Intestines Microanatomy Notes 2024-25 PDF

Microanatomy of the Intestines and Anal Canal Rod D. Braun Page 1 of 33 MICROANATOMY OF THE INTESTINES AND ANAL CANAL Learning Objectives: 1. Describe the microanatomy and function of the small intest...

Microanatomy of the Intestines and Anal Canal Rod D. Braun Page 1 of 33 MICROANATOMY OF THE INTESTINES AND ANAL CANAL Learning Objectives: 1. Describe the microanatomy and function of the small intestine. Compare and contrast the role of the stomach and the intestine in digestion. Describe the mechanisms used by the intestine to increase surface area for digestion and absorption. Diagram the structure of an intestinal villus, including its blood vascular and lymph vascular patterns. Compare and contrast villi and microvilli. Describe the glands of the submucosa, their location along the small intestine and their function. Describe the cell types in intestinal epithelium and be able to identify them in LM and TEM. Describe the functional organization of the enterocyte, including the specializations of its apical surface, junctional complex, and role of its basolateral domain. Describe the role of the enterocyte in the digestion/intake of proteins, lipids, sugars, gastric intrinsic factor, minerals, and water. Describe the role of the enterocyte in the secretion of secretory IgA. Describe the function of the secretions released by enteroendocrine cells in the intestines. Describe the cell content of the mucosal glands (crypts of Lieberkühn) and the functions of the cells. Describe the elements of the immune system found in the lamina propria and the epithelium as well as their function. Distinguish between duodenum and jejunum or ileum at LM level (H&E). 2. Describe the microanatomy and function of the large intestine and appendix. Compare and contrast the mucosa of the colon and small intestine and distinguish at LM level (H&E). Compare and contrast the muscularis externa of the colon and small intestine. Compare and contrast the histological organization of the appendix to that of the colon and small intestine. 3. Describe the microanatomy and function of the anal canal. Describe the changes in the histological organization of the alimentary canal wall from the upper through the lower anal canal. Microanatomy of the Intestines and Anal Canal Rod D. Braun Page 2 of 33 Lecture Content Outline I. Small intestine A. Features B. Layers of wall C. Cell types in epithelium D. Cell types in intestinal glands (crypts of Lieberkühn) E. Gut-associated lymphatic tissue (GALT) F. Distinctive characteristics of small intestine II. Large intestine A. Features B. Colon C. Appendix D. Rectum III. Anal canal A. Mucosa B. Submucosa C. Muscularis externa D. Fibrosa (adventitia) IV. Review of alimentary canal (digestive tract) wall A. Stomach B. Small intestines C. Large intestines Microanatomy of the Intestines and Anal Canal Rod D. Braun Page 3 of 33 I. SMALL INTESTINE A. FEATURES 1. Plays major role in both chemical digestion of food and absorption of products of digestion (see Digestion and Absorption lecture). 2. Divisions a. Duodenum: shortest segment, ~25 cm long b. Jejunum: ~2.5 m long c. Ileum: ~3.5 m long Figure 1. Diagram illustrating the layers of the intestinal wall for different portions of the small and large intestines. Modified from Human Histology: A Microfiche Atlas, Erlandsen & Magney (1985). Microanatomy of the Intestines and Anal Canal Rod D. Braun Page 4 of 33 3. Like the rest of the digestive tract (alimentary canal), its wall has four layers (Figure 1): mucosa, submucosa, muscularis externa, and serosa/adventitia (detailed in Section IB). Figure 2.. Layers of the wall of the small intestine (jejunum here). Note large submucosal folds (plicae circulares, one labeled PC) and finger-like mucosal projections, the villi (V). Muc: mucosa; SubM: submucosa; ME: muscularis externa; L: central lacteal th (lymphatic vessel). Serosa would be at bottom of section. Plate 60 from Ross & Pawlina, 6 ed. (2011). 