Purpose: It is still unclear whether the longitudinal anal muscles or conjoint longitudinal coats (CLCs) are attached to the vagina, although such an attachment, if present, would appear to make an important contribution to the integrated supportive system of the female pelvic floor.
Materials and methods: Using immunohistochemistry for smooth muscle actin, we examined semiserial frontal sections of 1) eleven female late-stage fetuses at 28-37 weeks of gestation, 2) two female middle-stage fetus (2 specimens at 13 weeks), and, 3) six male fetuses at 12 and 37 weeks as a comparison of the morphology.
Results: In late-stage female fetuses, the CLCs consistently (11/11) extended into the subcutaneous tissue along the vaginal vestibule on the anterior side of the external anal sphincter. Lateral to the CLCs, the external anal sphincter also extended anteriorly toward the vaginal side walls. The anterior part of the CLCs originated from the perimysium of the levator ani muscle without any contribution of the rectal longitudinal muscle layer. However, in 2 female middle-stage fetuses, smooth muscles along the vestibulum extended superiorly toward the levetor ani sling. In male fetuses, the CLCs were separated from another subcutaneous smooth muscle along the scrotal raphe (posterior parts of the dartos layer) by fatty tissue.
Conclusion: In terms of topographical anatomy, the female anterior CLCs are likely to correspond to the lateral extension of the perineal body (a bulky subcutaneous smooth muscle mass present in adult women), supporting the vaginal vestibule by transmission of force from the levator ani.
Keywords: Anal canal; embryology; levator ani muscle; longitudinal anal muscle; rectum; smooth muscle.