Enterocytes, or intestinal absorptive cells, are simple columnar epithelial cells which line the inner surface of the Small intestine and Large intestine. A glycocalyx surface coat contains . Microvilli on the apical surface increase its surface area. This facilitates transport of numerous small molecules into the enterocyte from the intestinal lumen. These include broken down , , and , as well as water, , , and . Enterocytes also have an endocrine role, secreting hormones such as leptin.
Function
The major functions of enterocytes include:
-
Ion uptake, including sodium, calcium, magnesium, iron, zinc, and copper. This typically occurs through active transport.
-
Water uptake. This follows the osmotic gradient established by NaKATPase on the basolateral surface. This can occur diffusion or paracellularly.
-
Sugar uptake. Polysaccharidases and in the glycocalyx break down large sugar molecules, which are then absorbed. Glucose crosses the apical membrane of the enterocyte using the sodium-glucose cotransporter. It moves through the cytosol (cytoplasm) and exits the enterocyte via the basolateral membrane (into the blood capillary) using GLUT2. Galactose uses the same transport system. Fructose, on the other hand, crosses the apical membrane of the enterocyte, using GLUT5. It is thought to cross into the blood capillary using one of the other GLUT transporters.
-
Peptide and amino acid uptake. Peptidases in the glycocalyx cleave proteins to amino acids or small peptides. Enteropeptidase (also known as enterokinase) is responsible for activating pancreatic trypsinogen into trypsin, which activates other pancreatic zymogens. They are involved in the Krebs cycle and the and can be synthesized with lipase.
-
Lipid uptake. Lipids are broken down by pancreatic lipase aided by bile, and then diffuse into the enterocytes. Smaller lipids are transported into intestinal capillaries, while larger lipids are processed by the Golgi apparatus and smooth endoplasmic reticulum into lipoprotein chylomicra and exocytosis into lacteals.
-
Vitamin B12 uptake. Receptors bind to the vitamin B12-intrinsic factor complex and are taken into the cell.
-
Resorption of unconjugated bile salts. Bile that was released and not used in emulsification of lipids are reabsorbed in the ileum. Also known as the enterohepatic circulation.
-
Secretion of immunoglobulins. Immunoglobulin A from plasma cells in the mucosa are absorbed through receptor-mediated endocytosis on the basolateral surface and released as a receptor-IgA complex into the intestinal lumen. The receptor component confers additional stability to the molecule.
Clinical significance
-
Dietary fructose intolerance occurs when there is a deficiency in the amount of fructose carrier.
-
Lactose intolerance is the most common problem of carbohydrate digestion and occurs when the human body doesn't produce a sufficient amount of lactase enzyme to break down the sugar lactose found in dairy. As a result of this deficiency, undigested lactose is not absorbed and is instead passed on to the colon. There bacteria metabolize the lactose and in doing so release gas and metabolic products that enhance colonic motility. This causes gas and other uncomfortable symptoms.
-
Cholera toxin may increase the secretion or decrease the intake of water and electrolytes, leading to possibly severe dehydration and electrolyte imbalance.
-
Rotavirus selectively invades and kills mature enterocytes in the small intestine.
[Robbins and Cotran Pathologic Basis of Disease, Chapter 17, 749-819]
Stem cell aging
Intestinal stem cell aging has been studied in
Drosophila as a model for understanding the biology of stem cell/niche aging.
Using knockdown mutants defective in various genes that function in the DNA damage response in enterocytes, it was shown that deficiency in the DNA damage response accelerates intestinal stem cell aging, thus providing a better understanding of the molecular mechanisms of this aging process.
[
]
See also
-
List of human cell types derived from the germ layers
External links
-
— "Digestive System: Alimentary Canal — jejunum, goblet cells and enterocytes"