4. The mucosa of the small intestine has special features that serve to increase the surface area available for digestion and absorption (Figure 1). Surface area is increased by: a. Plicae circulares (valves of Kerckring) i. Permanent submucosal folds (Figure 2). ii. Remember: Since the submucosa folds, the overlying mucosa also folds and increases the epithelial surface area. b. Villi i. Finger-like mucosal folds that project into the lumen (Figures 2 and 3). Microanatomy of the Intestines and Anal Canal Rod D. Braun Page 5 of 33 ii. These greatly increase epithelial surface area (Figure 3). iii. Do not confuse these with microvilli, which are cytoplasmic projections of individual cells (see Figure 3. SEM of cross-section through small Section IC). intestine. Lu: lumen, Mu: mucosa, Su: submucosa, Mu: muscularis externa, Vi: villi. From Tissues and Organs, Kessel and Kardon (1979). iv. Characteristic of small intestine only! B. LAYERS OF WALL (Figures 1 and 2) 1. Mucosa (Figures 2 and 4) a. Villi: As noted already, the mucosa is characterized by the presence of villi (Figures 3 and 4). i. Villi are covered by a simple columnar epithelium containing primarily enterocytes (absorptive cells), as well as other Figure 4. Diagram of mucosa of small intestine. Note simple columnar epithelium, underlying lamina propria, cell types (see and muscularis mucosae. Modified from Figure 17.18b th from Ross & Pawlina, 6 ed. (2011). Section IC). Microanatomy of the Intestines and Anal Canal Rod D. Braun Page 6 of 33 ii. Underlying the epithelium of the villi is a core of loose connective tissue, continuous with the rest of the lamina propria (Figures 4 and 5). b. Intestinal glands (crypts of Lieberkühn) i. Mucosal glands of the intestine are called Figure 5. LM of mucosa, including villi in small intestine (ileum) and crypts of Lieberkühn (intestinal glands). Muscularis mucosae at bottom. From Meyer (1970). intestinal glands or crypts of Lieberkühn. ii. Simple tubular glands that open at the base of the villi and empty into the lumen between adjacent villi. iii. Glands extend through the lamina propria to the muscularis mucosae (Figures 4 and 5). c. Lamina propria i. Delicate connective tissue that forms the core of the villus and surrounds the glands (Figure 5). ii. Contains blood and lymphatic vessels (Figure 4). Microanatomy of the Intestines and Anal Canal Rod D. Braun Page 7 of 33 iii. Network of fenestrated capillaries runs just under the epithelium (Figures 4 Figure 6. Lamina propria within villus of small intestine. Note and 6). presence of many plasma cells and a capillary just under epithelium at lower left. From Human Histology: A Microfiche Atlas, Erlandsen & Magney (1985). iv. Central lacteal (lymphatic vessel): blind-ending vessel that begins near tip of villus, runs through the central core of the villus, and drains to a plexus in the lamina propria and submucosa (Figures 2 and 4). v. Lymphatic infiltration is common - may be diffuse (Figures 5 and 6) or nodular. This is part of the gut-associated lymphatic tissue (GALT), which is found throughout the entire gastrointestinal tract (see Section IE). d. Muscularis mucosae: thin layers of smooth muscle, deep to glands (Figure 5). 2. Submucosa (Figure 2) a. Coarse collagenous connective tissue (dense irregular). b. Ganglion cells of submucosal (Meissner’s) plexus are present. Microanatomy of the Intestines and Anal Canal Rod D. Braun Page 8 of 33 c. Submucosal glands are present in duodenum ONLY (Figures 1 and 7). i. Glands are called Brunner's glands. Figure 7. Longitudinal section through wall of duodenum, showing presence of Brunner’s glands (BGl) in submucosa (SubM) and duct (D) connecting gland to lumen. Muc: mucosa, ii. They are ME: muscularis externa with inner circular (C) and outer longitudinal (L) layers, V: villi, MM: muscularis mucosae, S: th compound serosa. Plate 59 from Ross & Pawlina, 6 ed. (2011). branched tubular glands that produce an alkaline mucus, which buffers the acidic chyme entering from the pyloric stomach and raises its pH. iii. The ducts of Brunner’s glands empty into the crypts of Lieberkühn. 