1 Surgical Anatomy of the Colon, Rectum, and Anus
Abstract
This chapter will review the surgical anatomy of the colon and rectum, including general configuration, course relations, peritoneal coverings, and ileocecal valve in the colon, and peritoneal relations, fascial attachments, fascia propria, Waldeyer’s fascia, Denonvilliers’ fascia, lateral ligaments of the rectum, histology, muscles of the anorectal region, anorectal spaces, arterial supply, venous drainage, lymphatic drainage, and innervation.
1.1 Introduction
Although often thought of as a single organ, the colon is embryologically divisible into two parts. The transverse and right colon are derived from the midgut and are supplied by the superior mesenteric artery, while the distal half of the colon is derived from the hindgut and receives blood from the inferior mesenteric artery.
The large bowel begins in the right lower quadrant of the abdomen as a blind pouch known as the cecum. The ileum empties into the medial and posterior aspect of the intestine, a point known as the ileocecal junction. The colon proceeds upward and in its course is designated according to location as: ascending (right) colon, hepatic flexure, transverse colon, splenic flexure, descending (left) colon, sigmoid colon, rectum, and anal canal. The colon is approximately 150 cm long, and its diameter gradually diminishes from the cecum to the rectosigmoid junction, where it widens as the rectal ampulla, only to narrow again as the anal canal.
1.2 Colon
1.2.1 General Configuration
The colon differs from the small bowel in that it is characterized by a saccular or haustral appearance, it contains three taenia bands, and it has appendices epiploicae, a series of fatty appendages located adjacent to the tenae on surface of the colon. The taeniae are thickened bands of longitudinal muscle running along the colon from the base of the appendix. They merge in the distal sigmoid colon, where the longitudinal fibers continue through the entire length of the rectum. A study by Fraser et al 1 demonstrated that the longitudinal muscle forms a complete coat around the colon but is much thicker at the taeniae. The three taenia bands are named according to their relation to the transverse colon: taenia mesocolica, which is attached to the mesocolon or mesentery; taenia omentalis, which is attached to the greater omentum; and taenia libera, which has no attachment. These bands are about one-sixth shorter than the intestine and are believed to be responsible for the sacculations. 2 The transition from the sigmoid colon to the rectum is a gradual one. It is characterized by the taeniae coli spreading out from three distinct bands to a uniformly distributed layer of longitudinal smooth muscle that is thicker on the front and back than on each side, the loss of appendices epiploicae, and change in diameter.
1.2.2 Course and Alterations
The general topography of the colon varies from person to person, and such differences should be taken into account while reading the following discussion (▶ Fig. 1.1).
The vermiform appendix projects from the lowermost part of the cecum. From the ileocecal junction, the colon ascends on the right in front of the quadratus lumborum and transversus abdominis muscles to a level overlying the lower pole of the right kidney, a distance of about 20 cm. It is invested by peritoneum on its anterior, lateral, and medial surfaces. Superior to the colon is the undersurface of the right lobe of the liver, lateral to the gallbladder, and here the colon angulates acutely medially, downward, and forward, forming the hepatic flexure. Occasionally, there is a filmy web of adhesions extending from the right abdominal wall to the anterior taenia of the ascending colon, and this has been referred to as Jackson’s membrane.
The transverse colon is the longest (40–50 cm) segment of colon, extending from the hepatic to the splenic flexure. It is usually mobile and may descend to the level of the iliac crests or even dip into the pelvis. The transverse colon is enveloped between layers of the transverse mesocolon, the root of which overlies the right kidney, the second portion of the duodenum, the pancreas, and the left kidney. It contains the middle colic artery, branches of the right and left colic arteries, and accompanying veins, lymphatic structures, and autonomic nerve plexuses. This posterior relationship is of paramount importance because these structures are subject to injury during a right hemicolectomy if proper care is not exercised. In the left upper quadrant of the abdomen, the colon is attached to the undersurface of the diaphragm at the level of the 10th and 11th ribs by the phrenocolic ligament. The distal transverse colon lies in front of the proximal descending colon. The stomach is immediately above and the spleen is to the left. The greater omentum descends from the greater curvature of the stomach in front of the transverse colon and ascends to the upper surface of the transverse colon. The splenic flexure describes an acute angle, is high in the left upper quadrant, and therefore is less accessible to operative approach. It lies anterior to the midportion of the left kidney.
