New anatomical insight into the muscular structure of the anal canal: revealing Treitz muscle as a directional shift of the internal anal sphincter

Satoru Muro; Kumiko Yamaguchi; Naoko Inoshita; Yasuo Nakajima; Danyo Jennifer Edinam; Akimoto Nimura; Keiichi Akita

doi:10.3393/ac.2024.00647.0092

Articles

Page Path: HOME > Ann Coloproctol > Volume 41(6); 2025 > Article

Original Article Anorectal benign disease New anatomical insight into the muscular structure of the anal canal: revealing Treitz muscle as a directional shift of the internal anal sphincter: Satoru Muro¹, Kumiko Yamaguchi², Naoko Inoshita³, Yasuo Nakajima⁴, Danyo Jennifer Edinam¹, Akimoto Nimura⁵, Keiichi Akita¹; Annals of Coloproctology 2025;41(6):501-509.
DOI: https://doi.org/10.3393/ac.2024.00647.0092
Published online: December 29, 2025

¹Department of Clinical Anatomy, Graduate School of Medical and Dental Sciences, Institute of Science Tokyo, Tokyo, Japan

²Center for Innovative Teaching and Learning, Institute of Science Tokyo, Tokyo, Japan

³Department of Pathology, Moriyama Memorial Hospital, Tokyo, Japan

⁴Department of Colorectal Surgery, Moriyama Memorial Hospital, Tokyo, Japan

⁵Department of Functional Joint Anatomy, Biomedical Engineering Laboratory, Institute of Industry Incubation, Institute of Science Tokyo, Tokyo, Japan

Correspondence to: Satoru Muro, MD, PhD Department of Clinical Anatomy, Graduate School of Medical and Dental Sciences, Institute of Science Tokyo, 1-5-45 Yushima, Bunkyo-ku, Tokyo 113-8510, Japan Email: muro.fana@tmd.ac.jp

• Received: September 24, 2024 • Revised: June 27, 2025 • Accepted: July 30, 2025

This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/4.0/) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.

prev next

2,390 Views
150 Download
1 Web of Science
1 Crossref
1 Scopus

Full Article

Download PDF

Abstract
INTRODUCTION
METHODS
RESULTS
DISCUSSION
ARTICLE INFORMATION
REFERENCES

Abstract

Purpose
Understanding the muscular structure of the anal canal is crucial for the diagnosis and treatment of anorectal diseases. Treitz muscle is a vital yet poorly understood component. It supports the anal venous plexus and contributes to anal cushion formation. However, its anatomical details remain unclear, and various theories suggest different origins for its muscle bundles, which affects our understanding of the pathophysiology of hemorrhoids. In this study, we sought to clarify the origin and localization of Treitz muscle to provide an anatomical foundation for understanding anal function.
Methods
In this descriptive cadaveric study of 11 cadavers, we performed macroscopic examinations and immunohistological analyses on tissues from the anterior, lateral, and posterior walls of the anal canal. The origin and localization of Treitz muscle were qualitatively evaluated.
Results
Treitz muscle is a smooth muscle formed by a directional change in the muscle bundles of the internal anal sphincter, running longitudinally along its surface. A shift in the direction of muscle bundles originating from the internal anal sphincter, giving rise to Treitz muscle, was frequently observed in the anterolateral wall of the anal canal.
Conclusion
In summary, Treitz muscle, a smooth muscle extending from the internal anal sphincter, is considered part of the muscularis propria. Its directional shift was localized to the anterolateral wall, indicating that Treitz muscle is not uniformly distributed around the anal canal. This site-specific localization may influence the risk of hemorrhoids or cancer invasion depending on its anatomical position.
Keywords: Anal canal; Treitz muscle; Internal anal sphincter; Hemorrhoids; Rectal neoplasms

INTRODUCTION

Understanding the muscular structure of the anal canal is essential for diagnosing and treating anorectal diseases. One of the less explored muscle structures in the anal canal is Treitz muscle. Also known as the submucosal muscle of the anus, Treitz muscle was first described in 1853 by Treitz [1]. It is a smooth muscle that runs longitudinally between the mucosal epithelium and the internal anal sphincter. This muscle supports the anal venous plexus, contributes to the formation of anal cushions, and helps support the anal mucosa during defecation [2]. Treitz muscle is also deeply involved in the pathophysiological mechanisms of hemorrhoids [3]. Therefore, anatomical knowledge of Treitz muscle is essential for surgeons treating anorectal diseases.

