Section 10: Digestive System

Ronald A. Bergman, Ph.D., Adel K. Afifi, M.D., Paul M. Heidger, Jr., Ph.D.
Peer Review Status: Externally Peer Reviewed

Plates

See Figure 10 - B to use as a plate number "finder" for specific regions of the digestive system.

The digestive tract includes the mouth, pharynx, and digestive tube. Various regions of the digestive system display structural specializations related to their specific function. In addition, several glands deliver their secretory products into the digestive system, where regionally specific functions are performed.

The digestive process begins in the mouth as the food is chewed and ground to reduce the size of the pieces of ingested food. The chewing and grinding depends upon teeth, of which there are 32 in adult humans. These are located in the mandible and maxilla in two matched and bilaterally symmetrical arch- like rows. In the mandible (and maxilla), there are, in front, four incisors, two canines, four premolars, and six molars behind. One set of 20 non-permanent teeth precedes the permanent teeth, but the 12 molars are not replaced.

As the food is chewed, the salivary glands secrete saliva, which moistens it and initiates the digestion of starch and glycogen contained within the disrupted plant and animal cells, respectively (see Plates 209, 210, 211 and 212). The tongue moves the food within the mouth to the teeth and is essential in the chewing process. In addition, the sense of taste is received by, and transmitted to, the central nervous system from taste buds located on the tongue (see Plates 179 and 299). The tongue has four types of papillae (three of which contain taste buds): (1) filiform papillae are most numerous, conical in shape, partially keratinized, and devoid of taste buds. In some species other than man, such as the cat, the filiform are highly keratinized. (2) Fungiform papillae are dispersed among the filiform but are less numerous. These mushroom-shaped papillae have taste buds on their free surface. (3) Foliate papillae with taste buds on their sides are not common in humans but are in lagomorphs (rabbits). (4) Circurnvallate papillae arranged in a V-shape near the root of the tongue contain numerous taste buds arranged around the sides of the papilla. The papillae are surrounded by a trench, or moat, which is flushed by the secretion of serous glands (von Ebner) beneath the papillae.

The tongue directs the bolus of chewed food to the pharynx as an initial step in the involuntary reflex swallowing mechanism. The pharynx is a conical chamber through which both air and food pass.

The bolus of food passes into the esophagus, which conducts it to the stomach. The process is accomplished by muscular contraction and gravity. The muscularis of the esophagus contains both striated and smooth muscle in its middle third and is diagnostic for this organ (see Plate 188). The epithelium that lines the digestive system to this point is non-keratinized stratified squamous epithelium. An abrupt change in the epithelium takes place near the junction with the stomach, where the surface cells become columnar mucous secretory cells and the deep glands are termed cardiac glands (see Plate 189). Cardiac glands are found on both sides of the esophageal -gastric junction. The epithelium lining the stomach consists of tall, mucous-secreting surface cells, which also line invaginations of epithelium, the gastric pits, into which the gastric glands secrete. The gastric glands are composed of four epithelial cell types: (1) mucous neck cells, which protect the organ from the secretory products of stomach glands; (2) chief cells, which produce and secrete the enzyme pepsin; (3) parietal cells, which elaborate and secrete hydrochloric acid; and (4) argentaffin cells, which are unicellular endocrine glands. A table summarizing the location and function of some of these unicellular glands is given at the end of this summary (see Plates 190, 191, and 197). At the distal end of the stomach, mucous-secreting pyloric glands are found, and these are structurally similar to the cardiac glands.

The glands of the mucosa rest upon a basement membrane and a scanty lamina propria surrounding the gastric glands. Underlying this connective tissue layer is the thin muscularis mucosae, composed of two layers of smooth muscle fibers: an inner circular and an outer longitudinal layer. Beneath the mucosa, there is a loose connective tissue layer called the submucosa. It contains large blood vessels, lymphatics, and nerves. Surrounding the submucosa are three irregularly arranged layers of smooth muscle constituting the muscularis: an inner oblique, a middle circular, and an outer longitudinal layer. The distal opening of the stomach contains a valve, the pyloric sphincter, which is a special thickening of the circular layer of smooth muscle. The outermost covering of the stomach is termed serosa, because it is composed of a loose fatty connective tissue containing blood vessels and nerves covered by a mesothelium, the visceral peritoneum, or mesentery.

