Gastrointestinal tract

Gastrointestinal tract

Introduction

The function of the gastrointestinal system is to break down food for absorption into the body. This process occurs in five main phases: ingestion, fragmentation, digestion, absorption and elimination of waste products. Digestion is the process by which food is enzymatically broken down into molecules that are small enough to be absorbed into the circulation. As an example, ingested proteins are first reduced to polypeptides and then further degraded to small peptides and amino acids that can be absorbed.

The gastrointestinal system is essentially a muscular tube lined by a mucous membrane that exhibits regional variations, reflecting the changing functions of the system from mouth to anus. The mucous membrane is protective, secretory, absorptive or a combination of these in different parts of the tract (see Fig. 14.3). The muscle gives strength to the wall of the tract as well as moving the food along. Muscle is arranged somewhat differently in different areas of the tract. Because of its continuity with the external environment, the gastrointestinal system is a potential portal of entry for pathogenic organisms. As a result, the system incorporates a number of defence mechanisms which include prominent aggregations of lymphoid tissue, known as the gut-associated lymphoid system (GALT), distributed throughout the tract (see Ch. 11).

FIG. 14.1 Parts of the gastrointestinal tract
Ingestion and initial fragmentation of food occur in the oral cavity, resulting in the formation of a bolus of food. This is then conveyed to the oesophagus by the action of the tongue and pharyngeal muscles during swallowing. Secretion of saliva from major and minor salivary glands (see Ch. 13) aids fragmentation and lubricates the food for swallowing.
The oesophagus conducts food from the oral cavity to the stomach where fragmentation is completed and digestion begins. Initial digestion, accompanied by the intense muscular action of the stomach wall, converts the stomach contents to a semi-digested liquid called chyme. Chyme is squirted through a muscular sphincter, the pylorus, into the duodenum, the short first part of the small intestine. Digestive enzymes from a large exocrine gland, the pancreas, enter the duodenum together with bile from the liver via the common bile duct (see Ch. 15). Bile contains excretory products of liver metabolism, some of which act as emulsifying agents necessary for fat digestion. Duodenal contents pass along the rest of the small intestine where the process of digestion is completed and the main absorptive phase occurs. The middle segment of the small intestine is called the jejunum and the distal segment the ileum. There is no distinct anatomical boundary between these parts of the small bowel.
The liquid residue from the small intestine passes through the ileocaecal valve into the large intestine. Here, water is absorbed from the liquid residue, which becomes progressively more solid as it passes towards the anus. The capacious first part of the large intestine is called the caecum, from which projects a blind-ended sac, the appendix. The next part of the large intestine, the colon, is divided anatomically into ascending, transverse, descending and sigmoid segments, although histologically the segments are indistinguishable from one another. The terminal portion of the large intestine, the rectum, is a holding chamber for faeces prior to defaecation via the anal canal.

Development of the gastrointestinal tract

The GI tract is derived from endoderm, the innermost of the three layers forming the developing embryo. During fetal life, it is divided into three segments described as the foregut (with blood supply derived from the coeliac trunk), midgut (supplied by the superior mesenteric artery) and hindgut (supplied by the inferior mesenteric artery). These parts develop into the parts of the definitive GI tract. The foregut extends from the oesophagus down to the second part of the duodenum where the common bile duct enters the GI tract. The midgut extends to the junction of the middle and distal thirds of the transverse colon (known as Cannon’s point), and the more distal structures are derived from the hindgut. An awareness of the embryological development of the GI tract often facilitates our understanding of disease. For example, visceral pain arising from the different parts of the gut typically localises in distinct parts of the abdomen due to the pattern of its innervation. As a result, foregut pain is typically felt in the epigastrium, midgut pain in the periumbilical region and hindgut pain in the suprapubic area. This helps explains why the pain of acute appendicitis tends to begin in the periumbilical area (midgut origin) and only localises to the right iliac fossa later because of inflammation of the peritoneal surface.

FIG. 14.2 Structure of the gastrointestinal tract
The structure of the gastrointestinal tract conforms to a general plan that is clearly evident from the oesophagus to the anus. The tract is essentially a muscular tube lined by a mucous membrane. There are minor variations in the arrangement of the muscular component in different parts of the gut, but much more striking are the marked changes in the structure and therefore function of the mucosa in the different regions of the tract.
The gastrointestinal tract has four distinct functional layers: mucosa, submucosa, muscularis propria and adventitia.

