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Table of Contents
Table of Contents
10.1 Introduction
10.1 Introduction
To remain alive, humans must eat food. Food is the source of all of the energy that is needed to grow, repair, and maintain the human body. Specifically, each cell of the body requires its own personal share of food so that it can run its own chemistry of survival. The problem is that most food molecules available directly from the environment are too big to pass through cell membranes, and, additionally, they are organized into large clump-like masses that must be broken down before any absorption of the food molecules can take place. The preparations of the food for absorption, the actual absorption process itself, and the removal of indigestible wastes from the body all are performed by the digestive system.
The general sequence of events follows this pattern: first, a large food mass is taken into the mouth, in a process called ingestion (eating). Next, the food mass is snipped, then crushed, and pulverized by the teeth, a process called mastication (chewing). Saliva, which is comprised mostly of water, is mixed with the food in an initial attempt to dissolve the food molecules. This process is termed solubilization (dissolving). An enzyme that breaks down the large starch molecules into smaller glucose units is also mixed with the food, and this breakdown of macromolecules into smaller ones which are then able to cross cell membranes is called digestion. Very little carbohydrate digestion, however, occurs in the mouth, because the food is moved very quickly down the esophagus and into the stomach. The food mass is propelled through the digestive system by a series of rhythmical, directional contractions, called peristalsis. In the stomach the food is mixed with more gastric juice in an attempt to dissolve the molecules more completely, and a proteolytic enzyme (pepsin) is added to digest the tough fibrous proteins in the food. Carbohydrate and fat digestion are minimal in the stomach, and so is the absorption of the digested food molecules, being restricted mostly to drugs, alcohol, and short-chained fatty acids. After several hours of digesting in the stomach, the food is moved into the small intestine. Here the liver releases bile from its bag-like gall bladder and the pancreas releases its digestive juices onto the food. The small intestine itself releases digestive juices, so that all of the principal food molecules- fats, carbohydrates, and proteins-can be absorbed. It is therefore the small intestine that is primarily responsible for the absorption of food molecules, but all the way from the mouth the inner lining of the digestive tract releases fluids that dissolve the food and secretes mucus molecules that lubricate and protect the tract itself. The absorbed food molecules pass into the blood or lymphatic system, and are then delivered to all of the cells of the body. Indigestible molecules (waste) are pushed into the large intestine, or colon, where they are desiccated and consolidated by mucus. The final absorption of water and salts also occurs in the colon. The waste masses, now desiccated into a semi-solid state, are released under voluntary control into the environment by a process called defecation.
10.2 Tonsils
10.2 Tonsils
The tongue (image 10.2a) forms the floor of the oral cavity. It is composed primarily of skeletal muscle covered with a stratified squamous epithelium. It has two major divisions: the anterior body (what you see when you protrude your tongue) and a posterior root (which you can only see on inspecting the back of your oral cavity). This posterior portion of the tongue is devoid of papillae, but contains two types of tonsils. Numerous rounded nodules populate the dorsal part of the tongue and are collectively called the lingual tonsils. Tonsils are basically masses of Iymphoid tissue covered by a mucous membrane. The tonsils are strategically placed near the opening of the digestive system so that the lymphocytes in the tonsils can directly attack bacteria, viruses, and other disease- causing organisms. Thus tonsils are defensive organs.
The two palatine tonsils, roughly the size of baked beans, are at their maximum size during early childhood (as seen in image 10.2a and in image 10.2b) and are located beside the lingual tonsils. The surface of the palatine tonsil appears cratered, like the surface of the moon, and there are numerous microscopic crypts located along the outer epithelium of these tonsils. These crypts can be seen as invaginations in the surface epithelium. Image 10.2c shows the crypts and the tonsil epithelium (labeled "a" in image 10.2c). Mucous glands located below these crypts secrete onto the surface of the tongue. The epithelium in these crypts is also often discontinuous, allowing easy migration of white blood cells in and out of the tonsil.
Image 10.2a: Tongue illustration.
Image 10.2b: Tongue with palatine tonsils.
Image 10.2c: Tonsil epithelium (labeled "a").
