Contents | Introduction | Cells | Epithelia | Connective Tissue | Blood | Cartilage | Bone | Muscle | Nerves | Skin | Circulatory System | Respiratory System | Oral Cavity | Alimentary Canal | Pancreas Liver And Gallbladder | Urinary System | Immune System | Male Reproductive System | Female Reproductive System | Endocrine System | The Senses |Appendix | Glossary
The microanatomy of the alimentary canal is easy to comprehend if you remember that in all superb designs — be they architectural, automotive, or biologic — form follows function. The species homo sapiens is organized as a two-hole open tube. Food makes a linear journey from one of the tube to the other, with each segment of the alimentary canal performing a slightly different function. This process is reflected in (or, more correctly, permitted by) a series of successive microanatomic changes evident in tissue samples taken at strategic points along the canal's length.
In this chapter, we shall follow the path of swallowed food as it enters the esophagus; is propelled into the stomach, wherein it is churned, acid-hydrolyzed, and stored; passes through the pyloric sphincter into some 30 ft of small intestine, wherein it is enzymatically digested and absorbed; and finally enters the colon, where water is resorbed, leaving concentrated waste ready for expulsion.
The primary function of the digestive system is, of course, to remove needed nutrients from food, get them into the general circulation, and leave waste materials behind. To do this, the alimentary canal must secrete chemicals that aid food breakdown, absorb and transport nutrient molecules to blood and lymph, and propel food along its length. Given these three processes, understanding the nature of the basic histologic plan of the alimentary canal is simple. Figure 13-1, a cross section through the wall of the small intestine, shows the elements of the basic histologic plan of the alimentary canal.
A surface epithelium of short-lived cells comes into contact with the food and, where appropriate, secretes chemicals and absorbs nutrients. Directly beneath the epithelium lies a thin layer of loose connective tissue, the lamina propria. The lamina propria contains cells of the immune series that fight unwanted, possibly infective materials, and contains blood and lymph capillaries that not only take up digested nutrients, but also oxygenate and feed the epithelium.
The muscularis mucosae, difficult to identify in Figure 13-1, is usually a diffuse, ill-defined, gossamer-like skein of smooth muscle cells that imparts fine movements to the mucosa. Intestinal villi, for example, can shorten and wave about. Their movements result, in large part, from the efforts of the muscularis mucosae. Taken together, the epithelium, lamina propria, and muscularis mucosae form the mucosa.
The Submucosa is a bigger, tougher version of the Lamina Propria. A thick layer of dense connective tissue, the Submucosa serves several functions. It binds the Mucosa to a large muscle group, the muscularis extema; it contains a large population of immune cells, especially lymphocytes, that fight infection; it contains larger blood and lymph vessels whose finer branches enter the lamina propria; and, in the esophagus and duodenum, it can house submucosal glands.
The Muscularis Externa is a powerful group of smooth muscles set in bidirectional orientations. The inner circular layer constricts the lumen; the outer longitudinal layer shortens the tube. Together, as in a worm, they cause peristaltic movements that not only propel food, but also provide the motive force for venous return of blood from the gut.
The entire gut is wrapped in a collagenous bag, the Adventitia, which is a vascular, innervated extension of the mesenteries. The duodenum, applied against the wall of the body cavity, is covered by the Serosa, an extension of the parietal peritoneum.
Figure 13-1, Figure 13-2A. Light micrograph of a ctrodd section through the jejunum of the squirrel mondet showing the basic histolgic plan of the alimentary canal. 760 X
Figure 13-1B. Tracing of Fiure 13-1A drawn to clarify the major components of the wall of the alimentary canal.
After food is swallowed, it enters the esophagus, the first part of the alimentary canal. The esophagus is a short tube, some 10 inches long, that leads from the mouth to the stomach. Because the function of the esophagus - to deliver food from mouth to stomach - is relatively simple, its microanatomic architecture is relatively simple as well.
As in all segments of the alimentary canal, the wall of the esophagus is histologically subdivisible into four major regions: the Mucosa, the Submucosa, the Muscularis Externa, and the Adventitia. The Mucosa (MUC) and Submucosa (SUB) are illustrated at right. Figures A and B are matched light and electron micrographs of serial sections taken through the esophagus of a mouse, selected because its small size permits inclusion of most of the esophageal wall in a single cross section. The lumen (L) is at the top of the micrographs; the wall of the esophagus itself fills the rest of the field. The Mucosa is made up of the epithelium (EP), the Lamina Propria (LP), and the muscularis mucosae. The Mucosa lies atop the Submucosa, which, in turn, is surrounded by the Muscularis Externa. The Muscularis Externa is wrapped in the baglike Adventitia; neither the Muscularis Externa nor the Adventitia are evident in the figures at right.
