Anatomy of the smooth muscle structure in the female anorectal anterior wall: convergence and anterior extension of the internal anal sphincter and longitudinal muscle

Three-dimensional heterogeneity of smooth muscle fiber density anterior to the rectum in males: quantitative analysis with implications for transanal total mesorectal excision

PURPOSE

Transanal total mesorectal excision for rectal cancer in men poses a risk of urethral injury. The morphology of smooth muscle tissues around the rectum is reportedly characterized by differences in fiber density; however, quantitative analysis of these tissues for surgical applications is lacking. This study aimed to quantitatively analyze the histological properties of fiber density and the spatial extent of the smooth muscle anterior to the male rectum.

METHOD

This descriptive cadaveric study involving six adult cadavers was conducted at the Tokyo Medical and Dental University. Serial histological sections were prepared from tissues in the region anterior to the rectum, and immunostaining and 3D reconstruction were performed to evaluate the spatial distribution of the smooth muscle. Smooth muscle fiber densities were measured in different regions of the smooth muscle anterior to the rectum and statistically analyzed.

RESULTS

The three-dimensional heat map revealed a gradual change in fiber density within the smooth muscle anterior to the rectum, with lower density in the superior part and higher density in the inferior part. In mid-sagittal immunostained sections, the smooth muscle anterior to the rectum exhibited a significant difference in fiber density, averaging 23.22% ± 5.50% in the superior area and significantly higher at 46.99% ± 12.92% in the inferior area.

CONCLUSION

Heterogeneity in fiber density between the superior and inferior smooth muscle anterior to the rectum suggests that these differences could serve as landmarks, providing crucial positional information to avoid urethral injury during transanal total mesorectal excision.

Evaluating accuracy in artificial intelligence-powered serial segmentation for sectional images applied to morphological studies with three-dimensional reconstruction

Three-dimensional (3D) reconstruction is time-consuming owing to segmentation work. We evaluated the accuracy of the artificial intelligence (AI)-based segmentation and tracking model SAM-Track for segmentation of anatomical or histological structures and explored the potential of AI to enhance research efficiency. Images [obtained via computed tomography (CT) and magnetic resonance imaging (MRI)], anatomical sections from a Visible Korean Human open resource, and serial histological section images of cadavers were obtained. Six structures in the CT, MRI, and anatomical sections and seven in the histological sections were segmented using SAM-Track and compared with manual segmentation by calculating the Dice similarity coefficient. Segmented images were then reconstructed three dimensionally. The average Dice scores of CT and MRI results varied (0.13-0.83); anatomical sections showed mostly good accuracy (0.31-0.82). Clear-edged structures, such as the femur and liver, had high scores (0.69-0.83). In contrast, soft tissue structures, such as the rectus femoris and stomach, had variable accuracy (0.38-0.82). Histological sections showed high accuracy, especially for well-delineated tissues, such as the tibia and pancreas (0.95, 0.90). However, the tracking of branching structures, such as arteries and veins, was less successful (0.72, 0.52). In 3D reconstruction, high Dice scores were associated with accurate shapes, whereas low scores indicated discrepancies between the predicted and true shapes. AI-based automatic segmentation using SAM-Track provides moderate-to-good accuracy for anatomical and histological structures and is beneficial for conducting morphological studies involving 3D reconstruction.

Three-dimensional Analysis of the Distribution of Smooth and Skeletal Muscle Tissue Around the Female Urethra

INTRODUCTION AND HYPOTHESIS

The urethra is surrounded by layers of smooth muscle, including inner longitudinal and outer circler muscles, as well as the skeletal muscle of the external urethral sphincter. However, the extent of these muscles and their relationship with the levator ani (LA) remain unclear. This study aimed to clarify the composition of muscle layers around the female urethra and their three-dimensional arrangements.

