Histological Layers

The wall of the ureter is specialized to transport urine from the renal pelvis to the urinary bladder while tolerating repeated changes in luminal volume and pressure. Histologically, it consists of a folded mucosa, a substantial smooth muscle coat, and an outer adventitia.

The mucosa is lined by urothelium, also known as transitional epithelium, which forms a highly effective barrier between urine and the underlying tissues. Beneath the epithelium is a connective tissue lamina propria. Smooth muscle generates ureteric peristalsis, while the adventitia contains the vessels, lymphatics, nerves, and connective tissue that connect the ureter to surrounding structures.

Organization of the Ureteric Wall

The ureteric wall can be organized into three principal layers:

1. Mucosa, consisting of urothelium and lamina propria
2. Muscularis, composed of smooth muscle
3. Adventitia, composed of connective tissue containing vessels and nerves

Mucosa

The mucosa forms the innermost layer of the ureter and directly surrounds the lumen.

It consists of the urothelial lining and the underlying lamina propria. Unlike many portions of the gastrointestinal tract, the ureter does not have a distinct muscularis mucosae or a well-defined submucosa.

Mucosal Folds

When the ureter is relaxed, its mucosa forms prominent longitudinal folds that project into the lumen.

These folds allow the lumen to expand as a bolus of urine passes through the ureter.

Stellate Lumen

In transverse histological sections, the longitudinal mucosal folds commonly produce an irregular or stellate lumen.

This star-shaped appearance is a useful feature for identifying the ureter microscopically, particularly when combined with the urothelial lining and thick smooth muscle wall.

Urothelium

The luminal surface is lined by urothelium, traditionally called transitional epithelium.

Urothelium is a stratified epithelium specialized to accommodate stretching while maintaining a highly effective permeability barrier against urine.

Layers of the Urothelium

The urothelium contains several layers of cells that can be grouped into:

• Basal cells
• Intermediate cells
• Superficial umbrella cells

Basal Cells

Basal cells form the deepest portion of the urothelium and rest on the basement membrane.

They are generally small cells and contribute to maintenance and renewal of the epithelial lining.

Intermediate Cells

One or more layers of intermediate cells lie between the basal and superficial cell populations.

Their morphology changes as the urothelium stretches and relaxes.

Umbrella Cells

The luminal surface is formed by large umbrella cells. These cells are considerably larger than the deeper epithelial cells and may contain more than one nucleus.

Umbrella cells are highly specialized for maintaining the urinary barrier while permitting changes in surface area as the ureter distends.

Apical Surface of Umbrella Cells

The apical plasma membrane of umbrella cells contains specialized membrane proteins called uroplakins.

These proteins contribute to the formation of rigid membrane plaques that strengthen the luminal surface and reduce its permeability to components of urine.

Uroplakins

Uroplakins are integral membrane proteins characteristic of differentiated urothelial cells.

They organize into plaques within the apical membrane of umbrella cells and contribute to the permeability barrier of the urinary tract.

Barrier Function

Urine can contain high concentrations of ions, metabolic waste products, and other substances that could damage underlying tissues if they freely crossed the epithelial lining.

The urothelium therefore functions as a specialized barrier that limits movement of water and urinary solutes into the ureteric wall.

Urothelial Distensibility

The term transitional epithelium reflects the ability of the urothelium to change its microscopic appearance as the urinary tract expands.

When relaxed, the epithelium appears thicker and the superficial cells are rounded or dome-shaped. During distension, the epithelium becomes thinner and the superficial cells flatten.

Basement Membrane

The urothelium rests on a basement membrane separating it from the underlying connective tissue.

This interface supports epithelial organization and provides structural attachment between the urothelium and lamina propria.

Lamina Propria

Beneath the urothelium lies the lamina propria, a connective tissue layer containing collagen fibers, elastic components, fibroblasts, small blood vessels, lymphatic channels, and nerves.

The lamina propria supports the urothelium and contributes to the mechanical properties of the ureteric mucosa.

Connective Tissue of the Lamina Propria

The lamina propria contains relatively dense fibroelastic connective tissue.

Its collagen provides tensile strength, while elastic components permit deformation and recoil as the ureter repeatedly expands and contracts.

Absence of a Distinct Submucosa

The ureter does not possess a clearly defined submucosal layer comparable with that found in many gastrointestinal organs.

The lamina propria transitions toward the underlying muscular coat without a sharply separated submucosa.

Absence of Muscularis Mucosae

A distinct muscularis mucosae is also absent from the normal ureteric wall.

This feature helps distinguish the organization of the ureter from that of many tubular organs of the digestive system.

Muscularis

The muscularis forms a thick layer of smooth muscle surrounding the mucosa.

Its coordinated contractions generate the peristaltic waves that transport urine from the renal pelvis toward the urinary bladder.