The esophagus is lined by a stratified squamous epithelium. In humans this epithelium is nonkeratinized, whereas in rodents it is lightly keratinized. Because the epithelium serves to protect the esophagus from abrasion by the fast-moving swallowed bolus of food, the degree of keratinization of the epithelium is largely a reflection of the amount of roughage in the animal's diet. As is the case in all stratified squamous epithelia, the flat surface cells, or Squames, are periodically shed from the free surface and are replaced by the cells produced by the mitotic activity of the basal cell layer. In the figures at right, a single mitotic cell (arrow), caught in Metaphase, has been cut in a pair of adjacent serial sections and photographed by light and electron microscopy.
The epithelium rests on a highly cellular Lamina Propria made of loose connective tissue. Beneath the lamina propria is the Muscularis Mucosae, which is conspicuous in the human esophagus. In the mouse esophagus shown at right, however, the Muscularis Mucosae is not a distinct layer. Instead, it consists of scattered smooth muscle fibers, too small to be readily detected at this comparatively low magnification.
The entire Mucosa is supported by a strong Submucosa, a bed of dense connective tissue that contains Collagen fibers, elastic fibers, fat cells (A), and various vessels such as lymph capillaries (LC) blood capillaries (C), and postcapillary venules (V). The Muscularis Externa surrounding the Submucosa propels food down the esophagus by peristaltic contractions. Several striated muscle fibers of the Muscularis Externa (M) are evident in the lower left corner of Figure B. In the upper esophagus, the Muscularis Externa is made of striated muscle; in the lower esophagus, as in the remainder of the alimentary canal, it consists of Smooth Muscle.
Plate 13-1, Figures A and B. Matched pair of light and electron micrographs of serial cross sections taken through the mouse esophagus. A, fat cell (Adipocyte); C, capillary; EP, epithelium; L, lumen; LC, lymph capillary; LP, Lamina Propria; M, striated muscle fibers of Muscularis Externa (Figure B only); MUC, Mucosa; SUB, Submucosa; V, venule; arrow, cell in Mitosis. 850 X
After a swallowed mouthful of food has passed through the esophagus, it arrives in the stomach. The stomach wall, illustrated in the matched pair of light and electron micrographs at right, is markedly different in function - and hence structure - from the esophagus. Whereas the esophagus is a relatively simple conduit for food, the stomach is a highly complex, muscular holding tank that doubles as a blender. Its Mucosa houses glands that secrete enzymes and acids that accelerate the digestive process begun in the oral cavity. Although most absorption of digested food occurs further down in the alimentary canal in the intestines, some substances - water and alcohol, to name but two - are absorbed through the stomach wall and taken up into the general circulation.
The many functions performed by the stomach are reflected in its structural complexity. Initially, the microanatomy of the stomach wall can be difficult to comprehend. For example, histologic images often give the misleading impression that the surface of the stomach is thrown into small folds that resemble intestinal villi. In reality, the surface of the stomach is flat and perforated by conical invaginations called Gastric Pits. The Gastric Pits, in turn, are channels that lead to the gastric glands. The secretory products of the gastric glands, collectively called Gastric juice, contain proteolytic enzymes and acids and are quite corrosive. Consequently, the lining of the Gastric Pits and the surface of the stomach consist of special protective cells called surface Mucous Cells.
Figures A and B at right are matched light and electron micrographs of serial sections taken through the stomach wall of the mouse, here selected because its small size permits inclusion of an entire cross section on an EM specimen support screen. The lumen (L) is at the top of the field of view. The inner surface of the stomach, lined by surface Mucous Cells (S), makes contact with the lumen and its contents. In Figure B, the Mucus droplets that fill the apical pole of the cells are evident. Because the Gastric Pits (G) and the gastric glands tend to follow tortuous courses, some imagination is needed to see that the pits are continuous tunnels and the glands are simple tubes with cellular linings. The stomach, like the remainder of the alimentary canal, is lined by a simple Columnar Epithelium. The way in which this epithelium is bent, folded, and, in the case of the gastric glands, rolled into a tube can give the misleading impression that the epithelium lining the wall of the stomach is stratified.
The openings of several Gastric Pits (arrow) are evident in Figures A and B. Beneath the pits lie the gastric glands, whose lumens (*) open into the base of the pits. Close inspection of Figure B shows the profiles of several Gastric Gland lumens. Gastric glands are simple tubular glands whose walls are made up of several kinds of cells including Mucous Neck Cells, chief cells, and Parietal Cells. The chief cells (C), whose Cytoplasm is filled with Secretory Granules, make and release proenzymes such as pepsinogen. The Parietal Cells (P), also called oxyntic cells, secrete hydrochloric acid in surprisingly concentrated form. The tubular gastric glands are supported by the Lamina Propria which in the stomach is not the discrete layer it was in the esophagus. Beneath the gastric glands lies a well-developed Muscularis Mucosae (MM), under which lies the dense connective tissue of the Submucosa (SUB). The Submucosa, in turn, is surrounded by the contractile Muscularis Externa (ME), which, in the stomach, usually consists of three concentric layers: a circular, an oblique, and a longitudinal layer. The entire stomach is covered by the connective tissues of the Adventitia (A).
