The duodenum is the first major digestive and absorptive region of the intestine, receiving chyme from the stomach and mixing it with bile, pancreatic juice and enteric secretions.
They comprise from the outside to the inside:
• adventitia or serosa;
• longitudinal muscle layer;
• circular muscle layer;
• submucosa containing Brunner’s glands;
• muscularis mucosae;
• mucosal layer comprising the lamina propria and epithelial lining.
The epithelium rests on a basement membrane, on the loose connective tissue of the lamina propria, which is thrown up into finger-like villi and is indented into long, thin crypts (of Lieberkühn) from which new epithelial cells emerge. A thin layer of smooth muscle, the muscularis mucosa, separates the mucosa from the submucosa, which is thrown up in transverse folds known as plicae circulare. Branched tubular glands, called Brunner’s glands, are located in the submucosa and are connected to the lumen by narrow ducts. The lamina propria contains numerous fibroblasts, macrophages, lymphocytes, neutrophils, mast cells, vascular endothelial cells and other cells. An arteriole, venule and a lymphatic channel called a lacteal supply each villus. The arteriole and venule form a counter current circulation enhancing intestinal absorption.
Intrinsic enteric nerves ramify through the layers of the intestine, controlling motor and secretory function. The small intestinal epithelium contains a number of distinct cell types, all of which differentiate from stem cells located in the crypt. Enterocytes constitute most of the intestinal lining. They are columnar, with a round or oblong nucleus located centrally. On the luminal surface, microvilli, supported by an extensive network of cytoskeletal proteins, increase the surface area available for digestion and absorption. The surface of the microvilli are covered by glycoproteins and attached enzymes and mucins, forming a prominent brush border. Tight junctionslink adjacent enterocytes, so that the apical surface of the cell, and consequently the luminal surface of the intestine, is isolated from the basal surface. Thus, gradients of nutrients and electrolytes can be maintained and pathogens can be excluded. Enterocytes synthesize digestive enzymes and secrete them to the apical brush border. Goblet cells are specialized secretory cells that produce mucin. Cytoplasmic stores of mucin are not stained by conventional histochemistry and create the typical ‘empty goblet’appearance.
Paneth cells are found at the base of the small intestinal crypts. They are specialized for protein synthesis and secretion and contain antibacterial proteins such as lysozyme, phospholipase A2 and defensins. They may also have other, undefined, roles in intestinal health and disease. Entero-endocrine cellsare found predominantly near the crypt bases and produce many different enteric hormones. Stem cellsare located just above the Paneth cell zone. They retain the capacity to replenish the entire epithelium, by dividing to produce one daughter stem cell and one daughter cell that proliferates, differentiates and migrates up the crypt.
Structure
The duodenum extends from the pylorus, to the jejunum at the ligament of Treitz. It is approximately 30·cm long and ‘C’-shaped, facing the left, and is mostly retroperitoneal. The first part of the duodenum is called the bulb. The second part receives bile and pancreatic juice via the ampulla of Vaterand lies adjacent to the pancreason the left. The coeliac artery supplies the duodenum and venous drainage is via the superiormesenteric veininto the hepatic portal vein. The walls of the duodenum reflect the general organization of the intestinal wall.They comprise from the outside to the inside:
• adventitia or serosa;
• longitudinal muscle layer;
• circular muscle layer;
• submucosa containing Brunner’s glands;
• muscularis mucosae;
• mucosal layer comprising the lamina propria and epithelial lining.
The epithelium rests on a basement membrane, on the loose connective tissue of the lamina propria, which is thrown up into finger-like villi and is indented into long, thin crypts (of Lieberkühn) from which new epithelial cells emerge. A thin layer of smooth muscle, the muscularis mucosa, separates the mucosa from the submucosa, which is thrown up in transverse folds known as plicae circulare. Branched tubular glands, called Brunner’s glands, are located in the submucosa and are connected to the lumen by narrow ducts. The lamina propria contains numerous fibroblasts, macrophages, lymphocytes, neutrophils, mast cells, vascular endothelial cells and other cells. An arteriole, venule and a lymphatic channel called a lacteal supply each villus. The arteriole and venule form a counter current circulation enhancing intestinal absorption.