Despite its clinical significance, the anatomical details of Treitz muscle remain unclear. Several theories exist regarding the origin of its muscle bundles. One theory posits that it is a continuation of the rectal mucosal muscular layer [4], while another suggests that it is derived from the internal anal sphincter (IAS) [5, 6]. A third theory proposes that it is formed where the longitudinal muscle of the anal canal penetrates the IAS and extends subcutaneously [7–9].

Previously, we have identified a structure in which the muscle bundles of the IAS and longitudinal muscle change direction and converge to extend anteriorly at the anterior wall of the anorectal canal [10]. This observation suggests that the muscles of the anorectal canal do not always maintain strictly circular or longitudinal orientations but sometimes change direction to form expansive structures. Given these characteristics, Treitz muscle may be related to directional changes in the IAS. Therefore, we further investigated whether Treitz muscle is distributed uniformly or in a site-specific manner.

This study aimed to clarify the origin and localization of Treitz muscle within the anal canal. By elucidating these aspects, we aim to provide an anatomical foundation for understanding anal function and the pathophysiology of hemorrhoids, thereby offering new insights into their management and treatment.

METHODS

Ethics statement

This study was approved by the Board of Ethics of the Institute of Science Tokyo (No. M2018-006). All study procedures were conducted in accordance with all relevant guidelines and regulations. The bodies were donated in accordance with the Japanese Act on Body Donation for Medical and Dental Education (Act No. 56 of 1983). All donors had voluntarily declared their intention to donate their remains for educational and research purposes prior to death. Written informed consent was obtained after each donor received a clear explanation of the purpose and methods of using their body. Following death, the informed consent process was explained to the bereaved families, who raised no objections.

Specimen preparation

Eleven bodies (7 male and 4 female; mean age at death, 75.5 years [range, 56–88 years]) were donated to the Institute of Science Tokyo. All bodies were fixed by arterial perfusion with 8% formalin and preserved in 30% alcohol. Bodies with a history of pelvic organ surgery were excluded.

Macroscopic examinations

Macroscopic examinations were performed on 6 specimens. The pelvic region was excised en bloc. Two pelvises were sectioned in the median plane, and the lateral wall of the anal canal was dissected from the medial aspect. The remaining 4 pelvises were sectioned in the oblique coronal plane through the anorectal canal. This oblique coronal sectioning allowed for observation of the anterior and posterior wall structures without damaging the midline structures [10, 11]. A diamond-band pathology saw (EXAKT 312, EXKAKT Advanced Technologies) was used to cut the pelvis. The anterior and posterior walls of the anal canal were dissected from the posterior and anterior aspects, respectively. During dissection, the mucosa was removed to expose the muscular structure of the anal canal, consistent with our previous research methods [12, 13]. The origin and location of Treitz muscle were then observed.

Immunohistological analysis

Histological analyses were performed on the remaining 5 specimens. The pelvic region was sectioned in the midsagittal plane, and tissue blocks were collected from the anterior, lateral, and posterior walls of the anal canal. After paraffin embedding, tissue blocks were sectioned at 5-μm thickness. Sections were stained with hematoxylin-eosin, Elastica van Gieson, and Masson trichrome.

Additionally, immunostaining was performed using an anti-smooth muscle antibody to identify smooth muscle tissues. Slides were microwaved in 10 mM sodium citrate buffer (pH 6.0) for antigen retrieval. Endogenous peroxidase activity was inactivated by incubating tissues in methanol containing 0.3% H₂O₂ for 30 minutes. Nonspecific binding was blocked by incubation with phosphate-buffered saline containing 0.05% Tween 20 and 2.5% goat serum at room temperature for 30 minutes. Sections were then incubated overnight at room temperature with primary antibodies against smooth actin (ready-to-use actin, smooth muscle Ab-1, clone 1A4; Thermo Fisher Scientific) and skeletal myosin (1:200, NBP-1-89,707, myosin heavy chain 3 antibody, polyclonal; Novus Biologicals). After washing, the sections were incubated for 30 minutes at room temperature with peroxidase-conjugated anti-mouse immunoglobulin G (IgG; ready-to-use, MP-7452, ImmPRESS HRP goat anti-mouse IgG polymer; Vector Laboratories) and anti-rabbit IgG (ready-to-use, MP-7451, ImmPRESS HRP goat anti-rabbit IgG polymer; Vector Laboratories) as secondary antibodies. Immunocomplexes were detected using 3,3’-diaminobenzidine (Fujifilm Wako Pure Chemical) and counterstained with hematoxylin for 1 minute. This immunohistological method is similar to our previous techniques used for the rectoanal canal and is effective for detecting the distribution of smooth muscle tissues [14, 15].