Plate 13-2, Figures A and B. Matched pair of light and electron micrographs of serial sections through the stomach. A, Adventitia; C, Chief Cell; G, lumen of gastric pit; L, lumen of stomach; ME, Muscularis Externa; MM, Muscularis Mucosae; P, parietal cell; S, surface mucous cell; SUB, Submucosa; arrow, opening of gastric pit; *, lumen of Gastric Gland. 1,100 X
Although most digestion of food occurs in the intestines, some preliminary processing occurs in the stomach. Here, the Gastric juice secreted by gastric glands is mixed with the semisolid food received from the esophagus. Aided by the churning movements of the stomach's strong muscles, Gastric juice converts material received from the esophagus into a pulpy mass of liquid called Chyme that is suitable for delivery into the small intestine.
Gastric juice consists largely of secretions from two types of cells: chief cells (also called principal cells and zymogenic cells) and Parietal Cells. Both cell types inhabit gastric glands. Figure A, a lowmagnification electron micrograph, shows the chief and Parietal Cells in position within a Gastric Gland; Figures B and C depict individual chief and Parietal Cells in greater detail.
Chief cells manufacture and secrete the proenzyme pepsinogen. Pepsinogen, which is made of protein, is released into the lumen of the stomach, where, at acid pH, it is converted to form pepsin, a proteolytic Enzyme that cleaves peptide bonds. Because the Chief Cell is primarily a protein factory, its ultrastructure reflects the microanatomic features associated with the production of protein for export - a Cytoplasm rich in rough Endoplasmic Reticulum and Secretory Granules. In Figures A and B, the basal pole of each Chief Cell (Ch) is loaded with densely packed Cisternae of the rough Endoplasmic Reticulum (RER); the apical pole is packed with Secretory Granules (S). At lower left in Figure A, a group of chief cells is clustered about the tiny lumen (*) of a Gastric Gland.
Parietal cells are remarkably different in structure and function from chief cells. Parietal Cells are not protein factories: instead, they secrete hydrochloric acid at astonishingly high (0.2-M) concentrations. Electron micrographs of Parietal Cells (Figures A and C) show an electron-lucent Cytoplasm containing many Mitochondria (M, Figure C). These Mitochondria are essential for hydrochloric acid secretion. They provide energy in the form of ATP for the generation of hydrogen ion from carbon dioxide and water, a reaction mediated by the Enzyme carbonic anhydrase. Hydrogen ions are pumped out across the parietal cell's surface into the lumen of the Gastric Gland. To facilitate this process, the surface area of the Plasma membrane of each parietal cell is greatly expanded and is extensively invaginated to form a secretory canaliculus (Ca, Figure C).
Another class of cell, the enterochromaffin cell, (also called the argentaffin or argyrophil cell), is also present in the gastric glands (see Figures A and B). Enterochromaffin Cells (E), which require special stains for viewing with the light microscope, are endocrine cells, secreting several biologically active substances including the hormone Gastrin. Gastrin is extremely important to proper stomach function; it promotes pepsinogen secretion by chief cells, promotes acid secretion by Parietal Cells, and facilitates movements of the stomach wall. Taken together, the secretory products of the three types of cells in the gastric glands-chief cells, Parietal Cells, and Enterochromaffin Cells-all serve to prepare ingested food for entry into the next segment of the alimentary canal, the duodenum of the small intestine.
Plate 13-3 Figure A. Electron micrograph of gastric glands within the mouse stomach. C, capillary, Ch, Chief Cell; E, enterochromaffin cell; LP, Lamina Propria; MM, Muscularis Mucosae; P, parietal cell; RER, rough ER; S, Secretory Granules; *, lumen of Gastric Gland. 3,500 X
Figure B. Chief Cell within Gastric Gland. Ch, Chief Cell; E, enterochromaffin cell; RER, rough ER; S, Secretory Granules. 5,600 X Figure C. Parietal cell within Gastric Gland. Ca, secretory canaliculus; M, mitochondrion; P, parietal cell. 7,800 X 148
Once the stomach has converted swallowed food into the pulpy mass called Chyme, the pyloric sphincter - the muscular gateway into the intestine - opens. Chyme then passes from the stomach into the duodenum, the first segment of the small intestine. Here, digestion and absorption begin in earnest. In man, the entire intestinal tract measures some 30 ft in length. Most of this long tube is small intestine, which is anatomically divided into three segments, the duodenum, the jejunum, and the ileum. The wall structures of these three segments share the common plan presented in the overview. Despite their common pattern of organization, however, the duodenum, jejunum, and ileum do have distinct histologic features that render them identifiable in sectioned material.
