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

Abstract

Introduction Recent studies have revealed the extended nature of smooth muscle structures in the pelvic floor, revising the conventional understanding of the “perineal body.” Our aim was to clarify the three‐dimensional configuration and detailed histological properties of the smooth muscle structures in the region anterior to the rectum and anal canal in men. Materials and methods Four male cadavers were subjected to macroscopic and immunohistological examinations. The pelvis was dissected from the perineal side, as in the viewing angle during transperineal surgeries. Serial transverse sections of the region anterior to the rectum and anal canal were stained with Masson's trichrome and immunohistological stains to identify connective tissue, smooth muscle, and skeletal muscle. Results There was a series of smooth muscle structures continuous with the longitudinal muscle of the rectum in the central region of the pelvic floor, and three representative elements were identified: the anterior bundle of the longitudinal muscle located between the external anal sphincter and bulbospongiosus; bilateral plate‐like structures with transversely‐oriented and dense smooth muscle fibers; and the rectourethral muscle located between the rectum and urethra. In addition, hypertrophic tissue with smooth muscle fibers extended from the longitudinal muscle in the anterolateral portion of the rectum and contacted the levator ani. Conclusions The series of smooth muscle structures had fiber orientations and densities that differed among locations. The widespread arrangement of the smooth muscle in the pelvic floor suggests a mechanism of dynamic coordination between the smooth and skeletal muscles.

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

“…It is interesting that the seemingly unrelated EAS, STP, and LA surround the CG while extending anteromedially. In contrast, the DTP was observed as a plate-like structure of smooth muscle located in the deep perineal pouch, in agreement with our recent report 11,16 . In the present study, we showed that the DTP surrounds the medial and posterosuperior parts of the CG.…”

“…The DTP is a mysterious muscle; while there are reports that it does not exist 24,25 , www.nature.com/scientificreports/ some reports state that it is seen in 24.7%-100% of men and 16.7% of women 8,10,[26][27][28] . The DTP is classically considered to be comprised striated muscle 4 ; however, recent reports have indicated that it is comprised smooth muscle that traverses in the deep perineal pouch 11,16,[29][30][31][32] . The present study showed that the EUS surrounds the anterior and lateral sides of the urethra, and is formed from muscle bundles of the STPs, LA, and EAS extending into the deep perineal pouch.…”

“…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 .…”

“…Furthermore, previous studies have pointed out that the EUS and DTP are difficult to analyze, and that textbook descriptions are inconsistent 7,8 . There is still a debate concerning the existence and composition of the EUS and DTP [9][10][11] . Thus, the detailed anatomy of the muscles surrounding the CG remains unclear.…”

“…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.…”