全文获取类型
收费全文 | 128172篇 |
免费 | 22116篇 |
国内免费 | 3791篇 |
专业分类
154079篇 |
出版年
2023年 | 802篇 |
2022年 | 2061篇 |
2021年 | 3641篇 |
2020年 | 3967篇 |
2019年 | 6003篇 |
2018年 | 6517篇 |
2017年 | 6251篇 |
2016年 | 7427篇 |
2015年 | 8690篇 |
2014年 | 9422篇 |
2013年 | 10588篇 |
2012年 | 10016篇 |
2011年 | 9227篇 |
2010年 | 7977篇 |
2009年 | 6224篇 |
2008年 | 6641篇 |
2007年 | 5729篇 |
2006年 | 5158篇 |
2005年 | 4546篇 |
2004年 | 4384篇 |
2003年 | 4006篇 |
2002年 | 3399篇 |
2001年 | 2907篇 |
2000年 | 2573篇 |
1999年 | 1984篇 |
1998年 | 931篇 |
1997年 | 799篇 |
1996年 | 722篇 |
1995年 | 719篇 |
1994年 | 614篇 |
1993年 | 540篇 |
1992年 | 946篇 |
1991年 | 778篇 |
1990年 | 694篇 |
1989年 | 680篇 |
1988年 | 548篇 |
1987年 | 573篇 |
1986年 | 479篇 |
1985年 | 463篇 |
1984年 | 383篇 |
1983年 | 330篇 |
1982年 | 274篇 |
1981年 | 243篇 |
1980年 | 219篇 |
1979年 | 285篇 |
1978年 | 269篇 |
1977年 | 230篇 |
1976年 | 228篇 |
1974年 | 249篇 |
1973年 | 216篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
991.
GRP78, a master regulator of the unfolded protein response (UPR) and cell signaling, is required for inner cell mass survival during early embryonic development. However, little is known about its role in adult hematopoietic stem cells (HSCs) and hematopoiesis. Here we generated a conditional knockout mouse model that acutely deletes Grp78 in the adult hematopoietic system. Acute GRP78 ablation resulted in a significant reduction of HSCs, common lymphoid and myeloid progenitors, and lymphoid cell populations in the mutant mice. The GRP78-null induced reduction of the HSC pool could be attributed to increased apoptosis. Chimeric mice with Grp78 deletion only in the hematopoietic cells also showed a loss of HSCs and lymphopenia, suggesting a cell intrinsic effect. Analysis of GRP78 deficient bone marrow (BM) cells showed constitutive activation of all the major UPR signaling pathways, including activation of eIF2α, ATF6, xbp-1 splicing, as well as caspase activation. A multiplex cytokine assay further revealed alteration in select cytokine and chemokine serum levels in the mutant mice. Collectively, these studies demonstrate that GRP78 plays a pleiotropic role in BM cells and contributes to HSC survival and the maintenance of the lymphoid lineage. 相似文献
992.
NICOLAS FEAU MARIE‐JOSE BERGERON DAVID L. JOLY FABIEN ROUSSEL RICHARD C. HAMELIN 《Molecular ecology resources》2007,7(6):1222-1228
We report the discovery, characterization and validation of 118 single nucleotide polymorphisms (SNPs) for poplar leaf rust Melampsora medusae f. sp. deltoidae identified using a gene‐targeted approach in an expressed sequence tag (EST) library. We developed a genotyping assay using the iPLEX? primer extension method for two multiplex assays of 28 and 22 SNPs. 相似文献
993.
