全文获取类型
收费全文 | 101692篇 |
免费 | 872篇 |
国内免费 | 884篇 |
专业分类
103448篇 |
出版年
2022年 | 82篇 |
2021年 | 162篇 |
2020年 | 97篇 |
2019年 | 121篇 |
2018年 | 11964篇 |
2017年 | 10779篇 |
2016年 | 7658篇 |
2015年 | 942篇 |
2014年 | 649篇 |
2013年 | 908篇 |
2012年 | 4864篇 |
2011年 | 13385篇 |
2010年 | 12293篇 |
2009年 | 8535篇 |
2008年 | 10194篇 |
2007年 | 11791篇 |
2006年 | 689篇 |
2005年 | 911篇 |
2004年 | 1352篇 |
2003年 | 1412篇 |
2002年 | 1122篇 |
2001年 | 411篇 |
2000年 | 323篇 |
1999年 | 166篇 |
1998年 | 110篇 |
1997年 | 104篇 |
1996年 | 99篇 |
1995年 | 70篇 |
1994年 | 91篇 |
1993年 | 81篇 |
1992年 | 142篇 |
1991年 | 134篇 |
1990年 | 99篇 |
1989年 | 90篇 |
1988年 | 82篇 |
1987年 | 79篇 |
1986年 | 60篇 |
1985年 | 58篇 |
1984年 | 82篇 |
1983年 | 81篇 |
1982年 | 45篇 |
1981年 | 40篇 |
1980年 | 33篇 |
1979年 | 44篇 |
1978年 | 35篇 |
1975年 | 45篇 |
1973年 | 34篇 |
1972年 | 265篇 |
1971年 | 297篇 |
1970年 | 31篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
941.
Wei Fan Stefan A. W. Bouwense Ross Crawford Yin Xiao 《Journal of molecular histology》2010,41(1):51-60
Despite the important physiological role of periosteum in the pathogenesis and treatment of osteoporosis, little is known
about the structural and cellular characteristics of periosteum in osteoporosis. To study the structural and cellular differences
in both diaphyseal and metaphyseal periosteum of osteoporotic rats, samples from the right femur of osteoporotic and normal
female Lewis rats were collected and tissue sections were stained with hematoxylin and eosin, antibodies or staining kit against
tartrate resistant acid phosphatase (TRAP), alkaline phosphatase (ALP), vascular endothelial growth factor (VEGF), von Willebrand
(vWF), tyrosine hydroxylase (TH) and calcitonin gene-related peptide (CGRP). The results showed that the osteoporotic rats
had much thicker and more cellular cambial layer of metaphyseal periosteum compared with other periosteal areas and normal
rats (P < 0.001). The number of TRAP+ osteoclasts in bone resorption pits, VEGF+ cells and the degree of vascularization were found to be greater in the cambial layer of metaphyseal periosteum of osteoporotic
rats (P < 0.05), while no significant difference was detected in the number of ALP+ cells between the two groups. Sympathetic nerve fibers identified by TH staining were predominantly located in the cambial
layer of metaphyseal periosteum of osteoporotic rats. No obvious difference in the expression of CGRP between the two groups
was found. In conclusion, periosteum may play an important role in the cortical bone resorption in osteoporotic rats and this
pathological process may be regulated by the sympathetic nervous system. 相似文献
942.
Meili Shao Yong Wang Chunlai Wang Yang Guo Yonggang Peng Jiandong Liu Guangxing Li Huifang Liu Siguo Liu 《Acta veterinaria Scandinavica》2010,52(1):52
Background
Porcine contagious pleuropneumonia (PCP) is a highly contagious disease that is caused by Actinobacillus pleuropneumoniae (APP) and characterized by severe fibrinous necrotizing hemorrhagic pleuropneumonia, which is a severe threat to the swine industry. In addition to APP RTX-toxins I (ApxI), APP RTX-toxin II (ApxII), APP RTX-toxin III (ApxIII) and Outer membrane protein (OMP), there may be other useful antigens that can contribute to protection. In the development of an efficacious vaccine against APP, the immunogenicities of multicomponent recombinant subunit vaccines were evaluated. 相似文献943.
Annexins belong to a multigene family of Ca2+ dependent, phospholipid and cytoskeleton binding proteins. They have been shown to be upregulated under various stress conditions.
