"Describe the basic structure of the wall of the alimentary tract and indicate how lining epithelium varies in relation to different functions at various levels."

There is much that is similar in the structure of the walls at all levels of the gastrointestinal tact. The walls of the alimentary tract can be divided into four layers. The innermost of these is the mucosa, consisting of the lining epithelium, a thin lamina propria and the muscularis mucosae. The lining epithelium is the layer that shows the most adaptation to changes in function as the alimentary tract is descended, whether that function is conduction of the contents, secretion or absorption. The lamina propria is a highly vascular layer of loose connective tissue. In addition to the fibroblasts that go to construct it, it contains macrophages, aggregations of lymphoid tissue and lymphocytes that can be mobilised in response to any infection by the numerous micro-organisms that inhabit the alimentary tract.

The muscularis mucosae is the smooth muscle layer of the mucosa. It consists of two layers of muscle. The innermost muscle fibres running circumferentially, and the fibres of the outer layer running longitudinally. The muscularis mucosae allow the alimentary tract to change its lumen shape or return to its resting position after distension by passage of food.

The second layer of the alimentary tract is the submucosa. This is a moderately dense connective tissue containing many small blood vessels. IN some section of the alimentary tract such as the lining of the stomach the submucosa is the site of the body of various tubuloacinar glands. The submucosa is also the home of Meissners (or the submucosal) plexus. Theses aggregations of nervous tissue are responsible in part for the intrinsic motility of the gut.

Deep to the submucosa is the tunica muscularis. This is similar to the muscularis mucosae in that it consists of two layers of muscle with the inner fibres running circumferentially and the outer fibres running longitudinally. However these two layers are much thicker than the mucosae muscularis. Rather than being just smooth muscle throughout their length like the muscularis mucosae the tunica muscularis contains striated muscle in the upper third of the oesophagus. The tunica muscularis is also much thicker than the muscularis mucosae.

Between the two layers of the tunica muscularis lies a second plexus of sympathetic nerves, known as the myoenteric (or Auerbach’s plexus). This helps to control the peristaltic movements of the gut.

The outer coat of the gut is known as the serosa. It only exists from the diaphragm down. Above this point (i.e. the thoracic oesophagus) the oesophagus is incorporated into the surrounding tissues. The outer layer being an adventitia. For the abdominal oesophagus and beyond, the serosa consists of an outer wall of mesothelium, a squamous epithelium that is continuous with the thin mesentery supporting the intestines. Generally, a thin layer of loose connective tissue lies under the mesothelium. However adipocytes can accumulate in this layer.

The pattern described here is only general and depending on whether the gut has to perform conductive, secretory or absorptive functions at any point variation will occur.

The oesophagus has no other function but to transport food from the pharynx through to the stomach. The epithelium lining it is a stratified squamous epithelium designed to accommodate the passage of potentially abrasive food boli. In species that eat coarse grass and similar fodder the oesophageal epithelium may be of a keratinised form. In man a small degree of keratinisation is evidenced by the presence of keratohyalin granules in the cells of the superficial two or three layers (stratum corneum).

In order to ease the passage of food down the oesophagus there are mucus secreting gland arranged along it’s length. There are two main types of glands, the mucosal glands and the submucosal glands. The mucosal glands are more superficial and are limited to the lamina propria. They occur in limited numbers in the upper oesophagus and near its junction with the stomach. The mucosal glands are tortuous tubular glands lined by cuboidal or columnar epithelia. The small ducts drain into a large duct that usually empties into the oesophagus at a papilla. The submucosal are tubuloacinar glands. The cells in the acini contain large droplets of mucus.

The muscle layers show variation throughout the length of the oesophagus. At the pharyngeal end the muscularis mucosae are almost non-existent. Lower down near the stomach the muscularis mucosae form a continuous layer about 200-400 m m thick consisting mainly of longitudinal fibres. The tunica muscularis at the pharyngeal end is composed of striated muscle. This arrangement exists for the upper third of the oesophagus. For the middle third a layer of smooth muscle appears deep to the striated muscle. This takes over the entire tunica muscularis for the lower third of the oesophagus. These muscles help to pass any boli of food to the stomach. They are also partially responsible for restoring the original luminal shape of the oesophagus once the bolus has passed. They are aided in the by an abundance of collagen fibres in the submucosa.

The muscle at the lower end of the oesophagus also contribute to the gastroesophageal spincter, which has been termed a physiological sphincter. Although there is no thickening of the muscle at any point the action of the muscles at the lower end of the oesophagus causes an increase in luminal pressure at this point preventing reflux of the contents of the stomach.

The stomach is a big muscular bag designed to mix the food entering with a mixture of acid and enzymes. The semisolid food that was swallowed is effectively turned into a thick liquid for further chemical processing in the intestines. Whilst the stomach functions has a whole organ in this respect there are three regions that can be defined in terms of tissue structure. Mainly these differences are in the glands contained in the tissues. These glands emerge from shallow pits in the gastric mucosa known as foveolae.

