Embryologic considerations are important when considering the epithelium, innervation, vascular supply, and venous and lymphatic drainage of the anorectum. The rectum, endodermal in origin, is derived from the dorsal component of the cloaca, which is partitioned by the anorectal septum. The anal canal arises from an invagination of ectodermal tissue. The anorectum develops in the 8-week embryo from the fusion of the rectum and the anal canal, which occurs when the anal membrane ruptures. The dentate line marks the point of fusion and the transition from endodermal to ectodermal tissue.

The rectum is approximately 12-15 cm long. It extends from the rectosigmoid junction, marked by the fusion of the tenia, to the anal canal, marked by the passage into the pelvic floor musculature. The rectum lies in the sacrum and has three distinct curves resulting in folds that are visible on endoscopy and known as the valves of Houston. The proximal and distal curves are convex to the left, and the middle curve is convex to the right. The middle curve roughly marks the anterior peritoneal reflection, which is generally 6-8 cm above the anus.

Viewed from the abdomen, the rectum is seen to gradually transition from intraperitoneal to extraperitoneal beginning posteriorly at 12-15 cm from the anus. It becomes completely extraperitoneal at 6-8 cm from the anus. The rectum is “fixed” posteriorly, laterally, and anteriorly by the presacral or Waldeyer’s fascia, the lateral ligaments, and Denonvilliers’ fascia, respectively.

The anatomic anal canal starts at the dentate line, the junction of colorectal mucosa and anal mucosa, and ends at the anal verge, the junction of the anal mucosa with the perianal skin. However, the surgical anal canal is more clinically relevant. It extends from the muscular diaphragm of the pelvic floor to the anal verge.

 

 

The anal canal is a 3- to 4-cm-long collapsed slit that tilts posteriorly from the anal verge. The anal canal is “supported” by the surrounding anal sphincter mechanism, composed of the internal and external sphincters. The internal sphincter is a specialized continuation of the circular muscle of the rectum. It is an involuntary muscle that is normally contracted at rest. The structure and function of the external sphincter are controversial; however current evidence suggests that the external sphincter is the spout on a muscular funnel of continuous circumferential functional muscle mass that includes the external sphincter caudally and extends cranially to the conical puborectalis and levator ani muscles. The external sphincter is composed of voluntary striated muscle. The conjoined longitudinal muscle separates the internal and external sphincter. It is created by the aggregation of fibers from the longitudinal muscle of the rectum, fibers from the levator ani, and fibers from the puborectalis. Some fibers from this muscle become the corrugator cutis ani and insert on the perianal skin creating rugal folds and a puckered appearance. Other fibers traverse the internal sphincter and support the internal hemorrhoids as the mucosal suspensory ligaments.

The histologies of the rectum and anus are distinct. The rectum is composed of an innermost layer of mucosa that overlies the submucosa, the circular and longitudinal muscles, and in the proximal rectum, serosa. The mucosa is subdivided into three layers: (1) epithelial cells, (2) lamina propria, and (3) muscularis mucosa. The muscularis mucosa is a fine sheet of muscle containing a network of lymphatics. Lymphatics are essentially absent above this level, making the muscularis mucosa critical in defining metastatic potential of malignancies.

As the rectum enters the narrow musculature of the pelvic floor and becomes the anal canal, the tissue is thrown into folds known as the columns of Morgagni. At the lower end of the columns lie small pockets called crypts, some of which communicate with anal glands lying in or near the intersphincteric plane. The epithelium of the anal canal is composed of three types: colorectal mucosa is present in the proximal 2-3 cm; transitional epithelium is at and just above the dentate line; and anoderm, a squamous mucosa, is below the dentate line. The anoderm is a squamous mucosa rich in nerve fibers but lacking in secondary appendages (hair follicles, apocrine glands, and sweat glands). The anal verge marks the true mucocutaneous junction.

The pelvic floor is a consortium of funnel-shaped muscles that separates the pelvis and the perineum. It is composed of the levator ani and puborectalis muscles. The levator ani consists of two broad, thin, symmetric muscular sheets that originate around the pelvic sidewall and in the sacrospinous ligament that forms the principal support of the pelvic viscera. The puborectalis muscle originates on the posterior aspect of the pubis, forms a sling around the rectum, and returns to the posterior aspect of the pubis. The fibers of the puborectalis are situated immediately adjacent to and below the innermost component of the levator ani muscle, where they are intimately associated with the upper posterolateral fibers of the deep external anal sphincter. Thus, the puborectalis serves as a bridge between the broad sheet-like component of the funnel created by the levators and the narrow spout of the funnel created by the external anal sphincter. The puborectalis in the contracted state is responsible for the normal acute anorectal angle between the levators and the external sphincters. It is also responsible for the shelf that is normally palpable on digital examination as one passes from the distal narrow lumen of the “anus” to the more proximal capacious lumen of the “rectum.”

