The ureters are paired muscular ducts with narrow lumina that carry urine from the kidneys to the bladder.
An understanding of the anatomic relations of the ureters is critical to the practice of urology, as well as to the disciplines of gynecologic, vascular, and general surgery. The ureter serves as a critical landmark and is intimately involved with other vessels and organs, making accidental ureteral injury a dreaded consequence of surgery (see the image below). Knowledge of the microscopic anatomy and vascular supply of the ureter are essential during manipulation of the ureter. [1, 2, 3, 4]
The ureter is subject to natural variation such as duplication. Pathologic variants of the ureter are also prevalent and can manifest as urinary obstruction, renal failure, and infection. [5, 6, 7] Increasingly, aberrant ureteral anatomy can be identified on antenatal ultrasonography, enabling early medical and surgical intervention.
The mesoderm gives rise to the kidney, ureter, bladder, and urethra. The metanephros is the principle excretory unit starting at week 8 of gestation, eventually becoming the mature kidney. Metanephric development is contingent on the ingrowth of the ureteric bud, which arises from the distal posteromedial mesonephric duct.
Absence of the ureteric bud leads to renal agenesis, whereas incomplete ingrowth or ureteral atresia results in multicystic dysplastic kidney. The ureteric bud bifurcates with ingrowth into the metanephric blastema, leading to division of the calyces. Premature bifurcation may lead to incomplete duplication of the ureter or bifid pelvis. Other abnormalities of ureteric bud formation may lead to anomalies of number or termination (See Natural Variants and Pathophysiologic Variants). 
The ureter is roughly 25-30 cm long in adults and courses down the retroperitoneum in an S curve. At the proximal end of the ureter is the renal pelvis; at the distal end is the bladder. The ureter begins at the level of the renal artery and vein posterior to these structures. This ureteropelvic junction usually coincides with the second lumbar vertebra on the left, with the right being marginally lower.
The ureter then continues anteriorly on the psoas major muscle, crossing under the gonadal vein at the level of the inferior pole of the kidney. The ureters course medial to the sacroiliac joint and then curve laterally in the pelvis. The colon and its mesentery are associated anterior to the ureters. Specifically, the cecum, appendix, and ascending colon lie over the right ureter, and the descending and sigmoid colon lie over the left ureter. 
The ureter enters the pelvis, where it crosses anteriorly to the iliac vessels, which usually occurs at the bifurcation of the common iliac artery into the internal and external iliac arteries. Here, the ureters are within 5 cm of one another before they diverge laterally.
The ovarian vessels travel in the suspensory ligament of the ovary (infundibulopelvic ligament) and cross the ureter anteriorly and lateral to the iliac vessels. The ureters then course out to the ischial spines before coursing medially to penetrate the base of the bladder. The anteromedial surface of the ureter is covered by peritoneum, and the ductus deferens runs anteriorly. It travels with the inferior vesical neurovascular pedicle into the bladder. In females, the ureter runs posterior to the ovary and then deep to the broad ligament and through the cardinal ligament. The uterine artery crosses anteriorly in the rectouterine fold of peritoneum. 
Blood supply and lymphatic drainage
The vascular supply and venous drainage of the ureter is derived from varied and numerous vessels. One critical feature is that the arterial vessels travel longitudinally in the periureteral adventitia. In the abdominal ureter, the arterial supply is located on the medial aspect of the ureter, whereas in the pelvis, the lateral aspect harbors the blood supply . The upper ureter is supplied by the renal artery and by branches from the gonadal artery and aorta. The arterial supply of the middle ureter is derived from the common iliac and gonadal arteries. Finally, the distal ureter is supplied by branches of the common iliac and internal iliac branches, particularly uterine and superior vesical arteries. See the image below.
The venous drainage is paired with the arteries. Knowledge of this vascular supply is crucial in ureteral surgery, because a devascularized ureter is subject to complications of stricture and leak. Lymphatic drainage of the upper ureter joins the renal lymphatics to the lumbar nodes. The middle ureter drains to the common and internal iliac nodes. The lymphatic vessels of the pelvic ureter drain to the internal iliac and vesical nodes.
The ureter has an intrinsic pacemaker that governs peristalsis but also has autonomic inputs. Thoracolumbar preganglionic inputs synapse with aorticorenal and inferior and superior hypogastric sympathetic plexuses before innervating the ureter. Parasympathetic inputs derive from the S2-S4 segments. Mucosal irritation and luminal distention stimulate nociceptors whose afferents travel with sympathetic nerves and confer the visceral-type referred pain that results in the manifestations of ureteral colic. Pain or hyperesthesia may be sensed from the region of the ipsilateral ribs down to the scrotum or labia.
Close association of the abdominopelvic viscera places the ureter at risk for inflammatory, infectious, or malignant processes of the colon, appendix, oviducts, or ovaries. This may manifest as hematuria, pyuria, fistula, or obstruction. The mass effect of constipation, gravid uterus, or ovarian cysts may obstruct the ureter. The aorta and iliac vessels may exert deleterious effects on the ureter by mass effect or fibrotic reaction from the vasculopathy itself or by complications of the surgical management of aortoiliac disease.
The ureter has 3 physiologic narrowings: (1) the ureteropelvic junction, (2) the crossing over the iliac vessels, and (3) the ureterovesical junction. This is crucial in the manifestations of calculus disease. These narrowings may result in ureteral stones becoming trapped and obstructing at these specific levels. These narrowings may also limit retrograde instrumentation performed for diagnostic or therapeutic purposes.
