全文获取类型
收费全文 | 6054篇 |
免费 | 1148篇 |
国内免费 | 1123篇 |
专业分类
8325篇 |
出版年
2024年 | 90篇 |
2023年 | 261篇 |
2022年 | 361篇 |
2021年 | 451篇 |
2020年 | 370篇 |
2019年 | 377篇 |
2018年 | 315篇 |
2017年 | 285篇 |
2016年 | 311篇 |
2015年 | 256篇 |
2014年 | 374篇 |
2013年 | 392篇 |
2012年 | 282篇 |
2011年 | 308篇 |
2010年 | 217篇 |
2009年 | 250篇 |
2008年 | 316篇 |
2007年 | 350篇 |
2006年 | 309篇 |
2005年 | 237篇 |
2004年 | 265篇 |
2003年 | 234篇 |
2002年 | 200篇 |
2001年 | 159篇 |
2000年 | 116篇 |
1999年 | 122篇 |
1998年 | 119篇 |
1997年 | 106篇 |
1996年 | 106篇 |
1995年 | 115篇 |
1994年 | 91篇 |
1993年 | 83篇 |
1992年 | 57篇 |
1991年 | 66篇 |
1990年 | 46篇 |
1989年 | 38篇 |
1988年 | 35篇 |
1987年 | 31篇 |
1986年 | 28篇 |
1985年 | 27篇 |
1984年 | 23篇 |
1983年 | 18篇 |
1982年 | 22篇 |
1981年 | 18篇 |
1980年 | 29篇 |
1979年 | 11篇 |
1978年 | 8篇 |
1977年 | 13篇 |
1973年 | 7篇 |
1972年 | 6篇 |
排序方式: 共有8325条查询结果,搜索用时 0 毫秒
11.
Colony-stimulating factor 1 (CSF-1) selectively supports the survival, proliferation, and maturation of hemopoietic cells of the monocyte/macrophage lineage. Although the cellular receptor for CSF-1, (the c-fms protein) is a protein-tyrosine kinase activated by the binding of CFS-1, the role of phosphorylation of cellular proteins in CSF-1 signal transduction is poorly understood. Therefore, we examined the CSF-1-stimulated phosphorylation of cellular proteins in human BeWo choriocarcinoma cell line (known to express the c-fms protein). BeWo cells were metabolically labeled with 32Pi, stimulated with recombinant human CSF-1, and extracted with detergent. Phosphotyrosyl proteins were isolated from detergent extracts by affinity chromatography on a highly specific antibody to phosphotyrosine. Rapid phosphorylation of 170-kd protein, followed closely by the phosphorylation of a 56-kd protein, was observed in response to CSF-1. The 170-kd phosphotyrosyl protein bound to wheat germ agglutinin and was secondarily immunoprecipitated with a specific anti-fms serum, consistent with its identity as the CSF-1 receptor. Although purified human macrophages that proliferate in culture in response to CSF-1 are not generally accessible, CSF-1 did stimulate the phosphorylation of a 56-kd protein in intact mononuclear leukocytes from human peripheral blood. Thus, the BeWo cell line may represent a good model for the study of CSF-1-stimulated cellular protein phosphorylation. 相似文献
12.
Olwin BB 《Cytotechnology》1989,2(4):351-365
Heparin-binding growth factors modulate diverse biological activities including cellular proliferation, cellular differentiation, morphogenesis, and angiogenesis. Biochemical characterization for two members of the heparin-binding growth factor family, acidic and basic fibroblast growth factors, is extensive, while characterization of the remaining five members is forthcoming. Cell surface receptors have been identified for acidic and basic fibroblast growth factors, but little is known concerning their sites of action in vivo or the mechanisms involved in transducing the energy of growth factor binding to a biological response. An understanding of the biological basis for the diversity of the heparin binding growth factor family and the in vivo actions of these factors will prove a major challenge to future research efforts. 相似文献
13.
David R. Thompson Jurgen E. Pohl Yiu-Yu S. Tse Robert W. Hiorns 《International journal of biometeorology》1996,39(3):116-120
Analysis of the time of onset of chest pain in 2254 patients with a myocardial infarction admitted to a coronary care unit in Leicester during a 10-year period shows an association with temperature and humidity. During both the most cold and humid times of the year, the relationship is a strong one. A generalized linear model with a log link was used to fit the data and the backward elimination selection procedure suggested a humid, cold day might help to trigger the occurrence of myocardial infarction. In addition, cold weather was found to have a stronger effect on the male population while those men aged between 50 and 70 years were more sensitive to the effect of high humidity. 相似文献
14.
