this lecture provided a histological perspective on the different sections of the digestive system, focusing on tissue and cell types. the digestive system is introduced as one long tube which starts in the pharynx, becomes the esophagus, stomach, small intestine, large intestine, then rectum. within all of these sections, there are generally four different layers: the mucosa is the innermost layer and contains the epithelium (either stratified squamous or simple columnar), a loose or reticular CT lamina propria, and a layer of muscle named the muscularis mucosa. beneath this is the submucosa, which contains blood vessels and nerves (and in the small intestine, the meissner's plexus). beneath this is the muscularis externa, which is generally composed of an inner circular layer, middle plexus of ganglia ("myenteric" plexus), and outer longitudinal layer. finally, on the outside of the tube is the serosa (or adventitia in some places, such as the esophagus above the diaphragm), which is made up of connective tissue and mesothelium. all of the sections of the digestive system are made up of variants of these four layers depending on location and function.
the first section, the esophagus, is unique in that its epithelia is made up of non-keratinized stratified squamous (presumably to deal with the abrasion of food coming in, and non-keratinized because waterproofing isn't an issue), which is thrown into esophageal glands that begin the lubricating process. the muscularis layer is also unique in that the top part of the esophagus is mainly skeletal muscle, gradually blends with smooth muscle in the middle, and by the bottom is mainly smooth muscle. the esophagus ends in the esophageal-cardiac junction, where the epithelial layer transitions from the non-keratinized stratified of the esophagus to the simple columnar of the stomach.
the stomach has four sections, from top to bottom: the cardiac, fundic, body, pyloric regions. the mucosa of the stomach is thrown into gastric glands, which contain specialized secretory cells which aid in digestion: surface mucous cells and mucous neck cells secrete mucous near the tops and bottoms of the glands, respectively. parietal cells secrete HCl near the tops of the glands, and chief cells secrete pepsinogen near the base of the glands. the muscularis of the stomach is also unique in that it has an inner oblique layer, middle circular, and outer longitudinal layer. the fundus is the most representative region of the stomach, and the other three regions are variations on this region. for example, the epithelia in both the cardiac and pyloric regions contain only mucous cells, and the muscularis layers are tightened into the cardiac and pyloric sphincters on either end.
after the stomach comes the small intestine, which is divided into three regions, the duodenum, jejunum (the most representative region), and ileum. the intestinal mucosa is unique in that it is folded into large intestinal glands called "Krypts of Lieberkuhn", covered with columnar absorptive/secretory cells called enterocytes, with goblet cells scattered throughout. also present in the epithelium are enteroendocrine cells, which secrete hormone like substances, as well as Paneth cells, which secrete lysozymes to regulate the bacterial flora in the gut. the lamina propria of the intestine is well vascularized with type 2 fenestrated capillaries, with occasional lymph nodes called Peyer's Patches. additionally, there is often a "central lacteal" inside the villi, which is a large lymph vessel that aids in absorption of larger molecules. the muscularis of the small intestine generally contains an inner circular layer, followed by a nerve "net" called the myenteric plexus, followed by an outer longitudinal muscle layer.
the large intestine is similar to the small intestine, the main differences being: more goblet cells, lymph rich, but no microvilli, paneth cells, or enteroendocrine cells. it has two main sections, the colon and the rectum, and at the "end of the line", the rectal-anal junction, there are four notable changes: the transition from the simple columnar absorptive cells of the small intestine to the non-keratinized stratified squamous of the rectum, the patchy muscularis mucosa, the huge blood vessels in the submucosa, and the presence of skeletal muscle in the muscularis externa. (notice how this is similar to the esophagus)
the role of the pancreas and liver in digestion is then looked at: the pancreas is a mostly exocrine gland which secretes from compound acinar glands a variety of enzymes-- carboxypeptidase, chymotrypsinogen, trypsinogen for digestion of proteins, amylase for digestion of carbohydrates, lipase for digestion of lipids, and bicarbonate to neutralize the acidity from gastric digestion. it also functions as an endocrine gland with its "Islets of Langerhans", which contain alpha, beta, and delta cells which release glucagon, insulin, and somatostatin, respectively. the liver also functions as both an endocrine / exocrine gland which aids in digestion by releasing bile for digestion of fats (exocrine) and filtering and adding to blood (endocrine). the liver has many functions, such as processing of glycogen, fetal blood cell production, production of blood clotting factors, and detoxification of the blood.