RESULTS

Macroscopic examinations

When examining the lateral wall of the anal canal, the rectal and anal canal mucosae were first visualized, allowing identification of the pectinate line (Fig. 1A). After the removal of the mucosa, circular muscles were observed in the rectum, while several muscle bundles running longitudinally were seen in the anal canal and identified as Treitz muscle (Fig. 1B). Treitz muscle originated from the circular muscle (IAS) at the level of the anorectal angle, above the pectinate line (Fig. 1B). In the anterolateral wall of the anorectal canal, certain circular muscle bundles (IAS) changed direction, running either anteroinferiorly or posteroinferiorly. The anal venous plexus was observed near Treitz muscle. Upon removal of Treitz muscle, the full extent of the IAS became visible. No longitudinal fibers penetrating the IAS toward the lumen were observed.

When the anal canal was sectioned along its axis (oblique coronal plane), cross-sections of the IAS, longitudinal muscle, and external anal sphincter (EAS) in the canal wall were clearly visible (Fig. 2). In the anterior wall of the anal canal, after removing the mucosa, circular muscles were seen in the rectum, whereas Treitz muscle appeared inside the IAS (Fig. 2C). At the anterior midline, Treitz muscles descending from both sides merged and continued downward (Fig. 2C). In contrast, in the posterior wall of the anal canal, after removal of the mucosa, Treitz muscle was observed only in the posterolateral wall (Fig. 2E). Treitz muscle was not present in the posterior midline.

Immunohistological analysis

Histological examination of the lateral wall of the anal canal revealed the IAS, longitudinal muscle, and EAS. Additionally, muscle fibers running longitudinally along the luminal surface of the IAS were identified as Treitz muscle (Fig. 3A). Immunostaining confirmed that Treitz muscle consisted of smooth muscle fibers (Fig. 3B, C). Treitz muscle originated from the smooth muscle fibers of the IAS above the pectinate line (Fig. 3C). Beneath the mucosa, the muscularis mucosa continued from the rectum into the anal canal; however, Treitz muscle was observed as a completely separate structure, distinct from the muscularis mucosa and separated by subcutaneous tissue. A rich venous plexus was noted near Treitz muscle (Fig. 3C). At the lower end of Treitz muscle, the muscle fibers spread out, encasing the venous plexus bilaterally (Fig. 4A, B). The shape of the lower end of Treitz muscle varied among specimens: in some, fibers spread subcutaneously to cover the luminal surface of the EAS (Fig. 4A, B), while in others, they merged with smooth muscle fibers at the lower end of the IAS (Fig. 4C, D).

In the anterior wall of the anal canal, Treitz muscle ran longitudinally along the luminal surface of the IAS (Fig. 5A), and immunostaining confirmed it consisted of smooth muscle (Fig. 5B). Treitz muscle originated from the smooth muscle fibers of the IAS (Fig. 5B). A rich venous plexus was observed in close proximity to Treitz muscle.

In the posterior wall of the anal canal, Treitz muscle was present along the luminal surface of the IAS (Fig. 5C), and immunostaining confirmed its smooth muscle composition (Fig. 5D). Treitz muscle remained distinct from the muscularis mucosa, separated by subcutaneous tissue. The origin of Treitz muscle was not observed in the posterior wall of the anal canal. The lower end of Treitz muscle merged with smooth muscle fibers at the lower end of the IAS (Fig. 5D).

DISCUSSION

In this study, we clarified the origin and localization of Treitz muscle in the anal canal using a combination of macroscopic and immunohistological examinations. Treitz muscle is a smooth muscle formed by a directional change in the muscle bundles of the IAS, running longitudinally along its surface (Fig. 6). The directional shift of the muscle bundles originating from the IAS, which gives rise to Treitz muscle, is often observed in the anterolateral wall of the anal canal. Thus, muscle bundles descending from the anterolateral wall to the median anterior wall, as well as those descending from the lateral wall to the posterior wall, constitute Treitz muscle.