• Mucosa. The mucosa is made up of three components: the epithelium, a supporting lamina propria and a thin smooth muscle layer, the muscularis mucosae, which produces local movement and folding of the mucosa. At four points along the tract, the mucosa undergoes abrupt transition from one form to another: the gastro-oesophageal junction, the gastroduodenal junction, the ileocaecal junction and the rectoanal junction.

• Submucosa. This layer of loose collagenous connective tissue supports the mucosa and contains the larger blood vessels, lymphatics and nerves.

• Muscularis propria. The muscular wall proper consists of smooth muscle that is usually arranged as an inner circular layer and an outer longitudinal layer. In the stomach only, there is an inner oblique layer of muscle. The action of the two layers, at right angles to one another, is the basis of peristaltic contraction (see textbox).

• Adventitia. This outer layer of loose supporting tissue conducts the major vessels, nerves and contains variable adipose tissue. Where the gut lies within the abdominal cavity (peritoneal cavity), the adventitia is referred to as the serosa (visceral peritoneum) and is lined by a simple squamous epithelium (mesothelium). Elsewhere, the adventitial layer merges with retroperitoneal tissues.

Food is propelled along the gastrointestinal tract by two main mechanisms: voluntary muscular action in the oral cavity, pharynx and upper third of the oesophagus is succeeded by involuntary waves of smooth muscle contraction called peristalsis. Peristalsis and the secretory activity of the entire gastrointestinal system are modulated by the autonomic nervous system and a variety of hormones, some of which are secreted by neuroendocrine cells located within the gastrointestinal tract itself. These cells constitute a diffuse neuroendocrine system, with cells producing a variety of locally acting hormones found scattered along the whole length of the tract (see also Ch. 17).
Autonomic regulation of certain glandular secretions and the smooth muscle of the gut and its blood vessels is mediated by the enteric nervous system, comprising postganglionic sympathetic fibres and ganglia and postganglionic fibres of the parasympathetic nervous system, supplied by the vagus nerve. Contraction of the smooth muscle of the bowel is initiated by pacemaker cells known as interstitial cells of Cajal, modulated by the autonomic nervous system, particularly the parasympathetic nervous system. As in other organs of the body, parasympathetic efferent fibres synapse with effector neurones in small ganglia located in or close to the organ involved. In the gastrointestinal tract, parasympathetic ganglia are concentrated in plexuses in the wall of the tract. In the submucosa, isolated or small clusters of parasympathetic ganglion cells give rise to postganglionic fibres which supply the mucosal glands and the smooth muscle of the muscularis mucosae. This submucosal plexus, Meissner plexus, also contains postganglionic sympathetic fibres arising from the superior mesenteric plexus. Larger clusters of parasympathetic ganglion cells are found between the two layers of the muscularis propria, the postganglionic fibres mainly supplying the surrounding smooth muscle. This plexus is known as the myenteric plexus or Auerbach plexus.
Glands are found throughout the tract at various levels in its wall. In some parts of the tract (i.e. stomach, small and large intestine), the mucosa is arranged into glands that secrete mucus for lubrication among other things. In the lower oesophagus and duodenum, glands penetrate the muscularis mucosae to lie in the submucosa. The pancreas and liver are large glands draining into the gastrointestinal lumen but lying entirely outside its wall (see Ch. 15).

Motility of the gastrointestinal tract: peristalsis

Peristalsis is the primary mechanism by which food is propelled along most of the length of the GI tract, with some voluntary muscular action involved at both extreme ends of the process. The particular anatomical arrangement of smooth muscle in the wall of the GI tract is specialised to allow constriction of the luminal diameter (via the circular layer of muscle) as well as shortening of its length (via action of the longitudinal muscle layer). The coordination of this very complex mechanism acts to progressively squeeze food along and, in certain sites such as the stomach, also facilitates churning and mixing of the food to aid digestion. The autonomic nervous system is responsible for the control of this involuntary process, primarily via parasympathetic innervation of the gut.

Disorders affecting peristalsis

Certain disease states and drug treatments can interfere with the normal process of peristalsis. One uncommon condition called Hirschsprung’s disease is characterised by failure of migration of ganglion cells into the GI tract, usually in the rectum and distal colon. Patients with this disorder typically present with severe and chronic constipation and may develop progressive dilatation of the bowel. In its most extreme form, absolutely no ganglion cells are present in the bowel, and some patients develop aganglionic megacolon due to this failure of propulsion of food through the bowel.

A variety of drug treatments, including commonly used strong painkillers such as opiates, can produce severe constipation by interfering with normal peristaltic function.

