Physiological Pharmaceutics

146 Physiological PharmaceuticsFigure 7.2 Structure of the colon wallColonic structureThe wall of the colon is divided into four layers: the serosa, external muscular region(muscularis externa), submucosa and mucosa (Figure 7.2). The squamous epithelium of theserosa is covered with adipose tissue which forms distended fat pouches of peritoneum,known as appendices epiploicae. These are larger and more numerous in the distal half ofthe colon and are one of its distinguishing features. The serosa is absent from the rectumand anal canal.The muscularis externa consists of an inner circular muscle layer and an incompleteouter longitudinal layer composed of three separate 0.8 cm wide, longitudinal strips knownas teniae coli. These bands converge in the caecum at the root of the appendix. They travelthe length of the colon and eventually widen and join to form a continuous outerlongitudinal muscle layer which covers the rectum. Between the teniae coli is a thin layer oflongitudinal muscle which allows the inner circular muscle layer to bulge outwards. Thisoutward bulge is interrupted at intervals by contractions of the circular muscle, giving thecolon its characteristic sacculated appearance. These sacculae are also known as haustra andare more pronounced in the proximal half of the colon. Their size and shape varies with thecontractile activity of the circular muscle.Colonic mucosaThe colonic mucosa is divided into three layers: the muscularis mucosae, the lamina propriaand the epithelium. The muscularis mucosae is a layer of smooth muscle approximately 10cells thick which separates the submucosa from the lamina propria. The lamina propriasupplies structural support for the epithelium and is well supplied with blood vessels andlymphatics. It also contains numerous T lymphocytes, macrophages, plasma cells and somelymph nodules. These cells play an important role in the immune function of the colon, andhelp to protect it from bacterial invasion and attack.There are many histological similarities in the structure of the mucosa in the large andsmall intestine. The most obvious difference is that the mucosa of the large intestine isdevoid of villi. The colonic epithelium consists of a single layer of columnar cells lining thelumen, and is punctuated by numerous crypts, termed crypts of Lieberkühn. These are