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Homogeneous Substrate (homogeneous + substrate)
Selected AbstractsEffects of substrate geometry on growth cone behavior and axon branchingDEVELOPMENTAL NEUROBIOLOGY, Issue 11 2006Ginger S. Withers Abstract At the leading edge of a growing axon, the growth cone determines the path the axon takes and also plays a role in the formation of branches, decisions that are regulated by a complex array of chemical signals. Here, we used microfabrication technology to determine whether differences in substrate geometry, independent of changes in substrate chemistry, can modulate growth cone motility and branching, by patterning a polylysine grid of narrow (2 or 5 ,m wide) intersecting lines. The shape of the intersections varied from circular nodes 15 ,m in diameter to simple crossed lines (nodeless intersections). Time-lapse recordings of cultured hippocampal neurons showed that simple variations in substrate geometry changed growth cone shape, and altered the rate of growth and the probability of branching. When crossing onto a node intersection the growth cone paused, often for hours, and microtubules appeared to defasciculate. Once beyond the node, filopodia and lamellipodia persisted at that site, sometimes forming a collateral branch. At nodeless intersections, the growth cone passed through with minimal hesitation, often becoming divided into separate areas of motility that led to the growth of separate branches. When several lines intersected at a common point, growth cones sometimes split into several subdivisions, resulting in the emergence of as many as five branches. Such experiments revealed an intrinsic preference for branches to form at angles less than 90°. These data show that simple changes in the geometry of a chemically homogeneous substrate are detected by the growth cone and can regulate axonal growth and the formation of branches. © 2006 Wiley Periodicals, Inc. J Neurobiol 66: 1183,1194, 2006 [source] Similarities of stress concentrations in contact at round punches and fatigue at notches: implications to fretting fatigue crack initiationFATIGUE & FRACTURE OF ENGINEERING MATERIALS AND STRUCTURES, Issue 7 2000Giannakopoulos A linear elastic model of the stress concentration due to contact between a rounded flat punch and a homogeneous substrate is presented, with the aim of investigating fretting fatigue crack initiation in contacting parts of vibrating structures including turbine engines. The asymptotic forms for the stress fields in the vicinity of a rounded punch-on-flat substrate are derived for both normal and tangential loading, using both analytical and finite element methods. Under the action of the normal load, P, the ensuing contact is of width 2b which includes an initial flat part of width 2a. The asymptotic stress fields for the sharply rounded flat punch contact have certain similarities with the asymptotic stress fields around the tip of a blunt crack. The analysis showed that the maximum tensile stress, which occurs at the contact boundary due to tangential load Q, is proportional to a mode II stress intensity factor of a sharp punch divided by the square root of the additional contact length due to the roundness of the punch, Q/(,(b,,,a),,b). The fretting fatigue crack initiation can then be investigated by relating the maximum tensile stress with the fatigue endurance stress. The result is analogous to that of Barsom and McNicol where the notched fatigue endurance stress was correlated with the stress intensity factor and the square root of the notch-tip radius. The proposed methodology establishes a ,notch analogue' by making a connection between fretting fatigue at a rounded punch/flat contact and crack initiation at a notch tip and uses fracture mechanics concepts. Conditions of validity of the present model are established both to avoid yielding and to account for the finite thickness of the substrate. The predictions of the model are compared with fretting fatigue experiments on Ti,6Al,4V and shown to be in good agreement. [source] How does surrounding vegetation affect the course of succession: A five-year container experimentJOURNAL OF VEGETATION SCIENCE, Issue 4 2009ch Lanta Abstract Question: How does location and time of insertion affect the course of succession in experimental containers? Location: Bene,ov nad Lipou, ,eskomoravská vrchovina (Czech-Moravian uplands), Czech Republic Methods: We designed a 5-year container experiment in which plant succession started from scratch. Soil conditions were constant and all containers were filled with homogeneous substrate containing no propagules. We placed the containers in two contrasting habitats (meadow and floodplain) under identical climatic conditions but differing in surrounding vegetations and hence seed input. New containers were installed (and hence succession started) in two subsequent years, twice in each year (spring and autumn). We assume that the individual dates would lead to differences in propagule input and weather conditions. Results: Although both year and season of succession initiation considerably affected the initial species composition, we observed a pronounced convergence within the set of containers located in each habitat. However, the similarity of containers initiated at the same time but located in different habitats decreased over the course of succession. Final composition of the meadow and floodplain containers was therefore mostly determined by permanent seed input from their nearby neighborhood. Conclusions: This study demonstrated that propagule availability is an important determinant of the course of succession, and that differential seed input leads to different pathways of succession, even when all other environmental conditions are equal. [source] Interactions between neighboring algae and snail grazing in structuring microdistribution patterns of periphytonOIKOS, Issue 3 2001Masakado Kawata The micro-distribution of periphyton (filamentous algae) on homogeneous substrates was examined in experimental tanks with and without the pressure of grazing snails. The growth of periphyton attached to artificial substrate was estimated at a small spatial scale (9.3 mm×9.3 mm cells) by varying the number of grazers (0, 5, or 10 snails per tank), using image processing analysis without removing the periphyton. The results suggest that periphyton growth within a cell was negatively affected by the biomass of periphyton in the cell but was positively affected by the biomass of periphyton in neighboring cells. A semivariogram analysis indicated that spatial heterogeneity increased with increasing grazing pressure. The size of patches was not clearly related to the number of snails, but there was a tendency for relative patch size to increase with snail density. Computer simulations were also conducted to examine factors affecting the degree of spatial heterogeneity. The simulation studies indicated that snails should graze a site that was previously grazed in order to produce the observed spatial heterogeneity of periphyton. The results also indicated that the positive effects of neighboring periphyton on the growth of algae might create patches. The interactions among neighboring algae and snail grazing might be an important factor creating the spatial heterogeneity of periphyton even on homogeneous substrates. [source] | ||||||||||