The descending colon passes along the posterior abdominal wall over the lateral border of the left kidney, turns somewhat medially, and descends in the groove between the psoas and the quadratus lumborum muscles to its junction with the sigmoid at the level of the pelvic brim and the transversus abdominis muscle. 3 , 4 Its length averages 30 cm. The anterior, medial, and lateral portions of its circumference are covered by peritoneum. The distal portion of the descending colon is usually attached by adhesions to the posterior abdominal wall, and these adhesions require division during mobilization of this portion of the colon.
The sigmoid colon extends from the pelvic brim to the sacral promontory, where it continues as the rectum. Its length varies dramatically from 15 to 50 cm, and it may follow an extremely tortuous and variable course. It often loops to the left but may follow a straight oblique course, loop to the right, or ascend high into the abdomen. It has a generous mesentery and is extremely mobile. The serosal surface has numerous appendices epiploicae. The base of the mesocolon extends from the iliac fossa, along the pelvic brim, and across the sacroiliac joint to the second or third sacral segment; in so doing, it forms an inverted V. Contained within the mesosigmoid are the sigmoidal and superior rectal arteries and accompanying veins, lymphatics, and autonomic nerve plexuses. At the base of the mesosigmoid is a recess, the intersigmoid fossa, which serves as a valuable guide to the left ureter, lying just deep to it. The upper limb runs medially and upward, crossing the left ureter and iliac vessels; this is an extremely important relationship during resection of this part of the colon. The lower limb extends in front of the sacrum and also may be alongside loops of small bowel, the urinary bladder, and the uterus and its adnexa.
1.2.3 Peritoneal Coverings
The antimesenteric border of the distal ileum may be attached to the parietal peritoneum by a membrane (Lane’s membrane). 5 The cecum usually is entirely enveloped by peritoneum. The ascending colon is attached to the posterior body wall and is devoid of peritoneum in its posterior surface; thus, it does not have a mesentery. The transverse colon is invested with peritoneum. Its posterosuperior surface, along the taenia band, is attached by the transverse mesocolon to the lower border of the pancreas. The posterior and inferior layers of the greater omentum are fused on the anterosuperior aspect of the transverse colon. To mobilize the greater omentum or to enter the lesser sac, the fusion of the omentum to the transverse colon must be dissected. Because the omental bursa becomes obliterated caudal to the transverse colon and toward the right side, the dissection should be started on the left side of the transverse colon. Topor et al 6 studied 45 cadavers to elucidate surgical aspects of omental mobilization, lengthening, and transposition into the pelvic cavity. They identified that the most important anatomic variables for omental transposition were three variants of arterial blood supply: (1) in 56% of patients, there is one right, one (or two) middle, and one left omental artery; (2) in 26% of patients, the middle omental artery is absent; and (3) in the remaining 18% of patients, the gastroepiploic artery is continued as a left omental artery but with various smaller connections to the right or middle omental artery. The first stage of omental lengthening is detachment of the omentum from the transverse colon mesentery. The second stage is the actual lengthening of the omentum. The third stage is placement of the omental flap into the pelvis. The left colonic flexure is attached to the diaphragm by the phrenocolic ligament, which also forms a shelf for supporting the spleen. The descending colon is devoid of peritoneum posteriorly, where it is in contact with the posterior abdominal wall and thus has no mesentery.
The sigmoid colon begins at about the level of the pelvic brim and is completely covered with peritoneum. The posterior surface is attached by a fan-shaped mesentery. The lateral surface of the sigmoid mesentery is fused to the parietal peritoneum of the lateral abdominal wall and is generally known as the “white line of Toldt.” Mobilization of the sigmoid colon requires cutting or incising the lateral peritoneal reflection. The sigmoid colon varies greatly in length and configuration.
1.2.4 Ileocecal Valve
The superior and inferior ileocecal ligaments are fibrous tissue that helps maintain the angulation between the ileum and the cecum. Kumar and Phillips 7 found these structures to be important in the maintenance of competence against reflux at the ileocecal junction. In an autopsy evaluation, the ascending colon was filled with saline solution by retrograde flow, and in 12 of 14 cases the ileocecal junctions were competent to pressures up to 80 mm Hg. Removal of mucosa at the ileocecal junction or a strip of circular muscle did not impair competence to pressures above 40 mm Hg, but division of the superior and inferior ileocecal ligaments rendered the junction incompetent. Operative reconstruction of the ileocecal angle restored competence. It therefore appears that the angulation between the ileum and the cecum determines continence.