Previous histological studies using both adult and pediatric specimens have demonstrated the presence of Treitz muscle in all cases [2, 5]. Similarly, in the present study, Treitz muscle was identified in all specimens through macroscopic and histological analyses, suggesting a consistent anatomical structure. Despite this consistency, its precise origin has remained unclear. Historically, Treitz muscle was considered a continuation of the muscularis mucularis mucosa of the rectum [4]. Later studies described Treitz muscle as fibers of the longitudinal muscle that passed through the IAS and emerged subcutaneously [7–9]. Jit [5], however, denied continuity from the longitudinal muscle fibers, indicating instead that Treitz muscle is continuous with the IAS fibers, a view supported by Arakawa et al. [6]. Thomson [2] reported that Treitz muscle arises partly from the IAS and partly from the longitudinal muscle. Our findings suggest that Treitz muscle is, in fact, an extension of the IAS. This multifaceted body of evidence supports and reinforces the findings of Jit [5] and Arakawa et al. [6]. However, while previous research merely indicated continuity between Treitz muscle and the IAS, the present study demonstrates that this continuity is formed specifically by a directional shift in the muscle bundles of the IAS. Furthermore, we revealed that this directional shift occurs at the anterolateral wall of the anal canal, and that the presence, absence, and configuration of Treitz muscle vary among the anterior, lateral, and posterior walls. This indicates that Treitz muscle is not uniformly distributed around the circumference of the anal canal.

Currently, there are few reports on the localization of Treitz muscle within the anal canal. Jit [5] emphasized that the development of Treitz muscle varies depending on the location of the section within the same anal canal, indicating that it is not uniformly distributed circumferentially. Arakawa et al. [6] reported that Treitz muscle is not observed at the midline of the posterior wall of the anal canal, but is particularly thick in the 11–1, 4–5, and 7–8 o’clock positions, as noted by surgeons. This regional specificity aligns with our findings and is reasonably explained by the observation that the directional shift of muscle bundles from the IAS to Treitz muscle is localized to the anterolateral wall. Specifically, Treitz muscle arising from the anterolateral wall descends anteriorly to form thick bundles at the midline of the anterior wall (11–1 o’clock position), and posteriorly to form thick bundles at the lateral posterior wall (4–5 and 7–8 o’clock positions). Therefore, Treitz muscle bundles are less likely to form at the midline of the posterior wall (6 o’clock position). The formation of Treitz muscle by a directional shift of the IAS muscle bundles exemplifies a sophisticated adaptation to the complex function of defecation. The localization of this shift to the anterolateral anal canal clarifies fundamental aspects of its composition, origin, and distribution.

A detailed anatomical understanding of Treitz muscle is beneficial for the diagnosis and treatment of anorectal diseases. For example, stretching and disruption of Treitz muscle have been suggested as causes of hemorrhoids [2]. Repeated stretching can lead to muscle damage and prolapse, disrupting the anchoring and flattening function of the venous plexus provided by Treitz muscle [3]. As a result, secondary changes such as hypertrophy and congestion of vascular tissue can occur. In patients with hemorrhoids, the tissue supporting the anal cushion is destroyed and fragmented [16, 17]. Additionally, there have been reports of abnormal contraction of the IAS at ultraslow pressure in patients with hemorrhoids [18]. This finding is consistent with the observation that Treitz muscle is formed by a directional shift in the muscle bundles of the IAS. Surgical treatment for hemorrhoids is continually evolving [19, 20]. Detailed anatomical knowledge of Treitz muscle could lead to a deeper understanding of the pathophysiology of hemorrhoids and foster the development of new preventive and therapeutic strategies.

In the context of lower rectal cancer, the complex muscle layer structure of the anal canal complicates the pathological diagnosis of lower rectal cancer [21, 22]. Treitz muscle adds to this complexity. If cancer invades Treitz muscle, its interpretation as either a submucosal or muscularis propria structure can affect the diagnosis of tumor invasion as T1 or T2 [23], which may necessitate a change in the treatment plan. However, appropriate guidelines are not yet available. Our study anatomically classified Treitz muscle as part of the muscularis propria. Although Treitz muscle is also referred to as the “musculus submucosae ani,” this muscle is distinct from the muscularis mucosa, and its alternative name can therefore be confusing or inappropriate.

Considering that Treitz muscle extends and thickens immediately beneath the mucosa, careful consideration is needed when regarding it as pathologically equivalent to the muscularis propria, such as the IAS. In contrast to the typical muscularis propria, which uniformly encircles the anal canal, Treitz muscle exhibits regional specificity, as demonstrated in this study. Depending on the location of a cancerous lesion, the absence of Treitz muscle in certain areas of the anal wall may influence the risk of invasion. Therefore, pathological diagnosis of the anal canal should take into account variations in muscle layer structure based on the specific location of the tumor. As a regionally specific muscle structure unique to the anal canal, Treitz muscle requires the development of pathological guidelines grounded in detailed anatomical knowledge.

