Histo‐Topographic study of the longitudinal anal muscle

Abstract

The longitudinal anal muscle (LAM) has been described as a vertical layer of muscular tissue interposed between the circular layers of the internal (IAS) and external (EAS) anal sphincters. There is, however, no general agreement in the literature on its composition and attachments. The aim of this study was to investigate the histological structure, attachments, and topography of the LAM in order to evaluate its role in continence and defecation, thus enhancing knowledge of the surgical anatomy of this region. After in situ formalin fixation, the pelvic viscera were removed from eight male and eight female cadavers (age range: 52–72 years). Serial macrosections of the bladder base, lower rectum and anal canal, cervix and pelvic floor complex, cut in the transverse (six specimens) and coronal (six specimens) planes, underwent histological and immunohistochemical studies. Four specimens were studied using the E12 sheet plastination technique. The LAM was identified in 10/12 specimens (83%). Transverse and coronal sections made clear that it is a longitudinal layer of muscular tissue, marking the boundary between the internal and external anal sphincters. From the anorectal junction it extends along the anal canal, receives fibers from the innermost part of the puborectalis and the puboanalis muscles, and terminates with seven to nine fibro‐elastic septa, which traverse the subcutaneous part of the external anal sphincter, reaching the perianal dermis. In the transverse plane, the mean thickness of the LAM was 1.68 ± 0.27 mm. Immunohistochemical staining showed that the LAM consists of predominantly outer striated muscle fibers and smaller numbers of inner smooth muscle fibers, respectively coming from the levator ani muscle and from the longitudinal muscular layer of the rectum. The oblique fibers suggest that the LAM may represent the intermediate longitudinal course of small bridging muscle bundles going reciprocally from the striated EAS to the smooth IAS and vice versa. The spatial result is the helical course of striated and smooth muscle fibers between the EAS and IAS, which contribute not only to the narrowing but also to some shortening of the anal canal during sphincter contraction. Thus, rather than being a boundary, the LAM gives anatomical evidence of a functional connection between two muscle systems with different structures and topography. Clin. Anat. 21:447–452, 2008. © 2008 Wiley‐Liss, Inc.

Development of the external anal sphincter with special reference to intergender difference: observations of mid-term fetuses (15-30 weeks of gestation)

“…We were unable to find the pubo-analis, i.e., the innermost part of the puborectalis (Macchi et al 2008;Fröber et al 2001), even though an intergender difference has been reported. The present fetuses did not show a tight connection between the internal anal sphincter and conjoining longitudinal coat, in contrast to the adult morphology described by Arakawa et al (2004).…”

The Urethral Rhabdosphincter, Levator Ani Muscle, and Perineal Membrane: A Review

“…As well as the vascular wall, in the pelvic floor, smooth muscle and elastic fibers coexist (Figures 1(e), 2(b), 3(b), and 3(e)). For the anorectum, the LA insertion provides a smooth muscle-rich connective tissue band that is continuous with the conjoint longitudinal muscle coat [20] (Figures 4(c) and 4(d)) or longitudinal anal muscle [37]. The interface smooth muscles are also continuous with the rectourethralis muscle [38] (Figure 5(b)).…”

MRI Anatomy of the Anal Region

“…2,3,12 However, histologic studies suggest that the LRM muscle cannot be such a thick structure because it comprises so few smooth muscle fibers. 13 The results of 3-dimensional positioning in the current study indicate that this thick muscular layer is indeed the vertical levator (Fig. 2B) and that the vertical levator was mistaken for the LRM muscle in the study by Rociu et al 2,3,12 New MRI signs in the current study show that the caudal end of the vertical levator becomes a series of tendinous-like cords below the plane of the deep sphincter (Fig.…”