Protein pattern has played an important role in biosensors, bioMEMS, tissue engineering, fundamental studies of cell biology, and basic proteomics research. Here, we developed a straightforward and effective protein patterning technique using macroporous poly(2-hydroxyethyl methacrylate) (PHEMA) hydrogel micropatterns as a three-dimensional (3D) template for protein immobilization. Micropatterns of macroporous hydrogels with inverse opal structures were prepared on poly(ethylene glycol) (PEG)-coated silicon substrates by combining a colloidal crystal templating method with photopatterning. The resultant inverse opal hydrogel (IOH) micropatterns were modified with 3-aminopropyltriethoxysilane using the hydroxyl groups in PHEMA for the covalent immobilization of proteins. Proteins were selectively immobilized only on the hydrogel micropatterns, while the PEG regions served as an effective barrier to protein adsorption. Because of their highly ordered and interconnected 3D macroporous structures and large internal surface areas, protein loading in the IOH micropattern was about six times greater than that on a non-porous hydrogel micropattern, which consequently improved the protein activity. The porosity of the hydrogel micropatterns could be controlled using different sizes of colloidal nanoparticles, and using smaller nanoparticles produced hydrogel micropatterns with higher protein loading capacities and activities. To demonstrate the potential use of IOH micropatterns in biosensor systems, biotin was micropatterned on the hydrogels and the specific binding of streptavidin was successfully assayed using IOH micropatterns with better fluorescence signals and sensitivity than that of the corresponding non-porous hydrogel micropatterns. 相似文献
994.
995.
Protein secondary structure analyses from circular dichroism spectroscopy: methods and reference databases 总被引:3,自引:0,他引:3
Circular dichroism (CD) spectroscopy has been a valuable method for the analysis of protein secondary structures for many years. With the advent of synchrotron radiation circular dichroism (SRCD) and improvements in instrumentation for conventional CD, lower wavelength data are obtainable and the information content of the spectra increased. In addition, new computation and bioinformatics methods have been developed and new reference databases have been created, which greatly improve and facilitate the analyses of CD spectra. This article discusses recent developments in the analysis of protein secondary structures, including features of the DICHROWEB analysis webserver. 相似文献
996.
Species diversity of the genus Osmundea (Ceramiales,Rhodophyta) in the Macaronesian region 下载免费PDF全文
María Machín‐Sánchez Florence Rousseau Line Le Gall Valéria Cassano Ana I. Neto Abel Sentíes Mutue T. Fujii María Candelaria Gil‐Rodríguez 《Journal of phycology》2016,52(4):664-681
Species diversity within the genus Osmundea in the Macaronesian region was explored by conducting a comprehensive sampling in the Azores, the Canary, and the Madeira archipelagos. Toward identification, all specimens were first observed alive to verify the absence of corps en cerise, a diagnostic character for the genus and morphometric data were measured (thallus length and width, first‐order branches length and width, branchlets length and width, cortical cell length and width in surface view, cortical cell length and width in transverse section). Specimens were sequenced for COI‐5P (39 specimens) and three species delimitation methods (Generalized Mixed Yule Coalescent, Automatic Barcode Gap Discovery method, and Poisson Tree Processes) were used to assess the threshold between infra‐ and interspecific relationships. Subsequently, one or several sequences of plastid‐encoded large subunit of RuBisCO (21 specimens) per delimited species were generated to assess the phylogenetic relationships among Macaronesian Osmundea. Moreover, for each delineated species, vegetative and reproductive anatomy was thoroughly documented and, when possible, specimens were either assigned to existing taxa or described as novel species. This integrative approach has provided data for (i) the presence of O. oederi, O. pinnatifida, and O. truncata in Macaronesia; (ii) the proposal of two novel species, O. prudhommevanreinei sp. nov. and O. silvae sp. nov.; and (iii) evidence of an additional species referred as “Osmundea sp.1,” which is a sister taxon of O. hybrida. 相似文献
997.
Bone marrow-derived mesenchymal stem cells (BMSCs) are of particular interest in the field of tissue engineering because of
their potential to differentiate into osteoblasts, chondrocytes, and neuronal cells. In order to promote the differentiation
of BMSCs into specific cell types, appropriate scaffold biomaterials and bioactive molecules that can support the differentiation
of BMSCs into specific cell types are needed. We hypothesized that β-mercaptoethanol (BME), which has been reported to induce
the differentiation of BMSCs into neural-like cells, promotes BMSCs to differentiate into neural-like cells when BME is added
to polymeric scaffolds containing the BMSCs. We fabricated biocompatible film shaped scaffolds composed of poly(lacti-co-glycolic)
acid (PLGA) and various concentrations of BME to confirm that BME-promoted differentiation of BMSCs is concentration-dependent.