METHODS

Pelvises from five female cadavers were collected for histological analysis. The surrounding urethral tissue was serially sectioned and stained with Elastica van Gieson stain to identify the connective and muscular tissues. Immunohistological staining with an anti-smooth muscle antibody was performed to confirm the distribution of the smooth and skeletal muscles. Thereafter, muscle layers were three-dimensionally reconstructed on the basis of serial histological sections.

RESULTS

The female urethra was surrounded by four muscle layers arranged from inner to outer layers in the following order: longitudinal smooth muscle, circular smooth muscle, the external urethral sphincter (EUS), and an outermost smooth muscle layer. These muscles surrounded the urethra and partially extended anteriorly. Moreover, smooth muscle fibers extending from the vagina were found between the LA and EUS.

CONCLUSIONS

This study clarified the distribution of muscle tissue surrounding the female urethra, showing that the EUS is connected to the LA through the outermost smooth muscle layer. Thus, urination control likely involves both the urethral muscle layers and the pelvic floor muscles.

Two minutes is sufficient to characterize the viscoelastic properties of the human lower birth canal during the first stage of labor

BACKGROUND

The lower birth canal is the final constriction through which a fetal head must pass for delivery. Unfortunately, injuries to the lower birth canal tissues occur in up to 19 % of first-time vaginal deliveries due to the 300 % stretch required.

METHODS

This is a secondary analysis of data from 56 healthy nullipara recorded by a lower birth canal dilator during the first stage of labor. A four parameter Fractional Zener rheological Model was used to characterize the canal viscoelastic properties during the first stage of labor. We tested the (null) hypothesis that the Model constants identified during the initial 20 s ramp-and-5-min-hold dilation from 40 to 55 mm will not accurately predict the final hoop tension after 60 min of slower dilation.

FINDINGS

The null hypothesis was rejected in that when the four Model parameters were calculated for all nullipara from the 20-s-ramp-and-5-min-hold data, the relative fit error was 8 ± 4 %, and the relative prediction error after 60 min of dilation was 10 ± 5 %. Furthermore, when the Model constants were instead calculated from the 20 s ramp and only the initial 90 s of the 5-min-hold, the error was still acceptable: 13 ± 4 % for relative fit and 13 ± 18 % for relative prediction.

INTERPRETATION

Two minutes is sufficient to characterize canal viscoelastic properties during the first stage of labor and identify those with stiffer tissues at higher risk for a pelvic floor injury during delivery. This could form the basis for a clinical test predicting injury.

Skeletal Muscle Complex Between the Vagina and Anal Canal: Implications for Perineal Laceration

INTRODUCTION AND HYPOTHESIS

The anatomy of the skeletal muscles located between the vagina and anus is important during complex obstetric laceration reconstructions. We aimed to clarify the composition of skeletal muscles located between the vagina and anal canal and their three-dimensional configuration relevant to perineum repair.

METHODS

This observational study involved ten female cadavers. An anatomical dissection was performed to observe the muscles around the vagina and anal canal. Immunohistological analysis of the midsagittal section was performed to clarify the composition of the muscles, and dissection was performed to correspond to the cross-section. Wide-range serial sectioning and three-dimensional reconstruction were used to support these findings histologically and visualize the three-dimensional arrangement.

RESULTS

The region between the vagina and anal canal included the anterior part of the external anal sphincter, superficial transverse perineal muscle approaching from the lateral side, and levator ani, located cranially. They converge three-dimensionally in the median from each direction, forming a muscle complex between the vagina and anal canal.

CONCLUSIONS

The medial region between the vagina and anal canal in those giving birth includes a skeletal muscle complex formed by the confluence of the external anal sphincter, anterior bundle of the levator ani, and superficial transverse perineal muscle. In cases of severe perineal lacerations, these muscles could be injured. The anatomical knowledge that a part of the levator ani forms a muscle sling anterior to the anal canal is particularly important for obstetricians and gynecologists repairing obstetric lacerations and treating pelvic floor disorders.

