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The Digestive System
The gastrointestinal tract is a long, continuous tube of varying diameter extending from the lips to the anus. It functions: (1) to alter the food mechanically and chemically to a form which can be absorbed, (2) to perform the processes of absorption, (3) to form and secrete bile and digestive enzymes and, (4) to eliminate the unabsorbed residue. The detailed structure of the parts of the digestive tube is related to their specific functions, but the overall architectural plan, particularly from the esophagus to the recto-anal junction, is for the most part, the same. The wall of the tube, particularly from the esophagus to the recto-anal junction, usually consists of four layers: mucous membrane (mucosa), submucosa, muscularis externa, and either a serosa or adventitia, depending on location.
The mucous membrane (mucosa) lining the gastrointestinal tract always consists of (a) a superficial layer of epithelium attached to the basement membrane, (b) the lamina propria containing collagen, reticular and sometimes elastic fibers. In the G.I. it usually contains (c) the muscularis mucosae, a thin band of smooth muscle fibers with an inner circular and an outer longitudinally oriented layer except in the esophagus, where it is a single longitudinal layer.
The submucosa is a layer of loose connective tissue containing the large blood and lymph vessels and nerves, and lies between the mucosa and muscularis externa. The collagen fibers are coarser than those in the lamina propria.
The muscularis externa consists of smooth muscle in most of the digestive tract, but is striated muscle in the upper part of the esophagus. Along most of the tract the externa consists of an inner circular and outer longitudinal layer.
A thin layer of connective tissue covers the muscularis externa, and is called the adventitia. If, however, the connective tissue is also covered by mesothelium, the two constitute a serosa. Glands of various types are present in the mucosa, submucosa, or as separate structures (i.e., liver, pancreas) outside the digestive tube.
Autonomic nerve plexi and ganglia are present in the submucosa (Submucosal or Meissner's plexus) and between the muscle layers (Myenteric or Auerbach's plexus). The submucosal plexus is often difficult to identify, and you will be asked to locate it only on slides where it is easily demonstrable.
The dorsal surface of the tongue has projections called primary papillae. These are connective tissue projections covered by the surface stratified squamous epithelium. Connective tissue extends up into each primary papilla to form secondary papillae. Primary papillae and their epithelial cover may be pointed (filiform), rounded (fungiform), formed by deep indentations into the lamina propria, leaving the surface of the papilla at about the surface of the tongue (foliate and circumvallate papillae). Taste buds are oval structures within the epithelium. Don't confuse them with secondary papillae.
Slide #49, odd and even (in even numbered boxes only up to box 190) is the posterior aspect of the tongue illustrating circumvallate and some filiform papillae. Observe the taste buds on the lateral surface of the circumvallate papilla. In many slides, ducts of serous glands connect with the base of the trench around circumvallate papillae. Identify skeletal muscle fibers.
A tooth may be divided into 3 parts: (a) the crown which is exposed, (b) one to three roots which lie in the socket of the alveolar bone, and (c) the neck, where crown and root meet. Enamel forms the external layer of the anatomic crown of the tooth. Enamel, which is 9597% by weight of hydroxyapatite, is the hardest substance in the body. Dentin is composed of about 70% by weight of mineral, and is slightly harder than bone. The odontoblasts form an apparently continuous lining on the pulpal surface of the dentin, and play a role in maintenance and repair. The pulp is a loose, vascular, delicate connective tissue which is richly supplied with nerves, some of which make intimate contact with the odontoblasts. The cementum is the external layer of the root. It resembles bone in composition. The tooth is held in its socket in the alveolar bone by means of the periodontal membrane which serves on one side as the periosteum of the alveolar bone, and on the other side as the periosteum of the cementum.
Observe the closely packed dentinal tubules which give the dentin a striated appearance. Now examine the pulp cavity. The layer of cells apposed to the dentin consists of odontoblasts. The light staining pre-dentin adjacent to the odontoblasts is not mineralized. Identify other constituents of the pulp cavity. Locate the cementum, which is a thin layer of osseous tissue composing the outer layer of the root. Where the cementum is very thin, it will be acellular, and maintained by cells of the periodontal membrane. At the deepest part of the root, it is thicker and occasionally may have small Haversian systems. Examine the periodontal membrane. Observe in what direction Sharpey's fibers run. Examine the gingiva (gum).
