Ureteral Anatomy

Gary Faerber,¹ Amir H. Lebastchi,² & Rita P. Jen²

¹ Division of Urology, University of Utah School of Medicine, Salt Lake City, UT, USA

² Department of Urology, University of Michigan Health System, Ann Arbor, MI, USA

Introduction

The ureters are muscular tubes located in the retroperitoneum that deliver urine from the bilateral kidneys to the urinary bladder in the pelvis. They originate as extensions of the renal pelvis and average 22–30 cm in length and 1.5–6.0 mm in diameter in adults [1]. They are divided by convention into three sections: proximal, middle, and distal (Figure 38.1). Alternatively, they can be divided into abdominal, pelvic, and intramural portions. The proximal ureter begins at the renal pelvis at about L2 and ends at the level of the proximal sacrum. The middle ureter refers to the ureter from the proximal sacrum to the distal edge of the sacrum. The distal ureter is the portion of the ureter that runs from the distal edge of the sacrum and ends at the ureteral orifice.

Image described by caption. — **Figure 38.1** Normal intravenous pyelogram demonstrating course of ureter from kidney to bladder.

Embryology

During fetal development, three urinary systems with increasing complexity and function appear: the pronephros, the mesonephros, and the metanephros. The pronephros is a rudimentary group of cells with limited excretory function that appears in the cervical region during the fourth week of development which completely regresses by the end of that week [2, 3]. During the fourth week as the pronephros begins to regress, the mesonephros, which is derived from intermediate mesoderm, appears bilaterally on either side of what will be the spine [4]. The developing gonadal structures are just medial to the mesonephros, and the two developing organs together make up the urogenital ridge. At this point, further development of the mesonephros results in formation of the excretory tubules, capillaries, glomeruli, and Bowman’s capsule. A collecting system called the wolffian or mesonephric duct also forms. However, the mesonephros regresses like the pronephros.

The definitive urinary system is not formed until the permanent kidneys develop from the metanephric mesoderm in the fifth week [5]. The ureters and collecting system are derived from the ureteric bud, which grows from the wolffian duct. The wolffian duct otherwise regresses; however, in males, its remnants also contribute to structures of the genital system. Molecular signaling between the abutting ureteric bud and metanephric tissue results in development of the collecting system from the ureteric bud, specifically, the ureters, renal pelvis, calyces, and collecting tubules. The metanephric tissues become the glomeruli and nephrons.

At around 18 days, the ureter undergoes elongation, which results in redundancy and the formation of pleats that straighten as the ureter continues its elongation. As the ureter elongates it loses its lumen in the mid portion during days 37–40. Recanalization originates in the mid portion and extends cephalad and caudal to the ureteropelvic junction and ureterovesical junction, respectively. Hydrostatic pressure of secreted urine from the mesonephros is thought to contribute to recanalization. At nine weeks of development the metanephros, which will become the mature kidney, starts to produce urine. As this fetal urine drains into the bladder, patency of the ureter is maintained.

Anatomical course

The gentle sigmoid curve that the ureters take begins at the ureteropelvic junction (UPJ). They begin posterior to the renal vessels close to the renal hilum, lateral to the transverse processes of the lumbar vertebrae. The right ureter starts posterior to the second part of the duodenum and descends posterior to the ascending colon, cecum, appendix, and terminal ileum [1, 6]. The branches of the superior mesenteric artery and vein lie anterior to the right ureter. Both ureters lie just anterior to the psoas muscles, pass over the genitofemoral nerve, and under the gonadal vessels. The left ureter is attached to the mesocolon and passes under the sigmoid colon and its vessels, and travels behind the intersigmoid fossa. The left ureter also passes behind the inferior mesenteric artery in close proximity. The peritoneum covers the ureters anteriorly until they enter the pelvis.

As the ureters enter the pelvis, they also travel across the common iliac vessels and its branches. At the level of the ischial spine, the ureters lose their peritoneal layer and become completely retroperitoneal. The ureters further descend medially toward the bladder and pass anteriorly to the obturator neurovascular structures and posterior to the obliterated umbilical artery. In the male, the ureters pass behind the ductus deferens. In the female, the ureters pass under the ovarian and uterine vessels. In both genders, the ureters enter the bladder wall at an oblique angle at the ureterovesical junction (UVJ).

The caliber of the ureteral lumen varies along its course with average ranging from 1.8 to 3.0 mm [7]. There are three main areas of anatomical narrowing important during endoscopic procedures: at the UPJ, at the pelvic brim at the iliac vessels, and at the UVJ, with the UVJ being the narrowest of the three.