Of the three segments of the small intestine, the duodenum is the shortest. Measuring a mere 10 in. in length, the duodenum is highly efficient in the digestion and absorption of food. It receives digestive enzymes from the pancreas and Bile from the liver, which accelerate the digestion of materials in the intestinal lumen. The microanatomic organization of the duodenum reveals a number of structural specializations designed to increase its surface area, a feature crucial to an organ engaged in absorption.
First, the wall of the duodenum is thrown into large folds, the Plicae Circulares, which are visible to the naked eye. Second, the Mucosa of the duodenum is organized into a series of small projections, shaped like cactus leaves called villi, evident with the aid of the light microscope. Third, the surface of the epithelial cells atop the villi is highly modified to form Microvilli - tiny fingerlike projections of the Plasma membrane, visible at the level of the electron microscope. These three orders of folding-the Plicae Circulares, the villi, and the Microvilli - provide three stages of surface area amplification not only for the duodenum, but for the jejunum and ileum as well.
Figures A and B are a matched pair of light and electron micrographs of serial sections taken through several villi (V) and part of the Submucosa (SUB) of the duodenum. In these illustrations, the villi point to the right; the Submucosa is at the left. Each Villus is surrounded on all sides by the lumen (Lu) of the duodenum, normally filled with Chyme received from the stomach. The Villus is lined by a simple Columnar Epithelium (E) that may appear stratified when cut in oblique section. The free surface of the epithelium, as mentioned above, is covered by a prominent brush border made up of thousands of tiny Microvilli. The epithelium is supported by a connective tissue core, the Lamina Propria (LP), which contains prominent lymph capillaries called lacteals. A single Lacteal (L), cut along its length, is evident in the light (Figure A) and electron (Figure B) micrographs at right. Lacteals, with their thin walls and large-caliber lumens, often look like artifactual spaces under the light microscope. Because of their inherent low pressure they often collapse during tissue preparation and hence are hard to see. Lacteals are most conspicuous when filled with material recently absorbed by the Villus.
At the base of the villi lie the Intestinal Glands - simple coiled tubular glands often called the crypts of Lieberkuhn. The Intestinal Glands contain mitotically active stem cells that replace intestinal epithelial cells constantly shed from the tip of the Villus. Autoradiographic studies indicate that the life span of a typical intestinal epithelial cell is only 3 to 4 days. Beneath the villi and Intestinal Glands sits the Submucosa which in the duodenum contains conspicuous secretory glands called Brunner's Glands (BG). These will be illustrated at higher magnification on the following plate.
Plate 13-4, Figures A and B. Matched pair of light and electron micrographs of serial sections taken through the mouse duodenum. BG, Brunner's Glands; E, epithelium; IG, Intestinal Glands; L, Lacteal; Lu, lumen of duodenum; LP, Lamina Propria; SUB, Submucosa; V, Villus; *, brush border. Figure A, 550 X; Figure B, 675 X
Figure A is an electron micrograph of a longitudinal section through a single Villus of the duodenum. In this image, two morphologically distinct types of cells inhabit the simple Columnar Epithelium - columnar absorptive cells (C) and goblet cells (G).
Within the intestinal lumen (Lu), complex food-stuffs undergo extracellular digestion; that is, they are broken down into simpler components by digestive enzymes from the pancreas and by Bile from the liver. The end products pass from the lumen into the columnar absorptive cells, wherein further processing is carried out by the cytoplasmic machinery of the cells themselves. The nutrient materials are then transported from the columnar absorptive cells into the lamina propria (LP) that comprises the core of the Villus. The Lamina Propria is richly supplied with lymphatic capillaries (lacteals, L) and blood capillaries (*) that deliver the absorbed nutrients to the general circulation. The circulation, in turn, distributes nutrients to all of the body's cells.
Within the Lamina Propria, tiny bundles of autonomic nerves (N) innervate the Smooth Muscle fibers of the Muscularis Mucosae that move the villi. In addition, many cells of the immune system reside in the loose connective tissue of the Lamina Propria. Several Antibody-secreting Plasma cells (P) are present in Figure A. Since the intestinal epithelium is in direct contact with the "outside world," it is a prime portal for entry of pathogenic microorganisms and antigens. Consequently, the establishment of an immunologic first line of defense within the lamina propria does much to combat infection and disease that could otherwise run rampant throughout the tissues of the intestine or, worse yet, invade the general circulation.
