice may be intravesical (lateral or caudal to the normal site) or extravesical in the urethra, vestibule, or genitalia. Symptoms are influenced by gender and the site of the ureteral orifice, and may consist of urethral dribbling, vaginal “discharge,” epididymoorchitis, or pyelonephritis if reflux or obstruction are present. The greater the degree of ectopia in a lateral or extravesical location, the more likely it is that the corresponding renal unit will be dysplastic. The resected specimen thus may include a segmental or complete nephrectomy for dysplasia or pyelonephritis, or a distal ureter excised for reflux, obstruction, or ureterocele.

Fig. 3.3, Duplex ureter near point of confluence.

Refluxing Megaureter

Reflux from the bladder is the most common ureteral problem requiring surgical intervention. Patients usually present in early childhood with urinary tract infections and often already have renal scars (reflux nephropathy). Reflux may be unilateral or bilateral, and in about one-third of cases, the patients’ siblings have similar urologic abnormalities. Vesicoureteral reflux is caused by incompetence of the ureterovesical junction. There is a 2:1 predominance of females over males, possibly resulting from the additional mechanical support provided to the bladder by the prostate and seminal vesicles.

The affected ureters have abnormally short submucosal segments or deficiency of longitudinal fibers in the intramural segment, or both. Short submucosal segments are apparent to the urologist but difficult to demonstrate histologically. Deficiency of longitudinal fibers can be identified in longitudinal sections of the intramural segment and may appear to the urologist as an abnormally thin and translucent segment of distal ureter ( Fig. 3.4 ). Excision of the defective distal ureteral segment and reimplantation of the ureter is usually curative.

Fig. 3.4, Longitudinal section showing mucosal aspect of distal ureter in reflux. Note the thin wall.

Ureteropelvic Junction Obstruction

Ureteropelvic junction obstruction is the most common cause of ureteral obstruction and may present at any age. When occurring in childhood, it is frequently bilateral (16%), associated with other urologic malformations (15% to 20%), predominantly on the left side, and predominantly in boys; urinary tract infections are frequent. In contrast, cases presenting in adulthood are most often unilateral and occur in women. Treatment usually includes laparoscopic or robotic surgery. The two most common causes are defects in the muscularis (75%) and renal nonrotation associated with polar vessels (6% to 24%).

The obstructed ureteropelvic junction is characteristically funnel shaped ( Fig. 3.5 ). It may have a grossly visible area of thin muscle, a valvelike intraluminal protrusion of edematous mucosa or muscularis, or may be stenotic.

Fig. 3.5, Funnel-shaped zone of ureteropelvic junction obstruction.

The histologic appearance is varied. There may be segmental smooth muscle attenuation, often with a predominance of longitudinal fibers, diffuse lack of fascicular organization of pelvic muscles (i.e., dysplastic; see Ureteral Dysplasia section later in this chapter), segmental absence of smooth muscle, or stenotic lumen with normal muscle ( Fig. 3.6 ). “Valves” or “pleats” have also been described that probably result from herniation at the site of muscle abnormality.

Fig. 3.6, Ureteropelvic junction obstruction.

Renal polar blood vessels are common anatomic variants of the renal vasculature that usually do not obstruct the ureter because of the medial origin at the renal hilum. In congenitally nonrotated kidneys the pelvis is anterior, and polar vessels may cause significant ureteral obstruction.

Primary Megaureter

Primary megaureter is a nonrefluxing form of ureteral dilation. The gross appearance is distinctive ( Fig. 3.7 ), consisting of narrow and straight ureters immediately above the bladder that merge with segments that are fusiform and markedly dilated. This fusiform dilation differs from the tortuous appearance of ureteral dilation secondary to reflux or obstruction. In 80% of cases, there is functional obstruction at the level of the narrow segment that must be excised. In cross section the narrow segment shows predominance of circular fibers, hypoplasia, and fibrosis of the smooth muscle, or thickening of the periureteral sheath ( Fig. 3.8 ). The only abnormality of the dilated segment is smooth muscle hypertrophy. In the other 20% of cases the narrow segment of ureter has normal muscle, and the dilated segment above it has an almost complete absence of muscle. This has been referred to as dysplastic ureters (see Ureteral Dysplasia section later in this chapter) and is commonly associated with dysplastic kidney. However, this terminology may be misleading because there is no relationship with dysplastic (preneoplastic) urothelium.

Fig. 3.8, Primary megaureter segment showing smooth muscle hyperplasia.

Ureterocele

Ureterocele consists of congenital dilation of the distal ureter within the bladder ( Fig. 3.9 ). It may balloon into the bladder and occasionally protrudes into the urethra. Most ureteroceles occur in the upper pole ureter of a duplicated system in which the ureter usually passes dorsal to the lower pole ureter. Its dilated portion may undermine and distort the trigone, often resulting in obstruction or reflux of the normally situated lower pole ureter or, if the ureterocele is large, the contralateral ureter as well. Ureterocele rarely affects a single ureter.

Microscopically, the muscle of the wall of ureterocele varies from hypertrophic to atrophic or absent ( Fig. 3.10 ). Consistent with the usual ectopic location of the ureteral orifice associated with duplex kidneys, 70% of cases have segmental dysplasia of the upper pole of the kidney.

Fig. 3.10, Ureterocele showing thinning or lacking muscle in its wall.

Paraureteral Diverticulum

Herniation of the urinary bladder involving the distal ureter is called paraureteral diverticulum . It is usually congenital and detected in childhood, but may result from urethral or bladder neck obstruction at any age. Vesicoureteral reflux is commonly associated with paraureteral diverticulum. The location of the ureteral orifice within the diverticulum correlates with the risk for renal dysplasia ( Fig. 3.11 ). When the ureter opens into the dome of the diverticulum (a form of lateral ectopia) rather than near its orifice, the likelihood of renal dysplasia is great. There are few histologic studies of diverticula, but deficient ureteral muscle and sheath development have been reported.

Fig. 3.11, Bilateral paraureteral diverticula.

Ureteral Dysplasia

Ureteral dysplasia refers to ureters composed of infrequent smooth muscle cells lacking organization and failing to form fascicles. It is unrelated to urothelial dysplasia. Tokunaka et al. showed that involved muscle cells possess thin actin filaments but lack thick myosin filaments essential for normal contractility. Recognition of ureteral dysplasia is important because of the strong association with ipsilateral renal dysplasia (56% to 70% of cases).

Dysplastic ureters vary in appearance from atresia to dilation ( Fig. 3.12 ), and may have an anomalous location. Recognition of the dysplastic nature is not possible on gross examination, because ureters with normal muscle fascicle formation may have a similar appearance.

Fig. 3.12, Dysplastic megaureter associated with bilateral renal dysplasia.

Nonneoplastic Proliferative, Metaplastic, and Inflammatory Lesions

Nonneoplastic lesions of the ureter share similarities with those in the bladder, although inverted lesions may be more common in the ureter.

You're Reading a Preview

Become a Clinical Tree membership for Full access and enjoy Unlimited articles

Become membership

If you are a member. Log in here