The present study demonstrated that, in both genders, the urethral striated sphincter developed on the inferior or distal side of the site where the genital tracts merged with the urethra. The male fetal urethra changed its direction immediately inferior to the future prostatic colliculus. Thus, the usual model in which the striated muscle surrounds the urethra in the horizontal plane () does not correspond to the fetal morphology earlier than 13 weeks, or a CRL of 95 mm. Therefore, at these stages, the future membranous urethra seems to run horizontally along the antero-posterior axis, and the early male striated sphincter extends in the frontal plane (). In females, by contrast, there was no distal urethra, but the future vestibule opened inferiorly on the inferior side of the vaginal inferior end. Therefore, it appeared impossible for the female urethral striated muscle to "surround" this fissure-like structure corresponding to the future vestibule. Likewise, the urethrovaginal sphincter did not appear to reach the posterior midline area because of the inferiorly opened fetal vestibule. In fact, in adults, the vagina is tightly attached to the posterior wall of the urethra [7
]. However, we are unable to rule out the possibility that, before descent of the vagina, the urethral striated muscle extends posteriorly into the space between the vagina and the urethra.
Fig. 4 Schematic representations of the fetal urethral striated muscle. (A) The usual concept of the sphincter extending in a horizontal plane. (B) An actual image of the male sphincter extending in the frontal plane but not reaching the inferior aspect of the (more ...)
Experiments using an avian chimera system have shown that the perineal muscles originate from myotomal cells in common with the lower extremities [12
]. The urethral striated muscle likely originates from a precursor shared with the perineal muscles. In fact, the fetal urethral striated sphincter was located adjacent to the bulbospongiosus muscle, whereas in females, the vestibular bulb later separated the two. Sasaki et al. [13
] and Yamaguchi et al. [14
], studying mouse embryos, demonstrated an intimate topographical relationship between the bulbospongiosus and the external anal sphincter. The myogenic cells require an attachment or insertion. Mesenchymal tissues around the urethra inferior or distal to the vaginal end, as well as around the anorectum, seem to be favorable for myogenic cell migration and attachment. Conversely, the striated muscle does not provide a specific interfacial tissue for insertion to the vagina, but simply attaches to the latter [15
In large specimens of both genders, a membranous structure was seen connecting the crus penis or clitoris to the striated sphincter. A similar membranous structure was described by Kato et al. [8
] in fetuses at a much later stage than the present specimens. This might represent the earliest form of the urogenital diaphragm or the perineal membrane. Notably, the membranous structure separated the male striated sphincter from the bulbospongiosus, whereas the vestibular bulb played the same role in females. This topographical anatomy is maintained in adults in the urogenital diaphragm [16
] and the perineal membrane [8
A change in topographical anatomy is necessary for transition from the fetal morphology shown in to the adult form: the horizontal male urethra needs to change into the membranous urethra along the supero-inferior axis, while the urethrovaginal sphincter does so along the anteroposterior axis. The developing prostate is likely to contribute significantly to this change in males. In fact, in a CRL 94-mm male fetus with a definite prostate, Oelrich [17
] observed an adult-like course of the urethra as well as the striated muscle extending superiorly along the prostate. Thus, the change may occur at a stage slightly later than the present specimens. In contrast, a well-known diagram of the female perineum by Oelrich [6
] was based on a 27-year-old woman, and not on observations of fetuses. However, Kato et al. [8
] described the urethrovaginal sphincter at 20 weeks (CRL, >180 mm). During the hypothetical topographical change at the late stage, mechanical stress is likely to be responsible for differentiation of the membranous structure between the crus and the striated sphincter (see above) into the male urogenital diaphragm or the female perineal membrane. Subsequently, the female urethral striated muscle seems to extend posterolaterally along the superficial surface of the developed perineal membrane.