FIG. 14.3 Basic mucosal types in the gastrointestinal tract
(a) Squamous mucosa, H&E (MP) (b) Gastric type secretory mucosa, H&E (LP) (c) Intestinal type absorptive mucosa, H&E (MP) (d) Colorectal type absorptive/protective mucosa, H&E (MP)
Four basic mucosal types are found lining the gastrointestinal tract and these can be classified according to their main function:

• Protective. This type is found in the oral cavity, pharynx, oesophagus and anal canal and is illustrated in micrograph (a). The surface epithelium is of stratified squamous type and, although not keratinised in humans, it may be keratinised in some animals that have a coarse diet (e.g. rodents, herbivores). A stratified mucosal lining of this type is well suited to sites of potential frictional trauma, such as that associated with the passage of food during mastication and swallowing, or during the passage of faeces through the anal canal.

• Secretory. This type of mucosa occurs only in the stomach and is illustrated in micrograph (b). It consists of long, closely packed tubular glands that are simple or branched, depending on the region of the stomach. These glands act to produce various combinations of acid and digestive enzymes in order to facilitate digestion of food whilst also secreting mucus to protect the mucosa itself from injury.

• Absorptive. This mucosal form is typical of the entire small intestine and is illustrated in image (c). The mucosa is arranged into finger-like projections called villi which serve to dramatically increase surface area of the mucosa, with intervening short glands called crypts. In the duodenum, some crypts extend through the muscularis mucosae to form submucosal glands called Brunner’s glands. This is the major histological feature that differentiates the duodenum from the jejunum and ileum.

• Absorptive/protective. This form lines the entire large intestine and is shown in micrograph (d). The mucosa is arranged into closely packed, straight tubular glands consisting of cells specialised for water absorption, as well as mucus-secreting goblet cells to lubricate the passage of faeces.

FIG. 14.4 Components of the wall of the gastrointestinal tract
(a) Colon, H&E (HP) (b) Oesophagus, H&E (LP) (c) Colon, H&E (HP)
This series of micrographs illustrates the deeper layers of the wall of the gastrointestinal tract.
Micrograph (a) illustrates the muscularis mucosae MM, clearly demarcating the delicate lamina propria LP from the more robust underlying submucosa SM. This arrangement is typical of the whole of the gastrointestinal tract.
In most of the gut, the lamina propria consists of loose supporting tissue with a diffuse population of lymphocytes and plasma cells. The exception is the stomach which normally has few, if any, resident lymphoid cells. At intervals throughout the oesophagus, small and large bowels and appendix, prominent aggregates of lymphocytes with lymphoid follicles are found. There are also smaller numbers of eosinophils and histiocytes (see Fig. 4.21) to deal with any microorganisms breaching the intestinal epithelium until a specific immune response can be mounted. In the oesophagus, where the function of the mucosa is to protect against friction, the lamina propria is more collagenous than elsewhere and the muscularis mucosae is more prominent. The lamina propria is also typically rich in blood and lymphatic capillaries necessary to support the secretory and absorptive functions of the mucosa.
The muscularis mucosae consists of several layers of smooth muscle fibres, those in the deeper layers orientated parallel to the luminal surface. The more superficial fibres are oriented at right angles to the surface; in the small intestine, the fibres extend up into the villi (see Fig. 14.22). The activity of the muscularis mucosae keeps the mucosal surface and glands in a constant state of gentle agitation which expels secretions from the deep glandular crypts, prevents clogging and enhances contact between epithelium and luminal contents for absorption.
The submucosa consists of collagenous and adipose connective tissue that binds the mucosa to the main bulk of the muscular wall. The submucosa contains the larger blood vessels and lymphatics, as well as the nerves supplying the mucosa. Tiny parasympathetic ganglia PG are scattered throughout the submucosa, forming the submucosal (Meissner) plexus from which postganglionic fibres supply the muscularis mucosae.
The typical arrangement of the two layers of the muscular wall proper is seen in micrograph (b), which shows a longitudinal section of the oesophagus. The muscularis propria MP is made up of an outer longitudinal layer and a somewhat broader inner circular layer. There has been some artefactual separation of the layers in this micrograph, making them easier to visualise. The submucosa SM is separated from the lamina propria LP by the muscularis mucosae MM.
Micrograph (c) illustrates, at high magnification, the junction of outer longitudinal LM and inner circular CM layers of the muscularis propria in the large intestine. Between the layers, there are clumps of pale-stained parasympathetic ganglion cells of the myenteric (Auerbach) plexus. The two layers of the muscularis propria undergo synchronised rhythmic contractions that pass in peristaltic waves down the tract, propelling the contents distally. Peristalsis is initiated by the pacemaker cells, the interstitial cells of Cajal, but the level of activity is modulated by the autonomic nervous system, by locally produced gastrointestinal tract hormones and by other environmental factors. Parasympathetic activity enhances peristalsis while sympathetic activity slows gut motility.

