Maturation of the external urinary sphincter: a comparative histotopographic study in humans

The developmental anatomy of the striated urinary sphincter remains controversial and is scantly documented in children. We compared its structure and configuration in the fetus, infant and adult to determine anatomical differences among these groups. We removed 25 postmortem specimens from fetuses, infants and children, which were fixed and stained for histological study. Ages ranged from 14 weeks of gestation to 12 years postpartum. Transverse and mid sagittal sections were obtained from the bladder neck to the membranous urethra in male and the whole urethra in female subjects. At the level of the membranous urethra in male and mid urethra in female subjects the striated muscle fibers completely encircle the urethra and join behind it to form a tail-like structure that runs posteriorly towards the perineal body. This structure is mid sagittal in male and mediolateral in female subjects. At 3 to 4 months of life, at the level of the bulbourethral glands the tail disappears; the striated sphincter becomes horseshoe-shaped and its 2 branches bifurcate posteriorly to envelop these glands. The urethral striated sphincter consists of scantly dispersed muscle fibers in the fetus. In young infants it becomes well defined in both sexes with the presence of a tail-like structure that characterizes this age group. In older subjects this tail disappears and the sphincter assumes a horseshoe or omega-shaped configuration as splitting of the sphincter progresses caudo-cranially with development. We attempt to determine whether the ring configuration of the voluntary sphincter contributes to high voiding pressures that are reported to occur in some newborns and infants.