1. Approximately 21 ft. long: duodenum (one ft.), jejunum (eight ft.), and ileum (twelve ft.)

Transcription

1 IV. Small Intestines A. General features and functions 1. Approximately 21 ft. long: duodenum (one ft.), jejunum (eight ft.), and ileum (twelve ft.) 2. Functions: move forward chyme, continue digestion, absorb nutrients released by digestion 3. Structural specializations that increase surface area 500-fold a. plica circulares - permanent folds, involve mucosa and submucosa, increase surface area 2-3 times b. intestinal villi - finger-like projections, mm high, core of lamina propria, increase surface area 10 times c. microvilli - on absorptive cells, increase surface area 20-fold B. Mucosa 1. Intestinal villi - permanent evaginations of the mucosa a. diagnostic for small intestine b. covered with absorptive cells and goblet cells d. blind-ended lymphatic channels in lamina propria core of villi - called lacteals - important in absorption of products of lipid digestion 2. Intestinal glands (old name - crypts of Lieberkuhn) a. simple tubular glands formed by invagination of epithelium into the lamina propria, lie between villi b. composed of four cell types: goblet cells, absorptive cells, Paneth cells, and enteroendocrine cells c. cells in the basal half of glands and at bases of villi are mitotically active - regenerate the entire epithelial population every five days 3. Lamina propria a. highly vascularized with numerous connective tissue cells and lymphatics b. smooth muscle fibers extend from muscularis mucosa c. lymphatic nodules scattered along the tract, nodular aggregates known as "Peyer's patches"

2 C. Cell types of the small intestine epithelium 1. Intestinal epithelial cells (enterocytes) - dominant cell type a. histological appearance - tall columnar cell with a brush border b. well-developed surface glycocalyx - imparts resistance to proteolytic enzymes and contains enzymes which convert pancreatic proenzymes into active forms (most important - enterokinase - activates trypsinogen) c. cells bound together by junctional complexes d. responsible for the absorption of products of protein, carbohydrate, and lipid digestion - proteins absorbed as amino acids - carbohydrates absorbed as monosaccharides - lipids absorbed as monoglycerides and fatty acids (must be emulsified with bile)

3 2. Goblet cells a. look and function like goblet cells found elsewhere in the body b. produce mucinogen which when released converted to mucus when mixed with water c. mucus forms a protective and lubricating lining of small intestine d. relative proportion of these cells increases as you move distally along the GI tract 3. Paneth cells a. in bases of intestinal glands b. contain large eosinophilic granules c. secrete lysozyme - an antibacterial enzyme - breaks their cell walls 4. Enteroendocrine cells - a variety of cell types which are stimulated to release hormones in response to chyme entering duodenum from stomach a. release hormones toward basal lamina - hormones enter blood vessels in lamina propria b. secretin - its secretion triggered by acidity of chyme, this hormone stimulates secretion of bicarbonate by centroacinar cells of the pancreas

4 c. cholecystokinin (CCK) - its secretion triggered by fatty acids and L-amino acids in the duodenum, this hormone stimulates release of secretory granules by pancreatic acinar cells and also stimulates contraction of the smooth muscle in the wall of the gall bladder D. Submucosa 1. Plicae circulares (old name - valves of Kerckring) a. permanent ridges formed by mucosa and submucosa b. found only in the first 2/3 of the small intestine 2. Duodenal glands (of Brunner) - classified morphologically as compound tubular a. ducts empty into bases of intestinal glands b. produce alkaline mucus (for protection) c. stimulated to secrete by acidic chyme entering duodenum E. Outer layers 1. Muscularis externa - well-developed muscle layers 2. Serosa - adventitia plus visceral peritoneum V. Large Intestine A. General features and functions