10.3 Tongue
10.3 Tongue
The tongue is a large, flat muscular organ found in the mouth. The anterior portion, the body, is constructed primarily of skeletal muscle covered with a stratified squamous epithelium (image 10.3a). The muscles within the tongue are oriented in a variety of directions so that it is capable of manipulating food, and changing its shape during speech and swallowing. External muscles also attach the tongue to the hyoid bone, the mandible, and the styloid processes of the temporal bones. These muscles help the tongue move as an entire organ and make it even more versatile for mastication, speech, and swallowing.
The mucous membrane covering the tongue is outwardly folded into numerous small projections called papillae. The most common type of projection on the top surface of the tongue as seen in image 10.3b is called the filiform papilla. These papillae, which can measure up to 3 mm in length, increase the surface area of epithelium on the tongue, thereby retaining fluids more easily to assist in the sensation of taste. Numerous taste buds are scattered over the surface of the tongue, and the "papillated" surface insures more adequate and prolonged exposure of chemical stimuli on that surface.
The circumvallate papilla as seen in image 10.3c is another kind of papilla present on the surface of the tongue. A dozen or less in total number, they form a V-shaped line at the posterior portion of the body of the tongue. The third type of papilla is called "fungiform" because of its resemblance to a mushroom, which can be seen in the fetal tongue of image 10.3a. These are scattered among the smaller filiform papillae all over the surface of the tongue and appear as elevated, rounded bumps on the surface. This fetal tongue, however, differs markedly from that of the adult. Many individual surface epithelial cells can be seen, and only a few rounded tubercles that represent the forming fungiform papillae (labeled "FFu" in image 10.3a) are present on the tongue at this developmental stage. The filiform papillae are not yet differentiated, but are clearly seen as elevated, rounded projections from the surface.
Image 10.3a: Fetal tongue with stratified squamous epithelium and fungiform papillae (FFu).
Image 10.3b: Filiform papillae of the tongue.
Image 10.3c: Circumvallate papillae of the tongue.
10.4 Papillae of the Tongue
10.4 Papillae of the Tongue
The conical projections on the dorsal surface of the adult tongue are called filiform papillae (labeled "Fi" in image 10.4a). Note that it resembles a shag carpet rather than a flat, bumpy surface seen in the fetal stage of development. Image 10.4b shows filiform papillae (labeled "a" in image 10.4b) and fungiform papillae (labeled "b" in image 10.4b) occuring all over the surface, but are especially common near the tip of the tongue.
The circumvallate papilla (labeled "CP" in image 10.4c) consists of a wall, or vallum, surrounding the central papilla and an intervening furrow, or moat (arrows in image 10.4c). Taste buds are concealed beneath the papilla epithelium. Observe the opening, or pore, of a taste bud (labeled "OTB" in image 10.4c) on the surface of the circumvallate papilla. Image 10.4d is a light microscope slide of a papilla. Note that the majority of taste buds are positioned along the lateral surface of the central papilla, so that they open into the furrow. The arrow points to glands below the surface of the tongue.
Image 10.4a: Filiform papillae (Fi).
Image 10.4b: Filiform papillae (labeled "a") and fungiform papillae (labeled "b").
Image 10.4c: Circumvallate papillae (CP), intervening furrow (arrows), and opening of the taste bud (OTB).
Image 10.4d: Light microscope image of papillae with the arrow pointing to the glands below the tongue surface.
10.5 Salivary Glands
10.5 Salivary Glands
Over a quart of saliva is secreted into the mouth every day. Some of this saliva is secreted by very tiny glands scattered within and below the surface epithelium of the tongue, but most of the saliva, however, is secreted by three pairs of salivary glands. Image 10.5a shows large salivary glands that are principally of the compound tubuloalveolar type, being composed of cellular tubules and alveoli. The largest of the paired salivary glands, the parotid glands, are located below and anterior to the ear (image 10.5b). Each parotid gland has a duct that crosses and penetrates the muscles of mastication and empties into the cavity of the mouth opposite the second upper molar tooth.
The submandibular glands (sometimes called the submaxillary glands) lie medial to the angle of the mandible (image 10.5c). The ducts of the submandibular glands travel anteriorly under the floor of the mouth to open at the base of the frenulum of the tongue. The sublingual glands are located on the floor of the mouth within a fold of mucous membrane. Each sublingual gland has several small ducts that open onto the floor of the mouth (image 10.5d). All of the salivary glands are innervated and can be triggered to secrete copious amounts of saliva by the presence of food in the mouth.
Image 10.5a: Salivary glands.
Image 10.5b: Illustration of salivary glands.
