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1.
V. G. Borkhvardt 《Russian Journal of Developmental Biology》2000,31(3):154-161
The development of the fin and limb buds involves a balance of centrifugal (active) and centripetal (passive) mechanical forces,
the first of which acts to move the walls of these structures away from each other and the second of which holds them together.
When the volume of the mesodermal core increases, the generated force meets with the resistance of the basal membrane, and
as a result, the limb bud has a tendency to acquire a cylindrical shape. Collagen fibers, individual mesenchymal cells, and
their groups hold together the dorsal and the ventral wall of the limb bud, prevent the movement of these walls away from
each other, and in this way direct bud growth along the proximodistal and the anteroposterior axes. The balance of the forces
which stretch the ectodermal layer and those which constrain it has also been observed in the development of other body parts. 相似文献
2.
The association between the red macroalga Jania adhaerens J. V. Lamour. and the sponge Haliclona caerulea is the most successful life‐form between 2 and 4 m depth in Mazatlán Bay (Mexican Pacific). J. adhaerens colonizes the rocky intertidal area and penetrates into deeper areas only when it lives in association with H. caerulea. The aposymbiotic form of the sponge has not been reported in the bay. To understand the ecological success of this association, we examined the capacity of J. adhaerens to acclimate in Mazatlán Bay using transplant experiments. The transplanted aposymbiotic J. adhaerens did not survive the first 2 weeks; however, J. adhaerens when living in association with H. caerulea, acclimated easily to depth, showing no sign of mortality during the 103 d of the experiment. We conclude that the ability of J. adhaerens to colonize in deeper areas in this hydrodynamic environment may in part rely on the protection provided by the sponge to the algal canopy. Both species contribute to the shape of the associated form. Nevertheless, the morphological variation in the association appears to be dominated by the variation in J. adhaerens canopy to regulate pigment self‐shading under light‐limited conditions and/or tissue resistance under high hydrodynamics. Consequently, our results are consistent with light as the abiotic controlling factor, which regulates the lower depth distribution of the association in Mazatlán Bay, through limiting the growth rate of J. adhaerens. Hydrodynamics may determine the upper limit of the association by imposing high mass losses. 相似文献
3.
Marianne Bouvier 《American journal of primatology》1988,15(2):129-142
A new technique has been developed for estimating age in rhesus macaques (Macaca mulatta) that is based on mandibular measurements and has several advantages over presently available techniques. First, the new technique is relatively quick and inexpensive because a single measurement or radiograph suffices to produce an age estimate. Second, mandibular growth represents a more continuous phenomenon than either epiphyseal union or dental eruption; therefore, age estimates may be made throughout the period of growth without significant gaps in the process. Finally, the new technique may be used on incomplete or even fragmentary skeletal material because it requires that only a portion of the mandible be preserved. The new technique produces age estimates with a prediction error of ±5.08 months in males and ±7.29 months in females. These errors are only slightly higher than those found previously for dental eruption or epiphyseal union data. 相似文献
4.
1. As for some other spring‐feeding moths, adult flight of Epirrita autumnata (Lepidoptera: Geometridae) occurs in late autumn. Late‐season flight is a result of a prolonged pupal period. Potential evolutionary explanations for this phenological pattern are evaluated. 2. In a laboratory rearing, there was a weak correlation between pupation date and the time of adult emergence. A substantial genetic difference in pupal period was found between two geographic populations. Adaptive evolution of eclosion time can thus be expected. 3. Metabolic costs of a prolonged pupal period were found to be moderate but still of some ecological significance. Pupal mortality is likely to form the main cost of the prolonged pupal period. 4. Mortality rates of adults, exposed in the field, showed a declining temporal trend from late summer to normal eclosion time in autumn. Lower predation pressure on adults may constitute the decisive selective advantage of late‐season flight. It is suggested that ants, not birds, were the main predators responsible for the temporal trend. 5. Egg mortality was estimated to be low; it is thus unlikely that the late adult period is selected for to reduce the time during which eggs are exposed to predators. 6. In a laboratory experiment, oviposition success was maximal at the time of actual flight peak of E. autumnata, however penalties resulting from sub‐optimal timing of oviposition remained limited. 相似文献
5.