This study has several limitations. First, the donated bodies examined had a mean age greater than 70 years, reflecting a relatively older population. Although muscle atrophy and degeneration associated with advanced age cannot be completely ruled out, these factors likely have minimal impact on muscle continuity and localization. Second, this was an anatomical investigation and did not include pathological verification using patient specimens. Further pathological and oncological studies are necessary to inform the development of new diagnostic guidelines.

In conclusion, this study identifies Treitz muscle as an extension of the IAS and as a component of the muscularis propria. Its formation, resulting from directional shifts in muscle bundles, is localized to the anterolateral wall of the anal canal, underscoring its region-specific distribution and potential clinical significance. A more comprehensive understanding of the anatomical characteristics of Treitz muscle may enhance the accuracy of anorectal disease diagnoses, particularly in distinguishing between T1 and T2 stages of rectal cancer or in elucidating the structural basis of hemorrhoid development.

ARTICLE INFORMATION

Conflict of interest

No potential conflict of interest relevant to this article was reported.

Funding

This study was supported by the Japan Society for the Promotion of Science (JSPS) KAKENHI (No. JP19K23821, No. JP21K15329, No. JP21H03799).

Acknowledgments

The authors sincerely thank those who donated their bodies to science so that anatomical research could be performed. The results of this research can potentially increase the overall knowledge of humankind and improve patient care. Therefore, the donors and their families deserve great gratitude.

Author contributions

Conceptualization: SM, KY; Data curation: SM, KY; Formal analysis: all authors; Funding acquisition: SM, KA; Investigation: SM; Methodology: SM, KY; Project administration: SM; Visualization: SM; Writing–original draft: SM; Writing–review & editing: KY, NI, YN, DJE, AN, KA. All authors read and approved the final manuscript.

Fig. 1.

Macroscopic examination of the lateral wall of the anal canal. (A) The lateral wall of the anal canal is shown, highlighting the rectal and anal canal mucosa and the identification of the pectinate line (PL; arrows). (B) After the removal of the mucosa, the circular muscle (CM) is visible in the rectum, and Treitz muscle (asterisks) is observed in the anal canal. Treitz muscle originates from the CM (internal anal sphincter [IAS]) at the level of the anorectal angle, above the pectinate line (dashed arrows), as a result of a directional shift in the muscle bundles. This shift occurs in the anterolateral wall of the anorectal canal at the level of the anorectal angle. (C) Following removal of Treitz muscle, the full extent of the IAS is revealed, with no longitudinal fibers penetrating the IAS toward the lumen. EAS, external anal sphincter; LM, longitudinal muscle.

Fig. 2.

Macroscopic examination of the anterior and posterior walls of the anal canal. (A) The anal canal is sectioned along its axis (oblique coronal plane indicated by the red dotted line) to observe its anterior and posterior walls. (B) The anterior wall of the anal canal with mucosa intact. (C) Treitz muscle (asterisks) in the anterior wall of the anal canal after mucosal removal: Treitz muscle originates from the circular muscle (CM; internal anal sphincter [IAS]; dashed arrows). (D) The posterior wall of the anal canal with mucosa intact. (E) Treitz muscle (asterisks) in the posterior wall of the anal canal after mucosal removal: Treitz muscle is observed only on the posterolateral wall and is absent from the posterior midline. EAS, external anal sphincter; LM, longitudinal muscle.

Fig. 3.

Histological analysis of the lateral wall of the anal canal. (A) The lateral wall of the anal canal stained with Masson trichrome: the internal anal sphincter (IAS), longitudinal muscle (LM), and external anal sphincter (EAS) are observed. Muscle fibers running longitudinally along the luminal surface of the IAS are identified as Treitz muscle (asterisks). (B) Immunostaining confirms that Treitz muscle consists of smooth muscle fibers. (C) Magnified view of the dashed rectangular area in panel B. Treitz muscle originates from the smooth muscle fibers of the internal anal sphincter (arrowheads). A rich venous plexus (VP) is observed near Treitz muscle. CM, circular muscle; PL, pectinate line; MM, muscularis mucosa.

Fig. 4.

