title: Reappraisal of intergender differences in the urethral striated sphincter explains why a completely circular arrangement is difficult in females: a histological study using human fetuses.
abstract: To investigate why the development of a completely circular striated sphincter is so rare, we examined histological sections of 11 female and 11 male mid-term human fetuses. In male fetuses, the striated muscle initially extended in the frontal, rather than in the horizontal plane. However, a knee-like portion was absent in the female fetal urethra because, on the inferior side of the vaginal end, a wide groove for the future vestibule opened inferiorly. Accordingly, it was difficult for the developing striated muscle to surround the groove, even though there was not a great difference in width or thickness between the female vestibule and the male urethra. The development of a completely circular striated sphincter seems to be impossible in females because of interruption of the frontal plane by the groove-like vestibule. However, we cannot rule out the possibility that before descent of the vagina, the urethral striated muscle extends posteriorly.
authors: Department of Urology, National Hospital Organization Higashi-Hiroshima Medical Center, Higashi-Hiroshima, Japan.
authors: Hiroshi
authors: H
authors: Masumoto
authors: Atsushi
authors: A
authors: Takenaka
authors: Jose Francisco
authors: JF
authors: Rodríguez-Vázquez
authors: Gen
authors: G
authors: Murakami
authors: Akio
authors: A
authors: Matsubara