全文获取类型
收费全文 | 24000篇 |
免费 | 1703篇 |
国内免费 | 9篇 |
出版年
2023年 | 119篇 |
2022年 | 176篇 |
2021年 | 602篇 |
2020年 | 361篇 |
2019年 | 484篇 |
2018年 | 613篇 |
2017年 | 507篇 |
2016年 | 839篇 |
2015年 | 1264篇 |
2014年 | 1411篇 |
2013年 | 1736篇 |
2012年 | 2017篇 |
2011年 | 1985篇 |
2010年 | 1225篇 |
2009年 | 1076篇 |
2008年 | 1461篇 |
2007年 | 1358篇 |
2006年 | 1244篇 |
2005年 | 1157篇 |
2004年 | 1051篇 |
2003年 | 1006篇 |
2002年 | 825篇 |
2001年 | 200篇 |
2000年 | 183篇 |
1999年 | 181篇 |
1998年 | 146篇 |
1997年 | 137篇 |
1996年 | 116篇 |
1995年 | 120篇 |
1994年 | 103篇 |
1993年 | 109篇 |
1992年 | 101篇 |
1991年 | 100篇 |
1990年 | 111篇 |
1989年 | 97篇 |
1988年 | 97篇 |
1987年 | 79篇 |
1986年 | 70篇 |
1985年 | 88篇 |
1984年 | 74篇 |
1983年 | 73篇 |
1982年 | 52篇 |
1981年 | 68篇 |
1980年 | 50篇 |
1979年 | 55篇 |
1978年 | 53篇 |
1977年 | 57篇 |
1975年 | 49篇 |
1974年 | 50篇 |
1973年 | 48篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
991.
Nigel R. Franks James W. Hooper Mike Gumn Tamsyn H. Bridger James A. R. Marshall Roderich Groß Anna Dornhaus 《Swarm Intelligence》2007,1(2):81-94
Many decisions involve a trade-off between commitment and flexibility. We show here that the collective decisions ants make
over new nest sites are sometimes sufficiently flexible that the ants can change targets even after an emigration has begun.
Our findings suggest that, in this context, the ants’ procedures are such that they can sometimes avoid ‘negative information
cascades’ which might lock them into a poor choice. The ants are more responsive to belated good news of a higher quality
nest than they are when the nest they had initially chosen degraded to become worse than an alternative. Our study confirms,
in a new way, that ant colonies can be very powerful “search engines”. 相似文献
992.
Andrea Matta Maurizio Tomasella Anna Valente 《Flexible Services and Manufacturing Journal》2007,19(3):173-194
This paper presents an optimal solution, based on Markov decision theory, for the problem of optimal capacity-related reconfiguration of manufacturing systems, under stochastic market demand. Both capacity expansion and reduction are considered. The solution quantitatively takes into account the effect of the ramp-up phenomenon, following each reconfiguration, on the optimal policy. A closed-form solution is presented for when product demand is independently and generally distributed over time. A real case concerning a flexible manufacturing line in the automotive sector is shown, to prove that ignoring the ramp-up effect in the decision process can lead to significant increases in overall costs. 相似文献
993.
Fountain KJ Kloss A Garibyan I Blitshteyn B Brezzani A Kyostio-Moore S Zuk A Sacchiero R Cohen AS 《Journal of chromatography. B, Analytical technologies in the biomedical and life sciences》2007,846(1-2):245-251
An ion exchange high performance liquid chromatography method was developed for determining creatinine levels in both mouse and rat serum samples. Separation of creatinine from other serum components was achieved in 10 min using a 100 x 4.1-mm, 10 microm strong cation exchange column following acetonitrile precipitation of serum proteins. Incorporation of a guard cartridge placed in-line prior to the analytical column was employed to prevent interference from compounds used in renal disease animal trials. Creatinine levels in normal and diseased animals were accurately determined in the 0.01-10 mg/dL range, and average recovery of the method was approximately 85% for both mouse and rat serum. Addition of 0.5-1.0% acetic acid to the acetonitrile used for protein precipitation significantly improved creatinine recovery to above 97% in mouse serum. The method was used for routine preclinical diagnosis of rat and mouse model renal function, and for the evaluation of renal disease treatment efficacy. 相似文献
994.
