Trigone of the Bladder: Anatomy, Boundaries, and Clinical Significance

Definition

The trigone is a smooth, triangular, highly sensitive region of the bladder base, bounded by the two ureteric orifices and the internal urethral orifice. It is embryologically distinct (derived from absorbed mesonephric ducts) and histologically different from the rest of the bladder mucosa.

Boundaries

• Superior (base): Interureteric ridge (ridge of mucosa connecting the two ureteric orifices).
• Inferior (apex): Internal urethral orifice.
• Lateral: Lines connecting each ureteric orifice to the internal urethral orifice.

Key Features

• Mucosa is tightly adherent to detrusor muscle → smooth, non-distensible.
• Rich sensory innervation (pelvic plexus) → initiates voiding reflex when stretched.
• Ureteric orifices: oblique intramural course → physiological valve preventing vesicoureteral reflux.
• Uvula of bladder: slight elevation behind internal urethral orifice (more prominent in males, from median lobe of prostate).

Clinical Significance

• Trigone-sparing cystectomy: Preserves continence and ureteral implantation.
• TURBT: Tumors in trigone require caution to avoid ureteral injury.
• Neurogenic bladder: Trigone sensitivity altered in spinal cord injury.
• Cystoscopy landmark: Key orientation point.
• Inflammation (trigonitis): Causes urinary frequency, urgency, dysuria.

Embryology Note

The trigone originates from the mesonephric (Wolffian) ducts, while the rest of the bladder epithelium is endodermal (from urogenital sinus). This explains its distinct histology and innervation.