We generated transgenic cotton plants expressing mustard annexin (AnnBj1), which showed enhanced tolerance towards different abiotic stress treatments like sodium chloride, mannitol, polyethylene
glycol and hydrogen peroxide. The tolerance to these treatments was associated with decreased hydrogen peroxide levels and
enhanced total peroxidase activity, enhanced content of osmoprotectants- proline and sucrose in transgenic plants. They showed
higher retention of total chlorophyll and reduced TBARS in leaf disc assays with stress treatments, and decreased hydrogen
peroxide accumulation in the stomatal guard cells when compared to their wild type counterparts. They also showed significantly
enhanced fresh weight, relative water content, dry weight under stress. Treatment with sodium chloride resulted in enhanced
expression of genes for ∆-pyrroline-5-carboxylase synthetase in leaves, and sucrose phosphate synthase, sucrose synthase and
cellulose synthase A in the leaves and fibers of transgenic plants. The transgenic plants maintained normal seed development,
fiber quality and cellulose content under stress. 相似文献
944.
Nazim Fatès 《Swarm Intelligence》2010,4(2):91-115
The decentralised gathering problem consists in grouping in a compact cluster agents that are initially randomly scattered.
We propose a bio-inspired algorithm, the Reaction–Diffusion–Chemotaxis aggregation scheme, to group agents that have limited
abilities. The agents and their environment are described with a stochastic model inspired by the aggregation of the Dictyostelium discoideum cellular slime mold. The environment is an active lattice, whose cells transmit information according to a reaction–diffusion
mechanism. The agents are virtual amoebae; they trigger excitations randomly and move by following reaction–diffusion waves.
We demonstrate that despite its simplicity, this model exhibits interesting properties of self-organisation and is efficient
for gathering agents. Moreover, observations show that the system is robust to various perturbations, such as the presence
of obstacles on the lattice or noise in the movements of the agents. 相似文献
945.
Silvano Fares Elina Oksanen Mika Lännenpää Riitta Julkunen-Tiitto Francesco Loreto 《Photosynthesis research》2010,104(1):61-74
Plants are exposed to increasing levels of tropospheric ozone concentrations. This pollutant penetrates in leaves through
stomata and quickly reacts inside leaves, thus making plants valuable ozone sinks, but at the same time triggers oxidation
processes which lead to leaf injuries. To counteract these negative effects, plants produce an array of antioxidants which
react with ozone and reactive molecules which ozone generates in the leaf tissues. In this study, we measured the effect of
an ozone concentration which is likely to be attained in many areas of the world in the near future (80 ppb) on leaves of
the vertical profile of the widespread agroforestry species Populus nigra. Changes in (1) physiological parameters (photosynthesis and stomatal conductance), (2) ozone uptake, (3) emission of volatile
organic compounds (VOCs, i.e. isoprene, methanol and other oxygenated compounds), (4) concentration of antioxidant surface
compounds, and (5) concentration of phenolic compounds were assessed. The aim was to assess whether the defensive pathways
leading to isoprenoids and phenolics formation were induced when a moderate and chronic increment of ozone is not able to
damage photosynthesis. No visual injuries and minor changes in physiology and ozone uptake were observed. The emission of
isoprene and oxygenated six-carbon (C6) volatiles were inhibited by ozone, whereas methanol emission was increased, especially
in developing leaves. We interpret these results as suggesting an ontogenetic shift in ozone-treated leaves, leading to a
slower development and a faster senescence. Most surface and phenolic compounds showed a declining trend in concentration
from the youngest to the fully expanded leaves. Ozone reduced the concentrations of chlorogenic acid derivatives at the leaf
surface, whereas in total leaf extracts a metabolic shift towards few phenolics with higher antioxidant capacity was observed. 相似文献
946.
Background
Both bovine coronavirus (BCV) and bovine respiratory syncytial virus (BRSV) infections are currently wide-spread in the Swedish dairy cattle population. Surveys of antibody levels in bulk tank milk have shown very high nationwide prevalences of both BCV and BRSV, with large variations between regions. In the Swedish beef cattle population however, no investigations have yet been performed regarding the prevalence and geographical distribution of BCV and BRSV. A cross-sectional serological survey for BCV and BRSV was carried out in Swedish beef cattle to explore any geographical patterns of these infections. 相似文献947.
