Revisiting the Anatomy of the Rectum and the Anal Canal

3.1 Mucosa

The mucosal layer of the colon is the most metabolically and immunologically active. The mucosa consists of epithelium, lamina propria, and the muscularis mucosae.

3.2 Submucosa

The submucosa of the rectum is composed of smooth muscle fibers, connective tissue, vasculature, lymphatic channels, and various neural networks. The submucosal neural plexuses are the inner plexus (Meissner’s) which innervates the muscularis mucosae and the second is the outer submucosal plexus (Henle’s) which contributes to the innervation of the muscularis layer.

Clear distinction between the mucosa and submucosa are crucial for rectal cancer staging. Tumor invasion into the submucosa (T1 stage in TNM staging) indicates an early stage of cancer spread beyond the mucosal layer, specifically beyond the muscularis mucosae [13]. Tumor invasion into the submucosa increases the likelihood of metastasis through lymphatic spread to regional lymph nodes or distant metastases [13].

3.3 Muscular externa

The muscularis externa layer of the rectum consists of two smooth muscle layers. The first is the inner circular layer which facilitates constriction and segmentation movements of the rectum. The next is the outer longitudinal layer which aids in shortening and propelling colonic contents from rectum into the anal canal. Between these two layers exists a myenteric plexus (Auerbach’s) which innervates the two layers. Additionally, interstitial cells (of Cajal) which are specialized pacemaker cells that drive peristalsis are found throughout the muscularis propria. As mentioned above, distinction between the histological layers of the rectum is crucial for rectal cancer staging [13]. If the tumor is confined to the muscularis externa, it is classified as T2 in the TNM staging system. When the tumor breaches the muscularis externa and extends into the perirectal fat or mesorectal tissue, it is classified as T3. Further extension into adjacent organs or structures (such as the bladder, prostate, or peritoneum) is classified as T4 [13].

4.1 Internal anatomy and histology of the anal canal

The histology of the anal canal is continuous with the histological arrangement of the gastrointestinal tract i.e. mucosal, submucosal, muscularis, and serosa/tunica adventitia layers. The contents and composition of each of these layers corresponds to the location and function of the anal canal.

4.1.1 Mucosa of the anal canal

The mucous membrane of the anal canal has 10–15 permanent longitudinal folds called anal columns, joined at the bases to form anal valves at the level of the pectinate line. Histologically, there are four important transition zones in the epithelium of the anal canal – colorectal, anal transitional, squamous, and perianal zones (Figure 1) [11, 16].

1. The colorectal zone is approximately at the anorectal junction where a change in epithelium is seen from simple to stratified columnar epithelium. This zone is most susceptible to colorectal like adenocarcinomas (Figure 2) [11].

2. The anal transitional zone (ATZ) features stratified columnar or stratified cuboidal epithelium. Here the epithelium begins to transition towards a stratified squamous appearance (Figure 2). The ATZ extends from the dentate line to the new squamous columnar junction at the pectineal line. On anoscopy, the SCJ is the most important and readily identifiable landmark, typically visible as a clear line of tissue that is paler than the adjacent proximal rectal epithelium – this corresponds to the pectinate line. The boundaries between the colorectal and ATZ are not well demarcated. This zone, because of the volatile nature of transitioning cells, is susceptible to anal intraepithelial neoplasia and squamous cell carcinomas in the presence of oncogenic strains of HPV [11, 16, 17].

3. The squamous zone is composed of uninterrupted, non-keratinized stratified squamous epithelium. This extends towards the white line (of Hilton) (Figure 2) [11, 16, 17].

4. The perianal skin zone mirrors the histology of the skin - i.e., keratinized stratified squamous epithelium (Figure 2), with hair follicles, sebaceous glands, sweat glands, and apocrine glands (circumanal glands). Melanocytes are also seen in this zone. This extends from the white line towards the anal margin (i.e., perianal skin). Due to the nature of the histology, this zone is susceptible to hidradenitis suppurativa [18], melanomas [19], lichen planus [20], and benign fibroepithelial growths.