Part of the Submucosa of the duodenum is illustrated in Figure B. Here, the base of the intestinal epithelium-composed of the Intestinal Glands-is at left; the Submucosa is at right. The Submucosa is a thick layer of dense connective tissue that contains Collagen fibrils (Co), elastic fibers, cells of the immune series, lymph capillaries (L), blood capillaries (*), larger blood vessels such as arterioles (A), and nerves (N). In addition, the Submucosa of the duodenum contains Brunner's Glands (BG), which provide a convenient histologic marker for the duodenum, for they are not present in any other segment of the small intestine. Brunner's Glands perform several crucial functions. They secrete bicarbonate ions, which buffer the acidic Chyme received from the stomach. In addition, they secrete Mucus that lubricates the epithelial surface over which the lumenal contents pass.
Plate 13-5, Figure A. Electron micrograph of longitudinal section through a Villus of the duodenum. C, columnar absorptive cell; G, Goblet Cell; L, lymph capillary (Lacteal); LP, Lamina Propria; Lu, lumen of duodenum; N, nerve bundle; P, Plasma cell; blood capillary; arrow, brush border; dotted line, boundary between epithelium and Lamina Propria. 1,100 X
Figure B. Electron micrograph of upper part of the Submucosa of the duodenum. A, Arteriole; BG, Brunner's Glands; Co, Collagen fibrils; IG, Intestinal Glands (crypts of Lieberkuhn) at the base of the epithelium; L, lymph capillary; LP, Lamina Propria; N, nerve; *, blood capillary; dotted line, boundary between Lamina Propria and Submucosa. 1,300 X
Having run its course through the duodenum, the partially digested Chyme enters the next segment of the small intestine - the jejunum. Some 8 ft long, the jejunum interconnects the duodenum with the ileum. Figures A and B at right are low-power electron micrographs of thin sections taken through the jejunum of the squirrel monkey. Figure A shows parts of several villi (V) projecting into the lumen; Figure B illustrates the lower aspect of the intestinal wall including the Intestinal Glands (IG), the Lamina Propria (LP), the Submucosa (SUB), and part of the Muscularis Externa (ME).
The Villus depicted in the center of Figure A has a convoluted profile, indicating that the Villus was partially contracted at the time of fixation. Villi are motile; they pump up and down at a rate of 6 cycles/ min, thereby maximizing surface exposure to gut contents and forcing nutrient-laden blood and lymph through capillaries and lacteals toward the larger plexus of vessels deep within the Submucosa. Like the duodenum, the jejunum is lined with a simple columnar epithelium containing columnar absorptive cells (C) and goblet cells (G). The free surface is lined by a brush border (BB) - rows of tightly packed Microvilli that provide a thirtyfold increase in epithelial surface area. The simple Columnar Epithelium covering the Villus is in direct contact with the gut contents. The core of the Villus is composed of the Lamina Propria (LP) and some Smooth Muscle fibers (*) sent in from the muscularis mucosae. In Figure A, part of the interface between epithelium and Lamina Propria has been marked with a dotted line. Within the core, the Smooth Muscle fibers are evident as dark, wavy, thin cells. The lymph capillaries, or lacteals, appear as thin-walled vessels that follow a tortuous course; they are easily distinguished from capillaries, because capillaries contain red blood cells and lymphatic vessels do not.
The lower portion of the wall of the monkey jejunum is illustrated in Figure B. Here we see the basal regions of the Intestinal Glands (IG) as they descend into the depths of the extensive Lamina Propria (LP). Within the lamina propria, one can see profiles of lacteals (La), capillaries (C), and numerous cells of the immune series. Fibroblasts abound amidst the network of connective tissue, as do scattered Smooth Muscle cells (*) of the Muscularis Mucosae. It is important to remember that often the Muscularis Mucosae is present not as a discrete layer of tissue, but rather as a series of Smooth Muscle fibers scattered throughout the Lamina Propria.
Beneath the highly cellular Lamina Propria lies the Submucosa (SUB). Figure B clearly illustrates the dramatic difference between the appearance of the Lamina Propria and the Submucosa; whereas the lamina propria consists of loose connective tissue, the Submucosa is made up of dense connective tissue. The heavy sheets of Collagen that make up the bulk of the Submucosa are penetrated by lymph vessels (La), small arterioles (A), capillaries, and tiny nerve bundles. The Submucosa acts, in a way, like a tubular Tendon; it provides a substrate against which the large muscles of the Muscularis Externa (ME) can contract. The rhythmic contractions of the Muscularis Externa power the peristaltic waves that propel food distally down the length of the intestinal tract.