Smooth Muscle Arrangement

The orientation of ureteric smooth muscle is less sharply separated than the discrete muscular layers seen in some other hollow organs.

Nevertheless, a general regional arrangement can be recognized along the length of the ureter.

Upper Ureter

In much of the upper ureter, the muscularis is commonly described as having two principal layers:

• Inner longitudinal smooth muscle
• Outer circular smooth muscle

Inner Longitudinal Layer

The inner smooth muscle bundles are oriented predominantly along the longitudinal axis of the ureter.

Contraction of these fibers contributes to shortening and coordinated deformation of a ureteric segment during peristalsis.

Outer Circular Layer

The outer smooth muscle bundles have a predominantly circular orientation around the ureter.

Their contraction narrows the lumen and helps propel a bolus of urine distally.

Lower Ureter

In the distal ureter, an additional outer longitudinal smooth muscle layer becomes prominent.

The lower ureter is therefore often described as having inner longitudinal, middle circular, and outer longitudinal muscular layers.

Regional Muscle Arrangement

Region	Typical Muscle Organization
Upper ureter	Inner longitudinal and outer circular smooth muscle
Lower ureter	Inner longitudinal, middle circular, and outer longitudinal smooth muscle

Muscle Bundles

The smooth muscle layers should not be interpreted as perfectly uniform concentric sheets.

Individual muscle bundles may have oblique or interwoven orientations, producing a functional muscular network capable of coordinated propagation of contractions.

Peristaltic Function

The ureter does not function simply as a passive conduit. Its muscular wall actively transports urine through peristalsis.

Sequential contraction behind a urine bolus and relaxation ahead of it propel urine distally toward the urinary bladder.

Origin of Peristaltic Activity

Contractile activity begins within the upper urinary collecting system and propagates through the ureteric smooth muscle.

The intrinsic properties of urinary tract smooth muscle permit coordinated waves to travel along the ureter even without voluntary control.

Role of Autonomic Innervation

The ureter receives sympathetic and parasympathetic autonomic fibers, but normal peristaltic activity is fundamentally generated by intrinsic smooth muscle activity rather than depending entirely on extrinsic autonomic innervation.

Autonomic pathways can influence ureteric activity and also carry visceral sensory information.

Adventitia

The outermost layer of the ureter is the adventitia.

It consists of fibroelastic connective tissue that merges with the surrounding retroperitoneal and pelvic connective tissues.

Contents of the Adventitia

The adventitia contains:

• Arteries and arterioles
• Veins and venules
• Lymphatic vessels
• Autonomic nerve fibers
• Connective tissue fibers
• Adipose tissue in surrounding regions

Blood Vessels in the Adventitia

The ureter has a segmental arterial supply derived from nearby vessels along its course.

These arterial branches form longitudinal anastomoses within the adventitial and periureteric connective tissue.

Surgical Importance of the Adventitia

The adventitia is clinically important because it contains much of the vascular network supplying the ureter.

Excessive circumferential stripping of the ureter during surgery can damage these vessels and compromise blood flow to the ureteric wall.

Absence of a Serosa

Because most of the ureter is retroperitoneal or extraperitoneal, its external surface is generally covered by adventitia rather than serosa.

The adventitia blends with surrounding connective tissue and helps anchor the ureter while still permitting movement during peristalsis.

Histological Identification

A ureter can usually be recognized microscopically by the combination of several features:

• An irregular or stellate lumen
• Prominent longitudinal mucosal folds
• Urothelial lining
• Large superficial umbrella cells
• Fibroelastic lamina propria
• Absence of distinct muscularis mucosae and submucosa
• A thick smooth muscle coat
• An outer connective tissue adventitia

Ureter Compared with the Urinary Bladder

Feature	Ureter	Urinary Bladder
Epithelium	Urothelium	Urothelium
Mucosal folds	Prominent longitudinal folds when relaxed	Rugae in much of the empty bladder
Muscular wall	Organized ureteric smooth muscle bundles	Thick detrusor muscle
Main function	Active transport of urine	Storage and expulsion of urine
Typical lumen	Small and often stellate in section	Large and highly distensible

Ureter Compared with the Renal Pelvis

The renal pelvis and ureter are both lined by urothelium and possess smooth muscle within their walls.

The ureter develops a more substantial muscular wall as it continues distally from the renal collecting system and assumes its role as an active conduit toward the bladder.

Ureter Compared with the Urethra

The ureter maintains a urothelial lining throughout its course, whereas the epithelial lining of the urethra changes along its length.

The ureter is also characterized by a prominent smooth muscle wall specialized for peristaltic transport.

Histological Changes During Distension

When urine enters a ureteric segment, the lumen enlarges and the mucosal folds partially flatten.

The urothelium becomes thinner in appearance, and superficial umbrella cells flatten as their luminal surface area increases.

Histological Changes During Relaxation

After a urine bolus passes, the ureter returns toward its resting configuration.