1.3 Rectum
Although anatomists traditionally assign the origin of the rectum to the level of the third sacral vertebra, surgeons generally consider the rectum to begin at the level of the sacral promontory. It descends along the curvature of the sacrum and coccyx and ends by passing through the levator ani muscles, at which level it abruptly turns downward and backward to become the anal canal. The rectum differs from the colon in that the outer layer is entirely longitudinal muscle, characterized by the merging of the three taenia bands. It measures 12 to 15 cm in length and lacks a mesentery, sacculations, and appendices epiploicae. These definitions may evolve as magnetic resonance imaging (MRI) is increasingly utilized to define rectal anatomy.
The rectum describes three lateral curves: the upper and lower curves are convex to the right, and the middle is convex to the left. On their inner aspect, these infoldings into the lumen are known as the valves of Houston. 8 , 9 About 46% of normal persons have three valves, 33% have two valves, 10% have four valves, 2% have none, and the rest have from five to seven valves. 9 The clinical significance of the valves of Houston is that they must be negotiated during successful proctosigmoidoscopic examination and, more importantly, that they are an excellent location for a rectal biopsy, because the inward protrusion makes an easy target. They do not contain all the layers of the bowel wall, and therefore biopsy in this location carries a minimal risk of perforation. The middle fold is the internal landmark corresponding to the anterior peritoneal reflection. Consequently, extra caution must be exercised in removing polyps above this level. Because of its curves, the rectum can gain 5 cm in length when it is straightened (as in performing a low anterior resection); hence, a lesion that initially appears at 7 cm from the anal verge is often found 12 cm from that site after complete mobilization.
In its course, the rectum is related posteriorly to the sacrum, coccyx, levator ani muscles, coccygeal muscles, median sacral vessels, and roots of the sacral nerve plexus. Anteriorly in the male, the extraperitoneal rectum is related to the prostate, seminal vesicles, vasa deferentia, ureters, and urinary bladder; the intraperitoneal rectum may come in contact with loops of the small bowel and sigmoid colon. In the female, the extraperitoneal rectum lies behind the posterior vaginal wall; the intraperitoneal rectum may be related to the upper part of the vagina, uterus, fallopian tubes, ovaries, small bowel, and sigmoid colon. Laterally above the peritoneal reflection, there may be loops of small bowel, adnexa, and sigmoid colon. Below the reflection, the rectum is separated from the sidewall of the pelvis by the ureter and iliac vessels.
1.3.1 Peritoneal Relations
For descriptive purposes, the rectum is divided into upper, middle, and lower thirds. The upper third is covered by peritoneum anteriorly and laterally, the middle third is covered only anteriorly, and the lower third is devoid of peritoneum. The peritoneal reflection shows considerable variation between individuals and between men and women. In men, it is usually 7 to 9 cm from the anal verge, while in women it is 5 to 7.5 cm above the anal verge. The middle valve of Houston roughly corresponds to the anterior peritoneal reflection. The posterior peritoneal reflection is usually 12 to 15 cm from the anal verge (▶ Fig. 1.2).
The location of the peritoneal reflection has not been extensively studied in living patients. Najarian et al 10 investigated the location of the peritoneal reflection in 50 patients undergoing laparotomy. The distance from the anal verge to the peritoneal reflection was measured in each patient via simultaneous intraoperative proctoscopy and intra-abdominal visualization of the peritoneal reflection. The mean lengths of the peritoneal reflection were 9 cm anteriorly, 12.2 cm laterally, and 14.8 cm posteriorly for females, and 9.7 cm anteriorly, 12.8 cm laterally, and 15.5 cm posteriorly for males. The lengths of the anterior, lateral, and posterior peritoneal measurements were statistically different from one another, regardless of gender. These authors’ data indicated that the peritoneal reflection is located higher on the rectum than reported in autopsy studies, and that there is no difference between males and females. Knowledge of the location and position of a rectal carcinoma in relationship to the peritoneal reflection will help the surgeon optimize the use of peranal techniques of resection.
1.3.2 Fascial Attachments
The posterior part of the rectum, the distal lateral two-thirds, and the anterior one-third are devoid of peritoneum, but they are covered with a thin layer of pelvic fascia, called fascia propria or the investing fascia. At the level of the rectal hiatus, the levator ani is covered by an expansion of the pelvic fascia, which on reaching the rectal wall divides into an ascending component, which fuses with the fascia propria of the rectum, and a descending component, which interposes itself between the muscular coats forming the conjoint longitudinal coat. 11 These fibroelastic fibers run downward to reach the dermis of the perianal skin and split the subcutaneous striated sphincter into 8 to 12 discrete muscle bundles.