Barbara Hulanicka Anna Lipowicz Sawomir Kozie Alicja Kowalisko 《Economics & Human Biology》2007,5(1):48-60
The objective of this paper is to evaluate: (a) the association between BMI in childhood and adolescence and BMI at age 50, and (b) the association between timing of maturation at puberty and BMI at age 50 and the occurrence of high blood pressure. We explore whether differences in somatic and physiological factors observed among children with early or late onset of puberty indicates that this period of life could be critical for appearance of some CVD risk factors later in the life cycle. Our data include 135 males and 148 females, born in 1953, participants in the Wroc?aw Longitudinal Study. Data pertaining to their growth and several indices of sexual and skeletal maturation were collected yearly between age 8 and 18. Subsequently, their anthropometric traits were measured and cardiovascular health status was assessed at age 50. We find that BMI at distinct stages in the life cycle were positively correlated. Also BMI at 50 were positively associated with blood pressure at 50. Moreover, earlier maturation at puberty is associated with higher values of BMI at age 50. We also find that earlier pubertal maturation is an independent factor that influences the appearance of high blood pressure in adulthood. 相似文献
995.
Parsy CB Chapman CJ Barnes AC Robertson JF Murray A 《Journal of chromatography. B, Analytical technologies in the biomedical and life sciences》2007,853(1-2):314-319
As part of a study to purify the internal domain of HER2 (ICD) from recombinant expression, through metal immobilised affinity chromatography (IMAC), we encountered a contaminant, SlyD, a 29 kDa native E. coli protein. SlyD is a recurrent contaminant, with a histidine rich domain enabling binding to IMAC columns and thus co-elution with the target protein. Research has been carried out on this protein and its purification, however, no work mentions how to treat it as a true contaminant or describe procedures to isolate it from target proteins. In this report, we described a two-step chromatographic method for the purification of ICD, including IMAC as a capture step and size exclusion chromatography (SEC) as a polishing step. IMAC allowed us to purify ICD from bacterial crude with SlyD co-eluting. SEC then allowed us to resolve ICD from SlyD and achieve a purity greater than 95% for ICD. However, this method has been developed to accommodate any protein whose molecular weight is different enough from SlyD to be separated by SEC. 相似文献
996.
Varga L Bíró A Széplaki G Tóth L Horváth A Füst G Farkas H 《Journal of cellular and molecular medicine》2007,11(6):1377-1383
Hereditary angioedema (HAE) is a rare disorder caused by the deficiency of the C1-inhibitor gene (C1INH) and characterized by recurrent bouts of angioedema. Autoimmune disorders frequently occur in HAE. Previously we found, that danazol has an adverse effect on serum lipid profile: reduced high-density lipoprotein (HDL) and elevated low-density lipoprotein (LDL) cholesterol levels are associated with long-term prophylactic use, whereas total cholesterol levels are unchanged. Our aim was to study the anti-cholesterol antibody (ACHA) production in HAE patients and compare it with those of healthy blood donors, and to investigate the possible associations between ACHA levels and serum lipid profile alterations caused by danazol. Anti-cholesterol IgG levels were measured by ELISA and their correlation with serum concentrations of total cholesterol, HDL, LDL, triglycerides was determined in HAE patients receiving/not receiving danazol. Serum ACHA levels were significantly higher in HAE patients, compared to healthy blood donors (P<0.0001). Longterm danazol prophylaxis had no effect on serum ACHA levels in HAE patients. However, we found a significant, negative correlation between ACHA levels and serum total cholesterol (r=-0.4033, P=0.0200), LDL (r=-0.4565, P=0.0076) and triglyceride (r=-0.4230, P=0.0121) levels only in danazol-treated patients, but not in HAE patients who did not receive long-term prophylaxis. Patients with HAE have higher baseline ACHA levels compared to healthy subjects, and this might reflect polyclonal B-cell activation. The latter would be a potential explanation for the lack of an increased incidence of infectious diseases in HAE patients, but might lead to increased autoimmunity. 相似文献
997.
