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Small intestine (p.2)

  • length
  • major functions
  • Structural Specializations
  • The small intestine wall is extensively folded at multiple levels
  • The epithelium is characterized by

  • approximately 20 feet or 6 meters long.
  • major functions: further digestion, absorption, the production of GI hormones and immunological defense.
  • Structural Specializationsstructure dictates function. 
  • The small intestine wall is extensively folded at multiple levels: plicae circulares, villi & crypts, and microvilli.
  • The epithelium is characterized by villi & crypts.


Small intestine:
Plicae circulares (p.3)

  • folds of the entire mucosa into the lumen with a core of submucosal tissue.
  • circularly arranged around the lumen of the small intestine.
  • increase the surface area of the organ about three-fold,
  • most important as mechanical mixers.
  • the most extensive in the jejunum, where they can protrude as far as 8mm into the lumen.


Small intestine:
Villi (p.4-6)

  • Villi 
  • The lamina propria core contains diverse connective tissue structures: 
  • The muscularis mucosa

  • Villi
    • folds of the epithelium into the lumen with a core of lamina propria.
    • extend approximately 1mm out from the mucosa
    • increase the small intestinal surface area another 10-fold.
    • can be leaf or finger shaped (tallest in jejunum, shortest in ileum).
  • The lamina propria core contains diverse connective tissue structures:
    • white blood cells (lymphocytes, plasma cells, eosinophils);
    • lacteals (blind ending lymphatic capillaries);
    • an arteriole from the submucosal artery;
    • a rich fenestrated capillary network (permeable to macromolecules);
    • unmyelinated nerves;
    • scattered smooth muscle cells from the muscularis mucosa out to the tip of the villus (contraction forces lymph from lacteals into larger lymphatic vessels).
  • The muscularis mucosa passes under the base of the crypts, separating the mucosa from the submucosa.


Small intestine:
Villi (p.7-11)

  • The villus epithelium is composed of two cell types,
    • Goblet cells 
      • secrete /
      • as more food is absorbed from the intestinal lumen/
      • goblet cells increase in number from/
    • Enterocytes, 
  • Neighboring cells are connected by/
  • Occluding or tight junctions/
  • The claudins/
  • Adhering junctions and desmosomes/

  • The villus epithelium is composed of two cell types,
    • Goblet cells
      • secrete mucins to provide a protective coating for the digestive enzymes of the microvillar glycocalyx.
      • as more food is absorbed from the intestinal lumen, the need for lubrication increases.
      • goblet cells increase in number from only a few in the epithelium of the upper small intestine, to the predominant cell type in the large intestinal surface epithelium.
    • Enterocytes,
      • columnar absorptive cells,
      • display an apical striated border of microvilli.
      • major cell in coordination of the gut immune system & bacterial homeostasis.
      • contain pattern recognition receptors called Toll-like receptors that respond to bacteria components: peptidoglycans, lipopolysaccharides and flagellin.
        • These surface epithelial cells respond by secreting beta-defensin, cytokines and chemokines.
  • Neighboring cells are connected by junctional complexes.
  • Occluding or tight junctions
    • prevent intercellular passage of nutrients from the intestinal lumen to the interstitium,
    • prevent mixing of apical and basolateral plasma membrane proteins.
  • The claudins that form the intestinal tight junctions are permeable to water.
  • Adhering junctions and desmosomes physically link one cell to its neighbors.


Small intestine:
Crypts (p.4-5+12-15)

  • Crypts
  • Crypt enterocytes secrete/
    • The secretion of these anions causes/
  • Paneth cells
    • ?
    • secrete/
    • involved in/