3. Muscularis externa (Figure 7, see also Figure 2) a. Two layers of smooth muscle: inner circular and outer longitudinal b. Ganglion cells of the myenteric plexus lie between the smooth muscle cell layers. c. Contraction of the inner circular layer of smooth muscle leads to constriction of a ring of smooth muscle and a decrease in diameter of that segment of the small intestine. Microanatomy of the Intestines and Anal Canal Rod D. Braun Page 9 of 33 Figure 8. Cells in villus epithelium and glands (crypts of Lieberkühn) of small intestine. Modified from Human Histology: A Microfiche Atlas, Erlandsen & Magney (1985). d. Contraction of the outer longitudinal layer leads to a decrease in length as well as an increase in diameter of that segment of the small intestine. 4. Serosa or adventitia a. Outermost layer is a serosa when the portion of the small intestine is intraperitoneal: initial portion of duodenum, jejunum, and ileum (Figure 7). b. Outermost layer is an adventitia when the portion of small intestine is retroperitoneal: rest of duodenum. Microanatomy of the Intestines and Anal Canal Rod D. Braun Page 10 of 33 C. CELL TYPES IN EPITHELIUM (Figure 8) 1. Enterocytes a. Columnar absorptive cells b. Primary cell type covering villi c. Replaced every 5-6 days by undifferentiated cells in the base of the crypt of Lieberkühn (Figure 8). d. Tall, regular microvilli at Figure 9. LM of epithelium of two neighboring villi. Note pale-staining goblet cells and striated border of microvilli apical surface of cells on enterocytes. From Human Histology: A Microfiche Atlas, Erlandsen & Magney (1985). greatly increase plasma membrane surface area for absorption. i. In LM sections, they are visible as the striated border (Figure 9). ii. In TEM sections, they can be clearly identified as microvilli (Figure 10). iii. Actin microfilaments Figure 10. TEM of epithelium in villus. Note enterocytes with microvilli on apical surface. Goblet within the microvilli are cell is at upper right. Lamina propria is at bottom of image. Same orientation as Figure 9. bundled by fimbrin and villin and are linked to the plasma membrane by myosin I and calmodulin (see the Microanatomy of Epithelium and Glands lecture). Microanatomy of the Intestines and Anal Canal Rod D. Braun Page 11 of 33 iv. Microfilaments interact with other actin and intermediate filaments (cytokeratins) in the terminal web and are anchored into the junctional complex. v. Microvilli are covered with a glycocalyx surface coat that contains different digestive Figure 11. TEM of apical surfaces of 3 enterocytes. Note microvilli and associated glycocalyx on apical enzymes (Figure surfaces, as well as junctional complex between two cells on left. From Human Histology: A Microfiche Atlas, 11). Erlandsen & Magney (1985). d. Junctional complexes form a tight seal near the apical margin (Figures 10 and 11). e. Functions of enterocytes (Figure 12): i. Uptake of ions via ion channels. ii. Digestion and uptake of carbohydrates: α. Plasma membrane of microvilli is covered with a glycocalyx surface coat, which includes enzymes, such as oligosaccharidases and disaccharidases, e.g., lactase and sucrase, for terminal digestion of carbohydrates to monosaccharides (Figures 12 and 13). Microanatomy of the Intestines and Anal Canal Rod D. Braun Page 12 of 33 Figure 12. Some of the functions of enterocytes in the small intestine. MVB: microvillus border; FFA: free fatty acid; MG: monoglyceride; 12C-FA: 12-chain fatty acid; AA: amino acid; IF: gastric intrinsic factor. Figure F11-B6 from Human Histology: A Microfiche Atlas, Erlandsen & Magney (1985). β. Monosaccharides are then absorbed by active transport and move to the basal membrane. There they are exported by other Figure 13. TEM of immunolocalization of sucrase (black dots) in glycocalyx of enterocyte in villus. From Human Histology: A Microfiche Atlas, transporters into Erlandsen & Magney (1985). the lamina propria of the villus, where they are picked up by fenestrated capillaries. γ. Details of carbohydrate digestion will be provided in the Digestion and