Plate 13-6 Figure A. Longitudinal section through the tip of a Villus in the jejunum of the squirrel monkey. BB, brush border; C, columnar absorptive cell; G, Goblet Cell; L, lumen of intestine; LP, Lamina Propria; V, Villus; *, Smooth Muscle Fiber; dotted line, interface between epithelium and Lamina Propria. 1,300 X
Figure B. Electron micrograph of the lower aspect of the wall of the same jejunum. A, Arteriole; C, capillary; IG, intestinal gland; La, lymph vessel (Lacteal); LP, Lamina Propria; ME, Muscularis Externa; SUB, Submucosa; *, Smooth Muscle fibers. 800 X
Figures A and B at right are a matched pair of light and electron micrographs of serial sections taken through the lower region of the wall of the monkey jejunum. The Intestinal Glands (IG) are near the top of the field; the Lamina Propria (LP) and Submucosa (SUB) are in the middle; and the Muscularis Externa (ME) and Adventitia (A) lie at the bottom of the field.
One of the features of intestinal microanatomy most difficult to visualize is the upward projection of the villi from the intestinal surface in contrast to the downward projection of the Intestinal Glands beneath the intestinal surface. Figures A and B illustrate the portion of the Intestinal Glands that lies beneath the intestinal surface; the intestinal surface is not evident in the image. (To locate the relative positions of villi and Intestinal Glands, see Figure 13-1.)
The simple Columnar Epithelium that lines each Villus continues down through the intestinal surface as part of the epithelium that lines the Intestinal Glands. Each Villus contains a central Lacteal, or lymph capillary, that carries absorbed fats and other nutrients downward toward a plexus of lymph vessels deep in the Submucosa. Each of these central lacteals continues downward from the Villus, passes through the intestinal surface, and squeezes between adjacent Intestinal Glands en route to the Submucosa. Several of these central lacteals (La) are clearly evident in Figures A and B. When seen with the light microscope, as in Figure A, these vessels can easily be mistaken for empty spaces in the tissue. The electron micrograph in Figure B reveals that they are not empty spaces at all, but rather represent the distended lumens of lymph vessels lined by a very thin Endothelium. This delicate Endothelium is underlain by long, thin smooth muscle fibers (*), derived from the Muscularis Mucosae, whose contractions help to compress the lymphatic vessel and pump the chyle absorbed from the intestinal lumen downward along the length of the Lacteal.
This pumping motion imparted by the smooth muscles that lie along the length of the Lacteal not only moves the chyle through the Lacteal, but also compresses the adjacent Intestinal Glands. This action serves to move the secretions of the blind-ended tubular Intestinal Glands upward into the lumen of the jejunum.
The lacteals course through the highly cellular lamina propria that lies atop the Submucosa. In the figures at right, the Submucosa appears as a distinct layer of dense irregular connective tissue. Many arterioles (Ar) that penetrate the Submucosa give rise to capillary (C) networks that supply the Intestinal Glands and villi with blood. In addition, the Submucosa contains a fine network of autonomic nerves (collectively referred to as Meissner's Plexus) that innervate the smooth muscles that move the intestinal Mucosa.
A much larger group of autonomic nerves called Auerbach's plexus (AP) innervates the powerful smooth muscles of the Muscularis Externa. In the jejunum, the Smooth Muscle fibers of the Muscularis Externa are organized into two layers: the inner circular layer (IC) and the outer longitudinal layer (OL). The tissue at right has been cut in longitudinal section. Consequently, the Smooth Muscle fibers of the inner circular layer are shown in cross section; the fibers of the outer longitudinal layer are cut along their length. At the boundary between the inner circular and outer longitudinal layers, the nerve cells of Auerbach's plexus - autonomic neurons that assist in regulation of the peristaltic movements of the jejunum generated by the Muscularis Externa - are apparent.
At the bottom of the figure, the Adventitia is evident. The Adventitia, an envelope of loose connective tissue, wraps around the outside of the intestine and joins with itself to form the sheet of mesentery that suspends the small intestine from the dorsal body wall.
Plate 13-7, Figures A and B. Matched pair of light and electron micrographs of serial longitudinal sections taken through the lower half of the wall of the monkey jejunum. A, Adventitia; AP, Auerbach's plexus; Ar, Arteriole; C, capillary; IC, inner circular layer of muscularis externa; IG, Intestinal Glands; La, Lacteal; LP, Lamina Propria; ME, Muscularis Externa; OL, outer longitudinal layer of muscularis externa, SUB, Submucosa; *, Smooth Muscle in wall of Lacteal. 770 X
Figures A and B at right are low-power electron micrographs of the ileum, the most distal portion of the small intestine. The ileum, like the duodenum and jejunum, has villi that project into its lumen. The villi are lined by a simple Columnar Epithelium through which digested nutrients are absorbed.