Image 10.5c: Submandibular or submaxillary glands.
Image 10.5d: Sublingual gland.
10.6 Esophagus
10.6 Esophagus
Once food is swallowed, it passes from the oral cavity into the oropharynx and then on into the laryngopharynx. From there the food enters the esophagus. Once in the esophagus, the food will be propelled by peristaltic movements down into the stomach. Thus the function of the esophagus is to conduct food from the throat into the stomach, a role which has earned it the name "gullet". The pharynx shares a conduction function with the respiratory system by serving to pass air into the lungs, but the esophagus is limited only to the transport of food and drink, and exclusive digestive system function.
Starting at the top of the esophagus and continuing all the way to the anus, the wall of the digestive tube has the same basic arrangement of four layers, or tunics, (as seen in image 10.6a). Although the structure of the wall is modified in various regions of the digestive tract, the four basic layers present are, from the inner lumen (labeled "Lu" in image 10.6a) of the digestive tube outwards: the mucosa (labeled "Mu" in image 10.6a), the submucosa (labeled "Su" in image 10.6a), the muscularis (labeled "ME" in image 10.6a), and the adventitia (labeled "Ad" in image 10.6a), also called the serosa.
The mucosa, or mucous membrane, consists of three layers: an epithelium, a connective tissue lamina propria, and a muscularis mucosa. The submucosa consists of loose connective tissue filled with collagenous and elastic fibers. Blood vessels, nerves, and lymphatic vessels are also present in the submucosa. Mucous glands, called esophageal glands (image 10.6b), may be present in some regions of the submucosa. The muscularis layer consists of an inner circular and an outer longitudinal layer, with striated muscle predominating in the upper third of the esophagus. Both smooth muscle and striated muscle are found in the middle third, whereas only smooth muscle is found in the muscularis of the lower third of the esophagus. The outer adventitia layer is composed of loose connective tissue, and a complete serosa surrounds only that portion of the esophagus below the diaphragm.
Image 10.6a: Four layers of the digestive tube: lumen (Lu), mucosa (Mu), submucosa (Su), and muscularis (ME).
Image 10.6b: Esophageal glands.
10.7 Esophageal Mucosa
10.7 Esophageal Mucosa
In image 10.7a the epithelial portion of the mucosa consists of stratified squamous epithelium (labeled "SS" in image 10.7a). This epithelial type protects the esophagus against abrasion from coarse food in the lumen (labeled "Lu" in image 10.7a). The thin connective tissue lamina propria (labeled "LP" in image 10.7a) of the mucosa is also apparent. Near the junction with the stomach, mucous glands called cardiac glands may be present in the lamina propria. The outermost layer of the mucosa is a thin layer of muscle called the muscularis mucosa (labeled "MM" in image 10.7a). This longitudinal band of smooth muscle is present in much of the wall of the esophagus, although it is absent in the upper portion. The muscularis mucosa permits localized, independent movement within the mucous membrane. A portion of the submucosa (labeled "Su" in image 10.7a) is also shown above. In addition to collagenous and elastic fibers, blood vessels (labeled "BV" in image 10.7a) fibroblasts, macrophages, and lymphocytes may be present in this layer.
Image 10.7b shows lymphocytic aggregates (arrow) in the esophageal lamina propria. Image 10.7c show a normal esophageal mucosa. The epithelial layer (labeled "a" in image 10.7c) and the lamina propria (labeled "b" in image 10.7c) are identified.
Image 10.7a: Esophageal mucosa: stratified squamous epithelium (SS), lumen (Lu), lamina propria (LP), muscularis mucosa (MM), submucosa (Su), and blood vessels (BV).
Image 10.7b: Lymphocytic aggregates (arrow) in lamina propria.
Image 10.7c: Normal esophageal mucosa epithelial layer (labeled "a") and lamina propria (labeled "b").