Barbara Repetto Alberto Palenzona Simone Cagnacci Eugenio A. Debbia Anna Marchese 《Annals of microbiology》2007,57(3):439-442
Pseudomonas aeruginosa is an opportunistic pathogen, intrinsically resistant to many antibiotics and prone to acquire resistance against many drugs. It is assumed that agents that disorganise the structure of the outer membrane might allow the passage of other drugs into cell. To verify this hypothesis, ceftazidime (CAZ) has been tested in association with glycopeptides (GLYs) and azithromycin (AZI). Time-kill experiments were performed on a representative strain. CAZ in combination with GLYs showed 99, 90 and 10% of CFU/ml reduction in 33.9,52.5 and 13.6% of the cases, respectively; the addition of AZI increased the incidence of 99% CFU/ml reduction to 42% of the cases. Indifference was the most common finding, and additive/synergism in the other cases. Present findings demonstrated that CAZ favourably reacted with GLYs in the presence of AZI. 相似文献
998.
Fibroblast growth factor receptors (FGFRs) are a family of four transmembrane (TM) receptor tyrosine kinases (RTKs) which bind to a large family of fibroblast growth factor (FGF) ligands with varying affinity and specificity. FGFR signaling regulates many physiological and pathological processes in development and tissue homeostasis. Understanding FGFR signaling processes requires the identification of partner proteins which regulate receptor function and biological outputs. In this study, we employ an epitope-tagged, covalently dimerized, and constitutively activated form of FGFR1 to identify potential protein partners by MS. By this approach, we sample candidate FGFR effectors throughout the life history of the receptor. Functional classification of the partners identified revealed specific subclasses involved in protein biosynthesis and folding; structural and regulatory components of the cytoskeleton; known signaling effectors and small GTPases implicated in endocytosis and vesicular trafficking. The kinase dependency of the interaction was determined for a subset of previously unrecognized partners by coimmunoprecipitation, Western blotting, and immunocytochemistry. From this group, the small GTPase Rab5 was selected for functional interrogation. We show that short hairpin (sh) RNA-mediated depletion of Rab5 attenuates the activation of the extracellular-regulated kinase (ERK) 1/2 pathway by FGFR signaling. The strategic approach adopted in this study has revealed bona fide novel effectors of the FGFR signaling pathway. 相似文献
999.
Ovarian steroids modulate uterine receptivity in domestic species. Luteinizing hormone (LH) stimulates prostaglandin (PG)F(2alpha) release from the porcine endometrium. However, the combined action of LH and steroids on PGs secretion has not yet been studied in pigs. The aim of the present study was to examine the effect of estradiol (E(2)) and progesterone (P(4)) on basal and LH-stimulated PGF(2alpha) and PGE(2) secretion and cyclooxygenase-2 (COX-2) protein expression in porcine endometrial stromal cells obtained on days 12-13 of the estrous cycle. Cells were cultured for 48 h in a medium containing charcoal-stripped newborn calf serum alone or supplemented with 10 nM E(2) and/or 50 nM P(4). Then, the cells were incubated for 6 h in the presence or absence of LH (20 ng/ml). Long exposure of stromal cells to steroids had no effect on PGF(2alpha) secretion, but PGE(2) release increased in the presence of E(2) plus P(4) (p<0.05). Pre-incubation of cells with E(2) plus P(4) resulted in enhanced PGF(2alpha) (p<0.05) and PGE(2) (p<0.001) secretion. Moreover, LH increased PG(2alpha) secretion in control (p<0.05) and E(2)-treated stromal cells (p<0.01). LH tended (p=0.07) to elevate PGE(2) release only in cells pre-exposed to E(2) plus P(4). The expression of COX-2 protein was increased by LH (p<0.05), but not by steroids. These results confirm the stimulatory effect of LH on PGF(2alpha) secretion and COX-2 expression in porcine stromal cells before luteolysis. PG release from porcine endometrium seems to be controlled by ovarian steroids, however only E(2)-treated-treated cells responded to LH. 相似文献
1000.
Seasonal reproduction in birds is the adaptation to breed at the time of the greatest survival of young. This exact moment is mainly imposed by the photoperiod which stimulates the hypothalamo-pituitary-gonadal axis and starts breeding season. This article summarizes present knowledge concerning: 1/ perception and transduction of light into the biological signal; 2/ model of the avian photoperiodic response; 3/ seasonal changes in hypothalamic secretion of gonadotropin-releasing hormone (GnRH), vasoactive intestinal polipeptide (VIP), as well as the gonadotropin inhibitory hormone (GnIH); 4/ seasonal interactions between pituitary hormones; 5/ seasonal morphological and functional changes within the avian gonads. 相似文献