2V. Small intestineA. Stomach is for initiating digestion, but no absorption of nutrients occurs in this organ.B. Small intestine, will continue process of digestion and will also act to absorb nutrients, essentially completing the digestive process that was started higher in the tract (mouth - saliva; stomach - acid, pepsin, etc.).1. Small intestine is a long, narrow tube about 15 ft in length (4.6 meters).2. As food is digested and passes along length of small intestine, the digestion mixture is kept in contact with the wall of this organ (in part due to the muscularis mucosae).3. This allows the absorption of nutrients released by digestive processes.pathphys/digestion/smallgut/anatomy.html
3C. Small intestine consists of 3 segments. 1. Duodenum - short, 1 ft. ( centimeters) in length.2. Jejunum3. Ileum
4D. General gross structure of small intestine: 1. We find that the wall (mucosa and submucosa) of the entire small intestine is thrown into folds. These folds ARE NOT the intestinal villi. While relatively few of these folds are found in the duodenum and the ileum, they are very common in the jejunum.2. Ducts carrying secretions from mucus glands, the liver and pancreas empty into the duodenum (Secretions from the liver are stored in the gall bladder).3. The jejunum and ileum receive no ducts from other organs.jejunumileumduodenumhistology/imagedisplay.cfm?file=cell0208histology/imagedisplay.cfm?file=cell0197histology/imagedisplay.cfm?file=cell0192
5Folds upon folds upon folds E. General histological structure of wall of small intestine that applies to all 3 segments.Folds upon folds upon folds1. The folds in the wall of the small intestine are called plicae circularis (or valves of Kerckring).2. These folds involve both the mucosa and submucosa.3. As we discussed previously, the mucosa may be divided into 3 sub-layers,a. epithelial lining of lumenb. lamina propria - loose C.T.c. muscularis mucosae - smooth muscle4. The submucosa consists of C.T. that lies beneath the mucosa5. The mucularis externa of longitudinal and circular smooth muscle, and the serosa are also present, but are not part of the valves of Kerckring.jejunumhistology/imagedisplay.cfm?file=cell0188
67. These sub-folds are the intestinal villi. 6. Under higher magnification we would see that the surfaces of the valves of Kerckring are thrown into sub-folds that involve only the columnar epithelium and lamina propria of the mucosa.7. These sub-folds are the intestinal villi.a. The lamina propria (but not the muscularis mucosae) extends into these villi and contains a dense blood capillary network that is important in the absorption of nutrients as discussed below.b. A central lymph capillary called a lacteal is also present in each villus.Villi vs. intestinal glands
7F. Duodenum a. These are the duodenal or Brunner’s glands 1. At the beginning of the duodenum, coiled tubular glands located in the submucosa open into the base of intestinal crypts between adjacent villi.a. These are the duodenal or Brunner’s glandsb. They produce alkaline mucus that protects the duodenal epithelium from stomach acids.2. The crypts themselves are the intestinal glands or crypts. These extend into the lamina propria between adjacent villi.a. They are called the crypts of Lieberkuhn.b. The epithelium lining these glands is continuous with the epithelium of the villi.
8b. Once they reach the tip, they will be shed, or desquamated. 3. Cells in the lower half of these crypts constantly undergo mitotic divisions to give rise to cells that will replace the epithelium of the villi.a. So, cells will migrate from this mitotic region toward the tips of the villi.b. Once they reach the tip, they will be shed, or desquamated.c. The lining of the small intestine is completely renewed in this manner every 2-5 days.
10b. This greatly increases surface area for absorption. 4. Cell types present in the intestinal wall - mostly columnar cells that make-up much of the absorptive epithelium of the villi and a relatively few goblet cells.a. The columnar cells have dense microvilli at their apical ends (striated or brush border).b. This greatly increases surface area for absorption.c. These microvilli are very tiny and dense - one absorptive cell has about 3000 at its apical end.d. This means that there are on the order of 200 million microvilli covering every square millimeter of surface in the small intestine.
