Small Intestine

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I. MUCOSA is lined by the following: A. Surface absorptive cells

1. These cells are joined by juxtaluminal tight junctions.

2. They possess microvilli that are coated by filamentous glycoproteins called the gly-cocalyx.

a. The glycocalyx contains the following important enzymes: lactase, sucrase, maltase, peptidases, and enterokinase.

b. Enterokinase converts the inactive form (e.g., trypsinogen) of pancreatic enzymes to the active form (e.g., trypsin).

3. Surface absorptive cells absorb carbohydrates, protein, lipids, vitamins, calcium, and iron from the intestinal lumen and transport them to the blood or lymph.

a. Carbohydrates arc digested to monosaccharides (glucose, galactose, fructose). Glucose and galactose are absorbed using a Na+-dependent cotransporter. Fructose is absorbed via facilitated diffusion.

b. Proteins are digested into amino acids, dipeptides, and tripeptides. These are absorbed using a Na+-dependent cotransporter.

C. Triacylglycerols (the main fat in a human diet) are emulsified by bile salts and digested to fatty acids and monoacylglycerols.

(1) Long-chain fatty acids (>12 carbons), monoacylglycerols, cholesterol, and fat-soluble vitamins (A, D, E, and K) are packaged into micelles and absorbed.

(2) Within the surface absorptive cell, resynthesis of triacylglycerols occurs in the smooth endoplasmic reticulum (sER). Subsequently, the triacylglycerols, cholesterol, and fat-soluble vitamins are packaged with apoproteins into chylomicrons and released into the lymph.

(3) Short- and medium-chain fatty acids (<12 carbons) are not packaged into chylomicrons, hut are released directly into the portal blood. Xeni-cal is a drug used in the treatment of morbid obesity that blocks approximately 30% of dietary fat from being absorbed.

d. Water-soluble vitamins are absorbed using a Na+-dependent cotransporter. Vitamin B12 requires intrinsic factor secreted by parietal cells of the stomach (see Chapter 9, Figure 9-4 and Chapter 13).

e. Calcium is absorbed and requires 1,25 (OH)2 vitamin D3, which is produced by the kidney (see Chapter 22), to be fully absorbed.

f. Iron is absorbed as ferroheme (bound to hemoglobin or myoglobin) or as free Fe2+ and transported in the plasma by a protein called transferrin (see Chapter 1 V1I1B).

B. Goblet cells synthesize mucinogen, which is stored in membrane-bound granules.

C. M cells are antigen-presenting cells that overlie lymphatic follicles.

II. INTESTINAL GLANDS (CRYPTS OF LIEBERKUHN) contain the following types of cells:

A. Stem cells demonstrate a high rate of mitosis and replace surface absorptive cells and goblet cells every 3-6 days.

B. Paneth cells secrete lysozyme, which has antibacterial activity (Figure 14-1).

C. Enteroendocrine cells (Figure 14-2) are comprised of the following:

1. I cells secrete cholecystokinin (CCK) in response to small peptides, amino acids, and fatty acids within the gut lumen. CCK stimulates enzyme secretion from the pancreas and release of bile from the gallbladder.

2. S cells secrete secretin in response to hydrogen ions and fatty acids within the gut lumen. Secretin stimulates the release of bicarbonate from the pancreas and from the biliary tract.

3. K cells secrete gastric-inhibitory peptide (GIP) in response to orally administered glucose, amino acids, and fatty acids in the gut lumen. GIP stimulates insulin secretion from pancreatic islets. This explains why an oral glucose load produces higher serum insulin levels than an intravenous glucose load.

4. L cells secrete glucagon-like peptide-1 (GLP-1) in response to orally administered glucose, amino acids, and fatty acids in the gut lumen.

a. GLP-1 stimulates insulin secretion from pancreatic islets.

b. GLP-1 inhibits glucagon secretion from pancreatic islets.

C. GLP-1 may be an effective therapeutic agent for patients with type 2 diabetes because the stimulatory effect of GLP-1 on insulin secretion is preserved in these patients.

III. GUT-ASSOCIATED LYMPHATIC TISSUE (GALT)

A. M cells sample the intestinal contents and present a specific antigen to immature lymphocytes.

B. Mature lymphocytes differentiate into plasma cells that secrete specific immunoglobulin A (IgA) antibodies into the lamina propria.

C. The majority of IgA within the lamina propria enters the bloodstream and reaches the liver sinusoids.

D. IgA within the liver sinusoids binds to receptors on hepatocytes and is endocytosed. At the bile canaliculi, IgA is released into the biliary tract in combination with a portion of its receptor (secretory piece) arid is known as secretory IgA.