Absorption lecture. Microanatomy of the Intestines and Anal Canal Rod D. Braun Page 13 of 33 iii. Uptake of peptides and amino acids: α. Present in the glycocalyx of the apical (microvilli) membrane is the enzyme enterokinase, which activates trypsinogen (from pancreas) in the lumen of the duodenum to form trypsin. β. Trypsin, in turn, activates other pancreatic proenzymes. γ. The activated pancreatic enzymes and peptidases within the glycocalyx (brush-border peptidases) cleave proteins to amino acids or very small peptides. δ. Most of the small peptides are hydrolyzed to amino acids by cytosolic peptidases, producing amino acids that exit the enterocyte by facilitated diffusion. ε. In the lamina propria, the amino acids are picked up by fenestrated capillaries (Figure 12). ζ. Details of protein digestion will be provided in the Digestion and Absorption lecture. Microanatomy of the Intestines and Anal Canal Rod D. Braun Page 14 of 33 iv. Digestion and uptake of lipids: α. In the intestinal lumen, lipids are broken down by pancreatic lipase and other pancreatic enzymes in the presence of bile salts to free fatty acids, monoglycerides, and other small lipids, which are solubilized in micelles. The micelles are transported to the apical membrane of the enterocytes, where the lipids diffuse into the cells. β. Smooth ER resynthesizes triglycerides (Figure 14) and transfers them to Golgi complexes for further processing and incorporation into vesicles with protein components (apoproteins). These vesicles are called chylomicrons (Figure 15). Figure 14. TEM of apical portion of enterocyte, with microvilli at top. Lipids are stained with osmium (black). Note SER filled with Figure 15. newly TEM of synthesized portion of triglycerides enterocyte, (black circles). showing formation of chylomicrons in Golgi (G). LD: lipid droplet. Microanatomy of the Intestines and Anal Canal Rod D. Braun Page 15 of 33 γ. The vesicles (chylomicrons) migrate to the lateral margin, where they are released by exocytosis into the intercellular space (Figure 16). Figure 17. LM of lacteals within villi of rat small intestine after a meal rich in fat. From Human Histology: A Microfiche Atlas, Erlandsen & Magney (1985). Figure 16. TEM of lateral margin between two enterocytes, showing electron-dense chylomicrons. δ. The chylomicrons then move toward lymphatic vessels (lacteals) in the center of the villus (Figures 2 and 17). ε. Details of lipid digestion will be provided in the Digestion and Absorption lecture. v. Uptake of vitamin B12: α. Gastric intrinsic factor (IF) is produced by parietal cells in gastric glands in fundic stomach (see the Microanatomy of the Esophagus and Stomach lecture). Microanatomy of the Intestines and Anal Canal Rod D. Braun Page 16 of 33 β. IF forms a complex with B12 in the intestinal lumen, and this complex can be taken up by enterocytes in the ileum, since they have IF receptors (Figure 12). γ. In the enterocyte, vitamin B12 is liberated and transported across the basal membrane into a fenestrated capillary in the lamina propria of the villus (Figure 12). δ. Vitamin B12 is necessary for RBC production. ε. Details of vitamin B12 uptake will be provided in the Digestion and Absorption lecture. vi. Recycling of unconjugated bile salts, which are returned to liver hepatocytes (see the Microanatomy of the Liver, Gallbladder, and Pancreas lecture). vii. Absorption of water: α. Na+-K+ ATPase in the lateral plasma membranes moves Na+ ions into the intercellular spaces and water follows. Microanatomy of the Intestines and Anal Canal Rod D. Braun Page 17 of 33 β. During active absorption, lateral plications separate, enlarging the space. γ. Together, the small and large intestines absorb approximately 8.5 liters of fluid daily. δ. Most is absorbed by the epithelial cells of the small intestine (~6.5L). The rest is absorbed by the colon. Figure 18. Production and release of secretory IgA (sIgA) in the small intestine. From Human Histology: A Microfiche Atlas, Erlandsen & Magney (1985). viii. Release of secretory IgA at the luminal surface (Figure 18): α. IgA is synthesized by plasma cells in the lamina propria (Figure 6) as a dimer. Microanatomy of the Intestines and Anal Canal Rod D. Braun Page 18 of 33 β. IgA is taken up by receptor-mediated endocytosis at the basal surface of enterocytes. γ. IgA-receptor complex is transported up through the cell, and is then released at the apical margin with the stabilizing portion of the receptor still attached. δ. This complex, called secretory IgA (sIgA), is less easily degraded within the intestinal lumen. 