The close association of the ovarian vessels at the level of the pelvic brim and the uterine artery in the rectouterine fold render the ureter subject to injury during oophorectomy or hysterectomy, as they are just deep to the crucial vasculature (see the following image).
The ureter is roughly 3-4 mm in diameter. Like the renal pelvis, bladder, and proximal urethra, the ureter is lined with transitional cell epithelium, which consists of a short basal layer, one or more layers of columnar cells, and, most apically, umbrella cells. The umbrella cells are specialized to survive bathing in hypertonic urine and to stretch with distention of the lumen.
Deep to the epithelial layer is the lamina propria, an elastic connective tissue matrix. The thickest layer of the ureter is the muscularis, which is composed of smooth muscles oriented in an inner longitudinal and outer circular arrangement. Finally, the outer portion of the ureter is the adventitia, a fibrous layer that harbors the vascular supply.  See the following image.
Duplication of the ureter
Ureteral duplication is one of the most common congenital abnormalities of the ureter. Autopsy series have estimated the incidence to be 0.8% or 0.9%. 
Ureteral duplication ranges from Y duplication to complete duplication. The Y duplication likely represents divergence of the ureteric bud before meeting the metanephric blastema during development, whereas complete duplication reflects 2 ureteric buds developmentally. Clinically, the incidence of the Y variant is lower than 0.9%, as this duplication is often without consequence. Unilateral ureteral duplication is 6 times more common than bilateral duplication and has no clear sex preponderance or laterality preference. However, a hereditary role in ureteral duplication is evident.
One of the paramount anatomic relationships of duplicated ureters is dictated by the Weigert-Meyer law, which states that the orifice of the lower pole ureter occupies a more cranial and lateral position. In general, the upper pole ureter drains less renal parenchyma. The lower pole ureter is more susceptible to dilatation from vesicoureteral reflux than single ureters or upper pole ureters. The upper pole ureters are often subject to anomalies of termination such as ureterocele or ectopic orifice.
Rarer permutations of ureteral duplication include the inverted Y duplicated ureter in which 2 separate ureteric buds fused in development. A blind-ended ureteral branch may develop but is often easily visualized only on a retrograde study. Ureteral triplication has also been described but is far less common than duplication.
Ureteropelvic junction obstruction
Ureteropelvic junction obstruction is the most common congenital abnormality of the ureter. The laterality of the anomaly favors the left side and is more common in males. Etiologies of ureteropelvic junction obstruction include a hypoplastic segment of ureter, crossing lower pole accessory renal artery, and high insertion of the ureter on the renal pelvis. An angulation or kink is almost always present. The standard of care for symptomatic ureteropelvic junction obstruction is dismembered pyeloplasty, which can be accomplished using a minimally invasive robotic approach.
A ureterocele is a sacculation or cystic dilatation of the terminal ureter. It results from incomplete canalization between the urogenital sinus and ureteric bud. The cystic dilatation forms between the superficial and deep muscle layers of the trigone. Ureteroceles may be orthotopic but are more often ectopic and in the bladder neck or urethra. These are usually associated with the upper pole moiety of a duplicated system and may cause obstruction of an ipsilateral or contralateral ureteral orifice or the bladder outlet.
Ectopic ureteral orifice
An ectopic ureteral orifice often occurs in the upper pole of duplicated systems but can also occur in single ureters. The orifice is caused by delayed or failed separation of the ureteral bud from the mesonephric ducts. Posterior urethra, seminal vesicle, and ductus deferens are common locations for ectopic ureteral orifices in males, making urinary tract infection and epididymitis common presenting features. Ectopic ureteral orifices in females tend to be in the urethra, vagina, or perineum, making total incontinence a presenting symptom.
The finding of megaureter is the gross manifestation of a heterogenous group of pathologies. A megaureter may be obstructed or unobstructed, refluxing or nonrefluxing. Ureterovesical functional obstruction without stricture but with an aperistaltic segment is the classic description of the obstructed megaureter. Megaureter may also be associated with prune belly syndrome and other congenital syndromes.
The ureter may be absent or may end blindly caudad to the flank. The result is an absent or multicystic dysplastic kidney, which may involute. Observation is the best treatment. Hypertension is rare, but contralateral vesicoureteral reflux is common, making voiding cystourethrography (VCUG) a valid recommendation for these patients.
Ureteral diverticula are exceedingly rare. They can result from a blind-ended bifid ureter and are typically without consequence. Or, they can be a true congenital saccular diverticulum occurring at any portion along the ureter; these may be quite large and store a great volume of urine. Acquired ureteral diverticula may also occur, particularly in the setting of chronic distal obstruction.
The right ureter may pass posterior to the inferior vena cava before hooking medially anterior to it and then resuming its normal course to the pelvis. This abnormality is a vascular rather than ureteral embryologic phenomenon, with the subcardinal vein persisting and becoming the inferior vena cava rather than the supracardinal vein. For this reason, the term preureteral inferior vena cava has been favored in some texts. The proximal ureter is often dilated in this setting.
The retrocaval ureteral abnormality is rare (1 per 1500 by autopsy series) and is more common in males. The presentation is usually incidental or in the fourth or fifth decades of life, with obstruction. A similar abnormality in vascular development may lead to retroiliac ureter.