Age effects on urethral striated muscle I. changes in number and diameter of striated muscle fibers in the ventral urethra

OBJECTIVE: This study was undertaken to test the null hypothesis that the number of striated muscle fibers in the ventral wall of the female urethra remains constant with increasing age. STUDY DESIGN: The urethra and surrounding tissues from 25 female cadavers, mean age 52 years (±SD 18, range 15-80 years), were selected for this study. Each specimen was divided along the midsagittal plane, and a Masson trichrome histologic section was prepared. A systematic count of striated muscle fibers in the ventral wall was then obtained at each decile of urethral length. RESULTS: A decrease in the total number of fibers within the sampled area was found with increasing age. The mean of the total fibers across all urethrae was 17,423 (±SD 9,624, range 4,788-35,867). Over the life span, an average of 364 fibers (2%) were lost per year (95% CI 197-531; P < .001). Mean fiber density was 671 (± SD 296, range 228-1374) fibers/mm 2 and decreased by 13 fibers/mm 2 per year (95% CI 8-17; P < .001). The mean lesser fiber diameter was 24 μm and did not change significantly with age ( P = .3). CONCLUSIONS: The number and density of urethral striated muscle fibers decline with age. (Am J Obstet Gynecol 2002;186:351-5.)