The sacrum and coccyx are covered with a strong fascia that is part of the parietal pelvic fascia. Known as Waldeyer’s fascia, this presacral fascia covers the median sacral vessels. The rectosacral fascia is the Waldeyer’s fascia from the periosteum of the fourth sacral segment to the posterior wall of the rectum. 12 , 13 It is found in 97% of cadaver dissections. 13 Waldeyer’s fascia contains branches of sacral splanchnic nerves that arise directly from the sacral sympathetic ganglion and may contain branches of the lateral and median sacral vessels. This fascia should be sharply divided with scissors or electrocautery for full mobilization of the rectum (▶ Fig. 1.3). The posterior space below the rectosacral fascia is the supralevator or retrorectal space (▶ Fig. 1.4).
Anteriorly, the extraperitoneal portion of the rectum is covered with a visceral pelvic fascia, the fascia propria, or investing fascia. Anterior to the fascia propria is a filmy delicate layer of connective tissue known as Denonvilliers’ fascia. 14 It separates the rectum from the seminal vesicles and the prostate or vagina (▶ Fig. 1.5). Denonvilliers’ fascia has no macroscopically discernible layers. Histologically, it is composed of dense collagen, smooth muscle fibers, and coarse elastic fibers. 15 , 16 Its attachments have been surrounded by confusion and debates. Some authors believe it is adherent to the rectum, 16 , 17 , 18 , 19 while others note that it is applied to the seminal vesicles and the prostate. 15 , 20 , 21 , 22
Lindsey et al 23 designed a study to evaluate the anatomic relation of Denonvilliers’ fascia: whether it is attached on the anterior fascia propria of the rectum, or on the seminal vesicles and prostate. They prospectively collected 30 specimens from males undergoing total mesorectal excision for mid and low rectal carcinoma, with a deep dissection of the anterior extraperitoneal rectum to the pelvic floor. The anterior aspects of the extraperitoneal rectal sections were examined microscopically for the presence or absence of Denonvilliers’ fascia. In patients in whom the carcinoma was anterior, 55% of the specimens had Denonvillier’s fascia present. Conversely, when the anterior rectum was not involved with carcinoma 90% of the specimens contained no Denonvilliers’ fascia. The authors concluded that “when rectal dissection is conducted on fascia propria in the anatomic plane, Denonvilliers’ fascia remains on the posterior aspect of the prostate and seminal vesicles. Denonvilliers’ fascia lies anterior to the anatomic fascia propria plane of anterior rectal dissection in total mesorectal excision (TME) and is more closely applied to the prostate than the rectum.” This study has put the debates to rest; Denonvilliers’ fascia is more closely applied to the seminal vesicles and the prostate than the rectum.
1.3.3 Lateral Ligament
The distal rectum, which is extraperitoneal, is attached to the pelvic sidewall on each side by the pelvic plexus, connective tissues, and middle rectal artery (if present). 24 Histologically, it consists of nerve structures, fatty tissue, and small blood vessels. 25 Recently, the anatomical term lateral ligament has been a subject of debate. In dissection of 27 fresh cadavers and 5 embalmed pelves, Nano et al 26 found that lateral ligaments were extension of the mesorectum to the lateral endopelvic fascia. From their experience with anatomic dissection applied to surgery, several conclusions were drawn: lateral ligaments are extensions of the mesorectum and must be cut at their attachment at the endopelvic fascia for TME to take place.
Lateral ligaments contain fatty tissue in communication with mesorectal fat and possibly some vessels and nerve filaments that are of little importance. Insertion of lateral ligaments at the endopelvic fascia is placed under the urogenital bundle. The middle rectal artery courses anteriorly and inferiorly with respect to the lateral ligament. Lateral ligaments can be cut at their insertion on the endopelvic fascia without injuring the urogenital nervous bundle, which, however, should be kept in view during this procedure, because it crosses the middle rectal artery and fans out behind the seminal vesicles. The lateral aspect of the rectum receives the lateral pedicle, which consists of the nervi recti and the middle rectal artery.