The intestine is a hollow tube that extends in a coiled course between the pylorus and anus. The intestine, like the stomach, is composed of four distinct layers (in sequence beginning inside): the mucosa, submucosa, muscularis, and serosa (if the intestine is contained within a peritoneal fold or mesentery) or adventitia (if it is covered and lies behind the peritoneum [retroperitoneal] and is attached to the posterior abdominal wall). The stomach, jejunum and ileum, and transverse and sigmoid colon are intraperitoneal and have a mesothelial surface, and therefore a serosal layer, whereas most of the duodenum and the ascending and descending colon are retroperitoneal, do not have a mesothelial covering, and, hence, are covered by an adventitia.

The mucosa of the small intestine is covered by a simple epithelium that lines the distinctive villi and intestinal glands. Four types of lining cells are found throughout the intestinal mucosa (lining epithelium and glands): (1) simple columnar absorptive cells; (2) goblet cells producing protective mucus; (3) Paneth cells, which produce and secrete lysozyme capable of digesting bacterial cell walls; and (4) argentaffin or enteroendocrine cells, which produce a variety of hormones regulating activity of the digestive system. Additional information on the structure and function of these cell types is included in the legends of Plates 194, 196, and 197.

The intestinal mucosal glands are the pit-like crypts of Lieberkühn. Surrounding the intestinal glands and forming the core of each intestinal villus is the lamina propria, which characteristically contains scattered lymphocytes, lymphatic aggregates, plasma cells, eosinophils, other macrophages, mast cells, and smooth muscle fibers derived from the underlying muscularis mucosae (see Plates 29, 195, and 198). Many of the plasma cells and lymphocytes of the lamina propria produce antibodies, primarily immunoglobulin A (IgA), which are transported through the epithelium and secreted into the intestinal lumen. In varying amounts, cells producing IgM and IgG antibodies are also present. It must be remembered that there is only a single cell layer of epithelium that separates the external environment, which harbors bacteria and other pathogenic organisms, from the sterile internal milieu or environment. The lamina propria has a rich capillary network and a lymphatic capillary in the core of each villus called the central lacteal. Located in the outermost layer of the lamina propria is a band of smooth muscle, the muscularis mucosae, composed of inner circular and outer longitudinal layers.

A diagnostic feature of the duodenum is the presence in the submucosa of the mucous glands of Brunner, the ducts of which open into the lumina of intestinal glands (see Plate 192). The duodenum receives bile from the liver and digestive enzymes from the pancreas. Brunner's glands play the same protective role as the other mucous glands previously described. The jejunum and ileum, with the exception of the Brunner's glands, are basically similar to the duodenum, although they tend to have more diffuse lymphoid cell aggregates or nodules in their submucosa than does the duodenum. As in other regions of the gastrointestinal tract, a submucosal nerve plexus (of Meissner) is found (see Plate 201). The muscularis is composed of an inner circular and an outer longitudinal coat.

Between these two layers is the myenteric nerve plexus of Auerbach (see Plate 199), containing axons, terminals or synapses, and postganglionic cell bodies of the parasympathetic nervous system, as well as axons of the sympathetic nervous system passing through the plexus. The parasympathetic nervous system is excitatory and results in the contraction of smooth muscle, whereas the sympathetic system inhibits smooth muscle in the digestive system. The sympathetic system is excitatory for vascular smooth muscle.

The intestine is divided anatomically into two major subdivisions, the small and the large intestine. The small intestine, about 2 to 4 m in length in (living) humans, is further subdivided into three regions: (1) the proximal duodenum (structurally unique because of submucosal Brunner's glands); (2) the middle jejunum; and (3) the distal ileum. The large intestine, about 2 m long, is approximately twice the diameter of the small intestine and includes four anatomically defined regions: the cecum, appendix, colon, and rectum. The colon is regionally divided into ascending, transverse, descending, and sigmoid areas. The inner lining of the large intestine lacks the villi that characterize the small intestine (see Plates 205 and 207). The intestinal glands are' tightly packed and appear more conspicuous owing to the lack of villi, but they are composed of the same cell types as the small intestine. Only quantitative differences exist in epithelial cell populations.