10.8 Stomach
10.8 Stomach
As seen in image 10.8a, the stomach is divided into four areas: cardia, fundus, body, and pylorus. The cardia surrounds the lower esophageal sphincter. The rounded portion above and to the right of the cardia is the fundus. Below the fundus is the large central portion of the stomach, called the body. The narrow, inferior region is the pylorus (antrum). The concave medial border of the stomach is called the lesser curvature, and the convex lateral border is the greater curvature. The pylorus communicates with the duodenum of the small intestine via a sphincter called the pyloric sphincter (valve). In image 10.8b, the stomach wall consists of the same four layers as the esophagus, but the mucosa (labeled "Mu" in image 10.8b) and muscularis (labeled "ML" in image 10.8b) are thicker in the stomach. In the empty stomach, the mucosa and submucosa may be folded. These folds, called rugae (labeled "Ru" in image 10.8b), disappear when the stomach is full. The numerous small openings (arrows in image 10.8b) on the surface of the rugae lead into the gastric pits, which secrete the enzymes so important for gastric function. A thin muscularis mucosae (labeled "MM" in image 10.8b) can be seen just internal to the submucosa (labeled "Su" in image 10.8b). The outer muscularis (labeled "ML" in image 10.8b) and the thin serosa (labeled "Se" in image 10.8b) can also be seen.
The stomach functions in the production of hydrochloric acid, mucus, digestive enzymes, and hormones. The inner layer of the stomach is called the mucosa, or mucous membrane. The lining epithelium of this layer consists of simple columnar cells that synthesize and release abundant amounts of mucus. Goblet cells are not differentiated in the gastric epithelium. The gastric mucosa is further characterized by numerous invaginations of the lining epithelium into the lamina propria layer of the mucosa. Each gastric gland is thus a tubular down growth of epithelium into the lamina propria. These invaginations result in the presence of numerous unbranched or branched tubes consisting of two majors regions: gastric pits and gastric glands. The luminal surface and the wall of the stomach are illustrated in the low-magnification scanning electron micrograph of image 10.8b.
In the micrograph in image 10.8c, the following layers are present: (1) the mucosa, or mucous membrane, which consists predominantly of numerous gastric pits and glands, (2) the thin muscularis mucosa (arrow in image 10.8c), consisting of smooth muscle and also constituting a portion of the mucosa.
Image 10.8a: Diagram of stomach.
Image 10.8b: Mucosa layer (Mu), muscularis layer (ML), rugae (Ru), openings leading to gastric pits (arrows), muscularis mucosae (MM), submucosa (Su), and serosa (Se).
Image 10.8c: Mucous membrane and muscularis mucosa (arrow) layers of the stomach.
10.9 Gastric Mucosa
10.9 Gastric Mucosa
The stomach functions principally in the digestion of proteins. The inner lining of the stomach is called the mucosa, and it consists of a simple columnar epithelium which is periodically modified into tubular glands, called gastric pits. The inner surface of the stomach wall and the upper portion of the gastric pits are lined by mucous-secreting calls. The mucous functions primarily in the protection of the stomach wall itself: preventing self-digestion by the powerful pepsin enzymes of the stomach's secretion. Each gastric pit is a tubular invagination of the epithelium downward into the lamina propria. These simple and branched invaginations are lined with two types of secretory cells: (1) chief cells which secrete the inactive form of pepsin, and (2) parietal cells which secrete the activating hydrochloric acid. The luminal surface and the wall of the stomach are shown in the low-magnification scanning electron micrograph of image 10.9a. The following layers are identified: (1) the mucosa, or mucous membrane, which consists predominantly of numerous gastric glands (labeled "GG" in image 10.9a), the thin muscularis mucosa (labeled "MM" in image 10.9a), consisting of smooth muscle which constitutes an inner portion of the mucosa, (2) the connective tissue of the submucosa (labeled "Su" in image 10.9a) layer, and (3) the smooth muscle band that forms the muscularis layer (labeled "ME" in image 10.9a). The position of a (4) thin outer serosa (labeled "Se" image 10.9a) is also shown along with an an artery (labeled "Ar" in image 10.9a) and a vein (labeled "Ve" in image 10.9a).
The stomach wall thus consists of the same four layers as the esophagus, but the mucosa and muscularis externa are thicker in the stomach. These structures are also seen in the microscopic section of image 10.9b. The inner mucosa contains many long, tubular gastric glands oriented perpendicular to the stomach wall. A small amount of connective tissue, the lamina propria, forms a packing between these gastric glands and secures their orientation. A layer of smooth muscle, the muscularis mucosa, forms the third layer of the mucosa, and assists in moving mucus secreted within the gastric glands into the stomach lumen. The submucosa consists of loosely packed connective tissue cells and fibers together with blood vessels, nerves, and lymphatic vessels. Thus the submucosa is a nutritive and supporting layer. The muscularis externa consists of spirally arranged smooth muscle bundles oriented in three major directions: an external longitudinal layer, a middle circular layer, and an inner oblique layer. The middle layer is particularly thick in the region of the cardiac and pyloric orifices and acts as a sphincter in these regions. The serosa is very thin and consists of a small amount of connective tissue covered by a single layer of squamous mesothelial cells.