A study by Rutegård et al 25 on 10 patients who underwent total mesorectal excision for rectal carcinoma revealed that “the often thin structures, usually referred to as the lateral ligaments, seem to arise from the pelvic plexuses and bridge over into the mesorectum … they can be identified in almost every patient.” The authors contend that the lateral ligaments are real anatomical findings. This finding was supported by Sato and Sato 13 in the dissection of 45 cadavers.
Conversely, Jones et al 24 meticulously dissected 28 cadaveric pelves; they found insubstantial thin strands of connective tissues traversing the space between the mesorectum and the pelvic sidewall. These strands of connective tissues were no different from those one would expect to find in any areolar plane. They were often absent altogether. The pelvic plexuses were distinct from the middle rectal artery (if present) and had no association with the connective tissues. Jones et al 24 believed that the lateral ligament was nothing more than a surgical artifact that results from injudicious dissection.
When the rectum is pulled medially, the complex of middle rectal artery and vein, the splanchnic nerves, and their accompanying connective tissues form a bandlike structure extending from the lateral pelvic wall to the rectum. 27 This structure was most likely mistaken as the “lateral ligament” in the past. Whatever one would call the lateral attachment of the low rectum, the tissues need to be divided in full mobilization of the rectum.
1.3.4 Mesorectum
The posterior rectum is devoid of peritoneum and has no mesorectum. The term mesorectum is a misnomer and does not appear in the Nomina Anatomica, although it is listed in the Nomina Embryologica. 28 The word mesorectum was possibly first used by Maunsell in 1892 and later popularized by Heald of the United Kingdom. 28 In answering the critique of using this word, Heald answered, “… it was a surgical word used by the foremost of my surgical teachers when I was a young registrar. Mr. Rex Lawrice of Guy’s Hospital used to describe the process of dividing the mesorectum as was well described in Rob and Smith’s textbook of surgery at that time … no other word seems readily available to describe it.” 29
Total mesorectal excision implies the complete excision of all fat enclosed within the fascia propria, which Heald calls the “mesorectum.” This dissection is performed in a circumferential manner down to the levator muscles. 28 Bisset et al 30 preferred the term “extrafascial excision of the rectum.” The term mesorectum has now been used worldwide and appears well entrenched.
Canessa et al 31 studied the lymph nodes in 20 cadavers using conventional manual dissection. The starting point was at the bifurcation of the superior rectal artery and ending at the anorectal ring. They found an average of 8.4 lymph nodes per rectum; 71% of the lymph nodes were above the peritoneal reflection and 29% were below it. Dissection of seven fresh cadavers on the mesorectum by Topor et al 32 yielded 174 lymph nodes; over 80% of the lymph nodes were smaller than 3 mm. Fifty-six percent of the nodes were located in the posterior mesentery, and most were located in the upper two-thirds of posterior rectal mesentery. The translational importance of this information is attested to by the need to perform a complete or near complete as compared to incomplete total mesorectal excision (TME) when performing a curative protectomy for rectal cancer.
1.4 Histology
Knowledge of the microscopic anatomy of the large intestine is of paramount importance in understanding the various disease processes. This is especially true in the case of neoplasia, where the depth of penetration will dictate the treatment recommendation.
The innermost layer is the mucosa, which is composed of three divisions. The first is a layer of columnar epithelial cells with a series of crevices or crypts characterized by straight tubules that lie parallel and close to one another and do not branch (glands of Lieberkühn). The surface epithelium around the openings of the crypts consists of simple columnar cells with occasional goblet cells. The tubules are lined predominantly by goblet cells, except at the base of the crypts where undifferentiated cells as well as enterochromaffin and amine precursor uptake and decarboxylation (APUD) cells are found. The epithelial layer is separated from the underlying connective tissue by an extracellular membrane composed of glycopolysaccharides and seen as the lamina densa of the basement membrane when viewed by electron microscopy. 33 Abnormalities classified as defects, multilayering, or other structural abnormalities have been reported in many types of neoplasms, including those of the colon and rectum. These abnormalities are more common in malignant than in benign neoplasms. The second division of the mucosa is the lamina propria, composed of a stroma of connective tissue containing capillaries, inflammatory cells, and lymphoid follicles that are more prominent in young persons. The third division is the muscularis mucosa, a fine sheet of smooth muscle fibers that serves as a critical demarcation in the diagnosis of invasive carcinoma and includes a network of lymphatics. 34
Beneath the muscularis mucosa is the submucosa, a layer of connective tissue and collagen that contains vessels, lymphatics, and Meissner’s plexus. It is the strongest layer of the bowel. The next layer is the circular muscle, which is a continuous sheath around the bowel, including both the colon and the rectum. On the external surface of the circular muscle are clusters of ganglion cells and their ramifications; these make up the myenteric plexus of Auerbach. Unmyelinated postganglionic fibers penetrate the muscle to communicate with the submucosal plexus. The outer or longitudinal muscle fibers of the colon are characteristically collected into three bundles, called the taeniae coli; however, in the rectum these fibers are spread out and form a continuous layer. The muscularis propria is pierced at regular intervals by the main arterial blood supply and venous drainage of the mucosa.
