Coexistence of Dense and Sparse Areas in the Longitudinal Smooth Muscle of the Anal Canal: Anatomical and Histological Analyses Inspired by Magnetic Resonance Images

Abstract

Magnetic resonance images of the anal canal show small, circular, low-intensity areas arranged in a row and a high-intensity area surrounding them internally and externally in the longitudinal muscle layer that cannot be explained by current anatomical findings. The purpose of this study was to elucidate the detailed structure of the longitudinal smooth muscle of the anal canal and to interpret the magnetic resonance image of the longitudinal muscle. Specimens for macroscopic anatomy and histology were obtained from six and seven cadavers, respectively. The histological nature of the longitudinal muscle was examined by staining serial transverse and coronal sections of the lateral wall of the anal canal with Masson's trichrome stain and using immunohistochemistry for smooth and skeletal muscle fibers. Dense and sparse areas of smooth muscle fibers coexisted in the longitudinal muscle layer. The dense areas formed columnar muscle bundles approximately 1.0-1.5 mm in diameter, and they continued from the longitudinal muscle bundles of the rectum. The columnar muscle bundles of the longitudinal anal muscle were internally and externally surrounded by sparsely arranged smooth muscle fibers that ran longitudinally. The coexistence of dense and sparse areas of smooth muscle fibers suggests that the structure of the smooth muscle is optimized for its function. This histological nature is probably reflected in the magnetic resonance image of the longitudinal muscle as the coexistence of low-and high-intensity areas. Clin. Anat. 33:619-626, 2020.

The series of smooth muscle structures in the pelvic floors of men: Dynamic coordination of smooth and skeletal muscles

“…Anti‐smooth actin (ready‐to‐use Actin, Smooth Muscle Ab‐1, Clone 1A4; Thermo Fisher Scientific, Fremont, CA) and anti‐skeletal myosin (ready‐to‐use Myosin, Skeletal Muscle Ab‐2, Clone MYSN02; Thermo Fisher Scientific) were used for immunological staining to confirm the distributions of smooth and skeletal muscle fibers. The detailed procedures have been described in previous reports (Muro et al, 2014; Muro et al, 2018; Muro et al, 2019; Muro et al, 2020; Nakajima et al, 2017).…”

“…The anatomy around the rectum and anal canal has attracted increased attention since the development of anorectal surgery (Aigner et al, 2004; Kraima et al, 2016, Tsukada et al, 2016; Nakajima et al, 2017; Muro, Tsukada, Harada, Ito, & Akita, 2018; Muro, Tsukada, Harada, Ito, & Akita, 2019; Okada et al, 2019). We previously analyzed the muscle structure around the rectum and anal canal anatomically (Baramee, Muro, Suriyut, Harada, & Akita, 2020; Kato, Muro, Kato, Miyasaka, & Akita, 2020; Muro et al, 2014; Muro et al, 2018; Muro et al, 2019; Muro et al, 2020; Nakajima et al, 2017; Suriyut, Muro, Baramee, Harada, & Akita, 2020; Tsukada et al, 2016). Those studies revealed a very interesting distribution of smooth muscle in this region.…”

“…Our previous anatomical studies have clarified the sparse area of smooth muscle tissue in the region anterolateral to the rectum and anal canal in women and have described the coexistence of dense and sparse areas in the longitudinal muscle layer of the anal canal (Muro et al, 2019; Muro et al, 2020). These studies led us to suspect that the smooth muscle around the rectum and anal canal has diverse properties.…”

Anatomy of Cowper’s gland in humans suggesting a secretion and emission mechanism facilitated by cooperation of striated and smooth muscles

“…Anti-smooth actin (ready-to-use actin, smooth muscle Ab-1, clone 1A4; Thermo Fisher Scientific, Fremont, CA) and anti-skeletal myosin (ready-to-use myosin, skeletal muscle Ab-2, clone MYSN02; Thermo Fisher Scientific) were used for immunological staining to confirm the distribution of smooth and striated muscle fibers. The detailed procedures have been described previously 11,13,[15][16][17]20 .…”

“…We have reported the characteristic structure of the pelvic floor, in which the striated muscles share their muscle bundles and are continuous, and the smooth muscle occupies the space between them [11][12][13][14][15][16][17][18][19][20] . These studies suggest that striated and smooth muscles coordinate in anal function and pelvic floor support.…”