Image 10.9a: Luminal surface and wall of stomach: gastric glands (GG), muscularis mucosa (MM), Submucosa (Su), muscularis layer (ME), serosa (Se), artery (Ar), and vein (Ve).
Image 10.9b: Microscopic section of layers of stomach wall.
10.10 Gastric Pits
10.10 Gastric Pits
The surface of the gastric mucosa is shown in image 10.10a. As viewed from the gastric lumen after removal of the mucous coat, several gastric pits (labeled "GP" in image 10.10a) are apparent. The cell boundaries of the surface epithelial cells (labeled "EC" in image 10.10a) are distinct. Epithelial cells extend downward to line each gastric pit, which is continuous below with a gastric gland (marked by the arrow in image 10.10b). The cells secrete mucus and electrolytes, such as calcium phosphate, sodium and potassium chloride, and sodium and potassium bicarbonate. The concentration of hydrochloric acid is potentially high enough to damage cells and tissues, but it is thought that the large quantity of mucus normally produced by the stomach protects the gastric mucosa from damage. The parietal cells (labeled "a" in image 10.10c) produce the hydrochloric acid. The zymogenic or chief cells (labeled "b" in image 10.10c) produce the enzyme, pepsinogen, that digest proteins.
Image 10.10a: Surface of gastric mucosa with identified gastric pits (GP) and epithelial cells (EC).
Image 10.10b: Gastric gland (arrow).
Image 10.10c: Parietal cells (labeled "a") and zymogenic cells (labeled "b").
10.11 Pancreas
10.11 Pancreas
The pancreas (image 10.11a) is located between the stomach and the duodenum. The pancreas can be divided into endocrine and exocrine portions. In image 10.11b, the endocrine portion consists of small aggregations of cells, the islets of Langerhans (labeled "b" in image 10.11b), which secrete hormones such as insulin, glucagon, and somatostatin into the portal venous system. The glandular epithelial cells of the exocrine pancreas are organized into acini (labeled "a" in image 10.11b), which convey their secretion into a branching system of ducts (labeled "c" in image 10.11c).
In image 10.11c, the parenchyma of the gland is divided into lobules by thin connective tissue septa that contain ducts, blood vessels, nerves, and lymphatic vessels. Each of the serous acinus cells, the islets of Langerhans, (labeled "a" in image 10.11c) is also closely surrounded by a supportive meshwork of connective tissue. Adjacent acinar cells are joined by junctional complexes, and their secretion is released at the apical pole of the cell. The secretory product of the acinar cells consists of enzymes that, once activated, aid in the digestion of food in the duodenum. By way of the pancreatic duct, the secretion is eventually emptied into the duodenum.
Image 10.11a: Pancreas.
Image 10.11b: Islets of Langerhans (labeled "b"), glandular epitelial cells of the exocrine pancreas (labeled "a"), and ducts (labeled "c").
Image 10.11c: Parenchymal gland with Islets of Langerhans (labeled "a").
10.12 Liver Lobule
10.12 Liver Lobule
Because the liver is a gland, its organization and function are best understood by dividing it into small repeating units shown image 10.12a. Each unit is commonly referred to as a liver lobule (image 10.12b). The classic lobule is polyhedral in shape, and consists of interconnecting plates of hepatocytes (labeled "Hc" in image 10.12b) that radiate toward a central vein (labeled "CV" in image 10.12b) located in the middle of the lobule. The central vein (labeled "a" in image 10.12c) can also be seen in image 10.12c. The classic lobule is bound on the periphery by groups of ducts and vessels, the most prominent of which is the conducting portal vein (labeled "CPV" in image 10.12b). Conducting portal veins branch at right angles to form distributing portal veins (labeled "DPV" in image 10.12b), which appear to delineate the periphery of the classic liver lobule. An arterial blood supply (labeled "Ar" in image 10.12b) with similar subdivisions also surrounds each lobule, and follows a path adjacent to that of larger portal vessels. Together, the portal venous blood and the arterial blood supply each lobule and mix upon entering a network of small sinusoidal capillaries within the lobule. Mixed blood, traveling via sinusoids, flows between intersecting plates of liver cells toward the middle of the lobule, where it empties into the central vein. From the central vein, blood flows into tributaries of the hepatic veins, which finally exit the liver through the hepatic veins to join the inferior vena cava. The sublobular vein (labeled "SLV" in image 10.12b), a tributary of the hepatic vein, is shown at the top of the micrograph.
