Trigone

The trigone of the urinary bladder is a smooth triangular region located on the internal surface of the bladder base. It is defined by three openings: the right and left ureteric orifices at its superolateral angles and the internal urethral orifice at its inferior angle.

The trigone differs from most of the remaining bladder mucosa because its surface remains relatively smooth rather than forming prominent folds when the bladder is empty. Its location at the junction between the ureters, bladder, and urethra makes it an important anatomical region in the storage and passage of urine.

Location

The trigone lies on the internal aspect of the base, or fundus, of the urinary bladder. It occupies the region between the two ureteric openings superiorly and the beginning of the urethra inferiorly.

When viewed from within the bladder, these three openings define its characteristic triangular configuration.

Boundaries

The boundaries of the trigone are determined by the positions of the ureteric and urethral orifices.

• The right ureteric orifice forms the right superolateral angle.
• The left ureteric orifice forms the left superolateral angle.
• The internal urethral orifice forms the inferior angle.
• The line between the two ureteric orifices forms the superior boundary.
• The lateral boundaries extend from each ureteric orifice toward the internal urethral orifice.

Ureteric Orifices

The right and left ureteric orifices are slit-like openings through which urine enters the urinary bladder from the ureters.

They are located at the superior corners of the trigone and are separated from one another by the interureteric region.

Internal Urethral Orifice

The internal urethral orifice is located at the inferior angle of the trigone. It marks the transition from the urinary bladder into the urethra.

Urine passes through this opening when the bladder empties during micturition.

Interureteric Ridge

A transverse elevation extending between the two ureteric orifices is called the interureteric ridge, or interureteric fold.

It forms the superior boundary of the trigone and can be identified on the internal surface of the bladder.

Interureteric Fold

The terms interureteric ridge and interureteric fold are commonly used for the slight transverse elevation connecting the ureteric openings.

The structure results from the underlying muscular and ureteric relationships at the bladder base.

Shape

The trigone has an approximately triangular shape, with a broad superior boundary extending between the ureteric orifices and an inferior apex directed toward the internal urethral orifice.

Its dimensions and precise configuration vary with bladder filling and individual anatomy.

Surface Appearance

The trigonal surface is characteristically smooth. This feature contrasts with the folded appearance of much of the remaining bladder mucosa when the bladder is contracted.

The smooth appearance reflects the relatively firm attachment of the trigonal mucosa to underlying tissues.

Comparison with the Remaining Bladder Mucosa

When the urinary bladder is empty, most of its mucosa forms folds or rugae. These folds flatten as the bladder fills.

The trigonal mucosa is more tightly attached to the underlying muscular layer and therefore remains comparatively smooth even when the surrounding bladder wall is contracted.

Trigone and Rugae

Feature	Trigone	Remaining Bladder Mucosa
Surface when bladder is empty	Relatively smooth	Usually folded
Prominent rugae	Absent	Present over much of the bladder
Attachment to underlying tissue	Relatively firm	More mobile
Relationship to urinary openings	Contains the three openings defining its angles	Does not define the ureteric-urethral triangle

Urothelial Lining

Like the rest of the urinary bladder, the trigone is lined by urothelium, also called transitional epithelium.

This specialized epithelium provides a barrier between urine and the underlying bladder tissues while accommodating changes in bladder volume.

Underlying Connective Tissue

Beneath the urothelium lies connective tissue containing blood vessels, nerves, and extracellular matrix components.

In the trigone, the mucosal tissues are closely associated with the underlying muscular structures, contributing to the relatively fixed and smooth appearance of this region.

Muscular Relationships

The trigone has specialized relationships with smooth muscle derived from the bladder wall and the terminal portions of the ureters.

Muscular fibers associated with the intramural ureters and bladder base contribute to the structural organization of the trigonal region.

Detrusor Muscle

The detrusor muscle forms the major smooth muscle coat of the urinary bladder and lies deep to the trigonal mucosa.

Its fibers integrate with muscular structures around the bladder base, ureteric entrances, and bladder neck.

Intramural Ureters

Before opening into the bladder, each ureter passes obliquely through the bladder wall. This segment is referred to as the intramural ureter.

The oblique course creates a functional valve mechanism that helps limit backward flow of urine from the bladder into the ureters.

Ureterovesical Junction

The ureterovesical junction is the region where each ureter enters and traverses the bladder wall before opening at a ureteric orifice.

Its relationship to the superior corners of the trigone is important in maintaining normal unidirectional flow from the ureters into the bladder.

Anti-Reflux Mechanism

As bladder pressure rises, the oblique intramural portion of each ureter is compressed within the bladder wall. This helps prevent urine from flowing backward into the upper urinary tract.

The effectiveness of this mechanism depends on the anatomy of the ureterovesical junction, including an adequate intramural ureteric course.

Relationship to the Bladder Neck

The inferior apex of the trigone approaches the bladder neck, where the urinary bladder becomes continuous with the urethra.

This places the trigone immediately adjacent to structures involved in controlling the bladder outlet.

Trigone in Males

In males, the internal urethral orifice at the inferior angle of the trigone leads into the prostatic urethra. The bladder neck lies immediately superior to the prostate.