  • Crypts
    • folds of the epithelium that invaginate down into the lamina propria.
    • serve as precursors for enterocytes or goblet cells of the villi.
  • Crypt enterocytes secrete chloride ions and bicarbonate ions into the intestinal lumen.
    • The secretion of these anions, especially Cl-, causes the electrical drag of Na+ out of the cells as well.
    • the exit of all these ions together results in the osmotic flow of water.
    • The Na+ is reabsorbed while facilitating villar cotransport of nutrients.
    • The excess water secretion is reabsorbed by the small intestinal villi and large intestinal crypts.
  • At the base of the crypts are Paneth cells.
    • large, acidophilic cells that have abundant basal RER, and large apical storage granules.
    • secrete
      • lysozyme (degrades peptidoglycan coat of bacterial cell walls),
      • alpha-defensins (ion channels that insert into parsites & bacteria),
      • TNFα (proinflammatory cytokine).
    • involved in population control of gut flora.
      • the lumen of the small intestine is never completely sterile;
      • however, there exists a large resident bacterial population in the large intestine.
      • Paneth cells of the small intestine thwart its backward expansion.


Small intestine:
Enteroendocrine cells (p.17)

  • Enteroendocrine cells
  • APUD cells found in the small intestine include:
    • Vasoactive Intestinal Peptide (VIP)
    • Somatostatin (D1 cell)
    • Enteroglucagon (L cell)
    • Neuropeptide YY

  • Enteroendocrine cells
    • also scattered through the small intestinal crypts.
  • APUD cells found in the small intestine include:
    • Vasoactive Intestinal Peptide (VIP)
      • increases HCO3- and water secretion from the pancreas and the small intestine
      • relaxes the esophageal sphincter, fundic stomach and the gall bladder
      • decreases stomach acid
    • Somatostatin (D1 cell)
      • feeds back to the stomach to shut down histamine and gastrin release
      • stimlulates MMC that initiate in the duodenum.
    • Enteroglucagon (L cell)
      • stimulates hepatic glycogenolysis
    • Neuropeptide YY
      • a satiety signal secreted by the cephalic phase of food intake, as well as by food in the small intestine.


Small intestine:
Enteroendocrine cells:
APUD cells found in the small intestine include: (p.18-20)

  • Cholecystokinin (CCK) (I cell)
  • Secretin (S cell)
  • Glucose dependent Insulinotropic Peptide (GIP)
  • Motilin (Mo cell)

  • Cholecystokinin (CCK) (I cell)
    • mainly secreted from the duodenum by fats or proteins in the lumen.
    • indirectly stimulates the secretion of digestive enzymes from the pancreas and stimulates gall bladder contraction
  • Secretin (S cell)
    • released by the presence of acid in the duodenum
    • results in secretion of bicarbonate and water from the pancreas and Brunner’s glands of the duodenum
  • Glucose dependent Insulinotropic Peptide (GIP)
    • induces insulin secretion;
    • release is stimulated by glucose in lumen
    • increases lipid deposits via stimulation of lipoprotein lipase
    • Initially this hormone was called Gastric Inhibitory Peptide because of its inhibition of gastric acid production.
  • Motilin (Mo cell)
    • stimulates upper GI motility
    • secreted during the fasting state at intervals of approximately 100 minutes
    • thought to influence the migrating motor complex (MMC) or "housekeeping" waves.


Small intestine:
Microvilli (p.21-23)

  • Microvilli
  • glycocalyx
  • The digestive process that began in the stomach is continued in the small intestinal lumen via/
    • enterokinase
    • The pancreatic enzymes will reduce/
    • Enzymes of the glycocalyx/

  • Microvilli
    • folds of the apical plasma membrane on each individual enterocyte with a core of actin cytoskeleton.
    • increase the surface area for nutrient digestion and absorption by at least another 20-fold.
  • Extending from the microvilli is a thick glycoprotein coat called the glycocalyx.
    • contains hydrolytic enzymes: enterokinase (enteropeptidase), dipeptidases and disaccharidases.
    • These are intrinsic membrane proteins that span the bilayer and expose their enzymatic active sites toward the small intestinal lumen.
  • The digestive process that began in the stomach is continued in the small intestinal lumen via the pancreatic enzymes.
    • Note that the glycocalyx enzyme, enterokinase, converts the pancreatic enzyme trypsinogen to its active form trypsin, which in turn activates the remaining pancreatic enzymes.
    • The pancreatic enzymes will reduce most luminal proteins and carbohydrates to di/tripeptides and disaccharide forms.
    • Enzymes of the glycocalyx complete the protein and carbohydrate digestion and provide single amino acids and monosaccharides for the enterocyte transporters.