The innervation of the rectum is via the sympathetic and parasympathetic nervous systems. The sympathetic nerves originate from the lumbar segments L1-3, form the inferior mesenteric plexus, travel through the superior hypogastric plexus, and descend as the hypogastric nerves to the pelvic plexus.

The parasympathetic nerves arise from the second, third, and fourth sacral roots and join the hypogastric nerves anterior and lateral to the rectum to form the pelvic plexus, from which fibers pass to form the periprostatic plexus. Sympathetic and parasympathetic fibers pass from the pelvic and periprostatic plexi to the rectum and internal anal sphincter as well as the prostate, bladder, and penis. Injury to these nerves or plexi can lead to impotence, bladder dysfunction, and loss of normal defecatory mechanisms.

The internal anal sphincter is innervated with sympathetic and parasympathetic fibers. Both are inhibitory and keep the sphincter in a constant state of contraction. The external sphincters are skeletal muscles innervated by the pudendal nerve with fibers that originate from S2-4. Above the dentate line, noxious stimuli are experienced as ill-defined dull sensations conducted through afferent fibers of the parasympathetic nerves. Below the dentate line, the epithelium is exquisitely sensitive. Cutaneous sensations of heat, cold, pain, and touch are conveyed through the inferior rectal and perineal branches of the pudendal nerve.

The arterial supply of the anorectum is via the superior, middle, and inferior rectal arteries. The superior rectal artery is the terminal branch of the inferior mesenteric artery and descends in the mesorectum. It supplies the upper and middle rectum. The middle rectal arteries arise from the internal iliac arteries and enter the rectum anterolaterally at the level of the pelvic floor musculature. They supply the lower two-thirds of the rectum. Collaterals exist between the middle and superior rectal arteries. The inferior rectal arteries, branches of the internal pudendal arteries, enter posterolaterally, do not anastomose with the blood supply to the middle rectum, and provide blood supply to the anal sphincters and epithelium.

The venous drainage of the anorectum is via the superior, middle, and inferior rectal veins draining into the portal and systemic systems. The superior rectal veins drain the upper and middle third of the rectum. They empty into the portal system via the inferior mesenteric vein. The middle rectal veins drain the lower rectum and upper anal canal into the systemic system via the internal iliac veins. The inferior rectal veins drain the lower anal canal, communicating with the pudendal veins and draining into the internal iliac veins. There is communication between the venous systems. This allows low rectal cancers to spread via the portal and systemic systems.

Lymphatic drainage of the upper and middle rectum is into the inferior mesenteric nodes. Lymph from the lower rectum may also drain into the inferior mesenteric system or into the systems along the middle and inferior rectal arteries, posteriorly along the middle sacral artery, and anteriorly through channels in the retrovesical or rectovaginal septum. These drain to the iliac nodes and ultimately to the periaortic nodes. Lymphatics from the anal canal above the dentate line drain via the superior rectal lymphatics to the inferior mesenteric lymph nodes and laterally to the internal iliac nodes. Below the dentate line, drainage occurs primarily to the inguinal nodes but can occur to the inferior or superior rectal lymph nodes.

Kodner IJ et al: Colon, rectum, and anus. In: Principles of Surgery, 7th ed. Schwartz SI (editor). McGraw-Hill, 1999.

Nivatvongs S, Gordon PH: Surgical anatomy. In: Principles and Practice of Surgery for the Colon, Rectum, and Anus. Gordon PH, Nivatvongs S (editors). Quality Medical Publishing, 1999.

Welton ML, Varma MG, Amerhauser A: Colon, rectum and anus. In: Surgery. Basic Science and Clinical Evidence. Norton et al (editors). Springer, 2000.

Source: Mark Lane Welton, MD
Portions of this article are reprinted from Way LW, Doherty GM (eds): Current Surgical Diagnosis & Treatment, 11th ed. New York: McGraw-Hill, 2003