4.2 Muscularis layer of anal canal (anal sphincter complex)

The anal sphincter complex consists of both the internal and external anal sphincters which are separated by a conjoint longitudinal muscle layer. The anal sphincter complex plays a critical role in defecation and maintaining continence. This complex corresponds histologically to the muscularis layer of the anal canal.

5.1 The upper limit of the rectum

The upper limit of the rectum begins at the rectosigmoid junction (RSJ). The RSJ serves as an important landmark for procedures such as total mesorectal excision (TME), resection margins in rectal cancer surgeries, and colostomy planning. This junction is characterized by the absence of taeniae coli and omental appendages (epiploic appendages). The RSJ is found at the level of the third sacral vertebrae. The RSJ is said to be 2.8 cm long in adults and 0.7 cm in neonates [30]. Histologically, the RSJ also marks the transition from sigmoid colon mucosa to rectal mucosa. The RSJ can be identified via an array of modalities. Some examples include:

5.1.1 Identifying the RSJ on CT and MRI

The sigmoid take off (STO) has been suggested as an imaging landmark to identify the RSJ on CT and MRI [31]. The STO is defined as the junction between the mesorectum and sigmoid mesocolon [13]. The STO has also been defined as the region where the fixed mesorectum ends and no longer tethers the rectum to the sacrum, and thus the rectum becomes mobile [28, 32]. The STO is seen on a sagittal view where the sigmoid colon moves horizontally away from the sacrum (Figure 2). Axially, the sigmoid colon moves ventrally (Figure 3). Identification of the STO allows for accurate classification of sigmoid or rectal tumors, above and below the STO, respectively [31].

While the STO is a promising landmark for defining the RSJ and improving the sensitivity of rectal cancer diagnoses, it is not without its limitations. For example, Hazen et al. [31], aimed to evaluate the use of the STO in clinical practice and found that in the presence of complex anatomy and pathology, and/or suboptimal and inconsistent imaging conditions identification of STO was not feasible and requires extensive training and expertise. The authors also highlighted that without adequately addressing the abovementioned challenges, the risk of misclassification of tumors would have a downstream effect on treatment plans and ultimately prognosis. Similar observations have been corroborated by other studies [13].

5.1.3 Identifying the RSJ on barium enema studies

To identify the RSJ reliably, Rubesin et al. [35] highlight that the patient should be positioned laterally to avoid sigmoid loops from obstructing the RSJ view. Rubesin et al. recommend that the overhead view obtained with the tube angled about 30° caudad and with the patient in the prone position is optimal to observe the rectosigmoid junction [35]. To the author’s knowledge, no literature to date outlines landmarks to identify the RSJ that are specific to barium enema studies (Figure 4).

5.2 The lower limit of the rectum and upper limit of the anal canal

The lower limit of the rectum is even more controversial than the upper limit. The main classifications are anatomical and clinical (Figure 5).

5.3 Lower limit of the anal canal

The lower limit of the surgical anal canal is around the mucocutaneous junction of the non-keratinised squamous mucosa (anal verge) and the keratinised perianal skin in line with the intersphincteric groove [36]. The intersphincteric groove is approximately 2 cm below the pectinate line in adults. The anal margin, also referred to as the perianal skin extends radially for about 5 cm distal to the anal verge. This is the skin around the anus proper and is what is seen on clinical examination (Figure 5) [37].

6.1 Peritoneal relations of the rectum

The rectum is both intra and extraperitoneal. The upper two thirds of the rectum is classified as intraperitoneal given that its anterior and lateral aspects are covered by peritoneum. Meanwhile, the lower third is classified as extraperitoneal. It should be noted that in the middle third of the rectum, only the anterior aspect of the rectum is covered by peritoneum [26] It is important to understand the peritoneal relations of the rectum, particularly in the context of rectal perforations. Any perforations through a part of the rectal wall that is covered by peritoneum means that the perforated matter has access to the peritoneum, potentially leading to sepsis [30].