Perineal-First Approach in Robotic Abdominoperineal Resection

INTRODUCTION

Although abdominoperineal resection (APR) is required for rectal cancer invading the levator ani muscle, its curative outcomes remain poorer than those of other rectal surgeries.1-3 In particular, the anatomic complexity around the anterior wall of the rectum increases the technical difficulty during APR, resulting in a high frequency of margin involvement that causes local recurrence. In this video, we present the technical details of a robotic perineal-first APR approach.

METHODS

For a 46 year-old man, locally advanced rectal cancer invading the levator ani muscles was diagnosed. Although total neoadjuvant therapy (8 cycles of induction FOLFOXIRI followed by chemoradiotherapy 50.4 Gy) decreased the tumor size, invasion was suspected still to remain. Therefore, robotic APR was performed. Written informed consent was obtained from the patient. For the perineal-first approach, we created a circular incision around the anus, then divided the fat tissues of the ischiorectal fossa until the levator ani muscle was exposed on both sides. Posterior and anterior dissections were performed along the coccyx and external anal sphincter, respectively. After placement of a lap protector to maintain air-tightness, the robotic approach was initiated. Posterior dissection was performed along the coccyx, then was connected to the already-dissected space created earlier by the perineal approach. Next, the levator ani muscle was divided from the dorsal to the lateral side. Finally, anterior dissection was performed along the prostate, followed by division of the rectourethral muscle, the smooth muscle fibers running vertically. The creation of the already-dissected space on the perineal side offers advantages of robotic manipulation from the abdominal side, especially anterior dissection.

RESULTS

We performed robotic APR using the perineal-first approach for 17 consecutive patients (12 men and 5 women) between 2019 and 2023. All 17 patients achieved complete total mesorectal excision with negative margins. The mean time required for the perineal approach was about 25 min. In anterior dissection using the robotic approach, division of the smooth muscle fibers at the perineal body (i.e., rectourethral muscle in males4 or muscular intermingling in females5) was reproducibly performed in both males and females.

CONCLUSION

Robotic APR with a perineal-first approach can be advantageous in ensuring surgical margin safety (especially for the anterior aspect of the rectum).

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.

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.

International standardization and optimization group for intersphincteric resection (ISOG-ISR): modified Delphi consensus on anatomy, definition, indication, surgical technique, specimen description and functional outcome

AIM

Intersphincteric resection (ISR) is an oncologically complex operation for very low-lying rectal cancers. Yet, definition, anatomical description, operative indications and operative approaches to ISR are not standardized. The aim of this study was to standardize the definition of ISR by reaching international consensus from the experts in the field. This standardization will allow meaningful comparison in the literature in the future.

METHOD

A modified Delphi approach with three rounds of questionnaire was adopted. A total of 29 international experts from 11 countries were recruited for this study. Six domains with a total of 37 statements were examined, including anatomical definition; definition of intersphincteric dissection, intersphincteric resection (ISR) and ultra-low anterior resection (uLAR); indication for ISR; surgical technique of ISR; specimen description of ISR; and functional outcome assessment protocol.

RESULTS

Three rounds of questionnaire were performed (response rate 100%, 89.6%, 89.6%). Agreement (≥80%) reached standardization on 36 statements.

CONCLUSION

This study provides an international expert consensus-based definition and standardization of ISR. This is the first study standardizing terminology and definition of deep pelvis/anal canal anatomy from a surgical point of view. Intersphincteric dissection, ISR and uLAR were specifically defined for precise surgical description. Indication for ISR was determined by the rectal tumour's maximal radial infiltration (T stage) below the levator ani. A new surgical definition of T3isp was reached by consensus to define T3 low rectal tumours infiltrating the intersphincteric plane. A practical flowchart for surgical indication for uLAR/ISR/abdominoperineal resection was developed. A standardized ISR surgical technique and functional outcome assessment protocol was defined.

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.