Lower end of Treitz muscle in the lateral wall of the anal canal. (A) At the lower end of Treitz muscle, muscle fibers spread out, encasing the venous plexus (VP) on both sides (Elastica van Gieson). (B) Immunostaining for smooth muscle (corresponding to panel A). Some fibers of Treitz muscle spread subcutaneously to cover the luminal surface of the external anal sphincter (EAS). (C) Another specimen of the lateral wall of the anal canal stained with Elastica van Gieson. (D) Immunostaining for smooth muscle (corresponding to panel C). Some fibers (arrows) of Treitz muscle (asterisks) merge with the smooth muscle fibers at the lower end of the internal anal sphincter (IAS). LM, longitudinal muscle.

Fig. 5.

Histological analysis of the anterior and posterior walls of the anal canal. (A) The anterior wall of the anal canal stained with Masson trichrome. Treitz muscle (asterisks) running longitudinally is observed along the luminal surface of the internal anal sphincter (IAS). (B) Immunostaining of panel A indicates that Treitz muscle consists of smooth muscle. It originates from the smooth muscle fibers of the IAS (arrowheads). A rich venous plexus (VP) is observed near Treitz muscle. (C) The posterior wall of the anal canal stained with Elastica van Gieson. Treitz muscle is seen along the luminal surface of the IAS (asterisks). (D) Immunostaining of panel C demonstrates that Treitz muscle consists of smooth muscle fibers. The lower end of Treitz muscle (small arrows) merges with the smooth muscle fibers at the lower end of the IAS. EAS, external anal sphincter; LM, longitudinal muscle; PL, pectinate line; MM, muscularis mucosa.

Fig. 6.

Schematic illustration of the origin and localization of Treitz muscle in the anal canal. (A) Schematic view from the medial aspect showing the course of Treitz muscle on the lateral wall of the anal canal. Treitz muscle originates at the level of the anorectal angle on the anterolateral wall of the anal canal, arising from a directional change in the muscle bundles of the internal anal sphincter (IAS). Some bundles descend along the anterior wall, while others course from the lateral wall toward the posterolateral wall, forming multiple muscle bundles. (B) The origin of Treitz muscle is the directional shift of the IAS. Specifically, some superficial muscle bundles, which usually run in a circular orientation, undergo a change in direction and begin to run longitudinally, forming Treitz muscle. This origin is localized to the anterolateral region of the anal canal, from which muscle bundles descend anteriorly and posterolaterally due to this shift in orientation. As a result, Treitz muscle demonstrates region-specific distribution, being localized to the anterior and bilateral posterolateral regions of the anal canal. CM, circular muscle.

