Spatial distribution of smooth muscle tissue in the female pelvic floor and surrounding the urethra and vagina

Twisted orientation of the muscle bundles in the levator ani functional parts in women: Implications for pelvic floor support mechanism

This study presents a comprehensive investigation of the anatomical features of the levator ani muscle. The levator ani is a critical component of the pelvic floor; however, its intricate anatomy and functionality are poorly understood. Understanding the precise anatomy of the levator ani is crucial for the accurate diagnosis and effective treatment of pelvic floor disorders. Previous studies have been limited by the lack of comprehensive three-dimensional analyses; to overcome this limitation, we analysed the levator ani muscle using a novel 3D digitised muscle-mapping approach based on layer-by-layer dissection. From this examination, we determined that the levator ani consists of overlapping muscle bundles with varying orientations, particularly in the anteroinferior portion. Our findings revealed distinct muscle bundles directly attached to the rectum (LA-re) and twisted muscle slings surrounding the anterior (LA-a) and posterior (LA-p) aspects of the rectum, which are considered functional parts of the levator ani. These results suggest that these specific muscle bundles of the levator ani are primarily responsible for functional performance. The levator ani plays a crucial role in rectal elevation, lifting the centre of the perineum and narrowing the levator hiatus. The comprehensive anatomical information provided by our study will enhance diagnosis accuracy and facilitate the development of targeted treatment strategies for pelvic floor disorders in clinical practice.

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.

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.

Role of Ano1 Ca2+-activated Cl- channels in generating urethral tone

Urinary continence is maintained in the lower urinary tract by the contracture of urethral sphincters, including smooth muscle of the internal urethral sphincter. These contractions occlude the urethral lumen, preventing urine leakage from the bladder to the exterior. Over the past 20 years, research on the ionic conductances that contribute to urethral smooth muscle contractility has greatly accelerated. A debate has emerged over the role of interstitial cell of Cajal (ICC)-like cells in the urethra and their expression of Ca²⁺-activated Cl^- channels encoded by anoctamin-1 [Ano1; transmembrane member 16 A (Tmem16a) gene]. It has been proposed that Ano1 channels expressed in urethral ICC serve as a source of depolarization for smooth muscle cells, increasing their excitability and contributing to tone. Although a clear role for Ano1 channels expressed in ICC is evident in other smooth muscle organs, such as the gastrointestinal tract, the role of these channels in the urethra is unclear, owing to differences in the species (rabbit, rat, guinea pig, sheep, and mouse) examined and experimental approaches by different groups. The importance of clarifying this situation is evident as effective targeting of Ano1 channels may lead to new treatments for urinary incontinence. In this review, we summarize the key findings from different species on the role of ICC and Ano1 channels in urethral contractility. Finally, we outline proposals for clarifying this controversial and important topic by addressing how cell-specific optogenetic and inducible cell-specific genetic deletion strategies coupled with advances in Ano1 channel pharmacology may clarify this area in future studies.NEW & NOTEWORTHY Studies from the rabbit have shown that anoctamin-1 (Ano1) channels expressed in urethral interstitial cells of Cajal (ICC) serve as a source of depolarization for smooth muscle cells, increasing excitability and tone. However, the role of urethral Ano1 channels is unclear, owing to differences in the species examined and experimental approaches. We summarize findings from different species on the role of urethral ICC and Ano1 channels in urethral contractility and outline proposals for clarifying this topic using cell-specific optogenetic approaches.

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.