The outermost layer, which is absent in the lower portions of the rectum, is the serosa or visceral peritoneum. This layer contains blood vessels and lymphatics.
1.5 Anal Canal
The anal canal is the terminal portion of the intestinal tract. It begins at the anorectal junction (the point passing through the levator ani muscles), is about 4 cm long, and terminates at the anal verge. 35 , 36 This definition differs from that of the anatomist, who designates the anal canal as the part of the intestinal tract that extends from the dentate line to the anal verge.
The anal canal is surrounded by strong muscles, and because of tonic contraction of these muscles, it is completely collapsed and represents an anteroposterior slit. The musculature of the anorectal region may be regarded as two tubes, one surrounding the other (▶ Fig. 1.6). 37 The inner tube, being visceral, is smooth muscle and is innervated by the autonomic nervous system, while the outer funnel-shaped tube is skeletal muscle and has somatic innervation. This short segment of the intestinal tract is of paramount importance because it is essential to the mechanism of fecal continence and also because it is prone to many diseases.
The anatomy of the anal canal and perianal structures has been imaged using endoluminal magnetic resonance imaging. 38 Investigators found that the lateral canal was significantly longer than its anterior and posterior part. The anterior external anal sphincter was shorter in women than in men and occupied, respectively, 30 and 38% of the anal canal length. The median length and thickness of the female anterior external anal sphincter were 11 and 13 mm, respectively. These small dimensions explain why a relatively small obstetrical tear may have a devastating effect on fecal continence and why it may be difficult to identify the muscle while performing a sphincter repair after an obstetrical injury. The caudal ends of the external anal sphincter formed a double layer. The perineal body was thicker in women than in men and easier to define. The superficial transverse muscles had a lateral and caudal extension to the ischiopubic bones. The bulbospongiosus was thicker in men than in women. The ischiocavernosus and anococcygeal body had the same dimensions in both sexes.
Posteriorly, the anal canal is related to its surrounding muscle and the coccyx. Laterally is the ischioanal fossa with its inferior rectal vessels and nerves. Anteriorly in the male is the urethra, a very important relationship to know during abdominoperineal resection of the rectum. Anteriorly in the female are the perineal body and the lowest part of the posterior vaginal wall.
1.5.1 Lining of Canal
The lining of the anal canal consists of epithelium of different types at different levels (▶ Fig. 1.7). At approximately the midpoint of the anal canal, there is an undulating demarcation referred to as the dentate line. This line is approximately 2 cm from the anal verge. Because the rectum narrows into the anal canal, the tissue above the dentate line takes on a pleated appearance. These longitudinal folds, of which there are 6 to 14, are known as the columns of Morgagni. There is a small pocket or crypt at the lower end of and between adjacent columns of the folds. These crypts are of surgical significance because foreign material may become lodged in them, obstructing the ducts of the anal glands and possibly resulting in sepsis.
The mucosa of the upper anal canal is lined by columnar epithelium. Below the dentate line, the anal canal is lined with a squamous epithelium. The change, however, is not abrupt. For a distance of 6 to 12 mm above the dentate line, there is a gradual transition where columnar, transitional, or squamous epithelium may be found. This area, referred to as the anal transitional or cloacogenic zone, has extremely variable histology.