Small intestine:
Microvilli (p.24-25)

  • nutrient symporters
  • secondary active transporters
  • The majority of the lipids
  • Together, the microvilli and glycocalyx are responsible for/

  • The second major class of intrinsic proteins of the microvillus plasma membrane is the nutrient symporters.
    • These carriers cotransport amino acids or monosaccharides with Na+.
    • These are passive processes which utilize the existing Na+ electrochemical gradient within the enterocyte.
  • Since the apical nutrient symporters are driven by the Na+ gradient created by basolateral 3Na+/2K+ ATPase active transport pumps, the symporters are often referred to as secondary active transporters.
  • The majority of the lipids diffuse across the plasma membrane as 2-monoglycerides, cholesterol, lysolecithin and free fatty acids.
    • Fatty acids greater than 24 C bind an enterocyte carrier protein for absorption.
  • Together, the microvilli and glycocalyx are responsible for the absorption of several hundred grams of carbohydrate, 100 grams of fat, 50-100 grams of amino acids, 50-100 grams of ions, and 7-8 liters of water per day.


Small intestine:
GALT (p.26)

  • GALT
  • lymphoid cells 
  • seen in normal GI mucosa
    • The notable exception
    • Diffuse lymphocytes and solitary nodules/
    • Peyer’s Patches

  • GALT
    • an acronym for “gut associated lymphoid tissue.”
    • one component of the larger Mucosal Immune System.
  • lymphoid cells
    • transient,
    • always found throughout the GI tract lamina propria, submucosa and even the epithelium itself.
  • Lymphocytes, plasma cells, macrophages, eosinophils and mast cells can all be seen in normal GI mucosa.
    • The notable exception is the neutrophil, whose presence indicates mucosal injury or inflammation.
    • Diffuse lymphocytes and solitary nodules (clustered lymphocytes with or without a pale secondary center) are not unusual in any portion of the GI tract.
    • Unique aggregated nodules called Peyer’s Patches are found only in the ileum.


Small intestine:
GALT (p.26-28)

  • At the site of Peyer’s patches, the overlying villi/
  • follicle associating epithelium (FAE)
  • "M" cells
  • The sensitized T- cells/
  • T and B cells/

  • At the site of Peyer’s patches, the overlying villi are frequently absent.
  • The epithelium covering a patch is called the follicle associating epithelium (FAE).
  • The FAE is composed of specialized cells called "M" cells.
    • do not display a border of microvilli, like the neighboring enterocytes, but rather irregular small folds called “microplicae.”
    • pinocytose a representative sampling of intraluminal antigens and transcytose them across to intraepithelial antigen presenter cells (APC) on the basolateral surface.
    • cup shaped with the flat “base” facing the intestinal lumen.
      • The “cup” portion is filled with APCs and T-lymphocytes.
  • The sensitized T- cells migrate into the underlying lymphatic nodule, and activate the maturation of B lymphocytes, which exit the Peyer’s patch.
  • T and B cells migrate to lymph nodes, where they proliferate into T-helper, T-cytotoxic and plasma cells. 
    • This sensitization and proliferation is considered the afferent (sensory) limb of the Mucosal Immune System.


Small intestine:
The afferent (sensory) limb of the Mucosal Immune System (p.29-30)

  • The committed T-cells and plasma cells/
  • Once out in the systemic circulation, these cells/
  • alpha4beta7
  • Once in the lamina propria, plasma cells produce/
  • IgA

  • The committed T-cells and plasma cells leave the lymph nodes and continue along lymphatic drainage, eventually reaching the right side of the heart.
  • Once out in the systemic circulation, these cells extravasate into the organs of the Mucosal Immune System (GI, respiratory, reproductive).
  • The immune cells display an integrin, alpha4beta7, which serves as the “addressin” for MadCAM-1 (Mucosal adhesion, Cell Adhesion Molecule-1) on the endothelial cells of these organs.
  • Once in the lamina propria, plasma cells produce large quantities of IgA, a dimeric form of antibody.
  • IgA is the main immunoglobulin found in mucous secretions: tears, saliva, colostrum, GI tract, genito-urinary tract, prostate and respiratory tract.