The large intestine begins at the ileocecal valve, which is formed by two folds of the mucosa and submucosa and circular smooth muscle. The cecum, or initial part, is a blind pouch from which the worm-like (vermiform) appendix extends. The surface epithelium is primarily a simple columnar epithelium with some goblet cells. Lymphoid tissue in the form of solitary lymphatic nodules forms a conspicuous part of the lamina propria, particularly in the appendix. A distinctive feature of the cecum and colon, but not the appendix, is the arrangement of the smooth muscle of the muscularis. The inner muscular layer is circular, but the outer longitudinal layer is aggregated into three equally spaced bands termed taeniae coli (or tapeworm of the colon). Between the taeniae, the longitudinal layer is very thin but is, nevertheless, a complete layer. Usually, the three taeniae come together and terminate at the origin of the appendix; this may be a guide to the appendix when it is abnormally positioned. The rectum is similar in structure to the colon but lacks the taeniae or ribbon-like bands of longitudinal smooth muscle. At the rectal-anal junction, the surface epithelium changes to a non-keratinized stratified squamous epithelium, which then changes at the anal orifice to its keratinized form (epidermis). At this most distal end of the digestive tube, cutaneous glands (large apocrine glands) and hair appear. The submucosa has an extremely rich vascular supply (hemorrhoidal vessels), and the circular smooth muscle of the muscularis forms the internal anal sphincter.

Absorption of digested food, bile salts, and water is facilitated by the enormously increased surface absorptive area represented by overall tube length, folds within the tube (plicae circulares), villi, and presence of microvilli on absorptive cells. Microvilli are absent in some malabsorption diseases; hence, this underscores their critical role in proper absorption of digested food and fluid.

Associated with the digestive tube are eight major glands: the three paired salivary glands, pancreas, and liver.

The major salivary glands are the parotid, submandibular, and sublingual. These large glands are structurally compound tubuloalveolar, whose ducts open into the oral cavity. The parotid gland is composed of serous cells exclusively in humans, and is diagnostic; the submandibular and sublingual glands are mixed serous and mucous glands (see Plates 209, 210, 211 and 212 ). The submandibular gland has more serous than mucous gland cells, whereas the sublingual gland is composed primarily of mucous gland cells. The secretory alveoli of these glands are linked to the oral cavity by a system of ducts, which are divided into three segments termed (distally, starting at the secretory cells): intercalated ducts, lined proximally by squamous cells and distally by cuboidal cells; secretory ducts, lined by columnar cells with basal striations; and excretory ducts, which initially are lined by columnar cells and terminate as tubes lined by stratified squamous epithelium continuous with the same epithelium covering the oral cavity. The duct system is most conspicuous in the parotid gland and least conspicuous in the sublingual gland, in which some modifications appear. The intercalated ducts of the sublingual gland are composed primarily of mucous cells in the form of tubules and are linked to very short secretory ducts. The final excretory segment is similar in all three glands.

The pancreas serves both exocrine and endocrine functions. The exocrine or digestive enzyme producing part of the gland will be considered here, and the endocrine portion will be considered in the section on endocrine glands. The pancreas is a compound tubuloalveolar gland with purely serous alveoli. It is covered by a sheath of areolar connective tissue from which thin fibrous septa extend into the gland and subdivide it into many distinct lobules. The parenchyma is a glandular epithelium consisting of pyramidal cells, which secrete zymogen granules into a system of ducts (see Plates 26 and 213). An unusual feature of the pancreas is the existence of the centroalveolar or centroacinar cells, which are interposed between the secretory cells and the lumen of the alveolus (see Plate 213). The centroalveolar cells are in continuity with the intercalated or interlobar ducts composed of cuboidal epithelium. The intercalated ducts join the excretory or interlobar ducts lined with simple columnar epithelium. These ducts join the main excretory duct, which empties into the lumen of the duodenum. The stroma is composed of delicate connective tissue supporting the parenchymal cells and nerve fibers, blood, and lymphatic vessels.

