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21.
Aging of the global world population both in developed and developing countries, an unbalanced male-female ratio within this elderly population, increasing populations – including elderly – within urban areas, all will have a marked effect on the practice of dentistry. Furthermore, major epidemiological changes occurring in the dental field, including an increasing dentition life expectancy, increasing root caries prevalence, decreasing coronal caries rates in children, and the relative increase in the prevalence of periodontal problems will have further impact on the dental care system. Major changes need to be anticipated both at the level of the care provider, including the dentist, the hygienist and the dental assistant, and at the level of the potential patient, especially among the geriatric population, if the profession is to be prepared to adequately address these issues. 相似文献
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23.
Canopy gaps are important as entry points for new genotypes and new species into many types of vegetation, yet little is known about them in any type of vegetation but forests. Forest gaps are too large for manipulative experiments to be readily undertaken, and hitherto grassland gaps have been too small to be easily mapped. Preliminary results from mapping small (>1 cm) grassland gaps with a new fibre-optic device suggest that experiments need to be performed at a smaller physical scale than has hitherto been achieved. 相似文献
24.
Colostrum Zn concentrations were measured in eight randomly selected Holstein dairy cows. Overall mean Zn concentrations were highest within 12 h postpartum (257 +/- 14 microM, mean +/- SEM), fell to 141 +/- 8 microM by 24 h, and then declined at a linear rate of 30 microM/d during the following 48 h. Zn concentrations at 3 d (82 +/- 5 microM) were not different from 150-d milk samples (72 +/- microM). In a second experiment, 32 early-gestation cows were blocked by stage of lactation into four groups in a randomized block design and injected with 0, 15, 30, or 45 mg of dexamethasone. Milk and blood samples were collected at 0, 12, and 24 h after injection and analyzed for Zn, and for fat, protein, and lactose in milk. Cows administered 0 and 15 mg of dexamethasone showed no difference in milk Zn concentrations compared to pretreatment measurements; however, milk Zn concentrations in cows administered 30- and 45-mg doses increased significantly. Plasma cortisol decreased in the dexamethasone-treated cows. Plasma Zn and milk fat, protein, and lactose did not change. These data indicate that glucocorticoids can mediate Zn uptake and transport by the mammary glands of lactating cows and suggest that the high Zn concentration in colostrum could be a result of the preparturient surge of cortisol. 相似文献
25.
The characteristics of malate transport into aerobically grown cells of the purple photosynthetic bacterium Rhodobacter capsulatus were determined. A single transport system was distinguished kinetically which displayed a Kt value of 2.9 ± 1.2 μM and Vmax of 43 ± 6 nmol · min-1 · mg-1 protein. Competition experiments indicated that the metabolically related C4-dicarboxylates succinate and fumarate are also
transported by this system. Malate uptake was sensitive to osmotic shock and evidence from the binding of radiolabelled malate
and succinate to periplasmic protein fractions indicated that transport is mediated by a dicarboxylate binding protein. The
activity of the transport system was studied as a function of external and internal pH and it was found that a marked activation
of uptake occurred at intracellular pH values greater than 7. The use of a high affinity binding protein dependent system
to transport a major carbon and energy source suggests that Rhodobacter capsulatus would be capable of obtaining growth sustaining quantities of C4-dicarboxylates even if these were present at very low concentrations
in the environment. 相似文献
26.
Jonathan B. Losos 《Oecologia》1986,68(3):338-343
Summary Step-wise multiple regression was employed to probe the determinants of species diversity of day geckos (Phelsuma) in the Indian Ocean. Independent variables were area, elevation, and two measures of isolation. Distance from Madagascar and island height (an indicator of habitat diversity) were the two most important predictors of species richness. Similar studies on other taxa rarely find isolation to be a major factor. The relatively poor dispersal abilities of reptiles may explain why isolation, rather than attributes of the islands, are more important in this case. The regressions also indicate that habitat diversity (assumed to correlate with maximum island elevation) is more important than area per se in determining species diversity. These results agree with predictions of the equilibrium theory of island biogeography, but historical processes have also greatly influenced species richness. 相似文献
27.
Summary Settlement rates of the high intertidal barnacle, Balanus glandula, were monitored at three sites in the rocky intertidal zone in Central California simultaneously with measurements of larval concentrations in the adjacent water column. In both 1983 and 1984, settlement rates onto vacant substrate differed among the sites by nearly two orders of magnitude. For all sampling dates, this spatial variation in settlement mirrored the spatial distribution of Balanus glandula cyprid concentration in the water column. A perfect rank correlation was found between cyprid concentrations near a site and subsequent settlement. A noteworthy observation was that the sites switched rank in their settlement rates from 1983 to 1984. This change in settlement rankings matched a switch in rankings for cyprid concentrations.Settlement itself appears to be an important cause of the spatial pattern of cyprid concentrations. Comparing the rates of settlement to estimates of the number of cyprids available at a site suggests that settlement causes a large drain on the cyprid population as a water mass passes over successive sites. No consistent spatial patterns were found in the distribution of other major plankton groups (calanoid copepods) that are similar in size to Balanus cyprids but do not settle.The large differences in settlement rates among these sites were previously shown to be a leading cause of large differences in the structure of benthic barnacle populations. The close correspondence shown here between these large differences in settlement and differences in larval concentrations suggests that nearshore oceanic processes affecting larval arrival contribute to the control of benthic community structure. 相似文献
28.
Summary The use of low-pressure steam autohydrolysis in the pretreatment of corn stover and hybrid poplar has been assessed. In terms of yield of prehydrolyzed solids, minimal by-product formation and extent of subsequent enzymatic saccharification, the results of low-pressure steam pretreatment were found to be as good as or better than those reported for more severe pretreatment processes. Almost complete saccharification of the cellulose in the prehydrolyzed biomass solids was obtained within 24h with a commercial cellulase preparation — Celluclast. The presence of grinding elements (glass beads) during the enzymatic hydrolysis was found to increase the extent of saccharification by 40% to 50% over controls without any grinding elements. 相似文献
29.
Abstract. Early subcultures of human embryonic lung fibroblasts are exceptional, as they grow far beyond confluence before growth ceases: the stationary dish may well contain 3-10 monolayer equivalents. Maximal growth rates, however, occur at about one-sixth confluence when doubling times are 15-20 hr; a density at which cell contacts begin to become frequent. the fact that a slowing down of growth is first apparent at such low densities argues against this regulation being due to diffusion effects. Confirmation of the role of short-range or contact interactions in growth regulation comes from an experiment using mixed cultures of fibroblasts: this shows that growth inhibition is not carried by medium-borne influences but depends on short-range (<1 mm) interactions. Evidence that cells can escape the effects of such contact interactions and so divide comes from time-lapse studies of dense cultures: there is a burst of motility soon after a fresh-medium change, which is followed by a burst of mitosis × 20 hr later. A medium change to conditioned medium supplemented with 10% foetal calf serum leads to neither the burst of motility nor the subsequent burst of mitosis, although this medium is better able to support the growth of sparse cells than is fresh medium. Data are also presented to show that the amount of collagen deposited in superconfluent cultures affects their growth: the stimulation of collagen production with ascorbic acid leads to an unexpectedly low stationary cell density and rather less movement in the culture. This result suggests that the collagen stabilizes cell contacts that are responsible for growth inhibition. the question of why these cells grow more slowly as density increases cannot be answered directly by these experiments; nevertheless, the results suggest that cell contact affects the permeability of the cell membrane to medium. 相似文献
30.