CHAPTER 4
Colon: anatomy and structural anomalies

Bo Shen

Columbia University Irving Medical Center, New York Presbyterian Hospital, New York, NY, USA

The large intestine consists of the colon and rectum. The colon is a tubular structure of approximately 150 cm in length that courses through the abdomen, beginning at the right lower quadrant with the cecum that is linked to the ileum via the ileocecal valve (Figure 4.1), followed by the ascending colon. The latter becomes the transverse colon at the hepatic flexure (Figure 4.2). The transverse colon crosses the upper abdomen towards the spleen, where it becomes the descending colon at the splenic flexure and terminates as the sigmoid colon in the left lower quadrant. The sigmoid colon makes a sweeping turn in the pelvis, joining the rectum at the sacral promontory. The lumen of the colon is divided by haustral folds (Figure 4.3). The triangular haustral folds serve as landmarks for the transverse colon (Figure 4.4), helping the endoscopist to identify the descending and ascending colons.

The rectum is a tubular organ that is straight in its course towards the anus and measures 12–15 cm in size (Figure 4.5). The rectum is divided by the valves of Houston, i.e. superior, middle, and inferior rectal valves (Figure 4.6). The distal rectum continues inferiorly to become the anal canal. The anal canal measures 3.2–5.3 cm in men and 3–5 cm in women. It opens externally as the anus, an anterior–posterior slit that remains virtually closed at rest. Approximately in the middle of the anal canal, the distal rectal mucosa of endodermal origin transitions into lower (cutaneous) lining derived from ectoderm (Figure 4.7). This transition occurs at the dentate line, an area notable for tooth‐like mucosal protrusions pointing cephalad. The folds of the distal rectal mucosa form the columns of Morgagni, which in turn form pits known as anal sinus crypts. Within these crypts are located the openings of the anal glands, which secrete mucus for lubrication of the anal canal to allow easier passage of stool. The anal gland and its opening is a common source of cryptoglandular abscess or fistula. Internal hemorrhoids are the venous cushions located submucosally on the left lateral, right posterior, and right anterior aspect of the anal canal and are covered by a rather thin layer of anoderm.

The anal sphincter complex is formed by the muscles of the pelvic floor and the muscles located along the pelvic sidewall (Figure 4.8). The muscularis propria of the distal rectum is the main part of the internal anal sphincter (Figure 4.9). The major muscle that contributes fibers to the external sphincter is the levator ani. The midline of the pelvic floor has several openings through which pass the lower rectum, urethra, and either the dorsal vein of the penis in the male or the vagina in the female. The levator ani is supplied by the roots of the sacral nerves S2–S4 as well as the perineal branch of the pudendal nerve.

The histological section in Figure 4.10 represents all layers of the colonic wall. At the top of the section is the colonic mucosa, with lymphoid aggregate extending into the submucosa. The muscularis propria consists of a circular smooth muscle layer and longitudinal layer. The exterior surface is covered by the serosa. Simple columnar surface epithelium (shown at the top of Figure 4.11) forms mucosal crypts arranged in parallel “row of test tubes.” The lamina propria consists of the stromal elements investing the crypts and extending from the surface epithelium to the smooth muscle cells of the muscularis mucosae. Prominent vascular structures (arterioles, venules, and lymphatics) are noted within the submucosa. The colonic surface epithelium is a simple columnar cuboidal epithelium composed of absorptive and goblet cells residing on a basement membrane complex. Absorptive surface cells do not contain mucin within their cytoplasm and under normal hematoxylin and eosin (H&E) staining, their cytoplasm appears eosinophilic. The nuclei of absorptive cells are oval and located towards the basement membrane. Nuclei of absorptive cells are uniform in their location and size and oriented in parallel with the long axis of the cells. Goblet cells synthesize and store mucus, and secrete mucus granules by exocytosis. Because the cytoplasm of goblet cells is almost entirely filled with mucin, which does not stain with standard H&E stain, it appears to be vacant.

The anal transition zone (ATZ) is a junction area between squamous epithelia and columnar epithelia of the rectum (Figure 4.12). ATZ epithelium consists of 4–9 cell layers, with the surface cells arranged as a columnar, cuboidal, or polygonal layer, while the basal cells are small with their nuclei arranged perpendicular to the basement membrane. Within the ATZ, small areas of the mature squamous epithelium may be present, especially at the upper border of the anal canal. At the distal aspect of the ATZ, approximately at the level of the dentate line, the squamous epithelium becomes more uniform, indicating the beginning of the squamous zone. Squamous epithelium in this zone is nonkeratinized with short or no papillae.