2. Goblet cells (Figures 9, 10, and 19) a. Unicellular exocrine glands. b. Apical cytoplasm packed with granules of the glycoprotein mucinogen (Figure 19), which is Figure 19. TEM of goblet cell among enterocytes. From Human Histology: A Microfiche Atlas, released to form a coat of Erlandsen & Magney (1985). mucus. 3. Enteroendocrine cells (Figure 20) a. Contain basal granules that are released into the underlying lamina propria (Figure 20). b. Make up 1% of cell population. Microanatomy of the Intestines and Anal Canal Rod D. Braun Page 19 of 33 c. As noted in the Microanatomy of the Esophagus and Stomach lecture, there are more than a dozen types of enteroendocrine cells throughout the digestive tract. Some are listed in that lecture, and the functions of many of them will be discussed in the Regulation of the Alimentary Canal and Microanatomy of the Liver, Gallbladder, and Pancreas Figure 20. TEM of enteroendocrine cells. lectures. Note polarization of secretory granules near basal surface. From Human Histology: A Microfiche Atlas, Erlandsen & Magney (1985). D. CELL TYPES IN INTESTINAL GLANDS (CRYPTS OF LIEBERKÜHN) (Figure 8) 1. Immature enterocytes 2. Goblet cells (see above) 3. Enteroendocrine cells 4. Paneth cells a. Located in the base of the crypts of Lieberkühn (Figures 8 and 21). b. Contain prominent acidophilic granules Figure 21. LM of Paneth cells in base of crypts of Lieberkühn. Note large distinct eosinophilic (acidophilic) granules in (Figure 21). Paneth cells. From Human Histology: A Microfiche Atlas, Erlandsen & Magney (1985). Microanatomy of the Intestines and Anal Canal Rod D. Braun Page 20 of 33 c. Products of Paneth cells i. Lysozyme: degrades bacterial surface coats. ii. Defensins: increase membrane permeability of parasites and bacteria. 5. Undifferentiated stem cells a. Precursors of other cells in the epithelium and glands. b. Like Paneth cells, the undifferentiated stem Figure 22. PAS-hematoxylin-stained LM of crypts of Lieberkühn in mouse after injection of tritiated thymidine cells are located in the (black dots), which was incorporated by dividing, undifferentiated cells. From Human Histology: A Microfiche base of the crypts of Atlas, Erlandsen & Magney (1985). Lieberkühn (Figures 22). c. Cells undergo mitosis within the glands (Figure 22), then migrate toward an extrusion zone at tips of villi (Figure 8). d. Replace epithelium every 5-6 days. Paneth and enteroendocrine cells are replaced more slowly. E. GUT-ASSOCIATED LYMPHATIC TISSUE (GALT) 1. Lymphatic infiltration is common in small intestine and may be diffuse or nodular (Figure 23). Lymphatic tissue will be discussed in the Figure 23. LM of lymphatic nodules in ileum, part of gut- Microanatomy of the associated lymphatic tissue (GALT). From Meyer (1970). Microanatomy of the Intestines and Anal Canal Rod D. Braun Page 21 of 33 Lymphatic System lecture later in the curriculum. a. Diffuse lymphatic tissue: Loose collection of T and B lymphocytes, plasma cells, and antigen presenting cells (APCs). b. Nodular lymphatic tissue: Localized concentrations of B lymphocytes organized in lymphatic nodules. 2. This is part of the gut-associated lymphatic tissue (GALT), which is found throughout the entire GI tract. 