Microvillus, microvilli

The microvillar surface is coated with a layer of electron-dense amorphous material (glycocalyx). In H. contortus, helical filaments composed of contortin are associated with this layer and fill the spaces between the microvilli. There can be up to ten strands of contortin in each microvillus ... 

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. 

The microvillus membrane of the absorptive cell has a thickness of 9 5-11.5 nm, which is somewhat thicker than normal biological membranes. It is folded into microvilli (about 200,000/mm2 in the human jejunum [11]), and increases the surface of the intestine by a factor of 20. Microvilli contract, and, during contraction the fibers in the core slide down toward the underlying terminal web. In this region, the movement of dings and other molecules will not only be diffusive but also agitalive. Molecules passing the membrane are absorbed by the trans-cel lular" pathway. 

Fig. 8.6. Paricterotaenia paradoxa-. development of microtriches from microvilli in the polycephalic larva (not to scale), (a) The original microvillus. (b) and (c) The microvillus base swells and contains peripheral electron-dense material, (d) A developing microthrix on the posterior neck region of larva the original microvillus is still attached, (e) A mature, modified microthrix on the posterior neck region. (/) A developing microthrix on the anterior neck region, the original microvillus is still attached, (g) A mature microthrix on the anterior neck region. (After MacKinnon Burt, 1984.)...

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. 

Figure 11.12. Electron Micrograph of a Microvillus. Lactase and other enzymes that hydrolyze carbohydrates are present on microvilli that project from the outer face of the plasma membrane of intestinal epithelial cells. [From M. S. Mooseker and L. G. Tilney, J. Cell. Biol. 67(1975) 725.]...

Microvilli "Microvilli" is the pural form "Microvillus" is the singular form. Microvilli are finger-like projections on the outer-surface of the cell. Not all cells have microvilli. Their function is to increase the surface area of the cell, which is the area through which diffusion of materials both into, and out of, the cell is possible. 

Villin is a calcium-dependent actin-binding cytoskeletal protein that is found in the brush border of the intestine and in the proximal renal tubular epithelium. A brush border is characteristic of colorectal carcinomas and is recognized at the ultrastructural level by the presence of microvilli with a dense core of microfilaments, core rootlets, and surface glycocalyx. Up to 33% of pulmonary adenocarcinomas may demonstrate microvillus rootlets... 

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... 

Microvillus Fat droplets Pinocytotic vesicle Microvilli cutoff Terminal web... 

Figure 2. Apical part of a Lepisosteus oculatus (Holostei) - taste bud in longitudinal section. The sensory epithelium of the organ consists of two types of light cells and one type of dark cells. The light cell a (Cla) terminates with several short microvilli, the light cell b (Clb) bears one long and thick microvillus. The dark cells (Cd) are apically rich in electron-dense vesicles of different sizes and end with several small receptor villi. Jt - tight junction RA - receptor area. Bar 1 pm.

Microvillus (plural Microvilli). A minute projection from the surface of an animal cell. 

Crane (1966) has theorized that the brush border plasma membrane is the site of a mosaic of the enzymes associated with the microvillus. This was based on experiments of Eichholz and Crane (1965) who recovered a fraction of pure microvillous membranes by density gradient centrifugation of brush border homogenate which possessed the total activities of alkaline phosphatase, maltas and sucrase and various peptidases. Evidence in favor of this idea was also collected by Johnson (1967) who demonstrated the presence of knobs 60 A in diameter on the glycocalyx of the luminal side of the plasma membrane which contained the brush border invertase and maltase (see Fig. 6). These knobs could be removed entirely from the microvilli of hamster intestine by papain digestion the remaining membrane, however, still has the alkaline phosphatase incorporated into it (Eichholz, 1969 Oda and Seki, 1966). 

Recruitment of the pump into the canalicular membrane is thought to involve a fusion process of the cytoplasmic tubules into the microvilli which is followed by a process of eversion so that the cytoplasmic surface of the pump is on the inner surface of the microvillus. Retrieval of the pump involves inversion and presumably separation of the tubule membrane from the apical membrane, as shown in Figure 6. 

Figure 4. Electron micrograph of the apical portions of several intestinal absorptive cells in the fasted state. Absorptive cells are highly polar. The plasma membrane bordering on the lumen of the intestine is modified to form numerous microvilli (MV). Beneath the microvillus border is the terminal web (TW) zone of the cytoplasm which contains relatively few cytoplasmic organelles. The smooth endoplasmic reticulum (SER) is most prominent just below the terminal web. The RER extends from the region of the terminal web all the way to the supranuclear Golgi complex. Mitochondria (M) nucleus (N) plasma membrane (PM). X 15,940. Reduced 12% for reproduction. From Friedman and Cardell (1977) with permission of the publisher.

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