the basic cell type of the liver is called the hepatocyte, and they are arranged in roughly hexagonally shaped functional units called "lobules". on the borders of the lobules, on roughly every other corner of the hexagon, are "portal triads" which contain hepatic portal veins (which supply oxygen poor, nutrient rich blood to the liver from the intestine), hepatic arteries (supplying fresh blood to the liver), and bile ducts. in the center of the lobules are "central veins" which carry away deoxygenated blood. the last aspect of the liver that's looked at is the liver acinus theory, which states that blood flows from the portal triad laterally to the adjacent central veins, forming a diamond shaped functional unit overlayed on two lobules.
questions
1. what are the four primary layers in the entire digestive system?
2. what is the mucosa composed of?
3. what is in the submucosa?
4. what is the muscularis composed of?
5. what is the serosa composed of?
esophagus
6. describe the mucosa of the esophagus.
7. describe the muscularis of the esophagus.
8. describe the outer wall of the esophagus.
9. what is the esophageal-cardiac junction?
stomach- surface mucosa, mucosa neck, chief, parietal,
10. what are the four parts to the stomach?
11. describe the mucosa of the stomach.
12. what are some other cells that function in the mucosa of the stomach?
13. describe the muscularis externa of the stomach.
14. how does the cardiac region of the stomach differ from the body region?
15. how does the pyloric region of the stomach differ from the body region?
small intestine- lieberkuhn, enterocytes, paneth, meissner's, peyer's patch, brunner's
16. what are the three parts to the small intestine?
17. describe the mucosa of the small intestine.
18. what is unique about the surface of the mucosa of the small intestine?
19. what are the crypts of Lieberkuhn?
20. which digestive enzymes are secreted by the pancreas to aid in digestion in the small intestine?
21. what is secreted from the liver to help in digestion in the small intestine?
22. what are some of the cells in the mucosal epithelium of the small intestine?
23. describe the lamina propria of the small intestine.
24. describe the submucosa of the small intestine.
25. describe the myenteric plexus and what role it plays.
26. the serosa of the small intestine is the...
27. what are brunner's glands and what do they do? where are they found?
28. what are some distinguishing characteristics of the ileum?
large intestine-
29. describe the mucosa of the large intestine.
30. what are the four characteristics of the rectal-anal junction?
accessory glands- salivary glands, pancreas, liver,
31. what are the three types of salivary glands and what do they secrete?
32. describe the exocrine function of the pancreas.
33. describe the endocrine function of the pancreas.
34. name five major functions of the liver.
35. describe how the liver operates both as an exocrine and endocrine gland.
36. what is the hepatic portal vein?
37. what is the hepatic artery?
38. what type of capillaries are in the liver?
39. what is the basic cell type in the liver and what are they assembled into?
40. describe the architecture of the lobule.
41. what is the liver acinus theory? why is it the most likely scheme of blood flow in the liver?
answers
1. mucosa, submucosa, muscularis, serosa
2. epithelium (mostly simple columnar), lamina propria, muscularis mucosa
3. connective tissue with blood vessels and nerves, sometimes a submucosal plexus (meissner's)
4. inner circular layer of smooth muscle, middle plexus of ganglia (myenteric/auerbach's), outer longitudinal smooth muscle
5. CT, mesothelium
6. non-keratinzed stratified squamous epithelium, has esophageal glands for lubrication.
7. upper 1/3 made of skeletal muscle, middle 1/3 mix of skeletal and smooth muscle, lower 1/3 smooth muscle.