CM circular muscle layer E stratified squamous epithelium G seromucous glands LM longitudinal muscle layer LP lamina propria Ly lymphoid aggregate MM muscularis mucosae MP muscularis propria Sk skeletal muscle Sm smooth muscle SM submucosa PG parasympathetic ganglion

FIG. 14.5 Oesophagus
(a) Masson trichrome stain (LP) (b) Masson trichrome stain (HP)
The oesophagus is a strong muscular tube that conveys food from the oropharynx to the stomach. The initiation of swallowing is a voluntary act involving the skeletal muscles of the oropharynx. This is then succeeded by a strong peristaltic reflex that conveys the bolus of food or fluid to the stomach. Food and fluid do not normally remain in the oesophagus for more than a few seconds and reflux is usually prevented by a physiological sphincter at the gastro-oesophageal junction (see textbox).
Below the diaphragm, the oesophagus passes a centimetre or so into the abdominal cavity before joining the stomach at an acute angle. Sphincter control appears to involve four complementary factors: diaphragmatic contraction, greater intra-abdominal pressure than intragastric pressure being exerted upon the abdominal part of the oesophagus, unidirectional peristalsis and maintenance of correct anatomical arrangements of the structures.
Micrograph (a) shows the lower third of the oesophagus. In the relaxed state, the oesophageal mucosa is deeply folded, an arrangement that allows marked distension during the passage of a food bolus. The lumen of the oesophagus is lined by a thick protective stratified squamous epithelium E (see Fig. 14.3). The underlying lamina propria is quite narrow and contains scattered lymphoid aggregates Ly. The muscularis mucosae MM is barely visible at this magnification.
The submucosa SM is quite loose with many elastin fibres, allowing for considerable distension during passage of a food bolus. The submucosa also contains small seromucous glands G, similar to salivary glands, which aid lubrication and are most prominent in the upper and lower thirds of the oesophagus.
The muscularis propria is thick, and inner circular CM and outer longitudinal LM layers of smooth muscle are clearly distinguishable. Since the first part of swallowing is under voluntary control, bundles of skeletal muscle predominate in the muscularis propria of the upper third of the oesophagus. In the middle third of the oesophagus, there is gradual transition from striated to smooth muscle and, in the lower oesophagus, the muscularis propria consists entirely of smooth muscle.
Micrograph (b) shows part of the muscularis propria of the upper oesophagus at high magnification in the area of transition from skeletal to smooth muscle fibres. A bundle of smooth muscle fibres Sm is seen, with two skeletal muscle fibres Sk in their midst. Other skeletal muscle fibres are seen in transverse section in the lower right of the micrograph. The cross-striations of the skeletal muscle are just visible at this magnification. The collagen of the endomysial supporting tissue stains green with this method.

Barrett’s oesophagus

The importance of the physiological sphincter at the gastro-oesophageal junction is apparent when the consequences of malfunction are considered. Reflux through the sphincter allows gastric acid into the lower oesophagus, causing the well-known symptom of ‘heartburn’. With time, the epithelium of the lower oesophagus undergoes metaplasia, i.e. it converts to a columnar mucus-secreting form, a reaction that may well be protective. Barrett’s oesophagus is the term given to this metaplastic columnar epithelium of the lower oesophagus. This metaplastic epithelium is at high risk of developing dysplasia and invasive adenocarcinoma. Oesophageal carcinoma generally has a poor prognosis.

FIG. 14.6 Oesophago-gastric junction
H&E (LP)
At the junction of the oesophagus with the stomach, the mucosa of the tract undergoes an abrupt transition from a protective stratified squamous epithelium SE to a tightly packed glandular secretory mucosa GM.
The muscularis mucosae MM is continuous across the junction, although it is less easily seen in the stomach where it lies immediately beneath the base of the gastric glands. The underlying submucosa SM and muscularis propria MP continue uninterrupted beneath the mucosal junction. The muscularis propria does not form a defined anatomical sphincter, but rather a physiological sphincter mechanism as described in Fig. 14.5.

