Brush border microvilli

The urinary contents also can provide important information concerning the presence of nephrotoxicity. The presence or elevated levels of enzymes, protein, amino acids, glucose, blood, or casts in the urine can signal renal injury. Enzymuria results primarily from the loss of the brush border (microvilli)... 

The plasma membrane of the brush border microvilli is characterized by certain distinctive structural features, which may be related to the specialized functional properties that distinguish it from plasma membranes of other cells. The width of the microvillus membrane (measured by electron microscopy) is 10-11 nm, whereas the average eukaryotic plasma membrane is only 7-9 nm. This is probably due to the biochemical composition of the membrane, which is characterized by a high protein to lipid ratio (1.7 1) and a unique lipid composition. The cholesterol-phospholipid ratio and the molar ratio of glycolipid to phospholipid are both about 1 1, which is consistent with the low values for membrane fluidity determined in microvillus membranes. This should be compared with the corresponding ratios... 

Bode, F., Baumann, K., and Kinne, R., 1976, Analysis of the pinocytic process in rat kidney. II. Biochemical composition of pinocytic vesicles compared to brush border microvilli, lysosomes and basolateral plasma membranes, Biochim. Biophys. Acta 433 294. 

Heidrich, H. G., Kinne, R., Kinne-Saffran, E., and Hannig, K., 1972, The polarity of the proximal tubule cell in rat kidney. Different surface charges for the brush-border microvilli and plasma membranes from the basal infoldings, /. Cell Biol. 54 232. 

Kinne, R., Murer, H., Kinne-Saffran, E., Thees, M., and Sachs, G., 1975, Sugar transport by renal plasma membrane vesicles Characterization of the systems in the brush-border microvilli and basal-lateral plasma membranes, /. Membrane Biol. 21 375. 

Microvilli are microscopic projections found on the luminal surface of the absorptive cells. Each absorptive cell may have literally thousands of microvilli forming the brush border. These structures increase the surface area for absorption another 20-fold. Together, these three anatomical adaptations of the intestinal mucosa — plicae circulares, villi, and microvilli — increase the surface area as much as 600-fold, which has a profound positive effect on the absorptive process. 

EPEC causes a degeneration of the microvillus brush border, with cupping and pedestal formation of the plasma membrane at the sites of bacterial attachment and reorganization of cytoskeletal proteins [43, 44], Invasion has been observed in some clinical specimens, but the mechanism of how this bacteria produces diarrhea is not fully understood. Some possibilities include an increase in permeability and loss in microvilli leading to malabsorption. 

Figure 3.12 The mammalian gastrointestinal tract showing important features of the small intestine, the major site of absorption for orally administered compounds (A) liver (B) stomach (C) duodenum (D) ileum (E) colon (F) longitudinal section of the ileum showing folding, which increases surface area (G) detail of fold showing villi with circular and longitudinal muscles, (H) and (I) respectively, bounded by (J) the serosal membrane (K) detail of villi showing network of (L) epithelial cells, (M) capillaries, and (N) lacteals (O) detail of epithelial cells showing brush border or (P) microvilli. The folding, vascularization, and microvilli all facilitate absorption of substances from the lumen. Source From Ref. 1.

In addition to the highly organized arrangement of actin and myosin filaments in muscle (discussed in detail in Squire et al., 2005), there are also highly organized arrays of microtubules in the cilia and flagella of eukaryotes that either pass fluid across the cell surface or act as the propellers for spermatazoa (Fig. IB). Ordered arrays of actin also occur in microvilli on the surfaces of cells from the brush border of intestinal epithelia. Here, the highly cross-linked actin arrays appear to be tensioned by interaction with myosin at the bases of the microvilli in the terminal web (Fig. 2B). 

The cells lining the lumen of the intestine are polarized, that is they have two distinct sides or domains which have different lipid and protein compositions. The apical or brush border membrane facing the lumen is highly folded into microvilli to increase the surface area available for the absorption of nutrients. The rest of the plasma membrane, the basolateral surface, is in contact with neighboring cells and the blood capillaries (Fig. 5). Movement between adjacent epithelial cells is prevented by the formation of tight junctions around the cells near the apical domain. Thus any nutrient molecules in the lumen of the intestine have to pass through the cytosol of the epithelial cell in order to enter the blood. 

Villin is an example of a bundling protein. Villin is found in the microvilli of, for example, intestinal brush border cells (Fig. 5-30). The microvilli greatly increase the surface area of the cells, which is essential for effective absorption to take place. Each microvillus extends about 2 p.m into the lumen of the gut and is supported by 20 or so actin filaments tightly bundled by villin (and other proteins) at regular intervals. In a feature common to many actin-based networks, all the filaments in the bundle are oriented with their barbed ends in the same direction, in this case toward the tip of the microvillus where they terminate. Cross-linking of the actin filaments to the plasma membrane occurs via a second protein from the myosin-1 family (a relative of the well-known contractile protein myosin-II). This protein binds its head domain to the sides of the filaments and embeds its tail domain into the membrane. 

Final hydrolysis of di- and oligosaccharides is carried out by surface enzymes of the small intestinal epithelial cells, called the brush border, a term that comes from the appearance of the enterocytes, in which the luminal plasma membrane is enlarged by a regular array of projections called microvilli. The enzymes are not secreted into the lumen, but are embedded in the cell membrane, many of these enzymes can protrude into the intestinal lumen up to 10 gm, as they are attached to the plasma membrane by an anchoring polypeptide that has no role by itself in the hydrolysis. 

Q1 The mucosa is a mucous membrane which forms the innermost layer of the intestine. In the small intestine the mucosal surface area is increased greatly by folds and by villi, finger-like projections containing a core with a lymph capillary (lacteal) and blood vessels. Villi are covered by absorptive columnar epithelial cells whose luminal surface is further increased by microvilli (brush border) on which digestive enzymes and transport mechanisms for inorganic ions are located. 

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