The esophagus has a structure typical of the four layered "general plan" with a relatively distinct mucosa, submucosa, muscularis externa and adventitia. Cardiac glands of the lamina propria, occur only in the superior and inferior portions of the esophagus and are not present in your slides. The muscularis mucosae is a well developed layer of smooth muscle. Submucosal glands are found scattered in the connective tissue of the submucosa. In contrast to most of the remainder of the digestive tract, the muscularis externa of the upper part of the esophagus is composed of striated muscle. Proceeding inferiorly, this is gradually replaced by smooth muscle. The adventitia is a layer of connective tissue continuous with that of the neck and mediastinum.
The stomach is a sack-like organ with a relatively strong muscular wall. Its epithelial lining is simple columnar and mucus-secreting. The surface of the mucosa is thrown into folds called rugae when the stomach is empty. This is made possible by the looseness of the submucosa and the tonus of the muscles. The surface is marked by closely spaced pits or grooves, called gastric pits (foveolae), into each of which several glands secrete. Gastric pits are about one-half the depth of the mucosa in the cardiac region, two-thirds in the pyloric region and only about one-fifth of the depth of the mucosa in the fundus and body of the stomach. Glands were named for the region of the stomach they occupied (cardiac, fundic, and pyloric) but glands of the fundus and body are now generally called gastric glands. Cardiac and pyloric glands secrete mucus; gastric glands secrete the enzymes and acid of the stomach. All glands are confined to the mucosa.
Examine slide #52, (fundic stomach) visually and by reversed ocular. Note the rugae and observe the pale region on the luminal surface which represents the gastric pits. Examine the luminal surface with a low power lens. Note that surface mucous cells (simple columnar epithelium) line the gastric pits. Observe the gastric glands in the mucosa which open directly into the gastric pits. Identify the three types of gland cells (1) body chief cells which are basophilic cells that secrete pepsinogen; (2) parietal cells which are round, acidophilic cells responsible for HCl production (they may be sometimes binucleate); What does the deep acidophilia indicate? (see Explanation 3) and (3) neck mucous cells or neck chief cells, which are in the superior half of the gastric glands (but comprise virtually all of the cells of cardiac and pyloric glands). The lamina propria is confined to narrow regions between glands.
All segments of the small intestine have plica circulares, folds of submucosa which thus contain muscularis mucosal and save to increase surface area. Villi are finger-like projections of the lamina propria, upon and between plicae circulares. Simple tubular intestinal glands (crypts of Leiberkuhn) lie in the lamina propria, opening between the villi, and their epithelium is continuous with that of the villi. The epithelium of villi contains absorptive cells with numerous microvilli on their free surface, (giving it a striated appearance), goblet cells, and enteroendocrine cells. At the base of the crypts are Paneth cells (cells with large acidophilic granules). Crypts also contain enteroendocrine cells and undifferentiated cells, (cells which give rise to other cell types). We will begin examination of the small intestine with the simplest of the three regions.
Examine slide #56 visually and by reversed ocular. The plica circulares project into the lumen and have a submucosal core. Observe the finger-like projections of villi that cover the surfaces of the plica. On the microscope study cross and longitudinal sections of villi. Identify the goblet cells and columnar absorptive cells. Examine the surface striations due to microvilli on the absorbtive cells. Observe the cells and vessels in the core of the villus. The central lacteal may be collapsed and will be difficult to see. It may appear as two closely approximated rows of flat cells. The capillaries lie close to the basement membrane of the epithelium.