REFERENCES

1. Treitz W. Über einen neuen muskel am duodenum des menschen, über elastische sehnen, und einige andere anatomische verhältnisse [On a new muscle in the human duodenum, on elastic tendons, and some other anatomical relationships]. Vierteljahrs Prakt Heilkd (Prague) 1853;37:113. German.
2. Thomson WH. The nature of haemorrhoids. Br J Surg 1975;62:542–52. Article PubMed PDF
3. Gordon PH, Nivatvongs S. Principles and practice of surgery for the colon, rectum, and anus. 3rd ed. CRC Press; 2007.Article
4. Fine J, Lawes CH. On the muscle-fibres of the anal submucosa, with special reference to the pecten band. Br J Surg 1940;27:723–7. Article PDF
5. Jit I. Muscularis submucosae ani and its development. J Anat 1974;118:11–7. PubMed PMC
6. Arakawa T, Murakami G, Ohtsuka A, Goto T, Teramoto T. Variations in anal submucosal muscles in elderly Japanese subjects. Biomed Res 2004;25:45–52. Article
7. Fowler R. Landmarks and legends of the anal canal. Aust N Z J Surg 1957;27:1–18. Article PubMed
8. Goligher JC, Leacock AG, Brossy JJ. The surgical anatomy of the anal canal. Br J Surg 1955;43:51–61. Article PubMed PDF
9. Wilde FR. The anal intermuscular septum. Br J Surg 1949;36:279–85. Article PubMed PDF
10. Muro S, Tsukada Y, Harada M, Ito M, Akita K. Anatomy of the smooth muscle structure in the female anorectal anterior wall: convergence and anterior extension of the internal anal sphincter and longitudinal muscle. Colorectal Dis 2019;21:472–80. Article PubMed PMC PDF
11. Muro S, Tsukada Y, Harada M, Ito M, Akita K. Spatial distribution of smooth muscle tissue in the male pelvic floor with special reference to the lateral extent of the rectourethralis muscle: Application to prostatectomy and proctectomy. Clin Anat 2018;31:1167–76. Article PubMed PDF
12. Muro S, Yamaguchi K, Nakajima Y, Watanabe K, Harada M, Nimura A, et al. Dynamic intersection of the longitudinal muscle and external anal sphincter in the layered structure of the anal canal posterior wall. Surg Radiol Anat 2014;36:551–9. Article PubMed PDF
13. Muro S, Shoji S, Suriyut J, Akita K. Anatomy of muscle connections in the male urethra and anorectal canal. BJU Int 2024;133:752–9. Article PubMed
14. Muro S, Suriyut J, Akita K. Anatomy of Cowper's gland in humans suggesting a secretion and emission mechanism facilitated by cooperation of striated and smooth muscles. Sci Rep 2021;11:16705.Article PubMed PMC PDF
15. Muro S, Tsukada Y, Ito M, Akita K. The series of smooth muscle structures in the pelvic floors of men: dynamic coordination of smooth and skeletal muscles. Clin Anat 2021;34:272–82. Article PubMed PDF
16. Bernstein WC. What are hemorrhoids and what is their relationship to the portal venous system? Dis Colon Rectum 1983;26:829–34. Article PubMed
17. Haas PA, Fox TA, Haas GP. The pathogenesis of hemorrhoids. Dis Colon Rectum 1984;27:442–50. Article PubMed
18. Waldron DJ, Kumar D, Hallan RI, Williams NS. Prolonged ambulant assessment of anorectal function in patients with prolapsing hemorrhoids. Dis Colon Rectum 1989;32:968–74. Article PubMed
19. Altomare DF, Picciariello A, Pecorella G, Milito G, Naldini G, Amato A, et al. Surgical management of haemorrhoids: an Italian survey of over 32 000 patients over 17 years. Colorectal Dis 2018;20:1117–24. Article PubMed PDF
20. White I, Avital S, Greenberg R. Outcome of repeated stapler haemorrhoidopexy for recurrent prolapsing haemorrhoids. Colorectal Dis 2011;13:1048–51. Article PubMed
21. Muro S, Kagawa R, Habu M, Ka H, Harada M, Akita K, et al. Coexistence of dense and sparse areas in the longitudinal smooth muscle of the anal canal: anatomical and histological analyses inspired by magnetic resonance images. Clin Anat 2020;33:619–26. Article PubMed PDF
22. Kraima AC, West NP, Roberts N, Magee DR, Smit NN, van de Velde CJ, et al. The role of the longitudinal muscle in the anal sphincter complex: implications for the intersphincteric plane in low rectal cancer surgery? Clin Anat 2020;33:567–77. Article PubMed
23. Japanese Society for Cancer of the Colon and Rectum. Japanese classification of colorectal, appendiceal, and anal carcinoma: the 3d English edition [secondary publication]. J Anus Rectum Colon 2019;3:175–95. Article PubMed PMC

Figure & Data

References

Citations

Citations to this article as recorded by

Histological architecture of the intersphincteric region of the anal canal: implications for the anatomical basis of anal fistula pathways
Satoru Muro, Yasuo Nakajima, Akimoto Nimura, Keiichi Akita
International Journal of Colorectal Disease.2026;[Epub] CrossRef

Cite this Article

Cite this Article: export Copy Download Format; Close

Download Citation

Download a citation file in RIS format that can be imported by all major citation management software, including EndNote, ProCite, RefWorks, and Reference Manager.

Format:

RIS — For EndNote, ProCite, RefWorks, and most other reference management software
BibTeX — For JabRef, BibDesk, and other BibTeX-specific software

Include:

Citation for the content below
Citation and abstract for the content below

New anatomical insight into the muscular structure of the anal canal: revealing Treitz muscle as a directional shift of the internal anal sphincter

Ann Coloproctol. 2025;41(6):501-509. Published online December 29, 2025

DOI: https://doi.org/10.3393/ac.2024.00647.0092

XML Download

Figure

New anatomical insight into the muscular structure of the anal canal: revealing Treitz muscle as a directional shift of the internal anal sphincter

Fig. 1. Macroscopic examination of the lateral wall of the anal canal. (A) The lateral wall of the anal canal is shown, highlighting the rectal and anal canal mucosa and the identification of the pectinate line (PL; arrows). (B) After the removal of the mucosa, the circular muscle (CM) is visible in the rectum, and Treitz muscle (asterisks) is observed in the anal canal. Treitz muscle originates from the CM (internal anal sphincter [IAS]) at the level of the anorectal angle, above the pectinate line (dashed arrows), as a result of a directional shift in the muscle bundles. This shift occurs in the anterolateral wall of the anorectal canal at the level of the anorectal angle. (C) Following removal of Treitz muscle, the full extent of the IAS is revealed, with no longitudinal fibers penetrating the IAS toward the lumen. EAS, external anal sphincter; LM, longitudinal muscle.