6.2 Fascial relations of the rectum

The mesorectum covers the rectum and its surrounding fat. The mesorectum transmits the superior rectal artery (and its branches) and vein (and its tributaries), and lymphatics. Anterolaterally in the mesorectum, are branches of the inferior hypogastric plexus and the middle rectal arteries (where present). Dissection around the inferior hypogastric plexus must be carefully done to preserve urinary bladder and sexual functions [26]. There are suggestions that branches of the inferior hypogastric plexus and the middle rectal arteries are enclosed by the lateral ligaments, however, not much is known about the morphology, extent, and prevalence of these ligaments.

The mesorectum, with its fatty tissue predominantly deposited posteriorly, provides not only a protective role but also supplies vascular, lymphatic, and neural elements crucial for rectal function. The mesorectum itself is encased within the mesorectal fascia (fascia propria of the rectum). The mesorectal fascia covers the rectum down to the level of levator ani muscle [26] Surgically, the mesorectal fascia provides the resection plane for total mesorectal excision (TME). Posterior to the mesorectal fascia is the presacral space (retrorectal space). This space itself is anterior to the presacral fascia which some texts suggest is synonymous with Waldeyer’s fascia. The exact definition of Waldeyer’s fascia remains debated, with two predominant interpretations: (1) it is considered the presacral fascia, which forms the base of the retrorectal space and condenses into the rectosacral ligament; or (2) it is described as the rectosacral fascia, a dense connective tissue layer extending from the periosteum of the fourth sacral vertebra to the posterior rectal wall (Figure 6) [39]. Waldeyer’s fascia, however, remains a critical landmark during posterior rectal mobilization. It is suggested that improper blunt dissection may compromise the mesorectum, increasing the risk of cancer metastasis. Additionally, tearing of Waldeyer’s fascia from the sacrum can lead to significant bleeding from the presacral veins [39].

Anteriorly, in males the mesorectal fascia becomes coalesced with the rectoprostatic (rectovesical) fascia (Denonvilliers fascia), which is anterior to the rectovesical pouch. In females, the mesorectal fascia anteriorly combines with rectovaginal fascia (Denonvilliers fascia) to form the rectovaginal septum which is anterior to the rectouterine pouch (of Douglas). It has been suggested that the merging of the respective fascia potentially limits the spread of malignancy from one adjacent organ to the other (Figure 7) [40].

7.1 Superior rectal artery

The inferior mesenteric artery (IMA) branches from the aorta at the level of L3 anterolaterally to the left. It gives off branches to the distal segments of the colon and its pelvic continuation is the superior rectal artery (SRA). The SRA commences at the level of the pelvic brim and crosses the left common iliac vessels with the inferior hypogastric nerves on both sides. It pierces the mesorectum and gives off branches to the rectosigmoid junction, an upper rectal branch which then divides into right and left terminal branches (Figure 8) [41]. At the level of S3 the SRA enters the upper mesorectum and the right and left terminal branches that descend posterolaterally [1]. The right terminal branch of SRA is usually larger than the left and is considered the true continuation of the SRA (Figure 8) [41]. The left terminal branch of SRA is considered a collateral branch of IMA and supplies the anterior surface of the rectum [41]. The perforating arteries of the SRA branches enter the rectal submucosa and anastomose with branches of the middle and inferior rectal arteries. The SRA is the main artery that supplies the rectum and contributes significantly to the arterial supply of the internal hemorrhoids [26] The branches of SRA terminate usually above the anal valves [41]. The SRA also supplies two muscles of the anal sphincter complex i.e., the IAS and CLM. The SRA is closely associated with lymph nodes serving as the primary pathway for lymphatic drainage from the rectum and upper anal canal.

7.2 Middle rectal artery

The prevalence rate of the middle rectal artery (MRA) ranges from 12 to 97% [42]. The MRA is a direct branch off the anterior division of the internal iliac artery (Figure 8). The MRA is thought to provide the anatomical basis for lateral spread of rectal cancer [42]. The blood supply territories of the MRA are not clear.