D. James Morré 《Journal of bioenergetics and biomembranes》1994,26(4):421-433
An NADH oxidase activity of animal and plant plasma membrane is described that is stimulated by hormones and growth factors. In plasma membranes of cancer cells and tissues, the activity appears to be constitutively activated and no longer hormone responsive. With drugs that inhibit the activity, cells are unable to grow although growth inhibition may be more related to a failure of the cells to enlarge than to a direct inhibition of mitosis. The hormone-stimulated activity in plasma membranes of plants and the constitutively activated NADH oxidase in tumor cell plasma membranes is inhibited by thiol reagents whereas the basal activity is not. These findings point to a thiol involvement in the action of the activated form of the oxidase. NADH oxidase oxidation by Golgi apparatus of rat liver is inhibited by brefeldin A plus GDP. Brefeldin A is a macrolide antibiotic inhibitor of membrane trafficking. A model is presented where the NADH oxidase functions as a thiol-disulfide oxidoreductase activity involved in the formation and breakage of disulfide bonds. The thiol-disulfide interchange is postulated as being associated with physical membrane displacement as encountered in cell enlargement or in vesicle budding. The model, although speculative, does provide a basis for further experimentation to probe a potential function for this enzyme system which, under certain conditions, exhibits a hormone- and growth factor-stimulated oxidation of NADH. 相似文献
15.
Substance use disorders (SUDs) are highly prevalent and exact a large toll on individuals’ health, well-being, and social functioning. Long-lasting changes in brain networks involved in reward, executive function, stress reactivity, mood, and self-awareness underlie the intense drive to consume substances and the inability to control this urge in a person who suffers from addiction (moderate or severe SUD). Biological (including genetics and developmental life stages) and social (including adverse childhood experiences) determinants of health are recognized factors that contribute to vulnerability for or resilience against developing a SUD. Consequently, prevention strategies that target social risk factors can improve outcomes and, when deployed in childhood and adolescence, can decrease the risk for these disorders. SUDs are treatable, and evidence of clinically significant benefit exists for medications (in opioid, nicotine and alcohol use disorders), behavioral therapies (in all SUDs), and neuromodulation (in nicotine use disorder). Treatment of SUDs should be considered within the context of a Chronic Care Model, with the intensity of intervention adjusted to the severity of the disorder and with the concomitant treatment of comorbid psychiatric and physical conditions. Involvement of health care providers in detection and management of SUDs, including referral of severe cases to specialized care, offers sustainable models of care that can be further expanded with the use of telehealth. Despite advances in our understanding and management of SUDs, individuals with these conditions continue to be stigmatized and, in some countries, incarcerated, highlighting the need to dismantle policies that perpetuate their criminalization and instead develop policies to ensure support and access to prevention and treatment. 相似文献
16.
The relationship between birth weight and relative subcutaneous fat distribution at school age was considered in 131 boys and 106 girls 7 to 12 years of age. Relative fat distribution at school age was estimated with the ratio of the subscapular to triceps skinfolds (S/T) for the total sample, and with the ratio of the sum of two trunk (subscapular, midaxillary) to the sum of two extremity (triceps, medial calf) skinfolds (T/E) for subsamples of 102 boys and 63 girls. There were no sex differences in the S/T ratio (mm/mm), boys 0.62 ± 0.15, girls 0.63 ± 0.18; T/E ratio (mm/mm), boys 0.58 ± 0.13, girls 0.59 ± 0.16; and BMI (kg/m2), boys 17.1 ± 2.4, girls 16.9 ± 2.2. Second order partial correlations, controlling for age and the BMI or age and sum of skinfolds, between birth weight and the skinfold ratios are, respectively, ?0.22 and ?0.20 (p<0.01) for S/T and ?0.29 and ?032 (p<0.01) for T/E in girls, and ?0.18 and ?0.17 (p<0.05) for S/T and ?0.06 and ?0.6 for T/E in boys. Though low, the correlations suggest that as birth weight decreases proportionally more subcutaneous fat is accumulated on the trunk than on the extremities, more so in females than in males. Results of stepwise multiple regression analyses indicate that birth weight accounts for from 2% to 8% of the variance in relative subcutaneous fat distribution at school age. 相似文献
17.