3. Lymphoid elements in connective tissue of lamina propria and/or submucosa (Figure 23). a. Lymphatic nodules with B cells b. Diffuse lymphatic tissue 4. Nodular (follicular) GALT is covered by a specialized epithelium (Figure 24) that includes: a. M (microfold) cells i. Flattened cells with short apical folds of the plasma membrane. Figure 24. Top: LM of specialized epithelium overlying the dome of a Peyer’s patch. From Human Histology: A Microfiche Atlas, Erlandsen & Magney (1985). Bottom: Schematic of specialized epitheium showing ii. Take up antigens function of M cells. From “Vivo Pathophysiology” (http://www.vivo.colostate.edu/hbooks/pathphys/digesti and transport them on/smallgut/mcells.html). to underlying lymphocytes. Microanatomy of the Intestines and Anal Canal Rod D. Braun Page 22 of 33 b. Dendritic cells: sample and transport antigen from the lumen. c. Enterocytes: assist by degrading antigens 5. Nodular form of GALT is most prominent in the ileum, where it can be seen macroscopically as Peyer’s patches. F. DISTINCTIVE CHARACTERISTICS OF SMALL INTESTINE 1. Characterized by the presence of villi, not found elsewhere. 2. Portions of small intestine a. Duodenum: identified by the presence of Brunner's glands in the submucosa (Figures 25 and 26). b. Jejunum: no submucosal glands Figure 25. LM of wall of duodenum. Note Brunner’s gland in submucosa. From Meyer (1970). c. Ileum i. No submucosal glands (Figure 27). ii. Tends to show an increase in the number of goblet cells and the amount of lymphatic infiltration, but, for the purposes of this course, not distinguishable from jejunum. iii. Site of Peyer's patches. Microanatomy of the Intestines and Anal Canal Rod D. Braun Page 23 of 33 Figure 26. Low mag LM of wall of duodenum. Note Brunner’s glands Figure 27. Low mag LM of wall of ileum. Note absence of in submucosa. They empty into intestinal crypts. From Meyer (1970). Brunner’s glands in submucosa. From Meyer (1970). II. LARGE INTESTINE A. FEATURES 1. Plays major role in reabsorption of electrolytes and water and elimination of undigested food (feces). 2. Divisions a. Cecum, with projecting vermiform appendix b. Colon (ascending, transverse, descending, and sigmoid) c. Rectum d. Anal canal B. COLON Unlike the small intestine, the colon has no villi, only crypts of Lieberkühn Figure 28. SEM of cross-section through large (Figure 1). It looks like a flat surface intestine (colon). Lu: lumen, ML: muscularis externa, TG: crypts of Lieberkühn. From Tissues and Organs, with test tubes stuck into it (Figure 28). Kessel and Kardon (1979). Microanatomy of the Intestines and Anal Canal Rod D. Braun Page 24 of 33 The wall of the colon (cecum is similar) consists of the following layers (Figures 29 and 30): 1. Mucosa (Figure 31) a. Mucosa has a smooth surface (no villi) Figure 29. Diagram of the wall of the colon. Note taenia coli. Modified from Figure 15-16 in Atlas of Histology st (Cui, 1 ed., 2011). Figure 30. LM of wall of colon, with lumen at top. Note the taenia coli at bottom center and the absence of villi in the Figure 31. LM of mucosa in colon. Note absence of villi mucosa (top). From Meyer (1970). and straight crypts of Lieberkühn. From Meyer (1970). b. Epithelium is simple columnar with following cell types (Figure 31): i. Enterocytes with the primary role of reabsorption of electrolytes and water. Water is absorbed through action of the Na+-K+ ATPase transport system (see Section I.C.1.e). ii. Goblet cells: produce mucus Microanatomy of the Intestines and Anal Canal Rod D. Braun Page 25 of 33 c. Simple tubular glands (crypts of Lieberkühn, Figure 31) containing: i. Immature enterocytes ii. Goblet cells, which increase in number as the rectum is approached iii. Enteroendocrine cells iv. Undifferentiated stem cells: surface renewal every 5-6 days v. Note: Paneth cells are lost from the glands at the beginning of the cecum. d. Lamina propria with GALT e. Muscularis mucosae 2. Submucosa: no glands, but lymphatic tissue is usually present (Figure 30). 3. Muscularis externa (Figure 30) a. Continuous inner circular layer of smooth muscle b. Outer longitudinal layer of smooth muscle is organized into 3 distinct and separate bands, the taeniae coli (Figures 29 and 30). 