Cell proliferation increased as the BME concentration in the films increased at the early stage, and the proliferation rate
remained similar on the PLGA films for 3 weeks following the BMSC seeding. The expression of neuronal markers in differentiated
BMSCs was assessed by RT-PCR. At 2- and 3-week time-points, mRNA expression of neurofilament and neuron specific enolase was
significantly increased in PLGA/BME films containing 400 μM BME compared to PLGA films. Thus, we have identified BMSC-seeded
PLGA/BME films with 200 μM and 400 μM BME as potentially useful candidates for neural tissue engineering applications by promoting
BMSC proliferation and differentiation towards neural-like cells. 相似文献
998.
为了解我国北方不同草原类型中针茅根部内生真菌的群落结构及多样性变化,从新疆、甘肃、内蒙古3省(区)选择了6种不同草原类型(亚高山草甸、高山草甸、戈壁、荒漠草原、典型草原和草甸草原),进行针茅根部组织内生真菌的研究.共分离得到针茅根部内生真菌213株,根据序列的相似性(以97%为阈值),共获得51个真菌分类操作单元(OTUs),覆盖了4门7纲23科27属.在门的水平上子囊菌门真菌为绝对优势菌群,占分离真菌总数的93.4%,在各草原类型中均有分布;6种草原类型中针茅根部内生真菌的优势属差别较大,仅子囊菌门的镰孢菌属为各草原类型共有优势属,占分离真菌总数的41.3%,亚高山草甸的微结节霉属、高山草甸的Saccharicola和短梗霉属、戈壁的弯孢属和根霉属以及草甸草原的木霉属,为各草原类型中针茅根部内生真菌的优势属.高山草甸针茅根部内生真菌群落覆盖的门和属最多,Margalef丰富度指数和香农多样性指数最高,均匀度指数仅次于荒漠草原;而荒漠草原的Margalef丰富度指数最低,典型草原的多样性指数和均匀度指数最低.高山草甸和荒漠草原的内生真菌群落结构与其他草原类型之间的相似性系数都较低,分别为0.12~0.25和0.13~0.22,其他几种草原类型之间相似性相对较高,尤其是典型草原和草甸草原之间相似性系数为0.60.冗余分析(RDA)表明,海拔和纬度是影响6种草原类型中针茅根部内生真菌群落结构变化的主要环境因子. 相似文献
999.
Wood anatomy and carbon‐isotope discrimination support long‐term hydraulic deterioration as a major cause of drought‐induced dieback 下载免费PDF全文
Elena Pellizzari J. Julio Camarero Antonio Gazol Gabriel Sangüesa‐Barreda Marco Carrer 《Global Change Biology》2016,22(6):2125-2137
Hydraulic impairment due to xylem embolism and carbon starvation are the two proposed mechanisms explaining drought‐induced forest dieback and tree death. Here, we evaluate the relative role played by these two mechanisms in the long‐term by quantifying wood‐anatomical traits (tracheid size and area of parenchyma rays) and estimating the intrinsic water‐use efficiency (iWUE) from carbon isotopic discrimination. We selected silver fir and Scots pine stands in NE Spain with ongoing dieback processes and compared trees showing contrasting vigour (declining vs nondeclining trees). In both species earlywood tracheids in declining trees showed smaller lumen area with thicker cell wall, inducing a lower theoretical hydraulic conductivity. Parenchyma ray area was similar between the two vigour classes. Wet spring and summer conditions promoted the formation of larger lumen areas, particularly in the case of nondeclining trees. Declining silver firs presented a lower iWUE than conspecific nondeclining trees, but the reverse pattern was observed in Scots pine. The described patterns in wood anatomical traits and iWUE are coherent with a long‐lasting deterioration of the hydraulic system in declining trees prior to their dieback. Retrospective quantifications of lumen area permit to forecast dieback in declining trees 2–5 decades before growth decline started. Wood anatomical traits provide a robust tool to reconstruct the long‐term capacity of trees to withstand drought‐induced dieback. 相似文献
1000.