Part of the epithelium lining a Villus of the ileum is shown in Figure A. Here, two morphologically distinct types of cells, columnar absorptive cells (C) and goblet cells (G), lie side by side. The epithelial cells rest on a highly cellular Lamina Propria (LP); the brush border (BB) at their free surface is in direct contact with the lumen (L), which here appears as a narrow cleft between two closely spaced neighboring villi. The ileum has a much higher epithelial population of Mucus-secreting goblet cells than the duodenum has, reflecting a histologic trend within the small intestine - i.e., the population density of goblet cells increases distally along the intestinal tract, because as food travels down the intestinal tract, more and more nutrients are absorbed, and the mass of material left behind in the gut lumen becomes more compact and requires more Mucus to lubricate its passage. This histologic trend facilitates the identification of different regions of the small intestine.
The lower region of the wall of the ileum is illustrated in Figure B. Here, the basal portions of the Intestinal Glands (IG), the laminal propria (LP) that surrounds them, the Submucosa (SUB), and the Muscularis Externa (ME) are evident. The Submucosa of the ileum depicted in Figure B is different from that of the jejunum, illustrated in Plate 13-7. This difference is due not to specific differences in the construction of the jejunum and ileum, but rather to the different size of the animal of origin. The jejunum was obtained from a monkey, and the ileum, from the mouse. The smaller the animal, the more delicate the construction of its intestinal wall.
In figure B, a number of cells at the base of the intestinal glands have a distinct microscopic appearance. These cells are the Paneth Cells (P) - cells that contain many large, electron - dense granules. It is thought that these dense granules contain Lysozyme, an Enzyme that breaks down bacterial cell walls. Paneth Cells are believed to destroy certain intestinal bacteria by Phagocytosis. Although Paneth Cells can be found in all regions of the small intestine, they are normally most numerous in the ileum.
In addition to Paneth Cells, the Intestinal Glands of the ileum contain intestinal epithelial cells, goblet cells (G), and dividing cells (M). The intestinal epithelial cells secrete an alkaline fluid that passes from the glands into the gut lumen. The dividing cells provide replacements for all classes of epithelial cells that are periodically shed from the tip of the Villus. Autoradiographic experiments have shown that mitotic activity within the intestinal epithelium is restricted to stem cells that reside within the Intestinal Glands. Daughter cells migrate upward and differentiate into columnar epithelial cells or goblet cells. From birth to death, intestinal epithelial cells live some 3 to 4 days; consequently, the turnover of intestinal epithelial cells is high. It is no wonder that antimitotic drugs, such as those often administered in the course of cancer chemotherapy, have profound side effects on the digestive system.
Plate 13-8, Figure A. Electron micrograph of the simple Columnar Epithelium lining a Villus of the mouse ileum. C, columnar absorptive cell; BB, brush border; G, Goblet Cell; LP, Lamina Propria; L, lumen of ileum. 3,800 X
Figure B. Electron micrograph of the same ileum showing the area beneath the intestinal surface. A, Arteriole; G, goblet cells; IC, inner circular layer of Muscularis Externa; IG, Intestinal Glands; L, lumen of intestinal gland; LP, Lamina Propria; M, dividing cell; ME, Muscularis Externa; OL, outer longitudinal layer; P, Paneth Cells; SUB, Submucosa. 1,300 X
During the process of digestion and absorption, the ingested material finally reaches the colon of the large intestine. The microanatomy of the colon is clearly illustrated at right by a matched pair of light and electron micrographs. Figure A is a light micrograph, and Figure B, an electron micrograph, of serial cross sections taken through the entire wall of the colon of the mouse.
The large intestine looks very different from the small intestine. First, it has no villi at all. The surface of the colon is quite flat and is perforated by holes that represent the openings (arrow) of Intestinal Glands (IG). The colon is primarily concerned with water resorption, a straightforward process that does not require the elaborate motile, absorptive, and conductive machinery of the Villus. Second, the bulk of the Mucosa of the colon is taken up by the Intestinal Glands. These are simple straight tubular glands that have a blind end near the Submucosa (SUB) and an opening (arrow) at the intestinal surface. The lumen of the glands (Lu) can be followed for some distance within a single section because of the straight tubular nature of the colon's Intestinal Glands. Each intestinal gland is surrounded by a network of loose connective tissue, the Lamina Propria (LP), that contains Collagen fibers, elastic fibers, smooth muscle fibers, and cells of the immune series such as Antibody-producing Plasma cells (*).
The Mucosa of the colon is richly supplied with blood, evident in the micrographs at right. Here, an Arteriole (A) passing between the Lamina Propria and the Submucosa has been cut in longitudinal section. Although the Arteriole is initially a bit difficult to identify in the light micrograph (Figure A), the electron microscope at the corresponding area in Figure B clearly resolves the cross-sectional images of the Smooth Muscle fibers wound spirally around the Arteriole's contractile wall. Just above the Arteriole lies a cross-sectional image of a thin-walled vein (V) filled with red blood cells.