5 1. Anatomic divisions - ascending, transverse, and descending colon, and rectum B. Mucosa 2. Histologically, mucosa has a smooth surface - has no plicae or villi 3. Functions - dehydration of luminal contents (i.e. water recovery) and vitamin absorption 1. Epithelium tall columnar - absorptive, goblet cells, enteroendocrine cells 2. Intestinal glands - classified as simple tubular glands 3. Lamina propria - contains abundant connective tissue cells and lymph nodules C. Accessory coats 1. Muscularis externa - outer longitudinal layer composed of three distinct bands - termed taeniae coli 2. In rectum, longitudinal layer complete 3. Presence of serosa dependent on which area of colon you're examining

6 D. Appendix 1. Mucosa - resembles large intestine - i.e. no villi or plicae a. intestinal glands - simple tubular glands as in both the small intestine and colon b. prominent and numerous lymphoid nodules as in both the small intestine and colon 2. Accessory coats a. usual looking submucosa b. muscularis externa similar to small intestine (no taeniae as occurred in colon) E. Rectum, anal canal, and anus 1. Rectum a. its simple tubular glands are short and irregular, otherwise same as colon b. distal portion has 5-10 longitudinal columns - called the anal columns of Morgagni c. at end of columns is the pectinate line - marks end of rectum muscularis externa complete - no taeniae coli 2. Junction of the rectum and anal canal - the pectinate line a. mucosa epithelium is stratified non-keratinized squamous b. muscularis mucosa disappears c. hemorrhoidal venous plexus lies in the lamina propria d. inner circular layer of muscularis externa forms the internal anal sphincter, while outer longitudinal layer disappears

7 3. Anus a. lined by keratinized stratified squamous epithelium b. connective tissue components - include hemorrhoidal vessels, apocrine sweat glands, sebaceous glands, and hairs c. external anal sphincter formed by skeletal muscle - under control of spinal nerves VI. Pancreas A. General features - a compound acinar gland lies retroperitoneal near duodenum, weighs g, has slight regenerative capacity Stroma - loose CT a. has no capsule, finely lobulated by septa that partially subdivide organ b. conveys blood vessels, lymphatics, interlobular ducts, & excretory ducts Parenchyma - both an exocrine & endocrine gland a. exocrine component - compound acinar gland, secretion dependent upon hormones from the intestinal enteroendocrine cells (secretin & cholecystokinin) b. endocrine component - Islets of Langerhans, hormonal secretions play role in carbohydrate metabolism

8 B. Endocrine pancreas 1. Islets of Langerhans scattered throughout organ, islets are hormone-secreting cells associated with tufts of capillaries so in 3-D islets are like irregular cords 2. Beta cells - most numerous cell type, secrete Insulin which promotes the movement of glucose into cells, therefore its net effect is to lower blood glucose levels 3. Alpha cells - secrete Glucagon which elevates blood glucose levels 4. Delta cells - fewer in number, secrete Somatostatin C. Exocrine pancreas - purely serous gland, produces approx ml fluid/day that is slightly alkaline and contains about 15 enzymes and proenzymes 1. Acini - the main functional units a. organization and polarity - a single row of pyramidal cells resting on basement membrane, apical secretory granules and basal rough ER b. secretory products of acinar cells include: trypsinogen, chymotrypsinogen, DNAse, RNAse, amylase, and lipase c. release of enzymes dependent on cholecystokinin secreted by intestinal enteroendocrine cells

9 2. Ducts a. secretory canaliculi - first part of duct system, at the center of acini, granules are released into this space b. centroacinar cells - first non-acinar cell of duct system, lie within acini and line the initial (smallest) part of intercalated ducts, squamous or cuboidal shape produce the bicarbonate-rich component of the exocrine pancreas secretin - produced by intestinal enteroendocrine cells which in turn triggers their secretion of alkaline fluid c. intercalated ducts - lie within the pancreatic lobules, lined by simple cuboidal and low columnar epithelium d. interlobular ducts - lie in connective tissue septa, simple or stratified columnar epithelium e. excretory ducts - lie in connective tissue septa, stratified columnar epithelium f. two main ducts empty into duodenum main pancreatic duct (of Wirsung) opens into duodenum at the papilla (ampulla of Vater) whose opening is controlled by a sphincter (of Oddi) accessory duct (of Santorini)