Histology

The ureter can be separated into three histologic layers: mucosal layer, muscular layer, and adventitial layer. The innermost layer in the mucosa begins with the urothelium (transitional epithelium), which lines the lumen and is atop the lamina propria. The lumen of the decompressed ureter has a stellate (Figure 38.2) configuration that becomes progressively circular as it becomes distended with urine during peristalsis [8]. The lamina propria, composed of collagen fibers and fibrocytes, houses the microscopic vascular supply of the wall of the ureter and unmyelinated nerves. The muscular layer carries bundles of smooth muscle fibers that control peristalsis. These bundles of muscles are arranged longitudinally into subtle layers. The prominence of each muscle layer depends on the segment of ureter. In the abdomen the muscle layers are extremely thin and almost nonexistent on light microscopy [9]. In the pelvis, two layers of muscles have been identified: the outer circular layer and an inner longitudinal layer [10]. Additionally, as the ureter enters the bladder, the inner longitudinal layer becomes even more prominent and is thought to eventually contribute to the muscles fibers in the floor of the bladder to form the trigone. The adventitial layer, also called the ureteric sheath, carries two vascular networks that supply the ureteric wall. The inner network has perforating arteries that are continuous with the vascular network of the mucosal layer. The outer network contains vessels that run longitudinally along the course of the ureter. The adventitial layers also consist of collagen, fibrocytes, nerves, and muscle cells. All three layers contain unmyelinated nerve fibers.

Blood supply

Vascular supply to the ureters varies along its course. It relies on an anastomotic network of vessels in the adventitia, which is the richest in the proximal and distal segments. Although the vast majority of the blood supply to the ureters is provided by the abdominal aorta, renal, gonadal, common iliac, and internal iliac arteries, there is considerable variation to the contribution of vessels of origin for the remaining blood supply [11]. The upper third of the ureter is supplied mostly by branches of the renal artery with contributions from the gonadal and colic arteries. The vascularization of the mid third of the ureter is variable, with contributions from the aorta, iliac, and gonadal arteries. In some patients the mid ureter is supplied primarily by peritoneal vessels. The distal ureter receives its main arterial supply from the internal iliac, superior vesical, and inferior vesical artery. Additional ureteral vascular supply may be provided by branches from the renal capsular arteries, adrenal arteries, umbilical artery, aortic branches, uterine artery, and the middle rectal artery. These small arterial branches, which may be quite long in relation to their caliber, subdivide in ascending and descending branches and coalesce into the periureteric arterial plexus within the adventitia. The supplying dominant branches enter the ureter differently at various segments. In the abdomen, arterial branches supplying the ureter enter anteriorly and medially. In the pelvis, the arterial supply enters posteriorly and laterally. This is important when considering incisions into the ureter if ischemia and development of strictures are to be avoided.

The venous drainage of the ureters begins in the lamina propria and extends into the adventitia. This venous plexus drains proximally via the renal and gonadal veins and distally into the vesical venous plexus and uterine veins in females and gonadal veins.

Lymphatics

Lymphatic drainage of the ureter is dependent on the ureteral segment. The proximal ureter is drained by the ipsilateral renal lymphatics. In other words, they are drained by the aortic lymph nodes surrounding the gonadal artery. The middle ureter on the right is drained by the right paracaval and interaortocaval lymph nodes. The left middle ureter is drained by the left paraaortic lymph nodes. The pelvic ureters bilaterally are drained by the common iliac, the internal iliac, and external iliac lymph nodes.

Innervation

The innervation of the ureters can also be divided into proximal, middle, and distal. The proximal nerve supply begins in the renal and aortic plexuses. The middle ureter is supplied by the superior hypogastric plexus, and the pelvic plexus provides innervation to the distal ureter. Visceral pain fibers are referred from the proximal ureter to L2 spinal nerve mostly, but they can also be carried to T11, T12, and L1 by the thoracic and lumbar splanchnic nerves that are part of the sympathetic nervous system. Stimulation of these sensory nerves (usually due to acute dilation of the ureter) results in referred pain to the dermatomes of T12–L2, namely the back and sides of the abdomen, the scrotum (males) or labia majora (females) and superior part of the anterior thigh. Peristaltic waves for ureteral contraction are controlled by pacemaking cells in the minor calyces and renal pelvis and are not dependent on autonomic stimuli [4]. Each minor calyx possesses these pacemaker cells, which can initiate a sustained contraction. These contractions are propagated away from the renal pyramid and toward the renal pelvis [12]. The duration of the contractile wave is dependent on the amount of urine being transported. With low urine flow rates the calyceal generated peristalsis may fail to precipitate generation of a pelviureteric peristalsis. With high urine flow rates, each contractile wave that reaches the pelviureteric junction is sustained and results in generation of a ureteral peristaltic wave and thus distal propulsion of urine toward the bladder. Specifically, the ureteric muscular coat is stimulated by the stretch of muscle fibers due to urine and thus peristalsis occurs [13]. The ureter makes an average three peristaltic contractions in a minute. There is debate on whether or not autonomic ganglia are present in the ureteric wall along its entire course as they have only have been identified in the very distal ureter [14, 15].

On a molecular level, activation of various S alpha‐1‐adrenoceptors 1A and 1D as well as muscarinic receptors have been found to cause ureteral contractions and have thus been the target of pharmaceutical manipulation [13, 16]. Efferent and afferent innervation from cholinergic, adrenergic, nonadrenergic, and noncholinergic receptors have also been identified, though the roles of the latter are not all completely understood [17]. Distribution of these different receptors is not evenly distributed throughout the course of the ureter, with the lower ureter being more densely populated than the upper ureter.