The mouth (oral cavity) contains the tongue and teeth. The epithelium of the mucosa lining the cheeks and tongue is able to resist the wear and tear in-volved in chewing food, and in most regions it is a stratified, squamous (non-keratinised) epithelium. This epithelial surface is moistened by secretions from small serous and mucous glands in the submucosal layer and from salivary glands (Fig. 12.3) which drain their saliva into the oral cavity. Physical breakdown of food occurs in the mouth and it is mixed with salivary gland secretions which begin to break down large carbohydrate molecules.

There are three, paired salivary glands, the parotid, submandibular and sublingual glands. Each gland is invested by a connective tissue capsule, and connective tissue forms trabeculae which penetrate the glands and provide support for the blood vessels and nerves supplying the glands and for the ducts draining them. Each gland has secretory cells arranged as compound, tubuloacinar, exocrine glands (Chapter 3). The secretory acini drain into small ducts (lined by a simple cuboidal epithelium) which join together in a branching system; the larger ducts are lined by columnar epithelial cells, which may be stratified. The ducts drain the saliva into the oral cavity.

Two types of secretory cell, serous and mucous cells, are present in salivary gland acini (Fig. 12.3) and, respectively, they produce a watery secretion containing proteins which function as enzymes and a viscous mucus in which large carbohydrate complexes are major components. In addition, myoepithelial cells wrap around the acini and their contractions help to expel the secretions. The secretions assist the process of digestion by moistening and lubricating the food, and by providing enzymes. The main enzyme is amylase, which breaks down large carbohydrate molecules. Saliva is also responsible for lubricating and cleansing the oral cavity. It contains bactericidal substances such as lysozyme secreted by serous cells and may contain immunoglobulin A secreted by plasma cells.

The tongue is covered by a stratified, non-keratinised, squamous epithelium which on the undersurface is similar to the epithelium lining the mouth. The upper surface of the tongue is divided by a ‘V’-shaped groove into an anterior (two-thirds) and a posterior region developed from different parts of the embryo (see Mitchell B, Sharma R. Embryology: An Illustrated Colour Text. Elsevier: 2004). The stratified squamous epithelium on the upper surface is studded by prominent projections. In the posterior region, many of the projections are due to aggregations of lymphocytes deep to the epithelium. In addition, three distinct types of projection known as papillae are also described, and all assist in macerating food and resisting abrasion:

Deep to the mucosa of the tongue there are numerous, small serous and mucous glands which secrete onto the surface of the epithelium. The major component of the inner part of the tongue is skeletal muscle (see Fig. 5.7). The bundles of muscle cells are arranged in a complex three-dimensional meshwork and coordinated contraction of the muscle cells is important in chewing, swallowing and in sound production.

Swallowed food and drink pass via the pharynx to the oesophagus (gullet). The structure and function of the epithelium lining the component parts of the pharynx are described in Chapter 11. Aggregations of lymphoid cells are a prominent feature of the walls of the pharynx and form tonsils (Fig. 12.5). The lymphoid cells are important components of the immune mechanisms defending the alimentary canal (and the respiratory tract).

The major part of the oesophagus is in the thorax, but a small segment lies in the abdominal cavity and is continuous with the stomach. The structures in the walls of the oesophagus conform to the general plan for the gastrointestinal tract (Fig. 12.2). In the thorax, the connective tissue of the adventitia binds the muscularis externa of the oesophagus to adjacent structures. The muscularis externa consists of an outer, approximately longitudinal layer and an inner circular layer. In the upper part of the muscularis externa, striated voluntary muscle (Fig. 12.6) is present, whereas in the lower region it is replaced by smooth involuntary muscle. In the middle region there is a transition between the two types of muscle. Coordinated contraction of muscle cells in the different regions of the oesophagus ensures that swallowed food and drink normally pass to the stomach. In some regions, the submucosal tissue of the oesophagus contains serous and mucous glands in addition to blood vessels, nerves and aggregations of lymphoid cells. The epithelium of the mucosa lining the oesophagus is stratified and squamous (Fig. 12.6) and is able to resist friction from swallowed food and chemical attack by swallowed drinks, e.g. alcohol.

The stomach is a dilated portion of the gastrointestinal tract. It is described as having four regions: the cardiac region, the fundus, the body and the pylorus (Fig. 12.1). The cardiac region is continuous with the lower end of the oesophagus. The fundus is the part of the stomach lying closest to the diaphragm and is a site, in humans, where gas collects. The body of the stomach is the largest part of the stomach and the pylorus connects the body of the stomach to the duodenum. The stomach produces 2–3 litres/day of gastric secretions (juices) and the main constituents are mucus, water, hydrochloric acid and enzymes able to digest carbohydrates, fats and proteins. The mixture of gastric juices and ingested food and drink is known as chyme. The structure of the stomach conforms to the general plan (Fig. 12.2) with some modifications which relate to its functions.