Intrinsic enteric nerves ramify through the layers of the intestine, controlling motor and secretory function. The small intestinal epithelium contains a number of distinct cell types, all of which differentiate from stem cells located in the crypt. Enterocytes constitute most of the intestinal lining. They are columnar, with a round or oblong nucleus located centrally. On the luminal surface, microvilli, supported by an extensive network of cytoskeletal proteins, increase the surface area available for digestion and absorption. The surface of the microvilli are covered by glycoproteins and attached enzymes and mucins, forming a prominent brush border. Tight junctionslink adjacent enterocytes, so that the apical surface of the cell, and consequently the luminal surface of the intestine, is isolated from the basal surface. Thus, gradients of nutrients and electrolytes can be maintained and pathogens can be excluded. Enterocytes synthesize digestive enzymes and secrete them to the apical brush border. Goblet cells are specialized secretory cells that produce mucin. Cytoplasmic stores of mucin are not stained by conventional histochemistry and create the typical ‘empty goblet’appearance.
Paneth cells are found at the base of the small intestinal crypts. They are specialized for protein synthesis and secretion and contain antibacterial proteins such as lysozyme, phospholipase A2 and defensins. They may also have other, undefined, roles in intestinal health and disease. Entero-endocrine cellsare found predominantly near the crypt bases and produce many different enteric hormones. Stem cellsare located just above the Paneth cell zone. They retain the capacity to replenish the entire epithelium, by dividing to produce one daughter stem cell and one daughter cell that proliferates, differentiates and migrates up the crypt.
Function
Alkaline bile and pancreatic juices neutralize stomach acid. Powerful enzymes from the pancreas, which are activated in the lumen by autocatalysis and by the action of enterokinase released from duodenal enterocytes, support rapid and efficient digestion. The final stages of digestion occur in the brush border of enterocytes under the action of disaccharidases and peptidases. Bile salts emulsify fatty foods, allowing digestive enzymes to act more efficiently. Transport proteins in the apical membrane actively absorb sugars, amino acids and electrolytes into the enterocyte. Fatty acids and cholesterol enter by direct diffusion across the lipid membrane, are re-esterified intracellularly, complexed with apolipoproteins to form chylomicrons and released at the basolateral surface. The jejunum and ileum constitute the major digestive surfaces of the intestine, but iron and calciumin particular are preferentially absorbed in the duodenum. The small intestine is relatively free from resident bacteria and an antimicrobial environmentis maintained by the action of gastric acid and antibacterial substances produced by Brunner’s glands and Paneth cells. Biliary epithelial cells and enterocytes transport secretory dimeric immunoglobulin A(sIgA) into the lumen, which may also contribute to antimicrobial defence in the small intestine. Entero-endocrine cells in the duodenum secrete cholecystokinin and secretin in response to food, stimulating gallbladder contraction and pancreatic secretion, and inhibiting gastric motility. Thus, the duodenum participates in neuro-endocrine coordination of gastrointestinal function.Common disorders
Duodenal disorders may cause epigastric pain, diarrhoea, malabsorption, loss of weightand nutritional deficiencies. Bleeding ulcers may cause anaemia, haematemesis and melaena, the characteristic black tarry appearance of stool caused by partially digested blood. Cancer of the duodenum is extremely rare, while peptic ulcer and coeliac diseaseare common. Giardia lamblia is a protozoal pathogen that causes traveller’s diarrhoea, by adhering to and damaging the duodenal and jejunal epithelium, resulting in flatulence, diarrhoea and malabsorption. |
STRUCTURE OF HUMAN DUODENUM; PART OF DIGESTIVE SYSTEM |
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