The mucosa again becomes folded, the lumen becomes more irregular, and the urothelium appears thicker.

Urothelial Renewal

Under normal conditions, urothelium has a relatively low cellular turnover rate.

Following injury, however, epithelial cells can proliferate to restore the integrity of the urinary barrier.

Ureteritis

Ureteritis refers to inflammation of the ureter.

Inflammation may affect the urothelium, lamina propria, and surrounding tissues and can occur in association with infection, obstruction, calculi, or other pathological processes.

Effects of Ureteric Obstruction

Obstruction increases pressure proximal to the blocked segment and produces distension of the ureteric lumen.

Persistent obstruction can alter smooth muscle function and eventually produce structural remodeling of the ureteric wall.

Hydroureter

Hydroureter is abnormal dilation of the ureter, usually resulting from impaired urinary drainage.

Chronic dilation can alter the normal folded mucosal appearance and affect the organization and function of the muscular wall.

Ureteric Calculi and the Mucosa

A ureteric calculus can mechanically irritate the urothelial surface and produce local inflammation, edema, and sometimes bleeding.

The surrounding smooth muscle may contract vigorously in response to obstruction, contributing to ureteric colic.

Ureteric Stricture

A ureteric stricture is a pathological narrowing of the ureter that may develop after inflammation, ischemia, trauma, surgery, or other injury.

Fibrosis involving the ureteric wall can reduce luminal diameter and interfere with urinary drainage.

Ischemic Injury

The segmental vascular pattern of the ureter makes preservation of its adventitial blood supply important during surgery.

Loss of blood supply can produce ischemic injury, tissue necrosis, leakage, or subsequent fibrotic narrowing.

Urothelial Carcinoma

Urothelial carcinoma can arise from the urothelial lining of the ureter, although tumors of the upper urinary tract are less common than those of the urinary bladder.

The shared urothelial lining throughout much of the urinary collecting system explains why similar epithelial neoplasms can arise at different sites.

Layer-by-Layer Organization

Layer	Major Components	Primary Role
Urothelium	Basal, intermediate, and umbrella cells	Urinary barrier and distensibility
Lamina propria	Fibroelastic connective tissue, vessels, nerves	Structural support
Muscularis	Smooth muscle bundles	Peristaltic propulsion of urine
Adventitia	Connective tissue, vessels, lymphatics, nerves	Support, vascular supply, and attachment

Functional Integration of the Layers

The layers of the ureter function together rather than independently. The urothelium protects underlying tissues from urine, the lamina propria supports the mucosa and permits deformation, the muscularis generates propulsive contractions, and the adventitia supplies and anchors the ureter.

This organization allows the ureter to remain normally collapsed between urine boluses while rapidly expanding and contracting as urine travels toward the bladder.

Urine Transport Through the Ureteric Wall

1. Urine enters the ureter from the renal pelvis.
2. The lumen expands locally as urine enters a ureteric segment.
3. Mucosal folds flatten and the urothelium accommodates distension.
4. Smooth muscle contracts behind the urine bolus.
5. The contraction propagates distally as a peristaltic wave.
6. The urine bolus moves toward the urinary bladder.
7. The mucosa returns toward its folded resting configuration after the bolus passes.

Key Histological Features

• The ureter is lined by urothelium.
• The urothelium contains basal, intermediate, and superficial umbrella cells.
• Umbrella cells contain specialized uroplakin-rich apical membrane plaques.
• The mucosa forms longitudinal folds when the ureter is relaxed.
• The lumen commonly appears stellate in transverse sections.
• A fibroelastic lamina propria lies beneath the urothelium.
• A distinct muscularis mucosae is absent.
• A clearly defined submucosa is absent.
• The muscularis is composed of smooth muscle bundles.
• The upper ureter is generally described as having inner longitudinal and outer circular muscle layers.
• An additional outer longitudinal layer becomes prominent distally.
• The muscular wall generates ureteric peristalsis.
• The outer layer is adventitia rather than a complete serosa.
• The adventitia contains the ureteric vascular and neural networks.

Clinical Importance

The histological structure of the ureter reflects its two essential requirements: maintaining a highly resistant barrier against urine and actively transporting urine toward the bladder. The urothelium provides the barrier and accommodates repeated distension, while the smooth muscle coat generates coordinated peristaltic contractions.

The outer adventitia is particularly important surgically because it contains the longitudinal vascular network supporting the ureter. Damage to this tissue can compromise perfusion and contribute to ischemia or stricture formation. Pathological processes involving any layer can interfere with normal urine transport, including urothelial tumors, inflammation, fibrosis, calculi, and obstruction.

Understanding the histological layers of the ureter provides the foundation for studying urothelium, transitional epithelium, umbrella cells, uroplakins, ureteric peristalsis, hydroureter, ureteric obstruction, ureteric strictures, urothelial carcinoma, and urinary tract histology.