Development of a new histological identification method of human sinoatrial node suitable for immunohistochemical study

Histological identification of the human sinoatrial node (SAN) remains a challenge. Conventional identification methods, such as Lev's method, have certain limitations. The aim of our study was to develop a new histological identification method that could properly identify the sinoatrial node, applicable to the immunohistochemical study of intra-nodal structures. Thirty-nine human autopsied hearts were included in this study. The cases included 23 men and 16 women ranging in age from 20 to 99 years. The sinoatrial area from eight control samples was cut in the vertical section using the conventional Lev's method. In our new method, called the "En face one-block method," the sinoatrial node was cut in "En face" at the junction of the right border of the right appendage and superior vena cava, placed in one long cassette, and serially cut using a microtome. Immunostaining was performed using primary antibodies against CD31, podoplanin (D2-40), S-100, and other proteins. The average area of the SAN on the slide glass in our new method was 32.2 mm²_, which was significantly larger than that (3.59 mm²) of the control samples by Lev's method. The SAN area was positively correlated with age (r = 0.357; p = 0.026), especially in women (r = 0.626; p = 0.0095). The SAN group had significantly lower percentage of CD31-positive blood capillaries, higher percentage of podoplanin-positive lymphatic channels, and S-100-positive peripheral nerves. We successfully developed a novel cutting method applicable to immunohistochemical studies, with which we could provide a bird's-eye view of the sinoatrial nodes.

Short and long term results of anatomical reconstruction of perineal body and sphincter complex in obstetric anal sphincter injuries

OBJECTIVES

The effective way to reduce the risk of fecal incontinence (FI) in primary repaired obstetric anal sphincter injuries (OASIS) patients is to accurately detect the injury and provide complete anatomical reconstruction. The aim of the study was to evaluate the short-term and long-term results of OASIS cases that were diagnosed by an experienced surgical team and whose perineal body and anal sphincters were reconstructed separately.

MATERIAL AND METHODS

Sixteen patients that required consultations due to anal sphincter damage during vaginal delivery and underwent anatomical reconstruction due to Grade 3c and Grade 4 sphincter damage between 2007 and 2019 were included in the study. These cases were divided into three groups [Group 1 (≤12 months), Group 2 (12-60 months), Group 3 (≥60 months)] according to the time elapsed until anal manometry, and incontinence questionnaires were conducted in the postoperative period. Recto-anal inhibitory reflex (RAIR), mean resting (IB) and squeezing (SB) pressures were measured by anal manometry. Anal incontinence (AI) and FI rates were determined by questionnaires. Anal sphincter damage repair techniques (overlapping, end-to-end) were determined. These parameters were compared between the three groups.

RESULTS

Mean age of the patients was 27.5 (16-35) years. Six (37.5%) patients had Grade 3c, while 10 (62.5%) had Grade 4 injury. The overall mean RP and SP were 35 (26-56) mmHg and 67 (31-100) mmHg, respectively. Mean RP and SP were 46/67 mmHg, 33.5/75.5 mmHg, and 37.5/70.5 mmHg in Groups 1, 2, and 3 respectively. There was no difference between the three groups in terms of mean RP and SP (p= 0.691, p= 0.673). The rate of AI and FI in all patients were 18.75% and 12.5%, respectively while the rate of severe AI incontinence was 6%. Severe AI was observed in 1 (16.7%) case in Group 1, mild AI was observed in 1 (25%) case in group 2, and in 1 (16.7%) case in Group 3. RAIR was positive in all patients. In Group 1, 5 (83.3%) patients underwent overlapping repair, and in Group 3, 6 (100%) patients underwent end-to-end repair. This difference was statistically significant (p= 0.011).

CONCLUSION

In vaginal births, evaluation of anal sphincter damage, determination of perineal body structures and anal sphincters separately and performing anatomical reconstruction when needed significantly reduce the rate of FI in the short and long term.

