共查询到20条相似文献,搜索用时 15 毫秒
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Tabar V 《Cell Stem Cell》2011,9(6):490-491
The adenohypophysis secretes multiple hormones that control vital physiological functions. A recent article in Nature (Suga et al., 2011) describes a 3D protocol for the derivation of several endocrine pituitary cell types from mouse ESCs. 相似文献
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Ana Herrera Rosa Urich Elizabeth Rengifo Caín Ballestrini Armando González Williams León 《Trees - Structure and Function》2012,26(6):1759-1769
In Venezuela 30,000?km2 of land is covered by savannas, of which 410,000?ha have been planted with several species and hybrids of Eucalyptus for lumber and pulp production. Popular concern about possible diminutions in water availability of reservoirs near eucalypt plantations prompted our interest in measuring water use by these trees. Since these savannas are markedly seasonal, the response of species to seasonal drought is important. We aimed to compare the seasonal changes in single-leaf and whole-plant transpiration in a seasonally dry savanna with that of trees of E. urophylla in an experimental plantation. We also examined the seasonal changes in xylem water potential and stomatal response to air water-vapour saturation deficit (D). Transpiration in eucalypts and the dominant savanna species Trachypogon vestitus and Curatella americana was evaluated using measurements of leaf gas exchange in all three species, sap flux in eucalypts, microclimatic variables and allometric and photometric determinations of green area. In E. urophylla and T. vestitus, but not in C. americana, stomatal conductance (g s) proved sensitive to D. Integrated values of daily courses of transpiration rate were scaled to one ha in a preliminary approach to estimating ecosystem transpiration (E ha). The E ha of the savanna (the sum of E ha of T. vestitus and C. americana) was on average 2.4 times that of eucalypts during the daytime; when nocturnal eucalypt transpiration was included, the value was 1.9. The evapotranspiration calculated by the Penman–Montieth equation (ETc) of eucalypts was lower than the savanna all year round. The reference crop ET (ETo) varied little throughout the seasons, the highest value occurring in March. The ratio E ha/ETo for the savanna was on average near one during the dry season and almost two during the rainy season; the corresponding value for E. urophylla was 0.6 for both seasons. The ratio E ha/ETc was on average 0.8 for the species and the savanna. The cumulative E ha for the days of measurements was higher in the savanna than in the eucalypts during the daytime (39.8 and 17.3?mm, respectively), as was the cumulative ETc (37.5 vs. 20.3?mm). Measured and calculated cumulative ET in eucalypts, including nocturnal values, were 22.0 and 28.4?mm, respectively. At the leaf level, both eucalypts and trees of C. americana apparently may have accessed water from deep horizons, since their values of Ψ changed less seasonally than in T. vestitus. At the ecosystem level, the species that presented the largest changes in transpiration was T. vestitus, which markedly increased savanna transpiration during the rainy season. Our results suggest that, for the days of this study, and considering the environmental conditions of the ecosystems studied, the type of measurements and the scaling procedures, stands of E. urophylla transpire less water than the savanna. 相似文献
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Electron-ion collisions in a high-density plasma in strong electromagnetic fields are considered. The applicability condition for the approximate model of pair collisions in strong fields are determined. It is shown that this condition is identical to the condition for the plasma to be transparent. Investigations were carried out by the test particle method generalized to the case of several scattering centers. An accurate calculation of short-range collisions is provided by a “jump” method that is based on the exact solution to the problem of the motion of a particle in a Coulomb potential. This method can also be applied in other approaches to simulating a collisional plasma (such as particle-in-cell and molecular dynamics methods). 相似文献
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《Comptes rendus de l'Académie des sciences. Série III, Sciences de la vie》1998,321(7):553-555
Hereditary diseases have been reported with relatively high frequency in some small populations. Founder effect and genetic drift, associated or not with selective advantage of heterozygotes in case of recessive diseases, are the main explanations. Therefore, if we consider one population and one particular disease, only one deleterious allele should be observed.Determination of mutations has shown that in most cases the situation is more complex; more than one mutation is found among the patients. This finding can be explained by a multiple founder effect, with genetic drift and new mutations. 相似文献
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A. I. Matveev 《Plasma Physics Reports》2008,34(2):95-102
The spatial evolution of a Langmuir wave excited by external sources in a weakly inhomogeneous electron plasma in a longitudinal electrostatic field is considered. It is shown that, in a longitudinal electrostatic field, a Langmuir wave can only be amplified in an inhomogeneous plasma provided that the current of trapped electrons exceeds that of untrapped electrons. In this case, as the wave propagates through the inhomogeneous region where its phase velocity increases, some untrapped electrons become trapped in the wave potential wells. As a result, the current of trapped electrons increases and the wave is amplified. Moreover, in the regions where the bulk electrons are localized, the minima of the wave are amplified to a greater extent than its maxima. 相似文献
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Transurethral procedures were used in 620 of 677 cases in which prostatectomy was done (principally by residents supervised by a urologist) at a county hospital in a six-year period. Open operations were used in the other 57 cases. Results were classified as "excellent" in 46.8 per cent of the transurethral cases and as "good" in 36.3 per cent. 相似文献