James B. Hoying Carl A. Boswell Stuart K. Williams 《In vitro cellular & developmental biology. Animal》1996,32(7):409-419
Summary During angiogenesis, the microvasculature displays both vessel remodeling and expansion under the control of both cellular
and extracellular influences. We have evaluated the role of angiogenic and angiostatic molecules on angiogenesis in anin vitro model that more appropriately duplicates the cellular and extracellular components of this process. Freshly isolated microvessel
fragments from rat adipose tissue (RFMF) were cultured within three-dimensional collagen I gels. These fragments were characterized
at the time of isolation and were composed of vessel segments observed in the microvasculature of fatin situ (i.e., arterioles, venules, and capillaries). Fragments also exhibited characteristic ablumenally associated cells including
smooth muscle cells and pericytes. Finally, fragments were encased in an extracellular matrix composed of collagen type IV
and collagen type I/III. The elongation of microvascular elements was subsequently evaluated using morphologic and immunocytochemical
techniques. The proliferation, migration, and elongation of cellular elements in microvessel fragments from rat adipose tissue
was dependent on initial fragment density, matrix density, and required serum. Inclusion of endothelial cell growth factors
to microvessel fragments from rat adipose tissue 3-D cultures resulted in the accelerated elongation of tube structures and
the expression of von Willebrand factor in cells constituting these tubes. Molecules with reported angiostatic capacity (e.g.,
transforming growth factor and hydrocortisone) inhibited vessel tube elongation.
In vitro methods have been developed to evaluate numerous mechanisms associated with angiogenesis, including endothelial cell proliferation,
migration, and phenotypic modulation. Microvascular endothelial cell fragments described in this study represent anin vitro population of cells that accurately duplicate thein vivo microcirculatory elements of fat. The proliferation of cells and elongation of microvascular elements subsequently observed
in three-dimensional cultures provides anin vitro model of angiogenesis. Microvascular formation in this system results from pre-existing microvessel fragments unlike tube
formation observed when cultured endothelial cells are placed in three-dimensional gels. This form of tube formation from
cultured endothelium is more characteristic of vasculogenesis. Thus, the formation of microvascular elements from microvessel
fragments provides the opportunity to examine the mechanisms regulating angiogenesis in anin vitro system amenable to precise experimental manipulation. 相似文献
18.
用σ ̄(70)或σ ̄(38)和核心酶(E)组成的大肠杆菌RNA聚合酶全酶(Eσ)对四种启动子进行了体外转录。结果表明:lacUV5,rplJ主要被Eσ ̄(70)识别,katE主要被Eσ ̄(38)识别,fic在低盐浓度时被Eσ ̄(70)识别,在高浓度盐时被Eσ ̄(38)识别;Eσ ̄(38)转录特异性启动子所需酶量大于Eσ ̄(70)转录特异性启动子的酶量;在含有分别对σ ̄(70)或σ ̄(38)亲和性大的混和启动子的体外转录中,启动子之间不存在干扰和竞争,转录水平与启动子浓度成正相关。 相似文献
19.
I. W. J. Debyser 《International journal of primatology》1995,16(6):889-907
I review literature on juvenile mortality of captive prosimians in order to evaluate the available information on captive
breeding. Juvenile mortality includes abortion, premature mortality, stillbirth, and death of the unweaned young. Prosimian
juvenile mortality ranges between 25 and 45% in captive populations. It is generally lower in the Lemuroidea, particularly
the Cheirogaleidae, than in the Lorisoidea. Mortality is particularly high in the Lorisinae. Most mortality, including a high
stillbirth rate, occurs on the first day and during the first 10 days thereafter. Stress, maternal neglect and traumatic insults,
not infrequently linked to each other, are the most frequently reported causes of death. The percentage of congenital malformations
tends to be high in some colonies. Sex of the infant and parity seem to be important risk factors for juvenile mortality,
whereas litter size does not appear to be important. Based on few data, wild- caught females appear to have higher breeding
success than those born in captivity. Synchronized births in lemuroids and isolated births in Galagoare more likely to result in successfully weaned infants. 相似文献
20.
Annual gross primary productivity in mesotrophic Shahidullah Hall pond (Dhaka, Bangladesh) was 1383.35 g C m−2 y−1 (arithmetic mean). Daily primary productivity (between 1.6 and 6.8 g C m−2 d−1 was correlated with chlorophylla, day length and dissolved silica. Chlorophylla related significantly withk, incident light, SRP, alkalinity and conductivity. A negative correlation existed between biomass and rainfall. Productivity,
biomass, conductivity, alkalinity, and SRP increased after mid-winter.k, I
k
andZ
eu varied according seasonally.P
max related directly with temperature. Seasonal variation of ∝
B
was 0.0049–0.0258 mg C (mg chla mmol PAR)−1 m−2. Q10 was 2.12, community respiration 1334.99 g C m−2 y−1, and the underwater light climate 186.43μE m−2 s−1. 相似文献