The MRA is thought to contribute to the supply of the lower rectum and anal canal and thought to provide a collateral pathway with SRA and IRA. This thought process comes from the fact that the MRA perfuses the rectal stump significantly post ligation of the SRA [43]. Careful consideration should be taken when near the vicinity of the MRA to reduce risk of bleeding and consequently impairment of the surgical field [43]. The MRA perforates the inferior hypogastric plexus and the mesorectal fascia laterally.

Heinze et al., aimed to evaluate the anatomy of the middle rectal artery to facilitate surgical management. Out of 22 body donor dissections, they found the MRA to have originated from the internal pudendal artery in 45.5%, the gluteal artery in 22.7%, a common gluteal-pudendal trunk in 22.7%, or formed a trifurcation with these arteries in 9.1% of cases [42]. Where present, the MRA also gives off branches to the surrounding pelvic organs. The MRA perforating branches enter the rectal submucosa and anastomose with branches of the superior and inferior rectal arteries.

7.3 Inferior rectal artery

The inferior rectal artery (IRA) is the terminal branch of the internal pudendal artery. The internal pudendal arteries on the left and right sides enter the pudendal (Alcock’s canal) (Figure 8). The IRA then exits medially into the ischioanal fossae to supply the distal third of the rectum to supply the internal and external anal sphincters, anal canal, and perianal skin [26] There is bilateral communication between the pair of IRAs which leaves a watershed area in the dorso-caudal area of the rectal ampulla [26, 44, 45]. The watershed area potentially provides an anatomical rationale as to why anastomotic leaks are observed in the dorso-caudal rectal ampulla post low anterior resection of the rectum [44, 45].

Similar to the SRA and MRA, the IRA perforating branches enter the rectal submucosa and anastomose with branches of the middle and superior rectal arteries. Klosterhalfen et al. [45], outlined two main anatomical variants of the IRA. The first variant of the IRA gives no branches to the posterior commissure of the anal canal, whilst the second variant which is less common, supplies the posterior commissure. The authors concluded that where there is less perfusion to the posterior commissure (as seen in the first type of variant), makes the region more prone to ischemia and slower at healing if an injury such as anal fissure occurs [45].

8.1 Rectal venous plexus

The rectal venous plexus surrounds the rectum and has direct connections to local venous plexuses such as the vesical venous plexus in males and uterovaginal venous plexus in females [26]. The rectal venous plexus is divided into 2 parts based on the orientation relative to the histological layers of the rectum. The internal veins are found deep to the mucosa of the rectum and upper anal canal [26]. The external veins are found outside the muscularis layer.

8.2 Superior rectal vein

The unilateral superior rectal vein (SRV) drains the upper two-thirds of the rectum and the upper anal. The SRV eventually drains into the portal system via the inferior mesenteric vein. The superior rectal vein forms from the rectal plexus. The SRV runs along the upper mesorectum and the root of the sigmoid mesocolon. The SRV runs to the left of the SRA [26]. The SRV continues as the inferior mesenteric vein at the pelvic brim once the SRV crosses the left common iliac vessels. The pelvic brim is also the site where the abdominal ureter becomes the pelvic ureter. This close relationship is clinically significant because procedures involving the ligation or manipulation of the SRV, may pose a risk of inadvertent injury to the ureter.

8.3 Middle and inferior rectal veins

The lower third of the rectum and lower anal canal are drained by the middle and inferior rectal veins which enter the systemic venous circulation via the internal iliac veins. The venous drainage of the rectum provides several areas of hematogenous metastasis of cancer. For example, although rare, tumors in the lower rectum and anal canal can first metastasize to the lungs via systemic circulation without the involvement of hepatic metastases [44, 46]. In addition, the connection between systemic veins and the portal venous system provides a site for development of varicosities due to portal hypertension secondary to advanced liver disease.

