The Sigmoid Colon

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The sigmoid colon is a curved, S-shaped region of the large intestine and is the final segment of the colon. It transports fecal matter from the descending colon to the rectum and anus. Feces are stored in the sigmoid colon until they are ready to be eliminated from the body through the anal canal. The intestinal wall of the sigmoid colon also plays a small role in the absorption of water, nutrients and vitamins from feces.

The sigmoid colon is a hollow tube approximately 2.5 inches (7 cm) in diameter and 16 inches (40 cm) long. A band of smooth muscle tissue running the length of the colon contracts to form many small pouches called haustra in the walls of the sigmoid colon. Haustra help to increase the surface area of the colon, while their smooth muscle tissue helps to move feces toward the end of the colon. At its superior end, the sigmoid colon connects to the descending colon near the iliac crest in the left iliac region of the abdominal cavity. From this junction, the sigmoid colon curves medially and anteriorly toward the body's midline, before abruptly turning about 90 degrees to face inferiorly to form the rectum.

Four major tissue layers surround the hollow lumen of the sigmoid colon and provide its structure and function.

1. The innermost layer, the mucosa, is made of a mucous membrane consisting of simple stratified columnar epithelium and an underlying areolar connective tissue. The mucosa is the only tissue layer in contact with the contents of the sigmoid colon, acting as the interface between the colon and feces. Nutrients, vitamins and water present in feces are absorbed through the epithelium of the mucosa, while goblet cells produce thin, slick mucus to help feces move through the colon.
2. The next layer outside the mucosa is the submucosa, a layer of connective tissues and nerves that support the mucosa. Blood vessels in the submucosa transport nutrients to and from the sigmoid colon, while nervous tissue monitors the contents of the sigmoid colon to manage the defecation reflex.
3. The muscularis layer surrounds the submucosa and contains several layers of smooth muscle tissue. Contraction of smooth muscle tissue both forms the haustra of the sigmoid colon and propels feces out of the sigmoid colon and into the rectum during defecation.
4. The outermost layer of the sigmoid colon is covered by the serosa layer, which extends as a thin membrane known as the mesentery. Although it is made of thin simple squamous epithelial tissue, the serosa protects the sigmoid colon from external friction by giving it a smooth surface and secreting lubricating serous fluid into the abdominal body cavity. The mesentery secures the colon along the posterior body wall and provides blood vessels and nerves to the colon, while the serosa protects the colon from friction caused by the body's movement.

By the time digested food (known as chyme) reaches the large intestine, almost all of the available nutrients present in the chyme have been absorbed. Chyme is mixed with bacteria in the large intestine, which ferments the chyme to form feces. The fermentation process releases ions and vitamins from the chyme that are absorbed by the mucosa of the colon. Waves of peristalsis in the muscularis slowly push the feces from haustrum to haustrum through the ascending, transverse, and descending colon to the sigmoid colon. Feces entering the sigmoid colon have completed most of their bacterial fermentation and have had most of the water, ions and vitamins absorbed from them in the prior sections of the colon. Thus, the primary roles of the sigmoid colon are the storage of feces and the propulsion of feces into the rectum. The muscularis in the sigmoid colon plays an important role in controlling the expulsion of feces during defecation. Some absorption of water and vitamins K, B1, B2, and B12 also occurs as the feces are being stored in the sigmoid colon.

The sigmoid colon is clinically important. As it is the region of the colon closest to the body's exterior, the sigmoid colon is commonly used to screen for and prevent colon cancer. A sigmoidoscopy is a procedure wherein an endoscope is inserted through the anus and rectum and used to observe the mucosa of the sigmoid colon. Any growths (known as polyps) that are found during a sigmoidoscopy are removed and examined by a pathologist to check for abnormal cells. This procedure helps to detect and prevent colon cancer before it becomes incurable.