Fig. 2. Macroscopic examination of the anterior and posterior walls of the anal canal. (A) The anal canal is sectioned along its axis (oblique coronal plane indicated by the red dotted line) to observe its anterior and posterior walls. (B) The anterior wall of the anal canal with mucosa intact. (C) Treitz muscle (asterisks) in the anterior wall of the anal canal after mucosal removal: Treitz muscle originates from the circular muscle (CM; internal anal sphincter [IAS]; dashed arrows). (D) The posterior wall of the anal canal with mucosa intact. (E) Treitz muscle (asterisks) in the posterior wall of the anal canal after mucosal removal: Treitz muscle is observed only on the posterolateral wall and is absent from the posterior midline. EAS, external anal sphincter; LM, longitudinal muscle.

Fig. 3. Histological analysis of the lateral wall of the anal canal. (A) The lateral wall of the anal canal stained with Masson trichrome: the internal anal sphincter (IAS), longitudinal muscle (LM), and external anal sphincter (EAS) are observed. Muscle fibers running longitudinally along the luminal surface of the IAS are identified as Treitz muscle (asterisks). (B) Immunostaining confirms that Treitz muscle consists of smooth muscle fibers. (C) Magnified view of the dashed rectangular area in panel B. Treitz muscle originates from the smooth muscle fibers of the internal anal sphincter (arrowheads). A rich venous plexus (VP) is observed near Treitz muscle. CM, circular muscle; PL, pectinate line; MM, muscularis mucosa.

Fig. 4. Lower end of Treitz muscle in the lateral wall of the anal canal. (A) At the lower end of Treitz muscle, muscle fibers spread out, encasing the venous plexus (VP) on both sides (Elastica van Gieson). (B) Immunostaining for smooth muscle (corresponding to panel A). Some fibers of Treitz muscle spread subcutaneously to cover the luminal surface of the external anal sphincter (EAS). (C) Another specimen of the lateral wall of the anal canal stained with Elastica van Gieson. (D) Immunostaining for smooth muscle (corresponding to panel C). Some fibers (arrows) of Treitz muscle (asterisks) merge with the smooth muscle fibers at the lower end of the internal anal sphincter (IAS). LM, longitudinal muscle.

Fig. 5. Histological analysis of the anterior and posterior walls of the anal canal. (A) The anterior wall of the anal canal stained with Masson trichrome. Treitz muscle (asterisks) running longitudinally is observed along the luminal surface of the internal anal sphincter (IAS). (B) Immunostaining of panel A indicates that Treitz muscle consists of smooth muscle. It originates from the smooth muscle fibers of the IAS (arrowheads). A rich venous plexus (VP) is observed near Treitz muscle. (C) The posterior wall of the anal canal stained with Elastica van Gieson. Treitz muscle is seen along the luminal surface of the IAS (asterisks). (D) Immunostaining of panel C demonstrates that Treitz muscle consists of smooth muscle fibers. The lower end of Treitz muscle (small arrows) merges with the smooth muscle fibers at the lower end of the IAS. EAS, external anal sphincter; LM, longitudinal muscle; PL, pectinate line; MM, muscularis mucosa.

Fig. 6. Schematic illustration of the origin and localization of Treitz muscle in the anal canal. (A) Schematic view from the medial aspect showing the course of Treitz muscle on the lateral wall of the anal canal. Treitz muscle originates at the level of the anorectal angle on the anterolateral wall of the anal canal, arising from a directional change in the muscle bundles of the internal anal sphincter (IAS). Some bundles descend along the anterior wall, while others course from the lateral wall toward the posterolateral wall, forming multiple muscle bundles. (B) The origin of Treitz muscle is the directional shift of the IAS. Specifically, some superficial muscle bundles, which usually run in a circular orientation, undergo a change in direction and begin to run longitudinally, forming Treitz muscle. This origin is localized to the anterolateral region of the anal canal, from which muscle bundles descend anteriorly and posterolaterally due to this shift in orientation. As a result, Treitz muscle demonstrates region-specific distribution, being localized to the anterior and bilateral posterolateral regions of the anal canal. CM, circular muscle.

Fig. 1.

Fig. 2.

Fig. 3.

Fig. 4.

Fig. 5.

Fig. 6.

New anatomical insight into the muscular structure of the anal canal: revealing Treitz muscle as a directional shift of the internal anal sphincter