Significance of the broad non-bony attachments of the anterior cruciate ligament on the tibial side

Knowledge of the anatomy of the anterior cruciate ligament (ACL) is important to understand the function and pathology of the knee joint. However, on the tibial side of ACL, its structural relationships with the articular cartilage and lateral meniscus remain unclear. Furthermore, conventional research methods are limited to analyzing the bone attachments. We provide a comprehensive, three-dimensional anatomical description of the tibial side of the ACL that questions the principle that “a ligament is necessarily a structure connecting a bone to another bone.” In our study, 11 knees from 6 cadavers were used for macroscopic anatomical examinations, serial-section histological analyses, and three-dimensional reconstructions. The attachments of the tibial side of ACL consisted of attachments to the bone (102.6 ± 27.5 mm²), articular cartilage (40.9 ± 13.6 mm²), and lateral meniscus (6.5 ± 4.6 mm²), suggesting that the ACL has close structural relationships with the articular cartilage and lateral meniscus. Our study demonstrates that the tibial side of the ACL is not attached to the bone surface only and provides new perspectives on ligamentous attachments. Considering its attachment to the articular cartilage would enable more accurate functional evaluations of the mechanical tensioning of the ACL.

Anus-Preserving Surgery in Advanced Low-Lying Rectal Cancer: A Perspective on Oncological Safety of Intersphincteric Resection

The surgical management of low-lying rectal cancer, within 5 cm from the anal verge (AV), is challenging due to the possibility, or not, to preserve the anus with its sphincter muscles maintaining oncological safety. The standardization of total mesorectal excision, the adoption of neoadjuvant chemoradiotherapy, the implementation of rectal magnetic resonance imaging, and the evolution of mechanical staplers have increased the rate of anus-preserving surgeries. Moreover, extensive anatomy and physiology studies have increased the understanding of the complexity of the deep pelvis. Intersphincteric resection (ISR) was introduced nearly three decades ago as the ultimate anus-preserving surgery. The definition and indication of ISR have changed over time. The adoption of the robotic platform provides excellent perioperative results with no differences in oncological outcomes. Pushing the boundaries of anus-preserving surgeries has risen doubts on oncological safety in order to preserve function. This review critically discusses the oncological safety of ISR by evaluating the anatomical characteristics of the deep pelvis, the clinical indications, the role of distal and circumferential resection margins, the role of the neoadjuvant chemoradiotherapy, the outcomes between surgical approaches (open, laparoscopic, and robotic), the comparison with abdominoperineal resection, the risk factors for oncological outcomes and local recurrence, the patterns of local recurrences after ISR, considerations on functional outcomes after ISR, and learning curve and surgical education on ISR.

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.

Widespread anorectal lymphovascular networks and tissue drainage: analyses from submucosal India ink injection and indocyanine green fluorescence imaging

AIM

Abdominoperineal resection is associated with poor prognosis in patients with advanced lower rectal cancer. This study aimed to analyse the functional lymphovascular network and tissue drainage in the anorectal region.

METHODS

In this descriptive study, we performed microanatomical evaluations and intra-operative imaging analysis in a cadaver and patients with rectal cancer. Specimens with India ink injection were collected from a cadaver and from six patients who underwent abdominoperineal resection. Intra-operative indocyanine green fluorescence imaging was performed on four patients who underwent surgery for lower rectal cancer. India ink was injected into the submucosa at the dentate line of specimens. Tissue sections were examined by immunohistochemistry for D2-40 and CD31. Intra-operative indocyanine green was injected into the submucosa at the dentate line. Lymph flow was traced using a near-infrared camera system.

RESULTS

Fascia branching from the rectal longitudinal muscle layer extended to the posterior hiatal ligament and lateral endopelvic fascia connective tissue lamina on the surface of the levator ani muscle. The fascia contained veins labelled with ink in their lumina and initial lymphatics. Intra-operative indocyanine green fluorescence imaging revealed extensive lymph flow from the muscle layer of the anal canal to the hiatal ligament and endopelvic fascia along the longitudinal muscle layer fibres.