4. Serosa or adventitia (fibrosa): depends on location. Microanatomy of the Intestines and Anal Canal Rod D. Braun Page 26 of 33 C. APPENDIX 1. Similar to colon, except for a smaller diameter, and the presence of a continuous outer longitudinal layer of smooth muscle. i.e., there are no taenia coli (Figure 32). 2. Usually has more extensive lymphatic infiltration, especially more lymphatic nodules, than the colon. Figure 32. Transverse section of appendix. Note numerous large lymphatic nodules. Figure 17.30 th from Ross & Pawlina, 6 ed. (2011). 3. The appendix is covered by a serosa. D. RECTUM 1. Similar to the colon, but it has no taenia coli. 2. Surface epithelium is predominantly goblet cells. 3. Crypts of Lieberkühn lined predominantly by goblet cells Continued on next page. Microanatomy of the Intestines and Anal Canal Rod D. Braun Page 27 of 33 Figure 33. LM of anal canal. Mucosa of upper anal canal extends from top left until angled line marking the beginning of the lower anal canal. Lower anal canal is located between the angled and straight vertical lines. MM: muscularis mucosae, StS: stratified squamous nonkeratinized epithelium, StS(k): stratified squamous nonkeratinized epithelium, IAS: th internal anal sphincter, EAS: external anal sphincter. Modified from Plate 64 (upper panel) in Ross & Pawlina, 6 ed. III. ANAL CANAL The anal canal is the most distal portion of the alimentary canal and is about 4 cm long. It has the following layers: A. Mucosa 1. Upper anal canal resembles rectum of large intestine. a. Simple columnar epithelium b. Simple tubular glands (crypts of Lieberkühn): contain many goblet cells, as well as undifferentiated cells and a few enteroendocrine cells (Figure 33). 2. As the upper anal canal transitions into the lower anal canal, there are three changes in the mucosa (Box A in Figure 33, Figure 34). a. There is a transition from simple columnar to stratified columnar epithelium (sometimes stratified cuboidal then stratified columnar). Microanatomy of the Intestines and Anal Canal Rod D. Braun Page 28 of 33 b. Crypts of Lieberkühn are lost. c. Muscularis mucosae disappears. 3. Changes in epithelium Figure 34. LM of transition between upper and lower anal within the lower anal canal (Box A in Figure 34). Note: 1) Transition from simple columnar to stratified columnar epithelium; 2) Loss of crypts of Lieberkühn; 3) Loss of muscularis mucosae Modified canal: th from Plate 64 (lower left panel) in Ross & Pawlina, 6 ed. (2011). a. Epithelium will further transition to stratified squamous nonkeratinized epithelium (Box B in Figure 33). b. At the end of the anal canal, there is a transition of the stratified squamous nonkeratinized epithelium to the stratified squamous keratinized epithelium of thin skin (Box B in Figure 33, Figure 35). Figure 35. LM of transition between lower anal canal and external thin skin (Box B in Figure 34).. Note transition from nonkeratinized to keratinized stratified squamous epithelium. B. Submucosa: highly vascularized with Modified from Plate 64 (lower right panel) in Ross th & Pawlina, 6 ed. (2011). extensive plexus of hemorrhoidal veins. Microanatomy of the Intestines and Anal Canal Rod D. Braun Page 29 of 33 C. Muscularis externa (Figures 33, 36) 1. As the upper anal canal transitions to the lower anal canal, the inner circular layer of smooth muscle forms the internal anal sphincter. 2. Within the lower anal canal, the outer longitudinal layer of smooth muscle terminates in adjacent connective tissue. 3. An increasing layer of skeletal muscle forms the external anal Figure 36. LM of anal canal. Modified from Figure 17.37a in Histology: A Text and Atlas (Ross & Pawlina, sphincter (Figure 36). th 5 ed., 2005). D. Adventitia (Fibrosa): The anal canal is retroperitoneal, so it is covered by an adventitia. IV. REVIEW OF ALIMENTARY CANAL WALL (Figure 37) A. STOMACH 1. Rugae and mammilated areas are present, but there are no villi in the stomach! 2. Mucosal glands are simple branched tubular glands that empty into gastric pits. 3. Three poorly defined layers of smooth muscle in muscularis externa. Microanatomy of the Intestines and Anal Canal Rod D. Braun Page 30 of 33 Figure 37. Schematic of layers of the wall of the alimentary canal (digestive tract, GI tract), from the stomach through the large intestine (colon). Courtesy of Dr. Roberta G. Pourcho. B. SMALL INTESTINES 1. Finger-like mucosal projections into lumen are called villi and are only present in small intestine. 