Presence of smooth muscle continuous with the rectal and vaginal walls in the deep perineal space prompts reconsideration of the deep transverse perineal muscle.

The deep transverse perineal muscle was considered a striated muscle within the deep perineal space; however, recent studies suggest that it consists of smooth muscle. We investigated the histological composition and spatial distribution of the deep perineal space and its anatomical relationships with surrounding structures to understand its role in the perineum. Perineal muscle shape and location from nine cadavers (five males; average age, 79 years) were macroscopically examined. Serial sections were histologically analyzed using Masson's trichrome, Elastica Masson, and immunostaining of smooth muscle tissue, from which the structure was three-dimensionally reconstructed. An overall layered perineum structure was revealed in both sexes. A slender, band-like striated muscle was observed near the surface. Beneath this layer was a dense connective tissue membrane that provides separation. A triangular, plate-like shaped smooth muscle with transversely oriented fibers occupied the deep perineal space, which, unlike the striated muscle, was continuous with the rectal wall and was identified as the deep transverse perineal muscle. The smooth muscle was continuous with the longitudinal rectal muscle and, in females, with the vaginal muscular wall. Findings indicate that the deep transverse perineal muscle may be a distinct pelvic smooth muscle component.