10 VII. The Liver A. General features - largest gland, approximately 1500 g - receives a large volume of blood via portal vein and hepatic artery - metabolizes products of digestion, stores glucose, detoxifies 1. functions as an exocrine and endocrine gland a. synthesizes bile - needed for lipid absorption in the small intestine b. synthesizes plasma proteins 2. structural plan a. composed of interconnected cellular plates of hepatocytes which are disposed radially from the central vein b. hepatocytes are exposed to blood in hepatic sinusoids c. stroma - Glisson's capsule surrounds outside of liver, septa subdivide parenchyma into lobules, relatively little connective tissue within lobules B. Lobulation 1. Basic lobule is hexagon with a central vein at the center and a portal canal at each corner 2. Thin area of connective lies between adjacent lobules 3. Blood enters lobule from terminal branches of portal vein and hepatic artery and flows through the sinusoids to the central vein C. Lobulation patterns - three definitions exist to help explain the anatomy and function of the liver, these are not conflicting but rather complementary 1. Classical hepatic lobule - as described earlier, emphasizes endocrine function and changes in nutrition and pathologies a. anatomical limits - outlined by connective tissue septae b. a hexagon with portal canals at each corner c. based on blood flow - endocrine secretions of hepatocytes carried away in the sinusoids toward the central vein 2. Portal lobule - corresponds to lobular organization in exocrine glands a. anatomical limits ill-defined b. a triangle with a central vein at each corner and a portal canal at the center c. based on bile flow - drains toward bile duct located with the vascular supply in the center of lobule (as in other exocrine glands)

11 3. Liver acinus - defined as tissue supplied by a terminal branch of portal vein and hepatic artery and drained by initial segment of the bile duct system a. no anatomical limits - parenchyma is continuous with adjacent acini and their blood supply is shared b. diamond-shaped - involves two adjacent classical lobules D. Portal canal - formed of connective tissue plus blood vessels and ducts 1. Hepatic artery - brings 30% of blood to liver, carries oxygenated blood and brings products of fat digestion absorbed by small intestine to liver 2. Portal vein - brings 70% of blood to liver, this blood is deoxygenated but carries products of carbohydrate and protein digestion absorbed by small intestine 3. Bile duct - formed of simple cuboidal epithelium

12 4. Lymphatic channels - drain lymph produced by hepatocytes, this lymph contains a large concentration of what will eventually be plasma proteins, these channels drain into the thoracic duct E. Hepatocytes - six-sided cells - two surfaces free and four associated with other hepatocytes 1. Intercellular relationships - a. space of Disse - between free surface of hepatocytes and sinusoidal lining cells, contains blood plasma and microvilli of hepatocytes, area of exchange of metabolites b. bile canaliculi - dilated space between adjacent hepatocytes, from the first components of bile pathway c. cell junctions - adjacent hepatocytes joined together by a pair of zonule occludens junctions with the bile canaliculi between

13 2. Cytoplasm - contents vary with functional state rough ER and polysomes for protein synthesis - for production of hormones, plasma proteins, bile smooth ER moderately extensive, for detoxification of undesirable compounds a. mitochondria abundant giving cells a red appearance with H&E staining b. also contains lysosomes, glycogen, lipid droplets, and Golgi apparatus 3. Nucleus - almost half the cells are binucleate and most are polyploid, one or more prominent nucleoli