Changes involving the prostate or bladder neck can therefore influence urinary flow through the region adjacent to the trigone.

Uvula of the Bladder

In males, a slight elevation of the mucosa may occur immediately posterior to the internal urethral orifice. This is known as the uvula of the bladder.

It may become more prominent when the median region of the prostate enlarges and projects toward the bladder outlet.

Trigone in Females

In females, the inferior angle of the trigone leads directly toward the short female urethra.

The bladder base and trigone have close anatomical relationships with the anterior vaginal wall and surrounding pelvic connective tissues.

Relationship to the Urethra

The trigone funnels inferiorly toward the internal urethral orifice. During micturition, urine moves from the bladder cavity toward this opening and then enters the urethra.

Efficient emptying requires coordinated detrusor contraction and reduction of resistance at the bladder outlet.

Function

The trigone occupies a strategically important position in the urinary drainage pathway. It lies between the points where urine enters from the ureters and the point where urine leaves through the urethra.

Its smooth, relatively fixed surface helps maintain the anatomical relationships of these openings during changes in bladder volume.

Role in Urine Flow

Urine entering through the ureteric orifices accumulates within the bladder cavity. During micturition, detrusor contraction directs urine toward the bladder neck and internal urethral orifice.

The trigone therefore occupies the central anatomical region connecting upper urinary tract inflow with lower urinary tract outflow.

Role During Bladder Filling

During bladder filling, most of the bladder wall expands substantially. The trigone remains comparatively stable because its mucosa is more firmly attached to underlying structures.

This stability helps preserve the spatial relationship between the ureteric openings and the bladder outlet.

Role During Micturition

During micturition, contraction of the detrusor raises intravesical pressure and directs urine toward the internal urethral orifice.

Muscular activity around the bladder base and neck participates in coordinated opening of the urinary outlet.

Development

The development of the trigone is closely related to incorporation of the distal mesonephric ducts and ureters into the developing posterior bladder wall.

As development proceeds, the ureteric openings become separated from the openings of the mesonephric ducts and migrate to their definitive positions.

Embryological Origin

The urinary bladder develops predominantly from the vesical part of the urogenital sinus, which is endodermal in origin. The trigonal region has a distinctive developmental history because of the incorporation of mesonephric duct tissue into the posterior bladder wall.

Although mesodermal tissue contributes during formation of the trigonal region, the luminal epithelium of the mature bladder, including the trigone, becomes continuous with the endodermally derived urothelium.

Incorporation of the Mesonephric Ducts

During development, the caudal portions of the mesonephric ducts become incorporated into the posterior wall of the developing bladder.

This process contributes to the rearrangement of the ureteric and mesonephric duct openings and to formation of the anatomical region corresponding to the trigone.

Migration of the Ureteric Orifices

The ureters initially arise as ureteric buds from the mesonephric ducts. As the distal mesonephric ducts are incorporated into the bladder wall, the ureteric openings acquire separate entrances into the bladder.

The ureteric orifices ultimately occupy the superolateral corners of the trigone.

Development of the Internal Urethral Orifice

The inferior point of the mature trigone is formed by the internal urethral orifice, where the bladder cavity becomes continuous with the urethra.

Together with the repositioned ureteric openings, this establishes the characteristic triangular arrangement of the mature trigone.

Innervation

The trigonal region receives autonomic and visceral sensory innervation as part of the innervation of the urinary bladder.

Autonomic fibers reach the bladder through the inferior hypogastric and vesical plexuses, while sensory pathways convey information from the bladder wall toward the spinal cord.

Parasympathetic Supply

Parasympathetic fibers originate primarily from sacral spinal cord segments S2 to S4 and travel through the pelvic splanchnic nerves.

These pathways are particularly important for detrusor contraction during micturition.

Sympathetic Supply

Sympathetic fibers reach the bladder through hypogastric and pelvic autonomic pathways.

They contribute to storage-phase function and regulation of smooth muscle around the bladder outlet.

Sensory Innervation

Visceral afferent fibers from the bladder transmit information associated with distension and pathological stimuli.

The trigonal and bladder neck regions are clinically important because irritation in these areas can be associated with pronounced urinary urgency and frequency.

Blood Supply

The trigone receives blood through the vascular supply of the urinary bladder. The superior vesical arteries supply upper portions of the bladder, while inferior regions receive additional vessels that vary according to sex.

In males, inferior vesical arterial branches contribute importantly to the bladder base. In females, branches of the vaginal and uterine arterial systems contribute to the corresponding region.

Venous Drainage

Venous blood from the trigone drains through the vesical venous plexus and associated pelvic veins.

These veins communicate with other venous plexuses within the pelvis and ultimately drain principally toward the internal iliac veins.

Lymphatic Drainage

Lymphatic drainage from the bladder base and trigonal region is directed toward pelvic lymph nodes, particularly the internal iliac and related nodal groups.

Lymphatic pathways are clinically relevant to the spread of malignant disease involving the urinary bladder.