9.1 Visceral afferents

The visceral afferent pathway for most of the hindgut corresponds to the levels of the sympathetic motor division. While most of the hindgut afferents follow the sympathetic pathway, the rectum’s location below the peritoneal reflection (i.e., pelvic pain line), means that majority of its visceral afferents may travel via parasympathetic routes typically associated with pelvic viscera below the pelvic pain line. However, although minor, the upper aspect of the rectum which straddles the pelvic pain line, may have its visceral afferents follow the levels of sympathetic motor division. This may account for the blended sensory input seen in rectal distension and pain, where both noxious and innocuous stimuli are processed across different spinal levels [47, 48]. The convergence of these afferents within the spinal cord may explain the broad and poorly localized nature of rectal pain, as well as its tendency to mimic somatic or pelvic pathologies. Thus, afferent signals from the hindgut are received by afferent neurons from L1–L3 (via lumbar splanchnic nerves) and S2–S4 (via pelvic splanchnic nerves) dorsal root ganglia (Figure 9) [47, 48].

Visceral afferents in the rectum convey the sensation of rectal filling and are involved in reflex propulsive activity - a coordinated effort between the rectum, relaxation of pelvic floor, and anal sphincter complex that pushes feces through the rectum to the anal canal, into the external environment [26]. Feng and Guo [49] suggested that the lumbar splanchnic nerves predominantly carry high-threshold nociceptive signals, likely involved in detecting significant mechanical stress (e.g., distension) in the rectum and adjacent areas. They also suggested that the sacral nerves, in contrast, encode both nociceptive and innocuous sensations, which can contribute to a more complex pain presentation [48]. The diffuse nature of visceral and referred pain makes it challenging to pinpoint its exact source without additional diagnostic methods. Furthermore, the complex referral pattern of pain from the rectum (proctalgia fugax) and the overlap with innervation of adjacent structures in the pelvis might mimic conditions like lower back pain, pelvic pain syndromes, gynecological conditions, and/or biopsychosocial factors and should be considered accordingly.

9.2 Sympathetic motor innervation

The sympathetic preganglionic motor portion is conveyed from the lumbar splanchnic nerves (L1-L2/3) via the sacral splanchnic nerves (S1-S2). The cell bodies of the preganglionic sympathetic motor neurons originate from the intermediolateral columns L1-L2/3 and synapse in the inferior mesenteric plexuses. The preganglionic neurons also reach the sacral splanchnic nerves which then go onto to contributing to the superior (aka presacral nerve) and inferior hypogastric plexuses (Figure 9). The postganglionic neurons reach the rectal wall via neural networks that are intertwined along the corresponding arteries. The postganglionic sympathetic neurons from both sources release noradrenaline (norepinephrine) and thus, sympathetic innervation of the rectum results in inhibition of peristalsis by acting on the muscularis layer of the rectum [26].

9.3 Parasympathetic motor innervation

The hindgut receives parasympathetic motor innervation from the pelvic splanchnic nerves (S2-S4). The cell bodies of the preganglionic neurons originate from the lateral intermediate substance, found in S2, S3 and S4 segments of the spinal cord (i.e., the sacral parasympathetic nucleus). The preganglionic axons enter the inferior hypogastric plexus where they will either go directly to the rectum or ascend via the right and left hypogastric nerves to the superior hypogastric plexus to innervate some proximal segments of the hindgut or innervate adjacent pelvic organs (Figure 9). Parasympathetic stimulation via the release of acetylcholine causes colonic propulsion and the relaxation of the internal anal sphincter, both functions essential in defecation [26].

9.4 Somatic innervation of the anal canal

Of significance to pain perception and patient presentations of anorectal pathologies, is the change at the pectinate line in innervation from visceral afferent via superior rectal nerve (proximally) to somatic afferents via the pudendal (pelvic) nerve (distally) (Figure 9). The pudendal nerve also provides somatic motor innervation to the EAS.