Image 10.12d is a higher magnification of the components of the portal area, which supplies two or more adjacent lobules. They include branches of the portal vein (labeled "a" in image 10.12d), hepatic artery (labeled "b" in image 10.12d), and bile ductules (labeled "c" in image 10.12d) of different sizes. The bile ductules are distinguished from blood vessels at higher magnification by the presence of microvilli and cilia that project into their lumens. A single plate of hepatocytes, called the limiting plates, always surrounds each portal area.
Many anastomosing sinusoids between plates of hepatocytes are present within the lobule. In the living state, these sinusoids are packed with blood cells. Image 10.12e shows sinusoids (arrow). Kupffer's cells are also widely distributed within the hepatic sinusoids and are highly irregular in shape. Kupffer's cell processes may completely traverse the sinusoid. The cells are phagocytic (i.e. fixed macrophages) and are frequently located in regions where the sinusoids branch.
Image 10.12a: Liver lobules.
Image 10.12b: Liver lobule with hepatocytes (Hc), central vein (CV), conducting portal vein (CPV), distributing portal veins (DPV), arteries (Ar), and sublobular vein (SLV).
Image 10.12c: Central vein (labeled "a").
Image 10.12d: Components of portal area: portal vein (labeled "a"), hepatic artery (labeled "b"), and bile ductules (labeled "c").
Image 10.12e: Sinusoids (arrow).
10.13 Gallbladder
10.13 Gallbladder
The gallbladder is a pear-shaped sac located on the inferior surface of the liver. Bile produced by the liver is shunted to the gallbladder, where it can be stored and then concentrated before it is emptied into the duodenal portion of the small intestine. The topography of the luminal surface of the gallbladder is revealed in the low-magnification micrograph in image 10.13a. The folded gallbladder mucosa, which consists of a sheet of columnar epithelial cells (labeled "a" in image 10.13a) is anchored by a basement membrane to an underlying connective tissue layer, the lamina propria (labeled "b" in image 10.13a). The folded surface of the mucosa is brought about mainly by a folding of the underlying connective tissue layer. The epithelial layer follows these contours and, in addition, folds independently of the lamina propria to produce minor creases (arrows in micrograph in image 10.13b). Beneath the folded mucosa of the gallbladder lie interlacing sheets of smooth muscle. When stimulated, the muscle sheets within the gallbladder wall contract, emptying bile into a duct system leading to the small intestine.
In image 10.13c, the columnar cells and lamina propria are shown. These columnar cells absorb fluid (water) from the bile and help concentrate the bile before it is released into the intestine. Since major components of the bile (cholesterol) are poorly soluble in water, the concentrating process sometimes goes too far and gall stones can form due to crystallization of cholesterol (or bile salts) in the gallbladder. Image 10.13d is an example of gall stones formed in the gall bladder.
Image 10.13a: Folded gallbladder mucosa with columnar epithelial cells (labeled "a") anchored by lamina propria (labeled "b").
Image 10.13b: Epithelial layer folds independently of lamina propria, causing creases (arrows).
Image 10.13c: Columnar cells and lamina propria.
Image 10.13d: Stones formed in the gall bladder.
10.14 Small Intestine
10.14 Small Intestine
The tubular form of the small intestine is illustrated in image 10.14a in cross-section. The products of digestion are absorbed in the small intestine, which in humans is approximately 6 meters long. The small intestine is spatially divided along its length into the duodenum, jejunum, and ileum. Permanent folds of the small intestine are present in humans. These folds, called plicae circulares, or valves of Kerckring, are similar to those seen image 10.14b, but are absent in the scanning electron micrograph of image 10.14a. Many villi (labeled "Vi" in image 10.14a), actually folds of the mucosa layer, project 0.5-1 mm into the intestinal lumen (labeled "Lu" in image 10.14a). Villi are most widely distributed in the duodenum and least numerous in the ileum. They are long, broad, and leaf-like in the duodenum, tongue-like in the jejunum, and in the ileum they are more finger-like in shape. Clearly, the villi are a structural modification that serves greatly to increase the effective absorptive and secretory surface of the mucosa. The villi, in turn, are covered by a layer of simple columnar epithelial cells that have a striated apical border comprised of microvilli.