FIG. 14.7 Stomach
Food passes from the oesophagus into the stomach, a distensible organ, where it may be retained for 2 hours or more. In the stomach, the food undergoes mechanical and chemical breakdown to form chyme. Solid foods are broken up by a strong muscular churning action while chemical breakdown is produced by gastric juices secreted by the glands of the stomach mucosa.
There is little absorption from the stomach except for water, alcohol and some drugs. Once chyme formation is completed, the pyloric sphincter relaxes and allows the liquid chyme to be squirted into the duodenum.
In the non-distended state, the stomach mucosa is thrown into prominent longitudinal folds called rugae that allow distension after eating. Anatomically, the stomach is divided into four regions: the cardia, fundus, body (corpus) and pylorus (pyloric antrum). The pylorus terminates in a strong muscular sphincter at the gastroduodenal junction.
The mucosa of the entire stomach has a tubular glandular form, but there are three distinctly different histological zones:

• The cardia is a small area of mucus-secreting glands surrounding the entrance of the oesophagus. In some individuals the cardia measures only a few millimetres or may be incomplete or absent altogether.

• The mucosa of the fundus and body forms the major histological region and consists of glands that secrete acid-pepsin gastric juices as well as some protective mucus.

• The glands of the pylorus secrete mucus of two different types and there are associated endocrine cells which secrete the hormone gastrin.

FIG. 14.8 Body of the stomach
H&E (LP)
This micrograph illustrates the body of the stomach in the non-distended state. The mucosa M is thrown into prominent folds or rugae and consists of gastric glands that extend from the level of the muscularis mucosae MM to open into the stomach lumen via gastric pits or foveolae GP.
The muscularis propria comprises the usual inner circular C and outer longitudinal L layers, but the inner circular layer is reinforced by a further inner oblique layer O.
The submucosa SM is relatively loose and distensible and contains the larger blood vessels. The serosal layer, which covers the peritoneal surface, is thin and barely visible at this magnification.
The adipose tissue of the lesser and greater omentum is attached along the lesser and greater curvature of the stomach (not illustrated in this micrograph). Lymph nodes and large blood vessels lie within this omental fatty tissue.

FIG. 14.9 Body of the stomach: structure of the gastric glands
The mucosa of the fundus and body of the stomach consists of straight tubular glands that synthesise and secrete gastric juice. The gastric pits occupy about one-quarter of the thickness of the gastric mucosa and each has between one and seven gastric glands opening into it. Gastric juice is a watery secretion containing hydrochloric acid (pH 0.9–1.5) and the digestive enzyme pepsin, which hydrolyses proteins into polypeptide fragments. The stomach mucosa is protected from self-digestion by a thick surface covering of mucus, which is maintained at a higher pH than the gastric juice by the secretion of bicarbonate ions by the gastric surface mucous cells. The gastric glands contain a mixed population of cells:

• Surface mucous cells cover the luminal surface of the stomach and partly line the gastric pits. The cytoplasmic mucigen granules that pack these cells are stained poorly by the standard H&E stain. These cells have short surface microvilli and secrete protective bicarbonate ions directly into the deeper layers of the surface mucous coat.

• Neck mucous cells are squeezed between the parietal cells in the neck and base of the gastric glands. These cells have larger secretory granules and more polyribosomes than surface mucous cells.

• Parietal or oxyntic cells are distributed along the length of the glands but tend to be most numerous in the isthmus of the glands. These large rounded cells have an extensive eosinophilic (oxyntic) cytoplasm and a centrally located nucleus. Parietal cells secrete gastric acid as well as intrinsic factor, a glycoprotein necessary for the absorption of vitamin B₁₂ in the terminal ileum.

• Chief, peptic or zymogenic cells are located towards the bases of the gastric glands. Peptic cells are recognised by their condensed, basally located nuclei and strongly basophilic granular cytoplasm. This reflects their large content of ribosomes. These are the pepsin-secreting cells.

• Neuroendocrine cells, part of the diffuse neuroendocrine system, are also found in the base of the gastric glands. They secrete 5-HT (serotonin) and other hormones (see also Fig. 14.12).

• Stem cells are found mainly in the neck of the gastric glands. These undifferentiated cells divide continuously to replace all other types of cell in the glands. The maturing cells then migrate up or down as appropriate. These cells are not easily identified in sections of normal gastric mucosa but become very prominent with plentiful mitotic figures after damage to the mucosa has occurred, such as after an episode of gastritis (see textbox).

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