Ultrastructure of the trigone and its functional implications

Objective: To investigate the ultrastructure of the trigonal muscle (= superficial trigone), relate it to normal function, and identify any changes it may undergo in voiding dysfunction.

Materials and methods: 20 men (median age 67 years) with low-stage prostatic carcinoma, 10 with and 10 without bladder outlet obstruction, were selected by urodynamic evaluation. Trigonal biopsy was performed at radical prostatectomy, and processed for electron-microscopic study by standard procedures. Biopsies were evaluated independently by 2 examiners without prior knowledge of urodynamic data.

Results: Three obstructed and 1 unobstructed bladder had impaired detrusor contractility, and 1 obstructed bladder had detrusor overactivity. Compared to the previously investigated detrusor, the trigonal muscle had smaller compact bundles and fascicles with less collagen and more elastic tissue. Muscle cells had no or rare intermediate junctions that mediate mechanical coupling in normal detrusor, but predominant close cell appositions that mediate electrical coupling. Smooth muscle in most biopsies had widespread or focal features characteristic of the aged detrusor. None, however, had the features previously associated with obstructed detrusor, or detrusor with impaired contractility.

Conclusions: The trigonal muscle does not undergo structural changes as previously described in the detrusor in association with voiding dysfunction. Its contraction depends on electrical coupling of its muscle cells, and has a supportive role in normal micturition, mooring the terminal ureters to the bladder base, to allow efflux and guard against reflux of urine. Activation of volume and tension sensory neuroterminals may contribute to some storage and voiding micturition reflexes, and may be related to normal and abnormal perception of urge.