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V. B. Krasovitskii V. G. Dorofeenko V. A. Turikov V. I. Sotnikov 《Plasma Physics Reports》2006,32(1):25-31
The decay instability of a lser pulse propagating across an external magnetic field in a subscritical plasma is investigated analytically and numerically. It is shown that, when the relaxation of the pulse is taken into account, the hydrodynamic growth rate of the decay instability is slower than that obtained earlier in the constant-amplitude pump wave approximation. The results of numerical simulations by a particle-in-cell method demonstrate that an increase in the amplitude of the parametrically excited waves is accompanied by a decrease in their group velocity; in this case, up to 85% of the laser energy is converted into the energy of the plasma particles. It is found that, under resonance conditions, the magnetic field acts to increase the energy of the accelerated ions that escape from the plasma slab through its front boundary. 相似文献
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Messenger SL Molineux IJ Bull JJ 《Proceedings. Biological sciences / The Royal Society》1999,266(1417):397-404
The evolution of virulence was studied in a virus subjected to alternating episodes of vertical and horizontal transmission. Bacteriophage f1 was used as the parasite because it establishes a debilitating but non-fatal infection that can be transmitted vertically (from a host to its progeny) as well as horizontally (infection of new hosts). Horizontal transmission was required of all phage at specific intervals, but was prevented otherwise. Each episode of horizontal transmission was followed by an interval of obligate vertical transmission, followed by an interval of obligate horizontal transmission etc. The duration of vertical transmission was eight times longer per episode in one treatment than in the other, thus varying the relative intensity of selection against virulence while maintaining selection for some level of virus production. Viral lines with the higher enforced rate of infectious transmission evolved higher virulence and higher rates of virus production. These results support the trade-off model for the evolution of virulence. 相似文献
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BARRETTE JM 《Canadian journal of comparative medicine and veterinary science》1950,14(7):242-3; French transl, 244-5
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R S Hayes 《Journal of medical primatology》1986,15(5):367-371
An adult baboon with multiple organ symptomatology and tachycardia was successfully treated with one-time electrical cardioversion. The case is presented to demonstrate the need to include ECG monitoring in diagnostic assessment of primate animals with nonspecific symptoms. 相似文献
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Despite recent findings that mutualistic interactions between two species may be greatly affected by species external to the mutualism, the implications of such multi-species interactions for the population dynamics of the mutualists are virtually unexplored. In this paper, we ask how the mutualism between the shoot-base boring weevil Apion onopordi and the rust fungus Puccinia punctiformis is influenced by the dynamics of their shared host plant Cirsium arvense, and vice versa. In particular, we hypothesized that the distribution of the weevil's egg load between healthy and rust-infected thistles may regulate the abundance of the mutualists and their host plant. In contrast to our expectations we found that the dynamics of the mutualists are largely determined by the dynamics of their host. This is, to our knowledge, the first demonstration that the dynamics of a mutualism are driven by a third, non-mutualistic species. 相似文献
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The aim of the current study was to investigate the response of healthy older and young adults to a change in step direction from a forward to a lateral target in response to a sensory perturbation. Nine healthy older (75.1 ± 6.7 years; age range, 65–81 years) and nine young adults (27 ± 3.6 years; age range, 23–31 years) participated in the study. The sensory perturbation was a visual cue presented at random times while subjects stepped over an obstacle from quiet stance. For both young and elderly subjects there was an abrupt change in the slope of both shear ground reaction forces (GRFs) of the stance limb following the perturbation. The slope and peak of the change in GRFs was greater for the young subjects and the onset significantly earlier (205 ms compared to 271 ms). Changes in the GRFs were accompanied by an increase in bilateral gluteus medius and stance limb soleus activity. A late visual cue resulted in a delayed response for elderly subjects. These data show that a stepping response to a sensory perturbation was both delayed and of less magnitude for older adults which has implications for fall risk. 相似文献
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Trophic interactions in a shallow lake following a reduction in nutrient loading: a long-term study 总被引:13,自引:6,他引:7
After the diversion of a nutrient-rich inflow, the eutrophic lake, Alderfen Broad, initially showed reduced total phosphorus concentrations and phytoplankton populations, clear water and the establishment of submerged macrophytes. Internal P loading then increased, perhaps stimulated by the senescence of submerged macrophytes and exacerbated by the lack of flushing. Cyanophytes appeared in the summer of two years. As a consequence of poor recruitment of roach (Rutilus rutilus (L.)), the chief zooplanktivore, and a summerkill of the fish population, populations of large-bodied Cladocera (Daphnia hyalina/ longispina and ultimately D. magna) developed. In the long-term, these may have limited the further development of phytoplankton populations and clear water and submerged macrophytes returned. During this latter period, internal P release has remained high (> 380 µg l-1), thereby indicating the scope for biomanipulation even in eutrophic conditions. However, isolation of the lake has led to a decrease in water level (which through increased temperatures and lowered dissolved oxygen levels was probably responsible for the fish deaths) and further concentration of internal P load. Sediment is now being removed to reestablish greater water depth. 相似文献