The mammary gland undergoes hormonally controlled cycles of pubertal maturation, pregnancy, lactation, and involution, and
these processes rely on complex signaling mechanisms, many of which are controlled by cell–cell and cell–matrix adhesion.
The adhesion of epithelial cells to the extracellular matrix initiates signaling mechanisms that have an impact on cell proliferation,
survival, and differentiation throughout lactation. The control of integrin expression on the mammary epithelial cells, the
composition of the extracellular matrix and the presence of secreted matricellular proteins all contribute to essential adhesion
signaling during lactogenesis. In vitro and in vivo studies, including the results from genetically engineered mice, have
shed light on the regulation of these processes at the cell and tissue level and have led to increased understanding of the
essential signaling components that are regulated in temporal and cell specific manner during lactogenesis. Recent studies
suggest that a secreted matricellular protein, CTGF/CCN2, may play a role in lactogenic differentiation through binding to
β1 integrin complexes, enhancing the production of extracellular matrix components and contributions to cell adhesion signaling. 相似文献
948.
Nakazawa Y Sagane Y Kikuchi T Uchino M Nagai T Sato H Toeda K Takano K 《The protein journal》2010,29(8):598-608
We previously isolated Streptomyces racemochromogenes strain 10-3, which produces a phospholipase D (PLD) with high transphosphatidylation activity. Here, we purified and cloned
the PLD (PLD103) from the strain. PLD103 exerted the highest hydrolytic activity at a slightly alkaline pH, which is in contrast
to the majority of known Streptomyces PLDs that have a slightly acidic optimum pH. PLD103 shares only 71–76% amino acid sequence identity with other Streptomyces PLDs that have a slightly acidic optimum pH; thus, the diversity in the primary structure might explain the discrepancy observed
in the optimum pH. The purified PLD displayed high transphosphatidylation activity in the presence of glycerol, l-serine, and 2-aminoethanol hydrochloride with a conversion rate of 82–97% in a simple one-phase system, which was comparable
to the rate of other Streptomyces PLDs in a complicated biphasic system. 相似文献
949.
Geert Potters Marcel A. K. Jansen Nele Horemans Yves Guisez Taras Pasternak 《In vitro cellular & developmental biology. Plant》2010,46(3):289-297
A Nicotiana tabacum L. SR-1 leaf protoplast system was used to study the effects of dehydroascorbate and glutathione on cellular development. We found that dehydroascorbate is readily taken up by protoplasts and internally reduced to ascorbate. Concomitantly, cell division was inhibited and cell expansion stimulated. In this respect, dehydroascorbate counteracted auxin-mediated leaf protoplast development. In contrast to the effects of dehydroascorbate, glutathione-induced cell dedifferentiation, and this effect is similar to that mediated by high auxin concentrations. We conclude that dehydroascorbate and glutathione affect the auxin-mediated regulation of cellular development. Therefore, the biological role of these compounds extends beyond stress tolerance and defense. 相似文献
950.
Vedat Cinar Rasim Mogulkoc Abdulkerim Kasim Baltaci Ozgur Bostanci 《Biological trace element research》2010,133(1):29-33
This study was performed to determine how the calcium supplementation for a 4-week period affects the glucose and insulin
levels at rest and at exhaustion in athletes. This is a 4-week study performed on 30 healthy subjects varying between 18 and
22 ages. Subjects were separated into three groups: first group (group supplemented with calcium, sedentary group), second
group (calcium supplementations + exercise group), and third group (training group). Glucose and insulin parameters of the
groups were measured four times, at rest and exhaustion in the beginning of the research and at rest and exhaustion after
the end of 4 weeks application period. Exhaustion measurements both before and after the supplementations significantly decreased
in compared to rest measurements in terms of insulin (p < 0.05). Significant difference was not determined in the glucose values of groups. In terms of glucose, values increased
in all of the three groups occurred with exercise both before and after the supplementation by exercise and exhaustion (p < 0.05). The results of our study indicate that calcium gluconate supplementations for 4 weeks in sedentary subjects and
athletes did not significantly affect plasma insulin levels at rest and exhaustion. However, glucose levels were affected
by calcium supplementation and exhausting exercise in athletes. 相似文献