14 F. Hepatic sinusoids - a type of discontinuous capillary 1. Endothelium - gaps between adjacent cells that have a discontinuous basement membrane, space of Disse between endothelium and hepatocytes 2. Hepatic sinusoids - zonation pattern with respect to oxygenation, nutrition, and disease a. zone of permanent function - most metabolically active and least susceptible b. zone of varying activity - variable metabolic activity c. zone of permanent repose - least metabolically active and most susceptible to damage 3. Kupffer cells - resident macrophages of the liver, lie on endothelium and exposed to blood in lumen of sinusoid, filter out debris in plasma 4. Central veins - drain sinusoids, they eventually come together to form two or more hepatic veins, the latter join with the inferior vena cava G. Bile passages 1. Bile canaliculi - minute expansion of intercellular space between hepatocytes, form a three-dimensional network 2. Intrahepatic bile ducts a. canals of Hering - lie in septae between adjacent classical hepatic lobules, finest branch of bile duct b. ducts of portal canals - formed of simple cuboidal epithelium resembling the interlobular ducts of other exocrine glands 3. Hepatic duct - formed by the fusion of large bile ducts from different hepatic lobes, joins with cystic duct from gall bladder to form common bile duct

15 VIII. Gall Bladder A. General features and functions 1. Anatomic divisions - fundus, body, and neck (the latter continuous with the cystic duct) 2. Covered with peritoneum except on surface attached to liver 3. Functions a. storage (capacity of ml) and concentration bile b. bile's role in digestive process is to emulsify fat B. Epithelium 1. Pseudodiverticulae - due to convoluted architecture, resemble glands 2. Surface cells - simple columnar with brush border a. joined near apex by zonule occludens junctions - wide intercellular space below b. apical area of cytoplasm contains a high concentration of organelles - involved in active transport of sodium from lumen to intercellular space - this causes water to follow (i.e. an osmotic gradient) and thus helps to concentrate the bile C. Perimuscular connective tissue - rich blood supply and lymphatics D. Muscularis - irregular network of smooth muscle fibers, their contraction stimulated by cholecystokinin E. Serosa - adventitia-like connective tissue layer and visceral peritoneum except over part attached to liver

16 Gastrointestinal Tract & Glands Histology - Laboratory Guide Find the following among the images on the CD The following structures are common to all regions/subdivisions of the tubular digestive tract. Be sure you can identify them for each region: Adventitia Auerbach's plexus Lamina propria Meissner's plexus Mucosa Muscularis externa Muscularis mucosae Submucosa SUBDIVISIONS AND GLANDS Esophagus Esophageal glands proper Skeletal and smooth muscle Stratified squamous moist epithelium Gastro Esophageal Junction Stomach Cardiac region Fundic region Pyloric region Gastric glands (types) Cardiac glands Fundic glands Pyloric glands Gastric glands (regions) Body Gastric pits Neck Gastric glands (cells) Chief cells Enteroendocrine (DNES) cells Mucous neck cells Parietal cells Sheet gland Mucin Pyloric sphincter Gastro-Duodenal Junction Small Intestine (regions) Appendix Duodenum Ileum and jejunum Small Intestine (structures) Brunner's glands Enterocytes (absorptive cells) Enteroendocrine (DNES) cells Goblet cells Intestinal glands (crypts of Lieberkuhn)

17 Lacteals Paneth cells Plicae circulares Peyer's patches Villi Large Intestine (Colon) Enterocytes (absorptive cells) Goblet cells Intestinal glands (crypts of Lieberkuhn) Taeniae coli Appendix Rectum and Anus Anal canal (stratified squamous moist epithelium) Anal orifice Apocrine sweat glands External anal sphincter Hair follicles Internal anal sphincter Recto-Anal Junction Sebaceous glands Skin (stratified squamous dry epithelium) Pancreas Acinar cells Centroacinar cells Intercalated ducts Interlobular ducts Islets of Langerhans Serous acini Liver Bile canaliculi Bile canaliculus Bile duct Central vein Classical hepatic lobule Endothelial cells (endothelium) Hepatic artery Hepatic portal vein Hepatocytes Kupffer cells Portal canals Sinusoid (hepatic) Space of Disse Gall Bladder Apical organelles Infoldings (Pseudodiverticulae)