E. Secretory IgA enters the intestinal lumen with the bile.

IV. REPAIR (REGENERATION) involves stem cells (see II A).

V. CLINICAL CONSIDERATIONS

A. Celiac disease is a hypersensitivity to gluten and gliadin protein found in wheat and other grains.

1. Upon ingestion of gluten-containing foods, a large number of lymphocytes, plasma

Figure 14-1. (A) Light micrograph of numerous Paneth cells at the base of an intestinal gland with conspicuous secretory granules oriented toward the lumen (L). (B) Electron micrograph of Paneth cells. Paneth cells secrete a protein callcd lysozyme, and therefore, contain secretory granules (S), Golgi complexes (G), and rough endoplasmic reticulum (rER) typically found in protein-secreting cclls. L = lysosomcs; MV = microvilli; N = nuclcus. (Reprinted with permission from Satoh Y, Yamano M, Matsuda M, et al.: Ultrastructure of pancreatic cells in the intestine. J Election Microsc Tech 16(1 ):69, 1990. © 1990, John Wiley & Sons, Inc. Reprinted by permission of Wi-ley-Liss, Inc., a division of John Wiley & Sons, Inc.)

Figure 14-1. (A) Light micrograph of numerous Paneth cells at the base of an intestinal gland with conspicuous secretory granules oriented toward the lumen (L). (B) Electron micrograph of Paneth cells. Paneth cells secrete a protein callcd lysozyme, and therefore, contain secretory granules (S), Golgi complexes (G), and rough endoplasmic reticulum (rER) typically found in protein-secreting cclls. L = lysosomcs; MV = microvilli; N = nuclcus. (Reprinted with permission from Satoh Y, Yamano M, Matsuda M, et al.: Ultrastructure of pancreatic cells in the intestine. J Election Microsc Tech 16(1 ):69, 1990. © 1990, John Wiley & Sons, Inc. Reprinted by permission of Wi-ley-Liss, Inc., a division of John Wiley & Sons, Inc.)

Villus

Intestinal

Enzyme secretion

Release of bile

Release of HC03"

Release of HCO3-

Insulin secretion

Inhibition of glucagon secretion

Villus

Intestinal

Enzyme secretion

Release of bile

Release of HCO3-

Release of HC03"

Insulin secretion

Insulin secretion

Inhibition of glucagon secretion

Figure 14-2. Diagram of hormonal secretion from intestinal glands and their actions. Note that hydrogen ions, glucose, small peptides, amino acids, and fatty acids within the lumen of the intestinal gland stimulate the 1 cells (1), S cells (S), K cells (K), and L cells (L). CCK = cholecystokinin; GIP = gastric inhibitory peptide; GLP-1 = glucagon-like peptide 1; HCO,~ - bicarbonate; SEC = secretin.

cells, macrophages, and eosinophils accumulate within the lamina propria of the intestinal mucosa.

a. Cliadin antibodies are generally dctectablc in the blood.

b. These factors may contribute to the immunologic damage of the mucosa.

2. It is characterized clinically by chronic diarrhea, flatulence, weight loss, and fatigue.

B. Crohn disease (CD) is a type of inflammatory bowel disease involving the formation of a granuloma. The etiology of CD is unknown.

1. A classic feature of CD is the clear demarcation between diseased bowel segments located directly next to uninvolved normal bowel.

2. Neutrophils infiltrate the intestinal glands and ultimately destroy them, leading to ulcers. With progression of CD, the ulcers coalesce into long, serpentine ulcers (linear ulcers) oriented along the long axis of the bowel.

3. CD is characterized clinically by intermittent bouts of diarrhea, weight loss, and weakness.

4. Complications may include strictures of the intestinal lumen and the formation of fistulas.

C. Cholera is caused hy the gram-negative bacteria Vibrio cholerae, which produces an enterotoxin called cholera toxin.

1. Cholera toxin is an enzyme that catalyzes adenosine diphosphate (ADP) ribosy-lation of the as chain of Gs protein. This effectively increases cyclic adenosine monophosphate (cAMP) levels, which activates chloride ion channels of surface absorptive cells to secrete chloride ions into the lumen; sodium ions and water follow.

2. Certain strains of Escherichia coli produce toxins that cause traveler's diarrhea by a similar mechanism.

3. Both CD and traveler's diarrhea are characterized clinically by a severe, watery diarrhea.

D. Lactose intolerance is due to the absence of the enzyme lactase from the glycocalyx so that lactose cannot be digested to glucose.

1. The unabsorbed lactose within the lumen results in osmotic diarrhea.

2. Congenital lactase deficiency is a rare condition that becomes apparent in infants at the start of milk feeding.

3. Acquired lactase deficiency is generally due to rotavirus gastroenteritis, kwashiorkor, or old age.

4. Is characterized clinically by abdominal distention and an explosive, watery diarrhea.

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