Unicellular mucous gland cells, called goblet cells, are interspersed among the absorptive cells. Goblet cells are fewest in number in the duodenum, but increase greatly in number toward the ileum, and their secretion (glycoproteins and sulfated mucopolysaccharides) protects the epithelial layer, lubricates the digestive tube, and participates in the formation of feces. The villi contain a connective tissue core, or lamina propria. Large blood vessels, capillaries, a lymph vessel (lacteal), and some smooth muscle cells of the muscularis mucosa are located in the lamina propria. The submucosa (labeled "Su" in image 10.14a) and muscularis (labeled "Mu" in image 10.14a) layers of the small intestine can also be distinguished in the low-magnification scanning electron micrograph in image 10.14a. The circular region in the center of the intestinal sample in image 10.14b is an example of a carcinoma of the colon.
Image 10.14c is a microscopic section of the jejunum of the small intestines. Image 10.14d shows both gross and microscopic examples of healthy (left of image 10.14d) and pathologic (right of image 10.14d) intestinal epithelium.
Image 10.14a: Small intestine structures/layers: villi (Vi), lumen (Lu), submucosa (Su), and muscularis (Mu).
Image 10.14b: Plicae circulares (or folds) of the small intestines.
Image 10.14c: Microscopic section of jejunum of the small intestines.
Image 10.14d: Normal (left) verses pathologic (right) intestinal epithelium.
10.15 Intestinal Villi
10.15 Intestinal Villi
Villi (labeled "Vi" in image 10.15a) are broadest in the duodenum. Note the long leaf-like villi of the duodenum in the scanning electron micrograph on the left side image 10.15a. Compare these villi to the shorter, tongue-like villi of the jejunum in the micrograph on the right side of image 10.15a. In this right side, it is possible to discern several villi, which are elevations, or outward evaginations, of the mucosal floor (labeled "MF" in image 10.15a). Tubular invaginations of the mucosa floor occur at the bases of villi and are called intestinal glands, or crypts of Lieberkuhn. Intestinal villi are covered with a single layer of epithelial cells that is continuous with the layer of epithelium that lines the intestinal glands. In image 10.15b, the arrows are on the epithelium of a section of the jejunum. Intestinal glands in the duodenum are longer than those found in the jejunum and ileum, and possess a narrower lumen.
Image 10.15c is a cross-sectional view of the crypts (labeled "b" in image 10.15c) and Peyer's patches (labeled "a" in image 10.15c) which are lymphoid nodules found mainly in the ileum. Image 10.15d shows the central core of each villus (labeled "a" in image 10.15d) in the intestinal mucosa. Blood vessels, nerves, and a centrally located lacteal are contained within the connective tissue of the villus. The replacement rate of the absorptive cells of the mucosal epithelium is approximately 2-4 days.
Image 10.15a: Villi of the duodenum (left) and jejunum (right).
Image 10.15b: Epithelium of the jejunum (arrows).
Image 10.15c: Crypts (labeled "b") and Pey'er's patches (labeled "a").
Image 10.15d: Central core of the villus (labeled "a") in the intestinal mucosa.
10.16 Tubular Glands
10.16 Tubular Glands
The distribution of the numerous tubular intestinal glands (labeled "TG" in image 10.16a) of the mucosa of the colon is shown in image 10.16a. The luminal opening of the tubular gland (labeled "Lu" in image 10.16a) is also shown, along with the connective tissue (labeled "CT" in image 10.16a) of the lamina propria around the tubular glands. These glands secrete copious amounts of mucus into the colonic lumen where the sticky mucus functions as a "consolidator" for the feces. The mucus comes from goblet cells which line the glands, but there are also absorption cells, which help absorb salts and water, interspersed with the goblet cells. Image 10.16b shows a microscopic section of tubular glands using a special mucus stain, which shows the goblet cells as light-blue bulbs. Image 10.16c is another view of a goblet cell of a tubular gland or "colonic pits" (arrow in image 10.16c).
