Introduction

The ureters are bilateral thin (3 to 4 mm) tubular structures that connect the kidneys to the urinary bladder, transporting urine from the renal pelvis into the bladder. The muscular layers are responsible for the peristaltic activity that the ureter uses to move the urine from the kidneys to the bladder.

Embryologically, the ureter originates from the ureteric bud, which is a protrusion of the mesonephric duct, a part of the genitourinary system development.

The ureters begin at the ureteropelvic junction (UPJ) of the kidneys, which lie posteriorly to the renal vein and artery in the hilum[1]. The ureters then travel inferiorly inside the abdominal cavity. They pass over (anterior to) the psoas muscle and enter the bladder on the posterior bladder aspect in the trigone.

Three areas along the path of the ureter are clinically significant for renal stones lodging. These areas are: the ureteropelvic junction (UPJ), the ureterovesical junction (UVJ), and the crossover of the common iliac arteries. The UPJ is where the pelvis of the kidney transitions into the ureter and the UVJ is where the ureters enter the bladder.

The blood supply to the ureter is segmental. The upper ureter closest to the kidneys receives blood directly from the renal arteries. The middle part is supplied by the common iliac arteries, branches from the abdominal aorta, and the gonadal arteries. The most distal part of the ureter receives blood from branches of the internal iliac artery.

T12 through L2 provide innervation to the ureters, creating a ureteric plexus. Pain may refer to T12-L2 dermatomes.

Due to its location, the ureter can be damaged in colon and rectal surgery and gynecologic surgeries. 

Structure and Function

The ureteric wall is composed of three main of tissue: inner mucosa, middle muscle layer and outer serosa. The lining of the inner layer is transitional epithelium. Deeper to it is the lamina propria, which is combined with the epithelium make up the mucosal lining. The next deeper layer of tissue is the smooth muscle layer or lamina propria. An inner longitudinal and an outer circular layer comprise the smooth muscle layer of the ureter.

The path of the ureter is along the anterior edge of the psoas muscle, which is the general area where the gonadal vessels cross anteriorly to the ureter a third of the way to the bladder. The ureter crosses over the common iliac arteries, showing the anatomical landmark of the bifurcation of the common iliac vessels into internal and externa iliac vessels. The ureters finally enter on the posterior wall of the bladder where they incorporate into the trigone[2]. The ureters have specific anatomic relationships dependent upon which side of the body. The right ureter lies in close relationship to the ascending colon, cecum, and appendix. The left ureter is close to the descending and sigmoid colon.[1]

The nomenclature of the ureter is based on its anatomic relationship to surrounding structures. The abdominal ureter is the segment of the ureter that extends from the renal pelvis to the iliac vessels. The pelvic ureter extends from the iliac vessels to the bladder[3]. There is an alternative method of ureteral nomenclature: upper, middle, and lower segments. The upper ureter extends from the renal pelvis to the upper border of the sacrum. The middle ureter continues from the upper to lower borders of the sacrum. The distal ureter continues from the lower border of the sacrum to the bladder.

Embryology

In week 4, the development of the urinary tract (Kidney, Ureter, Bladder) begins with pronephros, mesonephros, and metanphros, which form from the nephrogenic cord. 

The stalk of the ureteric bud, which is a diverticulum from the mesonephric duct, gives rise to the ureter. The ureteric bud is regulated by different molecular pathways including GDNF-RET, BMP4(bone morphogenic protein 4), and Gremlin. There is a fine balance between ureteric bud growth and ureteric stalk elongation. This concerted effort to suppress ureteric bud development while also stimulating the elongation of the ureteric stalk is achieved by BMP4. And Gremlin secretion inhibits BMP4 and thus stimulates ureteric bud development. This finely orchestrated balance ensures that a single ureteric bud develops from each nephrogenic cord. [4]

The bud branches near the cranial aspect into the collecting tubules which become confluent and form the major calyces.[1]

Disruptions in the embryologic development can cause conganital abnormalitis of the kidney and urinary tract (CAKUT). 

Blood Supply and Lymphatics

The ureters receive their blood supply from multiple arterial branches[5]. In the upper or abdominal ureter, the arterial branches stem from the renal and gonadal artery, abdominal aorta, and common iliac arteries. In the pelvic and distal ureter, the arterial branches come from the vesical and uterine arteries, which are branches of the internal iliac artery. The arterial supply will course along the ureter longitudinally creating a plexus of anastomosing vessels. This is of clinical significance because it allows for safe mobilization of the ureter during surgery when proper exposure from surrounding structures is crucial.

The venous and lymphatic drainage of the ureter mirrors that of the arterial supply. The lymphatic drainage is to the internal, external, and common iliac nodes[6]. The lymphatic drainage of the left ureter is primarily to the left para-aortic lymph nodes while the drainage of the right ureter primarily drains to the right paracaval and interaortocaval lymph nodes.

Nerves

The exact role of the innervation of the ureter is unclear, but the innervation for ureteral peristalsis originates from the intrinsic smooth muscular pacemaker sites. Within the renal collecting system, the minor calyces are the location for the pacemaker sites.[7]

There is preganglionic sympathetic input from T10 through L2[8]. The aorticorenal, superior, and inferior hypogastric autonomic plexuses give rise to the postganglionic fibers.

S2 through S4 provide parasympathetic innervation to the ureter.

Muscles

The ureter is made up of 3 layers: innermost mucosa, muscularis, and the outer adventitia.

The mucosa is lined with circular transitional epithelium. The keratin in this layer is responsible for the waterproof propereties. 

The musclaris layer is made up of 2 longitudinal layers and a circular layer in the middle. The peristaltic motion of the ureter arises from the continuous smooth muscle layer from the ureter to the minor renal calyces, where the pacemaker for ureteric persitalsis is thought to arise 

The adventitia is made up of dense collagen and elastic fibers

Physiologic Variants

Abnormalities of the ureteric bud give rise to duplications of the abdominal ureter[9]. Abnormal division, specifically incomplete division of the metanephric diverticulum results in a bifid ureter with a divided kidney, while a complete division of the metanephric diverticulum results in a bifid ureter with a double kidney.[10]

An ectopic ureter does not enter the urinary bladder and can open into the bladder neck or the prostatic urethra in males[11]. In females, the ectopic ureter can open into the vestibule or vagina. This presents as urinary incontinence due to the direct communication between the urinary system and the vagina, causing continuous leakage of urine[12].

Surgical Considerations

The most common causes of ureteral injury are iatrogenic[13]. The overall incidence of iatrogenic ureteral injury varies between 0.5% to 10%. The most common type of procedure responsible for iatrogenic injury to the ureter is a hysterectomy (54%) due to the proximity of the uterine artery to the distal ureter[14][15]. As the ureter courses into the pelvis, it nears the infundibulo-pelvic ligament where it courses below to the uterine artery. Ureteral injuries may present with flank pain, ileus, hematuria, and prolonged high drain outputs. Elevated laboratory levels include BUN and creatinine.[16]

Clinical Significance

There are 3 specific areas of narrowing along the ureter. These areas of narrowing poise significant clinical sequelae when dealing with ureteral calculi[17]. The first is the ureteropelvic junction or UPJ. This is the area where the renal pelvis tapers into the proximal ureter. The second region of narrowing occurs where the ureter crosses the iliac vessels. The narrowing is due to the extrinsic compression of the iliac vessels on the ureter and the angle of the ureter as it enters the pelvis. The ureterovesical junction or UVJ is the third site of ureteral narrowing. A retrospective study of 94 patients presenting to the emergency department for colic found that 60.6% of stones were located at the UVJ.[18]