Variations in anal submucosal muscles in elderly Japanese subjects

A histological investigation of the anal submucosal muscle was performed on 73 specimens obtained from 30 elderly Japanese cadavers. On immunohistochemistry, the submucosal muscle (maximum thickness of the muscle layer, 0.3-5.0 mm) was positive for smooth muscle actin and desmin. This smooth muscle was accompanied by abundant elastic fibers, and clearly differed from the rectal muscularis mucosae. It was present around the entire circle of the anal canal wall, except for the mid-dorsal portion, and covered the internal aspect of the internal anal sphincter. The sphincter consistently issued several muscle fibers to the submucosal muscle. The distal end of the rectal muscularis mucosae almost corresponded with the level of the squamous-columnar epithelial junction. The muscularis mucosae either intermingled with (11/30) or did not co-exist with (19/30) the submucosal muscle. The submucosal muscles were usually more sparsely distributed than the internal sphincter, but they sometimes (5/30) included a dense, plate-like part adjacent to the lateral portion of the sphincter. Physiologically, the anal submucosal muscle seemed to provide a cushion for smooth evacuation; however, under pathological conditions, hemorrhoidal venous plexuses developed in this area. For functional preservation during hemorrhoidectomy, restricted treatment of the submucosal layer, based on individualized presurgical evaluations of its morphology, is required.