The Large Intestine
The large bowel is the final part of the gastrointestinal system and its primary function is the reabsorption of water and small nutrient particles. The intestinal wall has four primary layers:
- Serosa or adventitia: Outer layer responsible for keeping the digestive tract in the right position inside the body.
- Muscularis externa: Composed of a continuous inner layer of circular muscle and a discontinous outer layer of longitudinal muscle, which are responsible for the motility of the lumen contents.
- Submucosa: Connective tissue located between the layer of circular muscle and the mucosa.
- Mucosa: Inner layer of the intestinal wall, comprising a single layer of columnar epithelium (surface epithelium), connective tissue (lamina propria) and an outer muscle layer (lamina muscularis mucosae). The mucosa is characterised by the presence of numerous glands, the crypts of Lieberkühn, which are invaginations of the surface epithelium into the lamina propria. The structure of the normal mucosa is shown in Figure 1.
Figure 1: Section of normal colon. A = intestinal lumen; B = lamina propria; C= lamina muscularis mucosae; D = blood vessel; E = surface epithelium; F = Goblet cell; G = stem cells; H = crypt lumen; I = submucosa. Image taken from the School of Medicine (University of Leeds).