The development of the external urethral sphincter in humans

Objective To assess the hypothesis that during fetal development, the exter nal urethral sphincter changes from a concentric sphincter of undifferentia ted muscle fibres to a transient ring of striated muscle which regresses ca udo-cranially in the posterior urethra during the first year of life, when the sphincter assumes its omega-shaped configuration. Materials and methods The anatomy and development of the external urinary s phincter was assessed in human males and females during fetal life. Plastic -embedded sections (transverse. sagittal and frontal planes: 300-700 mum) o f the pelvis of 31 females and 31 males (9 weeks of gestation to newborn) w ere stained with azure II/methylene blue/basic fuchsin and viewed at x 4-80 . The sections of interest were taken from the bladder neck to the perineum . The sections of the membranous urethra were reconstructed three-dimension ally using a computer program. Results In both male and female an omega-shaped external sphincter was appa rent in all specimens at >10 weeks of gestation. In the early fetal period (ninth week), there was undifferentiated mesenchyme: in this period the mes enchyme was more dense in the anterior part and loose in the posterior part of the urethra. In females, there was a close connection between the ureth ra and the anterior wall of the vagina. Conclusion The omega-shaped configuration of the external urethral sphincte r was recognisable from 10 weeks of gestation in both sexes. There was no s uggestion of a change from a cylindrical to an omega-shaped sphincter in th e fetal period to birth. Also, a transient 'tail' posterior to the sphincte r was not apparent. The rectovesical septum was well developed in neonates. There is no reason to assume that the development of the. septum leads to an apoptosis of muscle cells in the posterior part of the external sphincte r in males after birth, The anatomical development of the external sphincte r does not explain transient outlet obstruction during fetal life. The func tion of the muscle may change during development because of neuronal matura tion.