Cystoscopic Appearance

The trigone is an important landmark during cystoscopy. The ureteric orifices, interureteric ridge, and internal urethral opening provide recognizable reference points when examining the interior of the bladder.

The regular arrangement of these structures helps orient the examiner within the bladder cavity.

Identification of the Ureteric Orifices

During cystoscopy, the ureteric orifices can be identified at the superior corners of the trigone. Urine may be observed entering the bladder intermittently from these openings.

Identification of the orifices is important during diagnostic and operative procedures involving the bladder and distal ureters.

Trigonal Irritation

Inflammation or irritation involving the trigonal region may produce urinary symptoms such as urgency, increased frequency, and discomfort associated with bladder filling or emptying.

These symptoms reflect the important sensory and functional relationships of the bladder base and outlet.

Trigonitis

The term trigonitis is used clinically for inflammatory or metaplastic changes involving the trigonal mucosa.

Its clinical significance depends on the underlying pathological process and associated urinary symptoms.

Urinary Tract Infection

Inflammation of the bladder mucosa during urinary tract infection may involve the trigone as part of the broader cystitis process.

Mucosal inflammation can stimulate sensory pathways and contribute to urinary urgency, frequency, and dysuria.

Vesicoureteral Reflux

Vesicoureteral reflux is the abnormal backward passage of urine from the bladder into one or both ureters.

It may occur when the normal anti-reflux mechanism at the ureterovesical junction is inadequate, particularly when the intramural ureter is too short or its relationship with the bladder wall is abnormal.

Trigone and Vesicoureteral Reflux

Because the ureteric orifices form the superior corners of the trigone, abnormalities involving the trigonal region and ureterovesical junction can affect the effectiveness of the anti-reflux mechanism.

The location and configuration of the ureteric openings are therefore important in the anatomical assessment of reflux.

Congenital Abnormalities

Abnormal development or insertion of the ureters can alter the normal anatomy of the trigone. Examples include ectopic ureteric openings and duplicated ureters with altered relationships to the bladder base.

Such abnormalities may be associated with urinary incontinence, obstruction, infection, or vesicoureteral reflux depending on their anatomy.

Bladder Outlet Obstruction

Conditions affecting the bladder neck or structures immediately inferior to the trigone can increase resistance to urinary flow.

In males, enlargement of the prostate is a common anatomical cause of obstruction near the bladder outlet.

Bladder Tumors

Urothelial tumors can arise in or extend into the trigonal region. Because the trigone contains the ureteric orifices and lies near the bladder outlet, lesions in this location may affect urinary drainage.

Knowledge of trigonal anatomy is important during cystoscopic evaluation and surgical treatment of bladder lesions.

Relationship to the Ureteric Orifices During Surgery

Surgical procedures involving the bladder base require careful identification of the ureteric orifices to avoid injury to the distal ureters.

The trigone provides the key internal anatomical landmarks used to locate these openings.

Trigone at a Glance

Feature	Anatomical Detail
Location	Internal surface of the bladder base
Shape	Triangular
Superior angles	Right and left ureteric orifices
Inferior angle	Internal urethral orifice
Superior boundary	Interureteric ridge
Surface	Relatively smooth
Epithelium	Urothelium
Major relationship	Ureters, bladder neck, and urethra

Key Anatomical Features

• The trigone is a triangular region on the internal surface of the bladder base.
• Its superior corners are formed by the right and left ureteric orifices.
• Its inferior corner is formed by the internal urethral orifice.
• The interureteric ridge connects the two ureteric openings.
• The trigonal mucosa is relatively smooth and firmly attached to underlying tissues.
• Unlike much of the empty bladder, the trigone does not form prominent rugae.
• The trigone is lined by urothelium.
• The intramural ureters enter at the superior corners of the trigone.
• The oblique intramural ureteric course contributes to the anti-reflux mechanism.
• The inferior trigone is closely related to the bladder neck.
• The trigone has a distinctive embryological relationship to incorporation of the mesonephric ducts.
• The mature trigonal surface is continuous with the urothelial lining of the remainder of the bladder.
• The trigone is an important landmark during cystoscopy.
• Abnormalities of the ureterovesical junction near the trigone can contribute to vesicoureteral reflux.
• Lesions of the trigone may affect the ureteric orifices or bladder outlet.

Clinical Importance

The trigone is clinically important because it contains the three openings that connect the urinary bladder with the ureters and urethra. Its ureteric orifices mark the entry points of the upper urinary tract, while the internal urethral orifice marks the beginning of the bladder outlet.

The trigone is also closely related to the ureterovesical anti-reflux mechanism. Abnormal anatomy of the intramural ureters or their insertion into the bladder can permit vesicoureteral reflux, potentially exposing the ureters and kidneys to elevated bladder pressure and ascending infection.

Its clearly identifiable boundaries make the trigone an important landmark during cystoscopy and bladder surgery. Understanding trigonal anatomy provides the foundation for studying ureteric orifices, the interureteric ridge, bladder neck anatomy, ureterovesical junctions, vesicoureteral reflux, cystoscopy, bladder development, urinary tract infection, and bladder outlet disorders.