Beneath the highly cellular Lamina Propria lies the dense connective tissue of the Submucosa. Details of the construction of the Submucosa are difficult to see at this relatively low magnification. At the junction between the Lamina Propria and the Submucosa, however, some dark, thin lines that represent Smooth Muscle fibers of the Muscularis Mucosae are evident (arrowhead). Although these fibers can be seen with the light microscope (Figure A), they are more clearly illustrated by electron microscopy (Figure B). Repeated comparison of light and electron micrographs of serial sections through the same structures facilitates identification of the somewhat blurry images inherent in light micrographs of cells and tissues.
Beneath the Submucosa lies the Muscularis Externa (ME), two sets of Smooth Muscle arranged in inner circular and outer longitudinal layers. Between the two layers is Auerbach's plexus (AP), a diffuse Ganglion of autonomic nerves that assists in the regulation of the peristaltic movements of the intestine generated by the Muscularis Externa.
Plate 13-9, Figures A and B. Matched pair of light and electron micrographs of serial cross sections through the entire wall of the mouse colon. A, Arteriole; AP, Auerbach's plexus; C, columnar absorptive cell; G, Goblet Cell; IG, intestinal gland; L, lumen of colon; Lu, lumen of intestinal gland; LP, Lamina Propria; ME, Muscularis Externa; S, artifactual space between Submucosa and Muscularis Externa; SUB, Submucosa; V, vein; *, Plasma cell; arrow, opening of intestinal gland; arrowheads, Smooth Muscle fibers of Muscularis Mucosae. 550 X
Figure A at right is an electron micrograph of the free surface of the colon. Here, the simple columnar epithelium that lines the colon's surface comes into direct contact with the contents of the lumen (L) of the large intestine. The lumen of the colon is continuous with the lumen (Lu) of each intestinal gland, here seen as an invagination of the epithelial surface. The surface of the colon is flat and lacks villi; the clefts evident in the tissue in Figure A are invaginations of the surface, rather than spaces between adjacent projections.
As in the small intestine, the epithelium that lines the colon is composed of columnar absorptive cells (C) and goblet cells (G). The free surface of the colon's epithelium is a brush border (BB) made up of thousands of closely packed Microvilli that greatly increase the surface area available for absorption. The apical surface of the Goblet Cell, too, bears a few short Microvilli; these disappear when Mucus is discharged onto the intestinal surface.
Goblet cells in several functional stages are evident in Figure A. Young goblet cells have a substantial amount of secretory Cytoplasm dedicated to the elaboration of the many mucigen droplets that eventually come to fill the apical pole of the cell. At maturity, the secretory Cytoplasm condenses and becomes dark-staining in the electron microscope. In the final stage of its life cycle, the Goblet Cell releases its Complement of mucigen droplets. Once outside of the cell, the mucigen droplets coalesce to form Mucus that coats the surface of the intestinal epithelium. A mature Goblet Cell (GD), caught in the act of releasing its Mucus into the lumen (Lu) of an intestinal gland, is evident at the right side of Figure A. Nearby, several dying cells (D) may represent empty goblet cells that have spent themselves.
The basal portion of several Intestinal Glands (IG) of the colon is illustrated in Figure B. Many goblet cells (G), quite different in appearance from those at the free surface of the epithelium, line the lumen (Lu) of each gland. Few columnar absorptive cells are evident, and Paneth Cells are quite rare in the colon's Intestinal Glands in healthy animals. A delicate Lamina Propria (LP) lies above a longitudinally sectioned Arteriole (A)-the same Arteriole photographed at lower magnification in Plate 13-9. Note the cross-sectional images of the many Smooth Muscle cells (SM) that are in the wall of the Arteriole.
Beneath the Lamina Propria and the Arteriole shown in Figure B, the images of Smooth Muscle of the muscularis mucosae (MM) are visible. The Muscularis Mucosae runs along the top of the Submucosa (SUB). A small space (S), an artifact of tissue preparation, facilitates identification of the boundary between the Submucosa and the Muscularis Externa (ME). Beneath the Muscularis Externa lies the Serosa, a thin layer of connective tissue and mesothelial cells that line the outer surface of the large intestine.
Plate 13-9, Figure A. Electron micrograph of the surface of the colon. BB, brush border; C, columnar absorptive cell; D, dying cell; G, goblet cell; GD, Goblet Cell discharging Mucus; L, lumen of colon; Lu, lumen of intestinal gland. 1,800 X
Figure B. Electron micrograph of the outer region of the wall of the mouse colon. A, Arteriole; G, Goblet Cell; IG, intestinal gland; Lu, lumen of intestinal gland; LP, Lamina Propria; ME, Muscularis Externa; MM, Muscularis Mucosae; S, artifactual space between Submucosa and Muscularis Externa; SM, Smooth Muscle in Arteriole wall; SUB, Submucosa. 1,900 X