The liver is the largest gland in the body. it is usually composed of four incompletely separated lobes covered by a connective tissue capsule (Glisson's capsule) and incompletely invested by reflections of the peritoneum. The parenchyma of this organ is composed of lobules and an anastomosing series of hepatic cords. In histologic sections, the hepatic cords radiate outward from the central vein (terminal hepatic venule) much like the spokes of a wheel. The cells are actually arranged in sheets one and two cells thick. Located between the cellular sheets are sinusoids approximately 9 to 12 µm in width, which receive blood at the periphery of the lobule from the portal vein and hepatic artery and, after traversing the lobule, discharge the blood into the central vein (terminal hepatic venule) at the center of the lobule (see Plates 214 and 215). The sinusoids are lined with endothelial cells and contain the stellate cells of von Kupffer. Kupffer's cells are phagocytes belonging to the widely distributed system of fixed macrophages. At the interface between adjacent hepatic cells, minute bile capillaries (canaliculi) are formed, which drain toward the periphery of the hepatic lobule into a bile ductule in the portal canal (see Plates 214, 217, and 218). This structural unit of the liver, the hepatic lobule, is repeated throughout the liver. The smaller bile ducts converge within the liver to form the hepatic duct outside the liver. The gallbladder and its cystic duct join the hepatic duct to form the common bile duct, which drains into the lumen of the duodenum. The gallbladder stores up to 90 ml of bile between meals. Bile is secreted continuously by the liver, with a total daily output of about 500 ml, some of which is recycled once or twice during a meal rich in fats.

Histologists have defined the basic unit of structure and function in the liver. The classic unit is the hepatic lobule, which is a hexagonal unit with cellular plates separated by sinusoidal capillaries and with branches of a hepatic arteriole, a portal venule, and a bile ductule at two or more corners of the hexagon. The unit is completed by a central vein (terminal hepatic venule). Another way of considering function is a triangular or biliary unit, the corners of which are formed of three central veins (terminal hepatic venules). This approach emphasizes the secretion of bile and the biliary system; bile has its origin in canaliculi formed between cells located at the periphery of the triangular unit. Canaliculi drain into bile ductules at the center of the triangular unit (portal lobule).

The third unit of structure/function is the hepatic acinus. This acinus may be as large as one-sixth of the hepatic lobule, and its hepatocytes receive blood from one hepatic arteriole and one portal venule; bile from the same hepatocytes is delivered to the accompanying bile ductule. See diagram.

Superficially, all liver cells look alike, although their arrangement may differ. The sinusoids and cells nearest the blood supply (periportal) are highly branched, whereas those near the central vein (terminal hepatic venule) are straighter or more radial in appearance. There is good evidence for structural and functional diversity within a hepatic acinus, which is the smallest structural/functional unit in the organ. The hepatic acinus can be divided into three regions or segments of approximately equal size; these are termed zones. Zone 1 is nearest the blood supply (periportal), and zone 3 includes the terminal hepatic venule (central vein). Zone 2 is located between zones 1 and 3. Evidence for this zonal structural/ functional diversity follows. Mitochondria are larger in zone 1 but are more numerous in zone 3. It is also true that there is an oxygen gradient (from high to low) between zones 1 and 3. After a meal, glycogen will be stored first in the peripheral region, or zone 1, later in zone 2, and finally in zone 3. The reverse is true when glucose is needed: Zone 3 is depleted of its glycogen first, followed by zones 2 and 1. No zonal differences have been found in protein synthesis, but it is believed that zone 3 is actively involved in lipid metabolism.

The administration of phenobarbital results in proliferation of smooth endoplasmic reticulum in zone 3. Continued administration Will spread the effect to zone 2 and ultimately to zone 1. There is also evidence for zonal liver damage. Zone 3 is affected by acetaminophen, halothane, carbon tetrachloride, and other agents. There are fewer compounds that selectively damage zone 1. Bile salts in high concentration, allyl formate, and yellow phosphorus all damage zone I initially. The concept of zonal metabolism is under active investigation.

The following is a listing of cells found in the mucosa of the digestive system that belong to a diffuse endocrine system composed of single or small groups of cells. They are known as argentaffin cells, because they are selectively stainable with silver salts. They are unstained in hernatoxylin and eosin preparations but will take a dye if the tissue is fixed with solutions containing mercury salts (e.g., Zenker's fixative). See Plate 197.

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