The stomach is covered by a serosal (peritoneal) membrane. The surface squamous epithelial cells of the peritoneal membrane secrete small amounts of fluid (into the peritoneal cavity) and, as the stomach fills with food and drink, it moves relatively easily against adjacent structures. Within the connective tissue of the peritoneal membrane large accumulations of fat cells may be present. The muscularis externa of the stomach differs from the general plan in that it has three layers: an innermost oblique, a middle circular and an outer longitudinal layer. These muscle layers help prevent overdistension of the stomach and their contractions help mix the chyme and move it towards the small intestine. In the pyloric region of the stomach, at the gastroduodenal junction, the muscularis externa is thickened and functions as a sphincter. The sphincter is controlled by autonomic nerves and hormones which regulate the passage of chyme into the duodenum. In contrast, at the gastrooesophageal junction, stomach contents normally are not regurgitated into the oesophagus although a distinct muscular sphincter is not apparent.

The interior of the empty stomach is characterised by thick folds which run longitudinally. They are known as rugae and are formed by submucosal and mucosal layers. The rugae unfold as the stomach fills. They also help to channel chyme towards the pylorus and help to maintain a large interface between the contents of the stomach and the surface epithelium.

The epithelium of the gastric mucosa dips into the lamina propria and forms channels known as gastric pits (Fig. 12.7). All the epithelial cells on the surface of all regions of the stomach and lining all the gastric pits are columnar and they secrete mucus (Fig. 12.8). This mucus forms a thick protective layer which helps to ensure that the stomach cells are not damaged by gastric secretions. At the base of the pits, gastric glands extend as tubes deep into the lamina propria and reach the muscularis mucosae. The gastric glands secrete various substances which drain into the pits, and from there to the lumen of the stomach. In the cardiac and pyloric regions of the stomach gastric glands are coiled tubules lined by columnar cells which secrete mucus. Their appearance in routine H&E-stained sections resembles the mucous cells lining gastric pits. In the rest of the stomach (body and fundus) the gastric glands are straight tubules and the epithelium lining them contains several types of cell:

The small intestine has three parts: duodenum, jejunum and ileum. Digestion continues in the small intestine and involves digestive enzymes from secretory cells in the mucosal epithelium and, in the duodenum, in the submucosa. Secretions also enter the lumen of the duodenum from the pancreas and liver and these also aid digestion. The luminal contents, known as chyle, have a transit time along the small intestine of 3–4 hours and digestion is largely completed in that time. Most of the products of digestion, e.g. amino acids and monosaccharides, are absorbed across the epithelium lining the small intestine and pass into blood vessels; fatty acids and triglycerides pass into lacteals (small, blind-ended lymph vessels).

The wall of the small intestine conforms to the generalized structural plan of the gastrointestinal tract (Fig. 12.2) though there are features which characterise each part. The epithelium of the mucosa lining the small intestine contains:

There are several structural features of the small intestine that facilitate contact between luminal contents and the mucosal epithelium and thus aid digestion and absorption of the molecules produced by enzymic activities:

The life span of epithelial cells in the mucosa of the small intestine is less than a week; cells at the tip of villi die and are rapidly replaced. It is the mitotic activity (Fig. 12.11) of stem cells in the epithelium at the base of intestinal glands which ensures that new cells are produced. Some of the new cells differentiate and become columnar epithelial cells, others become goblet cells, yet others remain as stem cells. The new columnar and goblet cells migrate from the intestinal glands along the epithelium and up to the tip of the villus where, in turn, they die.

The first part of the small intestine, the duodenum, is characterised by the presence of glands in the submucosal layer (Fig. 12.14). These glands (Brunner’s glands) make and secrete alkaline mucus which helps neutralise the acidic chyme entering from the stomach. Secretions from neuroendocrine cells in the duodenum inhibit the action of gastrin and help control the flow of gastric contents to the duodenum.

Within the walls of the small intestine there is a variety of immunocompetent cells, including lymphocytes (mainly T cells), plasma cells (mainly secreting IgA), eosinophils, mast cells and macrophages. Clusters of immune cells (GALT) are present and usually are in the submucosal layer (Fig. 12.14) or the lamina propria. The ileum, however, is characterised by the presence of large accumulations of immune cells which are visible to the unaided eye as white oval-shaped regions. These are known as Peyer’s patches. They are present in the lamina propria and may extend into the submucosa and are restricted to the gut wall opposite to the attachment site of the peritoneal membranes.

The large intestine comprises the colon, appendix, rectum and the upper part of the anal canal (Fig. 12.1). Its structure largely conforms to the general plan (Fig. 12.2). The mucosa is similar along the length of the large intestine, and goblet cells (Figs 12.15 and 12.16) are far more common than in the small intestine. The mucosal epithelium dips into the mucosa and forms intestinal glands which reach the muscularis mucosae (Fig. 12.16). This arrangement increases the area of interface between the luminal contents and the epithelial cells. Columnar epithelial cells, absorbing mainly water, are present mainly on or near the luminal surface, and stem cells and a few neuroendocrine cells are also present in the epithelium. Villi and submucosal glands are not present in the large intestine.

The muscularis externa of the large intestine differs from the general plan in that although it has an inner circular layer its outer longitudinal layer is arranged in three bands (taeniae coli). Peristaltic contractions of the muscularis externa move the contents in the lumen towards the anus. Although the large intestine is much shorter than the small intestine, the transit time through the large intestine takes relatively longer at up to about 48 hours. During transit, water and salt absorption occurs and the remnants form feces. The mucus secreted by goblet cells in the epithelium is important in lubricating the movement of feces along the large intestine.