Basal membrane polarity

In general, the specific constituents of milk are synthesized from small molecules absorbed from the blood. These precursors are absorbed across the basal membrane but very little is known about the mechanism by which they are transported across the membrane. Since the membrane is rich in lipids, and precursors are mostly polar with poor solubility in lipid, it is unlikely that the precursors enter the cell by simple diffusion. It is likely, in common with other tissues, that there are specialized carrier systems to transport small molecules across the membrane such carriers are probably proteins. 

Mogami H., Nakano K., Tepikin A.V., and Petersen O.H. 1997 Ca2+ flow via tunnels in polarized cells recharging of apical Ca2+ stores by focal Ca2+ entry through basal membrane patch. Cell 88, 49—55. 

Podocytes are polarized cells, so one can differentiate between luminal and abluminal (basal) membrane domains (the basal domain corresponds to the sole plates of the foot processes, which are embedded in the basement membrane). The slit diaphragm forms the border between the luminal and the abluminal membranes. 

The cytoskeleton is involved in the maintenance of cell shape and cytoplasmic processes (e.g., microvilli). In polarized epithelial cells, distinct cyto-cortical cytoskeletal complexes are associated with the apical and basal-lateral domains of the plasma membrane (Rodriguez-Boulan and Nelson, 1989 Mays et al., 1994). 

Electron microscopic evaluation of infected Caco-2 monolayers grown on membranes, under conditions that induced cellular polarization, prompted the conclusion that the larvae occupy the cytoplasm of cells they invade (ManWarren et al., 1997). Apical and basal plasma membranes appeared to be preserved in infected cells (Fig. 6.3). These findings reproduced the observations of Wright (1979) in his examination of intestinal tissues from infected mice. In contrast, when Li et al. (1998) performed similar experiments in HT29 monolayers grown on plastic, they concluded... 

The taste bud is a polarized structure with a narrow apical opening, termed the taste pore, and basolateral synapses with afferent nerve fibers. Solutes in the oral cavity make contact with the apical membranes of the TRCs via the taste pore. There is a significant amount of lateral connectedness between taste cells within a bud both electrical synapses between TRCs and chemical synapses between TRCs and Merkel-like basal cells have been demonstrated to occur [39]. Furthermore, there are symmetrical synapses between TRCs and Merkel-like basal cells [39]. In addition, these basal cells synapse with the afferent nerve fiber, suggesting that they may function in effect as interneurons [39]. The extensive lateral interconnections... 

An important observation is that certain viruses preferentially bud at different poles of their host cells. In MDCK-cell monolayers, VSV buds exclusively from the basal, or lateral, plasma membranes, and contains sialylated glycoproteins, whereas influenza virus buds exclusively from the apical plasma-membrane, and lacks neuraminic acid. The question arises as to whether glycosylation of viral glycoproteins is needed in order to determine the site of budding. An electron-microscope study revealed that polarity in the maturation sites of these viruses was maintained under conditions of inhibition of glycosyla-... 

Acetone, methyl ethyl ketone (2-butanone) and methyl isobutyl ketone (4-methyl-pentan-2-one) (6.8 mmol/kg bw for 3 days) increased the hepatotoxicity of carbon tetrachloride to Sprague-Dawley rats (Raymond Plaa, 1995a) this enhancement of toxicity was coincident with increased microsomal aniline hydroxylase activity (Raymond Plaa, 1995b). In addition to the effect on cytochrome P450, acetone, but not the other ketones, increased basal canalicular membrane fluidity, as measured by fluorescence polarization of 1,6-diphenyl-1,3,5 -hexatriene or 1 - [4-(trimethylammoniumphenyl)-6-phenyl] -1,3,5 -hexa-triene (Raymond Plaa, 1996). 

Careful observation at the borders of the enterocytes reveals a close juxtaposition of the membranes of adjacent epithelial cells, identified as zonulae occludens, located at the apical neck of the epithelial cells [168], The zonulae occludens is composed of apical complex the tight junctions and more basal structure underneath, the adherens junctions (also referred to as intermediate junctions) [169,170]. The adherens junctions are linked to the actomyosin ring at the cell circumference. This actin cytoskeleton, which forms a perijunctional area at the apical neck of the epithelial cells, is the cause for their polarized nature (Figure 1.12). The classical functions of tight junctions are the regulation of paracellular permeability and the restriction of apical basolateral intramembrane diffusion of lipids (cell polarity housekeeping). 

Normal NMJs have a generally polarized nerve terminal with accumulations of40-50 nm small clear vesicles near the presynap-tic membrane and mitochondria located farther away (Fig. 20.8). In mice, the terminal Schwann cell capping the nerve terminal can be difficult to resolve. The postsynaptic membrane has a series of junctional folds invaginating into the muscle fiber. At the mouth (crest) of each fold, the membrane appears electron dense because of the accumulation of AChRs. The synaptic cleft is pronounced and contains a visible basal lamina. 

Fig. 7.11. Comparison of the fine structure of the membranes of the eggs of Hymenolepis diminuta and H. nana. Note in H. nana (in contrast to H. diminuta) (a) the embryophore is thin and incomplete - a feature which may facilitate hatching in the gut of the definitive host (b) there is an additional polar filament layer between the oncospheral membrane and the basal lamina. Gc, Golgi complex ger, granular endoplasmic reticulum /, lipid bodies m, mitochondria n, nucleus v, vacuoles. (After Fairweather Threadgold, 1981a Holmes Fairweather, 1982.)...

Cells normally exhibit polarity that is, they have a top, bottom, left, and right sides. Most cells such as the epithelium present an apical pole that is characterized by many ruffles in the cell membrane termed microvilli and a basal pole that is in contact with a basement membrane. This polarity is very important because normal cell function can only be expressed if the cell has the correct orientation. 

Emonard, H. and Grimaud, J-A. (1990). Matrix metal-loproteinases a review. Cell Mol. Biol. 4, 195-203. Estreicher, A., Muhlhauser, J., Carpentier, J-L., Orci, L. and Vassalli, J-D. (1990). The receptor for urokinase plasminogen activator polarizes expression of the protease to the leading edge of migrating monocytes and promotes d radation of enzyme-inhibitor complexes. J. Cell Biol. Ill, 783-792. Evans, M.J. and Plopper, C.G. (1988). The role of basal cells in adhesion of columnar epithelium to airvray basement membrane. Am. Rev. Respir. Dis. 138, 481—483. 

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