Disposition of the striated urethral sphincter and its relation to the prostate in human fetuses.

OBJECTIVE To describe the arrangement of the muscle fibers of the striated urethral sphincter and its relationship with the prostate during the fetal period in humans. MATERIALS AND METHODS We analyzed 17 prostates from well preserved fresh human fetuses ranging in age from 10 to 31 weeks postconception (WPC). Transversal sections were obtained and stained with Gomori's trichrome and immunolabeled with anti alpha-actin antibody. RESULTS We found that the urethral striated sphincter (rabdosphincter) is located on the periphery of the smooth muscle and there was no merge between striated and smooth muscle fibers in any fetal period. In the prostate apex, the striated sphincter shows a circular arrangement and covers completely the urethra externally, whereas adjacent to verumontanum, it looks like a "horseshoe" and covers only the anterior and lateral surfaces of the urethra. Near the bladder neck, in fetuses younger than 20 WPC, we have found striated muscle fibers only at the anterior surface of the prostate, while in fetuses older than 20 WPC, the striated muscle covers the anterior and lateral surfaces of the prostate. CONCLUSIONS The urethral sphincter muscle covers the anterior and lateral surfaces of the urethra in all fetuses older than 20 WPC, close to the bladder neck and at the distal prostate. In the region of the prostate apex, the urethral sphincter covers completely the urethra circularly. The knowledge of the normal anatomy of the urethral sphincter in fetuses could be important to understand its alterations in congenital anomalies involving the base of the bladder, the bladder neck and the proximal urethra.