2. Mucosal glands are relatively short simple tubular glands that open at the base of the villi (crypts of Lieberkühn). 3. Submucosal glands are only present in duodenum (Brunner’s glands). a. Villi + submucosal glands = duodenum. b. Villi but no submucosal glands = jejunum or ileum Microanatomy of the Intestines and Anal Canal Rod D. Braun Page 31 of 33 4. Two well-defined smooth muscle layers in muscularis externa: inner circular and outer longitudinal. C. LARGE INTESTINES 1. Surface of mucosa is smooth. No villi in large intestines! 2. Mucosal glands are relatively long simple tubular glands (crypts of Lieberkühn). 3. Muscularis externa a. Two layers of smooth muscle in muscularis externa: inner circular and outer longitudinal. b. In colon, outer longitudinal layer is organized into three distinct and separate bands, the taeniae coli. References: Erlandsen, S.L. and Magney, J.E., Human Histology: A Microfiche Atlas, University of Minnesota Press: Minneapolis, 1985. Gartner, L.P. and Hiatt, J.L., Color Atlas of Histology, 3rd ed., Lippincott Williams & Wilkins: Philadelphia, 2000, Ch. 13. Kessel, R.G. and Kardon, R.H., Tissues and Organs: A Text-Atlas of Scanning Electron Microscopy, 1979. Kierszenbaum, A.L and Tres, L.L., Histology and Cell Biology: An Introduction to Pathology, 5th ed., Elsevier-Saunders, Philadelphia, 2020, Ch. 16. MacCallum, D.K., Michigan Medical Histology, University of Michigan, Ann Arbor, 2000. Microanatomy of the Intestines and Anal Canal Rod D. Braun Page 32 of 33 Meyer, D.L., Unpublished histology slide atlas, Department of Anatomy, Wayne State University School of Medicine, Detroit, 1970. Rhodin, J.A.G., Histology: A Text and Atlas, Oxford University Press: New York, 1974, Ch. 30. Ross, M.H. and Pawlina, W., Histology: A Text and Atlas, 6th ed., Lippincott, Williams, & Wilkins: Philadelphia, 2011, Ch. 17. Ross, M.H. and Pawlina, W., Histology: A Text and Atlas, 7th ed., Lippincott, Williams, & Wilkins, WoltersKluwer Health: Philadelphia, 2016, Ch. 17. (major source) Wismar, B.L. and Ackerman, G.A., A Visual Approach to Histology, Davis: Philadelphia, 1970. See next page for summary table and diagram. Microanatomy of the Intestines and Anal Canal Rod D. Braun Page 33 of 33 Layers in GI Tract Muscularis Fibrosa or Mucosa Submucosa Externa Serosa Portion of GI Muscularis Epithelium Glands Glands Layers Tract Mucosae Deep Mucosal or cardiac esophageal or IC/OL Esophagus SSNK Yes Fibrosa glands near ends esophageal Skeletal → Smooth proper Stomach - Simple columnar Cardiac Glands Yes None IO/MC/OL Serosa Cardia (surface mucous cells) Stomach - Simple columnar Fundic (Gastric, Yes None IO/MC/OL Serosa Fundus (surface mucous cells) Oxyntic) Glands Stomach - Simple columnar Pyloric Glands Yes None IO/MC/OL Serosa Pylorus (surface mucous cells) Small Intestine Simple columnar Intestinal (Crypts of Brunner’s Yes IC/OL Serosa - Duodenum (enterocytes + Goblet) Lieberkűhn) Glands Small Intestine Simple columnar Intestinal (Crypts of Yes None IC/OL Serosa - Jejunum (enterocytes + Goblet) Lieberkűhn) Small Intestine Simple columnar Intestinal (Crypts of Yes None IC/OL Serosa - Ileum (enterocytes + Goblet) Lieberkűhn) Simple columnar Intestinal (Crypts of IC/OL + Taenia Serosa or Large Intestine Yes None (enterocytes + Goblet) Lieberkűhn) Coli Fibrosa Upper Anal Upper: Simple columnar Intestinal (Crypts of IC → IAS (smooth) Yes None Fibrosa Canal (enterocytes + Goblet) Lieberkűhn) OL (stays smooth) IAS (smooth) → Lower Anal Stratified columnar → None Lost (No) None EAS (skeletal) Fibrosa Canal SSNK → SSK OL terminates Key: IC = inner circumferential IAS = internal anal sphincter OL = outer longitudinal EAS = external anal sphincter IO = inner oblique MC = middle circumferential Features of Mucosa in GI Tract (not to scale) RDB: 9/30/2024