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

Anatomy of muscle connections in the male urethra and anorectal canal

OBJECTIVES

To elucidate the male urethral muscular structure and its relationship with the anorectal canal muscles, as establishing an anatomical foundation for urethral function will contribute to the prevention, diagnosis, and treatment of urinary incontinence.

METHODS

Eight male cadavers were used. Using a multifaceted approach, we performed macroscopic anatomical examination, histological analysis of wide-range serial sectioning and immunostaining, and three-dimensional (3D) reconstruction from histological sections. In the macroscopic anatomical examination, pelvic halves were meticulously dissected in layers from the medial aspect. In the histological analysis, the tissue, including the urethra and anorectal canal, was serially sectioned in the horizontal plane. The muscular structures were reconstructed and visualised in 3D.

RESULTS

The membranous portion of the urethra had three muscle layers: the longitudinal and circular muscles (smooth muscle) and the external urethral sphincter (skeletal muscle). The circular muscle was connected posteriorly to the longitudinal rectal muscle. The external urethral sphincter had a horseshoe shape, with its posterior ends continuing to the external anal sphincter, forming a 3D ring-like sphincter.

CONCLUSION

This study revealed skeletal and smooth muscle connections between the male urethra and anorectal canal, enabling urethral compression and closure. These anatomical muscle connections suggest a functional linkage between them.

Significant relationship between musculoaponeurotic attachment of the abdominal and thigh adductor muscles to the pubis: implications for the diagnosis of groin pain

Groin pain is prevalent in orthopedic and sports medicine, causing reduced mobility and limiting sports activity. To effectively manage groin pain, understanding the detailed anatomy of supporting muscles is crucial. This study aimed to investigate the musculoaponeurotic attachments on the pubis and the relationship among intramuscular aponeuroses of abdominal and thigh adductor musculatures. Macroscopic analyses were performed in 10 pelvic halves. The bone morphology of the pubis was assessed in two pelvic halves using microcomputed tomography. Histological investigations were conducted in two pelvic halves. The external oblique aponeurosis extended to the adductor longus aponeurosis, forming conjoined aponeurosis, which attached to a small impression distal to the pubic crest. The gracilis aponeurosis merges with the adductor brevis aponeurosis and is attached to the proximal part of the inferior pubic ramus. The rectus abdominis and pyramidalis aponeuroses were attached to the pubic crest and intermingled with the gracilis-adductor brevis aponeurosis, forming bilateral conjoined aponeurosis, which attached to a broad area covering the anteroinferior surface of the pubis. Histologically, these two areas of conjoined aponeuroses were attached to the pubis via the fibrocartilage enthesis. Microcomputed tomography revealed two distinctive bone morphologies, a small impression and an elongated osseous prominence on pubis, corresponded to the two areas of conjoined aponeuroses. This study demonstrated close relationships between the aponeurotic attachment of the external oblique and adductor longus, and between the rectus abdominis, pyramidalis, gracilis, and adductor brevis. The findings of aponeurotic complexes would aid in diagnostic and surgical approaches for athletic groin pain.

Pelvic floor and perineal muscles: a dynamic coordination between skeletal and smooth muscles on pelvic floor stabilization

The purpose of this review is to present our researches on the pelvic outlet muscles, including the pelvic floor and perineal muscles, which are responsible for urinary function, defecation, sexual function, and core stability, and to discuss the insights into the mechanism of pelvic floor stabilization based on the findings. Our studies are conducted using a combination of macroscopic examination, immunohistological analysis, 3D reconstruction, and imaging. Unlike most previous reports, this article describes not only on skeletal muscle but also on smooth muscle structures in the pelvic floor and perineum to encourage new understanding. The skeletal muscles of the pelvic outlet are continuous, which means that they share muscle bundles. They form three muscle slings that pass anterior and posterior to the anal canal, thus serving as the foundation of pelvic floor support. The smooth muscle of the pelvic outlet, in addition to forming the walls of the viscera, also extends in three dimensions. This continuous smooth muscle occupies the central region of the pelvic floor and perineum, thus revising the conventional understanding of the perineal body. At the interface between the levator ani and pelvic viscera, smooth muscle forms characteristic structures that transfer the lifting power of the levator ani to the pelvic viscera. The findings suggest new concepts of pelvic floor stabilization mechanisms, such as dynamic coordination between skeletal and smooth muscles. These two types of muscles possibly coordinate the direction and force of muscle contraction with each other.

Novel combination method of wide-range serial sectioning and 3D reconstruction visualizing both macro-level dynamics and micro-level interactions in an attempt to analyze the female pelvic floor

The present report presents details of the method for combining wide-range serial sectioning and 3D reconstruction using an adult cadaver. For several decades, anatomists have utilized a variety of non-destructive three-dimensional (3D) visualization methods to complement gross anatomical analysis methods. These include vascular casting for the visualization of vascular morphology and micro-CT for the visualization of bone morphology. However, these conventional methods are restricted by the properties and sizes of the target structures. Here, we introduce a method to conduct 3D reconstruction based on wide-range serial histological sections from adult cadavers, which overcomes previous restrictions. An attempt at 3D visualization of the female pelvic floor muscles provides a detailed description of the procedure. Supplemental video and 3D PDF files allow multifaceted observation of 3D images. Wide-range serial sectioning visualizes morphology beyond the scope of conventional methods, while 3D reconstruction enables non-destructive 3D visualization of any structure that can be observed on a histological section, including skeletal muscle, smooth muscle, ligaments, cartilage, connective tissue, blood vessels, nerves, lymph nodes, and glands. The novel combination of both methods is instrumental in meso-anatomy, a discipline intermediate between macro-anatomy and micro-anatomy.

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

This study presents the detailed anatomy of the Cowper’s gland in humans. Elucidating the mechanism of secretion and emission of the Cowper’s gland requires analysis of the muscles around the Cowper’s gland. We hypothesized that the Cowper’s gland involves not only smooth muscle but also the striated muscles of the pelvic floor. Here, we provide comprehensive and three-dimensional anatomy of the Cowper’s gland and its surrounding structures, which overcomes the current local and planar understanding. In this study, seven male corpses of body donors were used to conduct macroscopic anatomy, histology, and three-dimensional reconstruction. The Cowper’s gland was surrounded laterally and posterosuperiorly by striated and smooth muscles, respectively. The striated muscle bundle was connected from the superficial transverse perineal muscle, levator ani, and external anal sphincter to the external urethral sphincter (rhabdosphincter). The smooth muscle was part of the deep transverse perineal muscle and entered between the bilateral Cowper’s glands and lobules. Our findings indicate that the secretion and emission of the Cowper’s gland in humans are carried out through the cooperation of striated and smooth muscles.

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

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.