Small intestine:
GALT (p.30-32)

  • poly-IgA receptor (Pig-r)
  • Once IgA binds to SC/
  • Eventually the transport vesicle/
  • Secretory IgA/
    • return “homing” of immune cells
  • M cells and IgA help to

  • The basolateral plasma membranes of the enterocytes contain a glycoprotein receptor for IgA called the poly-IgA receptor (Pig-r) with a detachable segment called the secretory component (SC).
  • Once IgA binds to SC,
    • the complex is endocytosed.
    • The secretory component remains with the IgA and is transcytosed to the apical surface of the enterocyte.
  • Eventually the transport vesicle fuses with the apical plasma membrane and releases secretory IgA into the intestinal lumen.
  • Secretory IgA binds antigens and enterotoxins, decreases the adherence of microbes to the epithelial surface, and neutralizes viruses and bacterial toxins.
    • This return “homing” of immune cells and the secretion and transport of IgA are considered the efferent (effector) limb of the Mucosal Immune System.
  • Clearly, M cells and IgA help to clear unwanted antigens before they even have the opportunity to breech the wall of the GI tract.


Small Intestinal Sections:
Duodenum (p.33-36+38)

  • duodenum
  • Duodenal villi
  • The most distinguishing feature of the duodenum
  • The duodenum co-ordinates with other organs of the GI system by/
  • The proximal small bowel is the site of absorption for:

  • duodenum
    • approximately 25 cm long
    • curves around the head of the pancreas.
  • Duodenal villi
    • leaf-like in shape (broad, flat);
      • this appearance varies in microscopic specimens depending on the plane of sectioning.
    • very few goblet cells in the epithelium, as there is yet little need for lubrication.
  • The most distinguishing feature of the duodenum is Brunner’s glands in the submucosa.
    • The only other portion of the GI tract which has glands in the submucosa is the esophagus.
    • Brunner’s glands are branched tubuloalveolar glands that produce a slightly alkaline mucus secretion to neutralize the acidic chyme arriving from the stomach.
  • The duodenum co-ordinates with other organs of the GI system by sensing luminal content and secreting multiple enteroendocrine hormones.
  • The proximal small bowel is the site of absorption for:
    • Folate
    • Calcium
    • Iron
    • Vit D
    • Vit Bs (not 12)


Small Intestinal Sections:
Jejunum and Ileum (p.37-40)

  • Jejunum
    • length
    • displays/
    • goblet cells
    • major area for/
  • Ileum
    • length
    • characterized by
    • There are only/
    • goblet cells
    • The terminal ileum is the site for absorption for:

  • Jejunum
    • 2.5 meters long.
    • displays the best developed plicae circulares and numerous, long, finger-like villi.
    • There are a medium number of goblet cells, more than in the duodenum.
    • major area for final digestion and absorption in the small intestine.
  • Ileum
    • longest portion of the small intestine (3.5 meters).
    • characterized by Peyer’s patches and short, club-like villi.
    • There are only shallow crypts and low plicae circulares.
    • The epithelium contains a large percentage of goblet cells.
    • The terminal ileum is the site for absorption for:
      • Vitamin B12 /intrinsic factor complex
      • bile acids
      • secretory-IgA


Large Intestine (p.41)

  • divided into/
  • The major functions of the large intestine
  • The mucosa of the large intestine appears/
  • In a histological section the mucosa may appear/
  • The epithelium is composed of/

  • divided into the cecum, appendix, colon, rectum, and the anus.
  • The major functions of the large intestine are the re-absorption of water and salts, and the elimination of undigested feces.
  • The mucosa of the large intestine appears smooth when viewed in a gross specimen; neither plicae circulares nor villi are present.
  • In a histological section the mucosa may appear folded due to post-fixation contraction of the muscularis externa.
  • The epithelium is composed of both enterocytes and goblet cells.


