Topography and Wall Structure
The urinary bladder (vesica urinaria) is a hollow muscular organ situated in the lesser pelvis. Anatomically, it is divided into the apex (apex vesicae), body (corpus vesicae), neck (cervix vesicae), and fundus (fundus vesicae). The trigone is located specifically on the fundus.
The bladder wall consists of several layers: the mucosa (tunica mucosa), submucosa (tela submucosa), muscular layer (tunica muscularis, which forms the detrusor muscle, m. detrusor vesicae), and an outer serosal or adventitial layer.
In the empty bladder, the mucosa forms random folds, whereas in the ureters the folds are longitudinal; both patterns are permitted by a loose submucosal layer. The trigone is an exception: the submucosa is entirely absent here. Consequently, the mucosa is firmly fused with the muscular coat, keeping the surface permanently smooth.
Borders of the Bladder Trigone
The region is triangular in shape, with its three corners defined by key anatomical orifices:
- Apex — points anteriorly and inferiorly. It corresponds to the internal urethral orifice (ostium urethrae internum).
- Base — faces posteriorly and superiorly. It is formed by the line connecting the right and left ureteral orifices.
- Interureteric fold (plica interureterica — a mucosal ridge that bridges the two ureteral orifices across the base of the trigone.
Sphincter Apparatus, Blood Supply, and Innervation
The internal urethral sphincter (m. sphincter urethrae internus or m. sphincter vesicae) is an involuntary smooth muscle sphincter surrounding the proximal urethra. The voluntary external urethral sphincter (m. sphincter urethrae) surrounds the membranous urethra (pars membranacea).
Blood supply to the urinary bladder is delivered via branches of the internal iliac artery (a. iliaca interna). Innervation is supplied by the pelvic autonomic plexuses, primarily the vesical plexus (plexus vesicalis).
Clinical Significance (Lieutaud's Triangle)
In clinical practice, the bladder trigone is frequently referred to as Lieutaud's triangle.
- Urothelial Carcinoma (Transitional Cell Carcinoma): This histology accounts for up to 90% of all bladder malignancies. Tumors most frequently arise within the trigone and along the posterolateral walls. Grossly, they may present as papillary exophytic lesions on a stalk (which often do not breach the basement membrane initially) or as sessile/flat plaques with mucosal thickening lacking papillary projections. Flat lesions are more frequently invasive and often represent carcinoma in situ (CIS), carrying a high potential for progressive anaplasia.
- Random Biopsy Mapping: In patients diagnosed with high-grade T1 tumors (T1G3) or carcinoma in situ (CIS), mapping biopsies are performed to assess disease extent. This includes sampling the trigone, apex, right wall, left wall, anterior wall, posterior wall, and prostatic urethra.
- Botulinum Toxin Type A Injection: To manage neurogenic detrusor overactivity and improve urodynamics, onabotulinumtoxinA is injected directly into the detrusor muscle. Typical therapeutic doses range from 100 to 300 units, providing clinical and urodynamic improvement lasting several months.