Microvilli, intestinal

Figure 2 (Left) Microscopic magnification (X46,500) of mucosal surface (S) associated with intestinal microvilli (V). (Right) Schematic of the microvilli illustrating the cytoskele-ton elements including junctional complexes. (From Ref. 76.)... 

Fujinaga, Y., Inoue, K., Nomura, T., Sasaki, J., Marvaud, J.C., Popoff, M.R., Kozaki, S. and Oguma, K., Identification and characterization of functional submits of Clostridium botulinum type A progenitor toxin involved in binding to intestinal microvilli and erythrocytes, FEBS Letters, 467, 179-183, 2000. 

Figure 1-2 Transmission electron micrograph of a dividing cell of Escherichia coli 0157 H7 attached to the intestinal epithelium of a neonatal calf. These bacteria, which are able to efface the intestinal microvilli, form characteristic attachments, and secrete shiga toxins, are responsible for -73,000 illnesses and 60 deaths per year in the U. S.10a After embedding, the glutaraldehyde-fixed tissue section was immuno-stained with goat anti-0157 IgG followed by protein A conjugated to 10-nm gold particles. These are seen around the periphery of the cell bound to the O-antigen (see Fig. 8-28). Notice the two microvilli of the epithelium. Courtesy of Evelyn A. Dean-Nystrom, National Animal Disease Center, USD A, Agricultural Research Service, Ames, IA.

Fujinaga Y, Inoue K, Nomura T, Sasaki J, Marvaud 1C et al. (2000) Identification and characterization of functional subunits of Clostridium botulinum type a progenitor toxin involved in binding to intestinal microvilli and erythrocytes. FEBS Lett 467 179-83 Fujita R, Fujinaga Y, Inoue K, Nakajima H, Kumon H et al. (1995) Molecular characterization of two forms of nontoxic-nonhemagglutinin components of Clostridium botulinum type a progenitor toxins. FEBS Lett 376 41 4... 

Vitamin B12 is special in as far as its absorption depends on the availability of several secretory proteins, the most important being the so-called intrinsic factor (IF). IF is produced by the parietal cells of the fundic mucosa in man and is secreted simultaneously with HC1. In the small intestine, vitamin B12 (extrinsic factor) binds to the alkali-stable gastric glycoprotein IF. The molecules form a complex that resists intestinal proteolysis. In the ileum, the IF-vitamin B 12-complex attaches to specific mucosal receptors of the microvilli as soon as the chymus reaches a neutral pH. Then either cobalamin alone or the complex as a whole enters the mucosal cell. 

Bundles of parallel actin filaments with uniform polarity. The microvilli of intestinal epithelial cells (enterocytes) are packed with actin filaments that are attached to the overlying plasma membrane through a complex composed of a 110-kD protein and calmodulin. The actin filaments are attached to each other through fimbrin (68 kD) and villin (95 kD). The actin bundles that emerge out of the roots of microvilli disperse horizontally to form a filamentous complex, the terminal web, in which several cytoskeletal proteins, spectrin (fodrin), myosin, actinin, and tropomyosin are present. Actin in the terminal web also forms a peripheral ring, which is associated with the plasma membrane on the lateral surfaces of the enterocyte (see Figure 5, p. 24). 

To date, there is very little known about if and how phytochemicals modulate the metabolism of GIT tissues other than the liver. Of particular interest are the xenobiotic metabolizing enzymes of the GIT, which are involved with transformation of drugs and toxins. Whereas the metabolic activities of the resident microflora dominate in the large intestine, mucosal enzyme activities are more important in the small intestine where bacterial densities are lower and the villi and microvilli increase the area of exposure. 

The third mucosal layer is that lining the entire length of the small intestine and which represents a continuous sheet of epithelial cells. These epithelial cells (or enterocytes) are columnar in shape, and the luminal cell membrane, upon which the microvilli reside, is called the apical cell membrane. Opposite this membrane is the basal (or basolateral) plasma membrane, which is separated from the lamina propria by a basement membrane. A sketch of this cell is shown in Fig. 5. The primary function of the villi is absorption. 

Figure 2 Comparison of intestinal epithelial cells in culture and in situ. (A) Human colon Caco-2 cells grown in culture for 16 days on a semiporous filter. (B) Epithelial layer of rat jejunum. AP, apical or luminal membrane B, basal or abluminal membrane BM, basement membrane G, goblet cell LS, lateral space mv, microvilli Nu, nucleus TJ, tight junction. Bars equal 10 pm. 

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. 

The gut wall within the small intestine is particularly well adapted for its role as an absorptive surface. Absorption rate is proportional to the area of the surface that is available for absorption. Thus, the internal surface of the small intestine is folded towards the lumen of the gut. This folding increases the surface area of the gut by approximately 3-fold. In this area, the gut wall is covered with many fingerlike projections called villi, and these provide a further 10-fold increase in surface area. In addition, the gut wall epithelial cells are polarized such that on the luminal surface there are millions of microvilli providing a further 20-fold increase in surface area for absorption. In all, these surface area modifications provide an absorptive area which is some 600-fold higher than would be provided by a simple cylinder. Thus, the estimated surface area of the human gut is approximately 200 m2 [1],... 

In an attempt to increase the biorelevance of the Ussing chambers technique even further, the use of simulated intestinal fluids (FaSSIF and FeSSIF) as transport media was recently evaluated [105], However, the potential difference collapsed to zero after 120 min when exposed to FaSSIF solution and permeability for mannitol was increased twofold. Electron micrographs revealed erosion of the villi tips and substantial denudation of the microvilli after exposure of the ileal tissue to FaSSIF and FeSSIF [105],... 

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