The first portion of the stomach encountered is the cardiac region. The glands in this region contain mostly mucus secreting cells indistinguishable form those of the gastric mucosa. There are, however, a few endocrine cells scattered through the glands and these are involved with the secretion of the polypeptide hormone gastrin. Gastrin stimulates secretion in the glands of the fundus and influences gastric motility.

The main body of the stomach is known as the fundus and contains gastric glands. These are by far the most abundant glands in the stomach. It is estimated that there are about 15 million of the simple branched tubular glands emerging from 3.5 million foveolae. From one to seven emerge in each foveola. The glands are composed of five cell types mucous neck cells, stem cells, chief cells, parietal cells and endocrine cells. Mucous neck cells are not solely found at the neck of the gland as their name suggests but are found singly throughout the gland. They secrete a more soluble mucus than that of the surface mucous cells.

Stem cells are also found in the neck of the gland. These are the only cells of the gland to show mitotic activity. Cells move up to form the mucous neck cells or to replenish the gastric mucosa which is constantly renewing itself. Alternatively cells created from the stem cells move downwards to create the remaining cell types in the gland.

The parietal cells are involved with the secretion of hydrochloric acid into the stomach cavity. They have an extensive system of canaiculae running through the cell. This is a membrane bounded region that is continuous with the apical cell membrane. These cells are normal found in the upper and middle portions of the gastric glands. In the lower third of the gland are to be found chief cells. These secrete pepsinogen, the precursor to the proteolytic enzyme pepsin.

The gastric mucosa of the empty stomach is a region containing many longitudinal folds called rugae. These flatten out when the stomach is filled. In the expanded stomach although the surface appears smooth it actually is crossed by numerous interconnect g furrows that bound convex areas 2-4mm in diameter. Each of these areas has numerous foveolae opening into it.

The epithelium of the gastric mucosa is composed of mucus secreting cells. The mucus serving to protect the epithelium from abrasion by ingested food or damage from the low pH of the stomach contents.

At the duodenal end of the stomach is the pyloric region. The glands here are more highly branched and tortuous than the gastric glands. Predominantly these also secrete mucus. In addition they secrete lysozyme which is effective at destroying bacterial walls.

In the small intestine the gut can begin to get on with its real job of absorbing nutrients. The epithelial lining is wrapped around many villi that increase it’s surface area. The epithelium is a simple columnar epithelium consisting of three cell types; absorptive cells, goblet cells and enteroendocrine cells. The absorptive cells or enterocytes are columnar with a centrally situated nucleus and a prominent brush border. Again, this brush border increases the surface area of the epithelium.

The goblet cells are the mucus secreting cells of the intestinal epithelium. As the mucus is a glycoprotein there is much endoplasmic reticulum in the base of the goblet cells. Towards the apical surface are many secretory granules ready to release mucus from the goblet cells’ cytoplasm.

The enteroendocrine cells are a diverse batch of cells as has been demonstrated by immunohistochemical techniques. In general one cell type secretes one hormone however there are some types that are known to secrete two or more different hormones.

The epithelium covering the villi extends into the intestinal glands or crypts of Lieberkuhn. The epithelium continues to about the upper half of the wall of the crypts. In the crypts can be found many cells in the process of mitosis. This rapid proliferation of cells is needed to replace the epithelial cells on the tips of the villi which are constantly being shed. In laboratory rodents the entire epithelial lining will be shed and replaced in two to three days. In humans it takes a little longer- from 3 to 6 days.

Should the intestinal epithelium be breached by any of the many micro-organisms that line the intestine there are many cell of the immune system to deal with them. Macrophages and lymphocytes abound in the lamina propria. In the small intestine they can be found in the lamina propria at the tips of the villi. In the colon they will be found immediately beneath the surface epithelium

Also to be found in the crypts are paneth cells. These are also known to secrete lysozyme. Further cell types include microfold cell (M cells) The take up macromolecules from the intestinal lumen and present them to thelymphocytes to stimulate an immune response.

The epithelium is further protected by secretion of a mucus of pH 8.1 - 9.3 by Brunner’s glands in the submucosa. This protects against the acidity of the gut contents at the proximal end and also provide the right environment for the pancreatic enzymes to work.

In the large intesine the same cell types remain except for the Paneth cells. The villi are no longer present but the crypts remain as absorption of nutrients is almost complete by this point. As the large intestine is descended the ratio of absorptive cells to goblet cells changes from 4:1 at the ilieal end to 1:1 at the colon. Again this is due to a decreased need for nutrient absorption.

Due to the potential roughness of the stools as the water is absorbed from them the cell turnover rate is as high as the rest of the intestine. Cells move up from the crypts of Leibekuhn onto the surface of the mucosa.

(c)1998 Nick Manville