CONCLUSIONS

The anorectal region contained widespread venous and lymphatic networks in proportion to its specific connective tissue framework around the longitudinal-muscle-layer-extending muscle bundles, which provides extensive networks for tissue fluid and cells.

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.

Essential anatomy for total mesorectal excision and lateral lymph node dissection, in both trans-abdominal and trans-anal perspective

BACKGROUND AND PURPOSE

Total Mesorectal Excisions (TME) is the standard treatment of rectal cancer. It can be performed under laparoscopic, robotic or transanal approach. Inadvertent injury to surrounding structure like autonomic nerves is avoidable, no matter which approach is adopted. Lateral lymph node dissection (LLND) is a less commonly performed pelvic operation involving dissection in an unfamiliar area to most general surgeons. This article aims to clarify all the essential anatomy related to these procedures.

METHODS

We performed thorough literature search and revision on the pelvic anatomy. Our cases of TME and LLND, under either laparoscopic or transanal approach, were reviewed. We integrated the knowledge from literatures and our own experience. The result was presented in details, together with original figures and intra-operative photos.

MAIN FINDINGS

Anatomy of pelvic fascia, autonomic nerve system, anal canal and sphincter complex are core knowledge in performing TME and LLND.

CONCLUSIONS

Thorough understanding of the pelvic anatomy enables colorectal surgeons to master these procedures, avoid complication and perform extended resection. On the other hand, surgeons can appreciate the complex pelvic anatomy easier by seeing the pelvis in opposite angles (transabdominal and transaanal view).

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

Data regarding urethral supporting structures are insufficient for understanding the mechanism of stress urinary incontinence. Whether smooth muscle fibers contribute to urethral support and pelvic floor support structures is unclear. This study aimed to clarify the histological structures and spatial distributions of smooth muscle tissues surrounding the urethra and vagina. Using cadaveric specimens, macroscopic anatomical and histological evaluations were conducted. Six female cadavers were used for macroscopic observations. Ten female cadavers were used for histological observations. Three pelvises were cut in a plane vertical to the urethra, and the other pelvises were cut in a plane parallel to the urethra and vagina to observe tissues surrounding the urethra and vagina. The major tissue component around the proper muscle layer of the urethra was smooth muscle tissue, which mediated among the urethra, pubis, and levator ani. Smooth muscle tissues laterally extended the smooth muscle fibers, both superiorly and inferiorly toward the levator ani, with a few fibers inserted in the levator ani. Smooth muscle was found between the urethral walls and pubic bones. Smooth muscle may contribute to the mechanism of pelvic floor support.

An anatomical study of the anterior wall of the hypopharyngeal and the cervical esophageal junction

OBJECTIVE

The upper esophageal sphincter plays a significant role by forming a physical barrier at the junction of the hypopharynx and the cervical esophagus. As few studies have focused on the ventral aspect of this junction, the contribution of the anterior wall of the cervical esophagus to upper esophageal sphincter function remains unknown. The purpose of this study was to examine the muscle fibers' arrangement at the junction, especially of those forming its anterior wall.

METHODS

Thirteen specimens from 13 Japanese cadavers were analyzed. Six specimens were dissected macroscopically, while the remaining seven were examined histologically.

RESULTS

The outer longitudinal layer of the proximal esophagus was well-developed anterolaterally. The uppermost fibers of the inner circular layer of the esophagus ascended longitudinally. The anterolateral part of the outer longitudinal layer and the uppermost region of the inner circular layer were continuous with the median tendinous band, in turn reaching superior to the cricoid cartilage. Histological analysis showed that the tendinous band was also continuous with the superior part of the perichondrium of the cricoid cartilage and with the aponeurosis of the transverse and oblique arytenoids.

CONCLUSION

The well-developed anterolateral region of the outer longitudinal layer and the uppermost fibers of the inner circular layer were continuous with the median tendinous band, which reached superior to the cricoid cartilage. The contraction of the proximal esophagus may result in protrusion of the anteromedian esophageal wall into the lumen, thereby supporting the closure of the upper esophageal sphincter.