A color change in the epithelium is also noted. The rectal mucosa is pink, whereas the area just above the dentate line is deep purple or plum color due to the underlying internal hemorrhoidal plexus. Subepithelial tissue is loosely attached to and radially distensible from the internal hemorrhoidal plexus. Subepithelial tissue at the anal margin, which contains the external hemorrhoidal plexus, forms a lining that adheres firmly to the underlying tissue. At the level of the dentate line, the lining is anchored by what Parks 39 called the mucosal suspensory ligament. The perianal space is limited above by this ligament and below by the attachment of the longitudinal muscle to the skin of the anal verge. The area below the dentate line is not true skin because it is devoid of accessory skin structures (e.g., hair, sebaceous glands, and sweat glands). This pale, delicate, smooth, thin, and shiny stretched tissue is referred to as anoderm and runs for approximately 1.5 cm below the dentate line. At the anal verge, the lining becomes thicker and pigmented and acquires hair follicles, glands, and other histologic features of normal skin. 2 In this circumanal area, there is also a well-marked ring of apocrine glands, which may be the source of the clinical condition called hidradenitis suppurativa. Proximal to the dentate line, the epithelium is supplied by the autonomic nervous system, while distally the lining is richly innervated by the somatic nervous system. 40
1.5.2 Anal Transitional Zone
The anal transitional zone (ATZ) is interposed between uninterrupted colorectal-type mucosa (columnar) above and uninterrupted squamous epithelium (anoderm) below, irrespective of the type of epithelium present in the zone itself. 41 The ATZ usually commences just above the dentate line. Using computer maps of histology, Thompson-Fawcett et al 42 found that the dentate line was situated at a median of 1.05 cm above the lower border of the internal sphincter. This is much smaller than that reported in the study by Fenger, 41 which portrayed the ATZ extending 0.9cm above the dentate line. Fenger used the traditional alcian blue stain. This results in overestimation of the length of the ATZ because the pale blue staining is due to staining of superficial nuclei of both squamous anoderm and transitional epithelium rather than staining of mucin-producing cells in the transitional epithelium. 42 The ATZ is much smaller than commonly thought.
The histology of the ATZ is extremely variable. Most of the zone is covered by ATZ epithelium, which appears to be composed of four to nine cell layers—the basal cells, columnar, cuboidal, unkeratinized squamous epithelium, and anal glands. The ATZ epithelium contains a mixture of sulfomucin and sialomucin. The mucin pattern in the columnar variant of the ATZ epithelium and in the anal canal is of the same type and differs from that of colorectal-type epithelium. The findings of a similar mucin pattern in mucoepidermoid carcinoma and in some cases of carcinoma arising in anal fistulas as well as in carcinoma suspected of arising in anal glands might indicate a common origin of the neoplasm in the ATZ epithelium.
Histochemical study shows that endocrine cells have been demonstrated in 87% of specimens. Their function is unknown. Melanin is found in the basal layer of the ATZ epithelium in 14% of specimens. Melanin cannot be demonstrated in the anal gland but is a constant finding in the squamous epithelium below the dentate line, increasing in amount as the perianal skin is approached. The melanin-containing cells in the ATZ seem a reasonable point of origin for melanoma, as do the findings of junctional activity and atypical melanocyte hyperplasia in the ATZ.
The ATZ epithelium has a dominating diploid population, although there was a small hyperdiploid peak representing nuclei with a scattered volume considerably higher than that of the main diploid population. This was present regardless of the histologic variant (columnar or cuboid) of the ATZ epithelium. Tetraploid or octoploid populations are not found. 41
1.5.3 Anal Glands
The average number of glands in a normal anal canal is six (range, 3–10). 43 Each gland is lined by stratified columnar epithelium with mucus-secreting or goblet cells interspersed within the glandular epithelial lining and has a direct opening into an anal crypt at the dentate line. Occasionally, two glands open into the same crypt, while half the crypts have no communication with the glands. These glands were first described by Chiari in 1878. 44 The importance of their role in the pathogenesis of fistulous abscess was presented by Parks in 1961. 37
Seow-Choen and Ho 43 found that 80% of the anal glands are submucosal in extent, 8% extend to the internal sphincter, 8% to the conjoined longitudinal muscle, 2% to intersphincteric space, and 1% penetrate the external sphincter. The anal glands are fairly evenly distributed around the anal canal, although the greatest number is found at the anterior quadrant. Mild-to-moderate lymphocytic infiltration is noted around the anal glands and ducts; this is sometimes referred to as “anal tonsil.”
In an autopsy study of 62 specimens, Klosterhalfen et al 45 found that nearly 90% of specimens contained anal sinuses. In fetuses and children, more than half of the anal sinuses were accompanied by anal intramuscular glands penetrating the internal anal sphincter, while in adult specimens anal intramuscular glands were rare.
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