Study the intestinal crypts (of Lieberkuhn), simple tubular glands that extend from the surface (between villi) to the muscularis mucosae. At the mouth of the gland, the epithelium is continuous with and resembles that of the villi. Note that the columnar cells of the crypt are lower in height and have a poorly developed striated border. Goblet cells are present. Identify mitotic cells. At the base of the crypts are large Paneth cells with large acidophilic granules these may be seen better on slide #55. These cells, as well as enteroendocrine cells, are best seen in crypts on slides 55 and 57. Enteroendocrine cells have small acidophilic granules at the base of the cell, and a nucleus near the lumen. Identify the submucosa. Are glands present? Identify the muscle layer. Is this a longitudinal or cross-section? Between the muscle layers, locate the myenteric plexus. The serosa with its mesothelial lining may be seen on parts of this section.
Study slide #55 using the directions for the jejunum. In particular, use this slide for the study of Paneth and enteroendocrine cells of the intestinal crypt. Study the submucosal glands (Brunner's glands) of the duodenum. These glands have a mucous type of secretion. The muscularis mucosae may be disrupted by the submucosal glands which thus appear to extend into the lamina propria.
The layers of the small intestine are continued into the large intestine. There are no villi, so the surface of the mucosa is smooth with indentations at the openings of the crypts. The crypts are deeper than those in the small intestine, and contain many goblet cells as well as undifferentiated epithelial cells and some enteroendocrine cells. The lamina propria resembles that of the small intestine. Scattered lymphatic nodules are present and may extend into the submucosa. Leukocytes commonly penetrate the epithelium, as in the small intestine. The muscularis mucosae is well developed. The submucosa and the inner circular layer of the muscularis externa have no special peculiarities. The longitudinal layer is of variable thickness. Three thick longitudinal bands of muscle (taenia coli) are placed around the circumference of the colon. In the rectum, the outer longitudinal layer becomes uniform in thickness. In the anal region, crypts become shorter and may disappear. About 1.5 cm above the anus, the epithelium becomes stratified squamous. At the level of the external sphincter, hair and sebaceous glands and large apocrine circumanal glands appear. The lamina propria in this region contains a plexus of large veins.
Inspect slide #59 (colon) visually or by reversed ocular. Identify the muscular layers and note if the outer layer is thickened at one region to form the taenia coli. Not all slides demonstrate it. On the microscope, compare the general features with those of the small intestines. Note the absence of villi. Is the section on your slide a longitudinal or cross-section? Examine the mucosa. How do the cells on the surface differ from those in crypts? Where are mitotic cells found? The submucosa and the inner circular layer of muscularis externa generally resemble those of small intestine. Examine them and identify their constituents. The nerve plexi are well demonstrated on this slide. Locate them. Note the abundant adipose tissue of the serosa.
Rectum and Anal Canal
Examine a longitudinal section through the recto-anal junction (Slide #60). Locate the mucosa and the junction between the simple columnar epithelium of the rectum proper and the stratified squamous epithelium of the anal canal. The simple columnar epithelium resembles that of the colon. Solitary lymph follicles may be present in the lamina propria. A serosa is not present at this level of the tract.
Study the anal canal lined with stratified squamous epithelium. Note the changes in the stratified squamous epithelium starting at the ano-rectal junction and proceeding caudally. The junction of simple columnar with non-cornified stratified squamous epithelium is the pectinate line. Proceeding distally, cornification of the epithelium occurs suddenly (line of Hilton) and the intercellular "adhesions" become better developed and easier to see. The muscularis mucosae gradually disappears. In the submucosa, strands of muscle appear (musculus submucosa ani). These strands may be traced through the circular muscle layer to the longitudinal layer from which they take their origin. The longitudinal layer of the muscularis externa becomes thinner and may have a few striated muscle fibers from the levator ani muscle. Observe the numerous blood vessels in the lamina propria and submucosa. The rectal veins are relatively small in this specimen, but they may be much larger and in extreme cases, form hemorrhoids.
Anal Canal and Exterior
Study a section of anal canal (slide #61). Examine the mucosa. Is the stratified squamous epithelium cornified? Are glands or ducts of glands present? Are hair follicles present? Note the general character of the lamina propria and submucosa. Some slides may contain a large duct near one end of the specimen, lined by pseudostratified or stratified epithelium. Examine the muscularis layers. Note the presence of striated muscle and its location.