The elongation rate, the gradient of the local elongation rate and the surface pH of maize roots were measured over 12 h. A data bank was constituted by storing these values. By sorting these results on the basis of different elongation rates, different classes of root were obtained. Two classes were chosen: the low-growth roots and the high-growth roots. The mean growth of these two root classes was stable with time and differed significantly from one another. The surface pH of the elongation zone was the same for the roots of these two classes, but the roots selected for their higher growth rate had a larger acid efflux in this zone. 相似文献
6.
Summary Between 3 Nov. 1983 and 9 Apr. 1984, six applications of fertiliser N (ammonium, nitrate or urea) were given to four autumn
sown (26 Oct. 1983)Vicia faba L cultivars, Banner Winter (BW) and Maris Beagle (MBg), cold tolerant cultivars normally sown in the autumn, and Herz Freya
(HF) and Maris Bead (MBd), cold sensitive cultivars more commonly sown in the spring. The effects of additional N were determined
by comparison with plants given zero-N (controls).
Application of N, regardless of form, had no effect on % emergence at the first sampling (15 Dec. 1983); >90% for BW, MBg
and HF, but only 40–60% for MBd. At this time the dry weight, carbon content and nitrogen content of all cultivars was approximately
20% less than that of the seed on planting. No more plants emerged after 15 Dec. 1983. Between 15 Dec. 1983 and 20 Feb. 1984,
all cultivars, regardless of N treatment, showed little change in dry weight, carbon content and nitrogen content but the
proportion of total plant dry weight, carbon content and nitrogen content in the cotyledons decreased while the proportions
in root, stem and leaf tissue increased. On 20 Feb. 1984 there were no N effects. All cultivars but especially BW and MBg,
showed progressive increases in dry weight, carbon content and nitrogen content during the period 20 Feb. 1984 to 8 May 1984.
Pooled results for all four cultivars indicated that on 8 May 1984, plants given ammonium and urea had a greater dry weight,
carbon content and nitrogen content than controls.
At harvest (1–3 Sep. 1984), BW and MBg outyielded (g dw seed m−2) HF and MBd. Pooled results for all cultivars indicated that application of N regardless of form gave increased yield and
an increased N concentration (mg N g−1 dw) in the seed. 相似文献
7.
Selective increase in cytokeratin synthesis in cultured rat hepatocytes in response to hormonal stimulation 总被引:3,自引:0,他引:3
I Leroux-Nicollet M Noel H Baribault R Goyette N Marceau 《Biochemical and biophysical research communications》1983,114(2):556-563
Addition of a combination of insulin, dexamethasone and EGF at seeding time to cultured rat hepatocytes in serum-free medium caused a selective increase in the biosynthesis of particular cytokeratin components. This increase was prominent during the first day in culture. No significant increases were detected in the absence of hormones or in the presence of either hormones added alone or in pairs, except in the case of insulin plus dexamethasone, which yielded an effect close to that obtained with the three factors. Interestingly, the latter condition also maintained a high level of albumin production over a 6-day period in culture. 相似文献
8.
Cell division in cell suspension cultures can be completely blocked by the growth retardant tetcyclacis at a concentration of 10-4 mol l-1. In rice cells it has been demonstrated that the growth inhibition can be completely overcome by application of cholesterol independent of the duration of pretreatment with tetcyclacis. In suspension cultures of maize and soybean, too, the effect of tetcyclacis on cell division was neutralized by adding cholesterol. Other plant sterols, stigmasterol, campesterol and sitosterol were active in a decreasing order. Modifications in the cholesterol perhydro-cyclopentanophenanthrene-ring system indicate that the hydroxyl group at C-3 and the double bond between C-5 and C-6 in ring B are required for the activity. In contrast, gibberellic acid as well as ent-kaurenoic acid could not compensate retardant effects. Likewise, tetcyclasis did not change the level of gibberellins in rice cells as shown by radioimmunoassay. Thus, it is concluded that in cell suspension cultures sterols play a more important role in cell division than gibberellins.Abbreviation GAx
gibberelin Ax 相似文献
9.
10.
Summary The content of fatty acids was analysed in an exudate from roots of pine seedlings grown axenically in vermiculite with a synthetic nutrient medium. The dominating fatty acdis were fewer in the exudate than in the roots. Unsaturated fatty acids were predominant. The total lipid fraction of the exudate promoted mycelial growth in two of the three ectomycorrhizal fungi tested. 相似文献