11e. Involvement of columnar cells in digestion - carbohydrates and peptides * Experimental evidence suggests that various enzymes such as disaccharidases and peptidases are associated with the microvillar surfaces of these cells.* These enzymes breakdown disaccharides and peptides to monosaccharides and amino acids that are easily absorbed across the plasmelemma of these cells.* This absorption is accomplished by active transport systems in the plasmellemma, so ATP is required.of Lieberkuhn
12f. Involvement of columnar cells in digestion - lipids * Lipids are also absorbed by the columnar cells of the intestinal epithelium.* This occurs mainly in the duodenum and the jejunum.* Lipids enter the digestive tract mainly in the form of di- and triacylglycerides, and phospholipids.* These are large hydrophobic molecules and the columnar cells are not able to absorb them directly.of Lieberkuhn
13* Here they are re-synthesized into triacylglycerides. * In the case of triacylglycerides, these lipids are broken down into their component parts (i.e. monoacylglycerides, glycerol, and fatty acids) by lipases and the bile acids secreted by the liver.* These are absorbed across the plasmelemma via diffusion (not active transport) and accumulate in the cisternae of the absorptive cells smooth ER.* Here they are re-synthesized into triacylglycerides.*The tryacylglycerides are surrounded by a thin layer of protein and , as a result, form particles called chylomicrons.* These are transferred to the golgi apparatus where they are packaged in membrane bound vesicles that are released into the cytoplasm.* These vesicles are exocytosed along the basal lateral walls of the absorptive cells, and then move into the lamina propria and enter the central lacteal in the villus.digestive/Intestine.htm#Crypts of Lieberkuhn
15a. These cells are interspersed between columnar absorptive cells. G. Other cell types present in the intestinal epithelium on villi and/or in crypts of Lieberkuhn1. goblet cellsa. These cells are interspersed between columnar absorptive cells.b. They are less abundant in the duodenum and increase in number toward the ileum.c. They produce mucus in the form of acid glycoproteins that provide lubrication for the intestinal tract.2. Paneth’s cells - found in cryptsa. These cells synthesize a zymogen that is a precursor of lysozyme.b. This enzyme is bacteriocidal and may act to regulate the composition of the bacterial flora associated with the mucosa.histology/imagedisplay.cfm?file=Cell0568
163. Enteroendocrine cells - in crypts 3. Enteroendocrine cells - in crypts3a. Argentaffin cells (old terminology)a. As in the stomach, these cells appear to have an endocrine function.b. They synthesize and release 5-hydroxytryptamine (serotonin), a neurohormone that will stimulate contraction of smooth muscle causing an increase in motility of the intestine.c. Argentiffin cells are most numerous in the basal portion of intestinal glands.
17b. Thus, APUD cells have an endocrine function. 3b. APUD (Amine Precursor Uptake and Decarboxylation) cells (old terminology) - in cryptsa. These cells take up amine precursors and decarboxylate them to amines for use in the synthesis of polypeptide hormones.b. Thus, APUD cells have an endocrine function.c. They produce various polypeptide hormones such as gastrin ( stomach - G cell), secretin (small intestine - S cell), and somatostatin (duodenum and pyloric stomach - D cell) that affect activities of the small intestine.
19VI. Jejunum and ileum A. Jejunum 1. Structure is similar to that described for the duodenum.2. Lacks Brunner's glands3. Lacks lymph nodules in the lamina propria4. Fingerlike villi are longer than in the duodenum or ileum5. Lacteals (lymph vessel in the lamina propria) are well developed to maximize fat absorption.
20B. Ileum1. The only unique diagnostic feature is the presence of many aggregations of lymphoid nodules in the lamina propria and submucosa that are called Peyer's patches.2. Peyer's patches can form protuberances in the wall of the ileum called domes.3. Domes are covered with a single epithelial layer called dome epithelium - M (microfold) cells.4. The dome epithelium is also called follicle-associated epithelium (FAE) and functions in the transport of antigens to underlying cells in the lymph nodules (follicles) of Peyer's patches.Dome epitheliumdigestive/images/ff843.jpg
21VI. Large intestine A. Specialized for 1. water absorption 2. Condensation and formationof the fecal mass3. Mucus production for lubrication and gives body to feces.
22C. No folds in the large intestinal wall, except in rectal region. B. The epithelium lining the large intestine is a simple epithelium of columnar absorptive cells with microvilli at apical ends and goblet cells.C. No folds in the large intestinal wall, except in rectal region.
23D. No villi are present; however, the epithelium does form invaginations, the entire walls of which compose the large intestinal glands - crypts of Lieberkuhn.E. These glands are composed almost entirely of columnar absorptive cells and goblet cells with a small number of enteroendocrine cells interspersed between - secretion product is mainly mucus.
241. Nodules may extend into submucosa. F. The lamina propria of the large intestine's mucosa is rich in lymphoid cells that may be aggregated into nodules.1. Nodules may extend into submucosa.G. A submucosa is present.H. The muscularis externa consists of an inner later of circular smooth muscle and 3 outer longitudinal bands of smooth muscle called the taenia coli.I. The serosal layer is characterized by protuberences of adipose tissue called appendices epiploicae.
25J. A short distance from anal opening, the lining of the tube shifts from columnar to a non-keratinized stratified squamous epithelium.K. Epithelial tissues as you enter the anal canal are a transition zone (like the lips) where the simple columnar epithelium of the rectal region changes to non-keratinized squamous epithelium, that then changes to the keratinized squamous epithelium of the skin.