Microtubules 9+2 pattern

Initiation and differentiation of microtubule patterns in the ciXiaXt Nassula. 

A rudimentary ciliary structure, consisting of a very short roodet, a basal body, an accessory centriole and a short cilium appears in the upper third of the epidermal cell (Fig. 6.2B). As in articulate brachiopod larvae, this rudimentary cilium extends into a small space left by the epidermal cell between its membrane and the embryonic setal surface. The microtubule pattern of the cilium could not be observed and its function remains unclear. 

Parallel arrays of microtubules are found in the axoneme of cilia and flagella of eukaryotic cells, and these are of constant pattern throughout the phylogenetic scale. 

Another example of parallel arrays of microtubules but with a much looser pattern is provided by axons, showing the involvement of such arrays in axoplasmic transport. This is illustrated in Figure 3. 

The Cytoskeleton—Microtubules and Microfilaments Patterns of Arrangement of Actin Filaments in Animal Cells... 

TABLE 8-1 Major microtubule cytoskeletal proteins of the nervous system Protein Expression pattern and distribution... 

Cholera toxin This protein catalyzes the covalent insertion of an ADP-ribosyl moiety derived from NAD into an unidentified site of tubulin, based on SDS—gel electrophoresis and peptide fragmentation patterns (Hawkins and Browning, 1982). Because only about 0.1% of the tubulin dimers appear to become modified, the association with microtubule control remains tenuous... 

Vinblastine, vincristine, and structurally related analogs inhibit microtubule polymerization by 50% at concentrations in the range 0.1-1 xM, and the process of tubulin addition to preformed microtubules, at steady state, is comparably sensitive to inhibition by these agents (5). As shown in Table I, the differences in values for inhibition of steady-state tubulin addition by vinblastine, vincristine, vindesine, and vinepidine were relatively small, but the pattern of activity in the tubulin addition system did not parallel that observed when the compounds were evaluated for effects on the proliferation of B16 melanoma cells in vitro. Vinepidine was more than twice as potent as vinblastine as an inhibitor of steady-state tubulin addition but nearly 10-fold less potent than vinblastine as an inhibitor of cell growth (i). 

Vincristine resistance has been studied in Chinese hamster ovary cell lines cells resistant to vincristine also are resistant to vinblastine and vindesine. Suggestions were made that, in cells with relatively low levels of drug resistance, at least two prominent mechanisms of resistance can occur (22). In the first instance, cellular resistance may be attributable to membrane alterations that are reversible, functionally, by treatment with verapamil. In the second, resistance has been postulated to be due to an altered sensitivity of tubulin to the effects of the drugs the primary basis for postulating an altered interaction with tubulin was that a subgroup of cells resistant to vincristine showed enhanced sensitivity to taxol, a drug that can stabilize microtubules. It should be emphasized that differential sensitivities of tubulins from different tumor cells to the effects of vincristine or vinblastine has been proposed as a basis for the susceptibilities of cells to the cytotoxic effects of such drugs (23). Differences have been described in the electrophoretic patterns for tubulins obtained from vin-... 

Although taxanes bind to p-tubulin promoting microtubule polymerization and stabilization of the spindle complex, they serve to cause a sustained mitotic block at the metaphase/anaphase boundary. This block will occur at a lower concentration than that which is required to increase the microtubule mass (10). However, it is not completely clear how this interaction with microtubules translates into cell death. Morphologic features and the characteristic DNA fragmentation patterns seen in the setting of apoptosis have been documented in tumor cells after therapy with taxanes (10). These observations are accompanied by the phosphorylation of Bcl-2, an anti-apoptotic protein, changing the cellular balance between Bax and Bcl-2 to a status that favors apoptosis (11). 

Sankararaman, S., Menon, G. I., and Kumar, P. B. S. (2004). Self-organized pattern formation in motor-microtubule mixtures. Phys. Rev., 70, 31904-18. 

A typical cross section of a cilium shows a ring formed by nine pairs of microtubules and two central tubules, i.e., the so-called nine + two pattern. Each doublet contains an A and a B subfibril with an inner and an outer dynein arm (a complex protein with ATPase activity) located on the A subfibril with radial spokes extending toward the central doublet. The ciliary membrane, which is an extension from the cell membrane of the epithelial cell, encloses the microtubules. The motion of the cilia is dependent on the sliding of the outer doublets past one another with the energy provided by adenosine triphosphate (ATP) through dynein ATPase activity. 

Early work on spermatogenesis has been reviewed in the first edition (796). More recent work has been reviewed by Euzet et al. (196), Davis Roberts (171), Ubelaker (889) and Lumsden Specian (462). Ultrastructure studies have shown that cestode spermatozoa generally (but see below) have the same pattern of organisation in all orders. The chief features are (a) a long thread-like body (b) an elongated nucleus (c) cortical microtubules underlying the plasma membrane (d) the absence of... 

Akum BF, Chen M, Gunderson SI, Riefler GM, Scerri-Hansen MM, et al. 2004. Cypin regulates dendritic patterning in hippocampal neurons by promoting microtubule assembly. 

UUoa L, Montejo de Garcini E, Gomez-Ramos P, Moran MA, Avila J (1994) Microtubule-assodated protein MAP IB showing a fetal phosphorylation pattern is present in sites of neurofibriUary degeneration in brains of Alzheimer s disease patients. Brain Res Mol Brain Res 26 113-122... 

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