Trigone of the urinary bladder

The trigone of the urinary bladder is a smooth triangular region of the internal urinary bladder. It is defined by three points: the two ureteral orifices and the internal urethral orifice. This area is clinically significant due to its role in urinary tract infections and its embryological development.

Anatomy

Diagram of the bladder showing the trigone

The trigone is located on the posterior wall of the bladder. It is a smooth, triangular area that is distinct from the rest of the bladder wall, which is typically folded and rugose. The apex of the trigone is directed towards the bladder neck, where the internal urethral orifice is located. The base of the triangle is formed by the interureteric ridge, which connects the two ureteral orifices.

Boundaries

The boundaries of the trigone are defined by:

• The two ureteral orifices, where the ureters enter the bladder.
• The internal urethral orifice, where the bladder opens into the urethra.
• The interureteric ridge, a fold of mucous membrane that runs between the ureteral orifices.

Histology

The trigone is lined by a specialized type of epithelium known as transitional epithelium, or urothelium. This epithelium is unique to the urinary tract and is capable of stretching to accommodate varying volumes of urine. Beneath the epithelium, the trigone has a dense layer of connective tissue and smooth muscle fibers that contribute to its smooth appearance.

Function

The primary function of the trigone is to act as a funnel, directing urine from the ureters into the urethra. The smooth muscle of the trigone helps maintain the position of the ureteral orifices and prevents reflux of urine back into the ureters during bladder contraction.

Clinical significance

Diagram showing the trigone and surrounding structures

The trigone is clinically significant for several reasons:

• Urinary tract infections (UTIs): The trigone is a common site for the development of UTIs due to its proximity to the urethra and its role in urine flow.
• Vesicoureteral reflux: This condition, where urine flows backward from the bladder into the ureters, can be associated with abnormalities in the trigone.
• Bladder cancer: The trigone is a common site for the development of bladder cancer due to its constant exposure to urine.

Embryology

The trigone develops from the mesonephric ducts, which are part of the embryonic kidney system. This is in contrast to the rest of the bladder, which develops from the urogenital sinus. This difference in embryological origin is reflected in the distinct histological characteristics of the trigone compared to the rest of the bladder.

Related pages

• Urinary bladder
• Ureter
• Urethra
• Urinary tract infection