Colonic mucosa

The colonic mucosa is divided into three layers: the muscularis mucosae, the lamina propria and the epithelium. The muscularis mucosae is a layer of smooth muscle approximately 10 cells thick which separates the submucosa from the lamina propria. The lamina propria supplies structural support for the epithelium and is well supplied with blood vessels and lymphatics. It also contains numerous T lymphocytes, macrophages, plasma cells and some lymph nodules. These cells play an important role in the immune function of the colon, and help to protect it from bacterial invasion and attack.

There are many histological similarities in the structure of the mucosa in the large and small intestine. The most obvious difference is that the mucosa of the large intestine is devoid of villi. The colonic epithelium consists of a single layer of columnar cells lining the lumen, and is punctuated by numerous crypts, termed crypts of Lieberk├╝hn. These are responsible for the production and differentiation of the absorptive, goblet and endocrine cells that make up the colonic epithelial layer. The goblet cells are responsible for the production of mucus which is important in minimising friction between the mucosal surface and the semi-solid luminal contents.

The mucosa is thrown into irregular folds known as plicae semilunares which, along with the microvilli of the absorptive epithelial cells, serve to increase the surface area of the colon by 10 to 15 times that of a simple cylinder of similar dimensions. The terminal portion of the digestive tract in most animals is distinguished from the rectum because of the transition of its internal surface from a mucous membrane layer (endodermal) to one of skin-like tissue (ectodermal). The anal opening is keratinized skin that has several folds while contracted. When open, the folds allow the skin to stretch without tearing. In the skin around the anal opening but not immediately adjacent to it are glands that give off perspiration. Both the upper and lower portions of the anal canal have circular and longitudinal muscle layers that allow expansion and contraction of the canal. The luminal surface of the rectum is covered by a membrane formed from columnar epithelial, endocrine and goblet cells.

Very few Paneth cells are found in the colon, although some exist in the caecum and ascending colon. It is believed that Paneth cells are involved in a variety of functions including secretion of digestive enzymes, production of trophic factors and elimination of heavy metals. They have abundant zinc-rich secretory granules which contain lysozyme, glycoprotein and other proteins.

The gut associated lymphoid tissue (GALT) is unevenly distributed throughout the GI tract. The lymphoepithelial regions of the colon are known as lymphoglandular complexes (LGC). They resemble sites in the small intestine which are associated with antigen sampling, but very little is known about this tissue.

Mucus

Mucus is produced by goblet cells and acts as a lubricant protecting the colon from abrasion from solid matter, particularly in the distal colon. Mucins are degraded by colonic bacterial flora. Much of the colonic mucin is sulphated, the degree of sulphation being greater in the distal colon and decreasing proximally towards the terminal ileum1. Histochemical studies have shown a relative depletion of colonic sulphomucins in both ulcerative colitics and patients with Crohn's disease2. Desulphation of the mucus will alter the net charge on the mucin and therefore change its physical properties, rendering it more susceptible to bacterial sialase attack3.

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