Large Intestine:
Mucosa (p.42-45)

  • composed of/
  • The epithelium
  • Appearance of Paneth cells in the colon/
  • H&E stained sections illustrate/
  • On the enterocytes/
  • The basolateral surface of the absorptive cells contains/
  • The lamina propria contains/
  • The lamina propria of the colon is unique, however, in its/
  • Lymphatic circulation/
  • GALT of the colon lamina propria includes/

  • composed of only crypts with no villi.
  • The epithelium is simple columnar with enterocytes, many goblet cells and a few enteroendocrine cells.
    • absence of Paneth cells.
  • Appearance of Paneth cells in the colon is thought to be metaplastic and reflects chronic mucosal injury.
  • H&E stained sections illustrate many goblet cells.
  • On the enterocytes, a glycocalyx is present, but it does not contain hydrolytic enzymes as in the small intestine.
  • The basolateral surface of the absorptive cells contains Na+K+-ATPase used to power a gradient for the retention of salts at the apical surface.
  • The lamina propria contains numerous fenestrated capillaries for the assimilation of reabsorbed fluids and ions.
  • The lamina propria of the colon is unique, however, in its lack of lymphatic capillaries.
    • This deficit contributes to the slow rate of metastasis of certain colon cancers.
  • Lymphatic circulation only extends as far in toward the lumen as the muscularis mucosa.
  • GALT of the colon lamina propria includes multiple (and frequently large) solitary lymphatic nodules, and isolated lymphocytes, plasma cells and eosinophils.


Large Intestine:
Muscularis Externa and Serosa/Adventitia (p.42-45)

  • Muscularis Externa
  • Serosa/Adventitia

  • Muscularis Externa
    • outer longitudinal layer
    • clustered into three bands called taeniae coli.
    • easily recognized at the gross anatomical level, where their constriction pulls the colon into sacculations called haustra.
  • Serosa/Adventitia
    • The ascending & descending colon are retroperitoneal and are covered by an adventitia.
    • The outer serosa of the rest of the colon often contains pendulous projections of fat.


Large Intestine:
Appendix (p.46)

  • epithelium
  • lumen diameter
  • lymphatic nodules
  •  debris

  • epithelium is identical to the colon's: crypts only, with numerous goblet cells.
  • However, the appendix has a very small lumen diameter.
  • There are lymphatic nodules clustered around the entire lumen (unlike the ileum where the Peyer's patches aggregate on the antimesenteric side of the lumen).
  • Frequently debris is preserved in the lumen.


Large Intestine:
Rectum (p.47-50)

  • rectal lumen
  • epithelial crypts
  • submucosa
  • muscularis externa

  • The rectal lumen is lined by large semilunar mucosal folds with cores of submucosal tissue called plicae transversales recti.
  • The epithelial crypts
    • contain absorptive cells and many goblet cells.
    • usually deeper than in the colon, and solitary lymphatic nodules are common.
  • The submucosa is characterized by large-lumened veins which aid in the reabsorption of water.
  • The muscularis externa
    • contains a continuous sheet of longitudinal smooth muscle, with no taeniae coli,
    • is bordered by an adventitia, as the rectum is embedded in retroperitoneal connective tissue.


Large Intestine:
Anus (p.50-52)

  • The mucosa
  • The epithelium
  • The submucosa
  • The circular layer of the muscularis externa smooth muscle
  • The external anal sphincter

  • The mucosa folds longitudinally into anal columns or anal valves.
  • The epithelium changes into stratified cuboidal or stratified squamous.
    • This change alone indicates the lack of absorption, and the increase in friction in the anus.
  • The submucosa
    • well vascularized,
    • contains the hemorrhoidal plexus (varicosities), nerves and Pacinian corpuscles (deep pressure sensation).
  • The circular layer of the muscularis externa smooth muscle thickens to form the internal anal sphincter.
  • The external anal sphincter, in contrast, is formed from skeletal muscle.