Three muscle slings of the pelvic floor in women: an anatomic study

The region anterior to the anal canal in women is composed of intertwined smooth and skeletal muscles. The present study aimed to clarify skeletal muscle morphology in the anterior region of the anal canal. The pelvic floor muscles of 28 pelvic halves from 16 female cadavers (mean age 79.75 years) were dissected from the inferior aspect to examine the perineal muscles, followed by midline transection and dissection from the inner surface to examine the pelvic outlet muscles. The bulbospongiosus muscle was found to be attached to the lateral surface of the external anal sphincter. The superficial transverse perineal muscle crossed superiorly to the bulbospongiosus and coursed medially toward its contralateral muscle bundle deep to the anterior portion of the external anal sphincter. The superficial transverse perineal muscle formed the middle sling. From the medial aspect, the anterior part of the levator ani was divided into anterior and posterior bundles to form the anterior and posterior slings, respectively. This study proposes that three muscular slings could be important in supporting the pelvic floor in women. In addition, this study shows that the anterior skeletal muscular wall of the anal canal is composed of the anterior muscle bundle of the levator ani, superficial transverse perineal, and proper external anal sphincter muscles.

Various significant connections of the male pelvic floor muscles with special reference to the anal and urethral sphincter muscles

The male pelvic floor is a complex structure formed by several muscles. The levator ani muscle and the perineal muscles are important components of the pelvic floor. The perineal muscles comprise the external anal sphincter, bulbospongiosus, superficial transverse perineal muscles, and ischiocavernosus. Although the connections of the muscles of the pelvic floor have been reported recently, the anatomical details of each muscle remain unclear. In this study, we examined the male pelvic floor to clarify the connection between the muscles related to function. Fifteen male pelvises were used for microscopic dissection, and three male pelvises were used for histological examination. On the lateral aspect, the perineal muscles were connected to each other. Bundles of the levator ani muscle extended to connect to the perineal muscles. In addition, the extended muscle bundle from the levator ani muscle and the perineal muscles surround the external urethral sphincter. On the medial aspect, the levator ani muscle and the external anal sphincter form the anterior and posterior muscular slings of the anal canal. The connection between the perineal muscles and levator ani muscle indicates a possible close relationship between the functions of the urethra and anus.

Interactive three-dimensional teaching models of the female and male pelvic floor

Controversies regarding structure and function of the pelvic floor persist because of its poor accessibility and complex anatomical architecture. Most data are based on dissection. This "surgical" approach requires profound prior knowledge, because applying the scalpel precludes a "second look." The "sectional" approach does not entail these limitations, but requires segmentation of structures and three-dimensional reconstruction. This approach has produced several "Visible Human Projects." We dealt with limited spatial resolution and difficult-to-segment structures by proceeding from clear-cut to more fuzzy boundaries and comparing segmentation between investigators. We observed that the bicipital levator ani muscle consisted of pubovisceral and puborectal portions; that the pubovisceral muscle formed, together with rectococcygeal and rectoperineal muscles, a rectal diaphragm; that the external anal sphincter consisted of its subcutaneous portion and the puborectal muscle only; that the striated urethral sphincter had three parts, of which the middle (urethral compressor) was best developed in females and the circular lower ("membranous") best in males; that the rectourethral muscle, an anterior extension of the rectal longitudinal smooth muscle, developed a fibrous node in its center (perineal body); that the perineal body was much better developed in females than males, so that the rectourethral subdivision into posterior rectoperineal and anterior deep perineal muscles was more obvious in females; that the superficial transverse perineal muscle attached to the fibrous septa of the ischioanal fat; and that the uterosacral ligaments and mesorectal fascia colocalized. To facilitate comprehension of the modified topography we provide interactive 3D-PDFs that are freely available for teaching purposes. Clin. Anat. 33:275-285, 2020. © 2019 Wiley Periodicals, Inc.

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

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. © 2019 Wiley Periodicals, Inc.