Image 10.16a: Tubular glands (TG) with its luminal opening (Lu) and connective tissue (CT).
Image 10.16b: Microscopic seciton of stained tubular gland.
Image 10.16c: Colonic pits (arrow).
10.17 Absorption Cells
10.17 Absorption Cells
The absorptive epithelial cells (labeled "AC" in image 10.17a) reside on a non-cellular basal lamina (labeled "BL" in image 10.17a), and their apical microvilli (labeled "MA" in image 10.17a) function to create a much greater surface area to facilitate absorption of nutrients. A light microscopic view of these cells is shown in image 10.17b. Note the vertically elongated absorption cells with their dense layer of microvilli (tip of arrow in image 10.17b) on their apical surface. These microvilli function to increase the surface area for absorption. Also observe the bulbous goblet cell (seen in the right upper corner of the epithelium in image 10.17b) which functions in the secretion of mucus (seen at the top right image 10.17b).
Image 10.17a: Absorptive epithelial cells (AC) with their microvilli (MA) on basal lamina (BL).
Image 10.17b: Light microscope image of absorptive epithelial cells with their microvilli (tip of arrow).
10.18 Appendix
10.18 Appendix
Towards the bottom of the cecum portion of the large intestine is a tubular evagination called the appendix (image 10.18a). The lumen of the appendix is usually small. The surface epithelium of the appendix is a mucous membrane consisting of columnar epithelium which is invaginated to form tubular crypts of Lieberkuhn. The epithelium is surrounded by connective tissue called the lamina propria. Underlying the lamina propria is a poorly developed muscularis mucosa. The submucosa is a thick layer of connective tissue, blood vessels, nerves and fat cells that underlies the muscularis mucosa. The arrow in image 10.18b points to a lymphatic nodule. Lymphatic nodules occur in large number in the lamina propria and submucosa of the appendix and serve a minor defensive function, but no major body function is assigned to the appendix, since it is considered to be a "vestigial" organ-that is, an organ that once had an important digestive function earlier in human evolution, but has now through natural selection regressed and shrunk to a small, non-functional structure.
Image 10.18a: Appendix.
Image 10.18b: Lymphatic nodule (tip of arrow).
10.19 Colon
10.19 Colon
A transverse section through a contracted portion of the colon wall is illustrated in the left of image 10.19a. Layers of the colon surrounding the central lumen (labeled "Lu" in image 10.19a) are highly folded, as is typical of an empty organ. The colon is made up of the same layers of the small intestine. They include a mucous membrane, or mucosa (labeled "Mu" in image 10.19a), a submucosa (labeled "Su" in image 10.19a), a muscularis externa (labeled "ME" in image 10.19a), and an outer serosa (labeled "Se" in image 10.19a). The surface of the mucosa adjacent to the lumen has a relatively smooth contour, since villi are lacking in the colon. The right of image 10.19a is a blow-up of the outlined part of the picture on the left. Image 10.19b shows the four basic layers of the colon: the mucosa (labeled "a" in image 10.19b), the submucosa (labeled "b" in image 10.19b), the muscularis externa (double arrows in image 10.19b), and the thin outer serosa.
Image 10.19a: Layers of the colon surroudning the lumen (Lu).
Image 10.19b: Layers of the colon: mucosa (labeled "a"), submucosa (labeled "b"), muscularis externa (double arrows), and serosa.
10.20 Rectum
10.20 Rectum
Image 10.20a is a transverse section of the colon and illustrates the lumen (labled "Lu" in image 10.20a) and the flexed mucosal surface. It also serves to illustrate the vast number of the tubular intestinal glands, since each of the small black dots represents an opening from a tubular gland into the lumen of the colon. Image 10.20b shows the pectinate line/anorectal junction/squamous-columnar (SC) junction. Notice the abrupt transition in epithelium from the columnar epithelial cells of the rectum to the squamous epithelium of the anus. A number of tubular glands are also shown. This is a very significant anatomical juncture since there is active mitosis of cells here and "border" disputes between the two different epithelia. This makes this area a common site for cancer development.
Image 10.20a: Colon illustrating the lumen (Lu).
Image 10.20b: Pectinate line/anorectal junction/squamous-columnar junction.
Created by Marissa Fukunaga, 2022Images by Bob Moeng, 2018Descriptions adapted from Bob Moeng, 2018
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