To research why the advancement of a totally circular striated sphincter is indeed rare, we examined histological parts of 11 feminine and 11 male mid-term human being fetuses. that before descent of the vagina, the urethral striated muscle tissue extends posteriorly. strong class=”kwd-title” Keywords: Urethal rhabdosphincter, Genital tract, Urogenital sinus, Colliculus, Human fetus Introduction In elderly women, the urethral striated muscle sphincter or rhabdosphincter (RS) does not show a completely circular arrangement, but is located on the pubic and/or lateral side of MUC12 the urethra [1, 2]. However, Perucchini et al. [3] have quantitatively Alisertib tyrosianse inhibitor demonstrated that a thin striated muscle layer remains along the vaginal side of the urethra, especially in young women. A recent detailed study using human female fetuses [4] has shown there is no striated muscle, but a connective tissue Alisertib tyrosianse inhibitor “raphe” along the vaginal side of the urethra. This embryology is absolutely consistent with Strasser et al. [1], but seems to rule out the possibility of a vaginal-sided striated muscle sphincter. However, Masumoto et al. [5] have reported that the urethral striated muscle is likely to extend to the vaginal side of the urethra when the vagina merges with the urethra at the higher position: at this stage, the vagina is in the process of descent to the future vestibulum. Thus, the embryological development of the striated sphincter in females is still unclear. In addition, with a few exceptions, the fetal development of the urethrovaginal sphincter (which is a well developed striated muscle in children and adults [2, 6, 7]) has not been well described [8], and there remains a further possibility that the urethrovaginal sphincter extends posteriorly to surround the thin fetal vagina. In both the fetal urethral and urethrovaginal striated sphincters, we have considered that the topographical anatomy during vaginal descent makes it difficult to understand this aspect of the developmental process. Does the descending vagina disturb the development of a striated sphincter, thus preventing a completely circular arrangement, as stated by Sebe et al. [4]? To address this issue, we examined the fetal topographical anatomy of the urethral RS with the aim of reappraising the existing concept of the difference in its development between the sexes. Materials and Methods The present study was performed in accordance with the provisions of the Declaration of Helsinki 1995 (as revised in Edinburgh 2000). We examined the paraffin-embedded histology of 22 mid-term fetuses (11 males and 11 females). The specimens varied in size and stage: the crown-rump lengths (CRL) of the male specimens were 27, 29, 29, 30, 39, 46, 48, 52, 84, 95, and 103 mm, while those of female specimens were 36, 48, 48, 62, 76, 78, 84, 94, 92, 100, and 103 mm. These sizes corresponded to ovulational ages of approximately 8-13 weeks [9]. All the specimens Alisertib tyrosianse inhibitor were part of the large collection kept at the Embryology Institute of the Universidad Complutense, Madrid, and were the products of miscarriages and ectopic pregnancies managed at the Department of Obstetrics of the University. Approval for the study was granted by the university ethics committee (approval number, B-08/374). Because of the nature of the specimens, we were unable Alisertib tyrosianse inhibitor to rule out the possibility that they contained pathology. However, no pathology was found in the developing umbilical vessels, liver, intestine, adrenal, and kidney in the specimens examined. Identification of gender was based on observations of the urogenital fold, including the primitive uterus. After routine methods for paraffin-embedded histology, the majority of the specimens had been cut nearly horizontally (anterior part, tilted inferiorly) at a thickness of 5 m and at intervals of 50 (100) m. Sagittal sections had been limited by those from male and feminine fetuses with a CRL of 103 mm, and had been cut at a thickness greater than 10 Alisertib tyrosianse inhibitor m because of the large size. According to the size of the specimen, 30-200 sections were necessary for full observation, including.