A skeletal-smooth muscle interface mediates levator ani attachment to the rectum and vagina: a macroscopic and histological study.

We aimed to clarify the attachment architecture between the levator ani and the rectum and vagina using integrated macroscopic and histological analyses of human pelvic floor specimens. Macroscopic dissection demonstrated whitish soft tissue at the interface between the levator ani and the lateral walls of the rectum and vagina. Histological and immune histochemical analyses of the corresponding regions revealed abundant smooth muscle components that appeared topographically continuous with the outer muscular layers of each organ within individual coronal sections. In both organs, the smooth muscle extended beyond the organ walls, spread over the superior and inferior surfaces of the levator ani, and interdigitated between skeletal muscle bundles, forming a serrated (saw tooth-like) interface. No dense connective tissue structures resembling tendons or aponeuroses were identified, indicating that levator ani-visceral attachment does not conform to a classical tendon-mediated enthesis. Distinct attachment patterns were observed between the two organs. At the rectal interface, a subset of levator ani muscle fibers coursed toward smooth muscle that appeared topographically continuous with the rectal longitudinal layer and formed an insertion-like arrangement. In contrast, at the vaginal interface, no insertion-like arrangement of levator ani muscle fibers into the vaginal wall was observed. Abundant vaginal smooth muscle extended laterally and enveloped adjacent levator ani muscle bundles. These findings provide a clearer anatomical characterization of a non-tendinous skeletal-smooth muscle interface between the levator ani and deformable pelvic visceral walls. This provides an anatomical basis for future studies investigating how the levator ani interacts mechanically with these deformable pelvic organs.