8. has adventitia, serosa seen only after tube penetrates diaphragm.
9. the end of the esophagus, an abrupt change from stratified squamous to simple columnar epithelium.
10. cardiac, fundus, body, pylorus
11. simple columnar epithelium with gastric glands for digestion, lamina propria which is richly vascularized, loose / reticular CT.
12. surface mucous cells, mucous neck cells secrete mucous on surface and at base of gastric pits. parietal cells secrete HCl at top of gland proper. chief cells secrete pepsinogen near base of gland.
13. inner oblique, middle circular, outer longitudinal. thickened at either end of stomach to form cardiac and pyloric sphincter.
14. cardiac region glands only have mucous cells, and there is an abrupt change from the esophagus.
15. pyloric region glands only have mucous cells. glands branch more and have deeper pits.
16. duodenum, jejunum, ileum.
17. simple columnar epithelium thrown into glandular tubes called intestinal glands (crypts of Lieberkuhn) and projections called villi.
18. they have a striated border formed from microvilli.
19. intestinal glands that both secrete and absorb.
20. trypsinogen, chymotrypsinogen, carboxypeptidase (for protein), amylase (for carbohydrates), lipase (for lipids)
21. bile salts (for lipids)
22. primarily composed of enterocytes which are columnar absorptive cells with microvilli, with mucous secreting goblet cells throughout, as well as enteroendocrine cells which are at the base of the glands and secrete hormone like substances, and paneth cells which secretes lysozyme to control bacterial flora.
23. well vascularized, type II capillaries, may have a central lacteal for absorption of larger molecules, may have lymph nodules called Peyer's patches.
24. may have a meissner's plexus, which assists in regulating secretion and blood flow.
25. a nerve net in the muscularis layer which helps coordinate peristaltic contractions of muscularis externa.
26. visceral peritoneum.
27. in the submucosa of the duodenum, secretes alkaline-mucoid material and urogastrone.
28. greatest amount of "GALT", Peyer's patches, fewer villi and more goblet cells.
29. simple columnar epithelia with intestinal glands but no villi. abundant goblet cells and lymph rich lamina propria, but no entero-endocrine or paneth cells.
30. epithelial transition from simple columnar to non-keratinized stratified squamous, patchy muscularis mucosa, huge blood vessels in submucosa, skeletal muscle in muscularis externa. transition, patchy, blood, skeletal.
31. parotid secretes mainly serous, submandibular secretes a mixture of mucous and serous, and sublingual secretes mostly mucous.
32. the pancreas' function is mainly to serve as an exocrine gland which secrete digestive enzymes (see question 20), composed of compound acinar glands. it also secretes alkaline bicarbonate in order to neutralize the acidity from gastric digestion.
33. 2% of the pancreas is used for endocrine function, in the form of "islets of langerhans" which are clumped glands which consist of alpha, beta, and delta cells which secrete glucagon, insulin, and somatostatin, respectively.
34. fetal hematopoesis (before shifting to bone marrow), glucagon processing, plasma protein production (such as clotting factors), bile production, and detoxification. blood, glucagon, clotting, bile, detox. detox clotting blood into a bile of glucagon.
35. exocrine because it dumps bile into the intestine, endocrine because it filters blood and adds to it.
36. the vein that brings oxygen poor but potentially nutrient rich blood into the liver from the GI tract.
37. the artery that brings fresh blood to the liver itself.
38. type three discontinuous.
39. basic cell type is the hepatocyte and they are arranged into lobules.
40. hexagonal shaped cell with a large central vein in the middle and a trio of vessels at every other corner of the border of the cell, called the portal area: containing hepatic portal vein (question 36), hepatic artery (question 37), and bile duct.
41. the liver acinus theory of blood flow says that blood flows laterally from the portal areas to the adjacent central vein (creating a diamond shaped functional unit overlayed on top of two lobules). this theory is backed up by the patterns of degradation during liver poisoning.
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