全文获取类型
收费全文 | 36098篇 |
免费 | 3070篇 |
国内免费 | 4061篇 |
专业分类
43229篇 |
出版年
2024年 | 131篇 |
2023年 | 531篇 |
2022年 | 1276篇 |
2021年 | 1922篇 |
2020年 | 1438篇 |
2019年 | 1670篇 |
2018年 | 1608篇 |
2017年 | 1155篇 |
2016年 | 1677篇 |
2015年 | 2398篇 |
2014年 | 2746篇 |
2013年 | 2950篇 |
2012年 | 3605篇 |
2011年 | 3182篇 |
2010年 | 1993篇 |
2009年 | 1829篇 |
2008年 | 2102篇 |
2007年 | 1767篇 |
2006年 | 1561篇 |
2005年 | 1325篇 |
2004年 | 1076篇 |
2003年 | 983篇 |
2002年 | 799篇 |
2001年 | 567篇 |
2000年 | 473篇 |
1999年 | 452篇 |
1998年 | 269篇 |
1997年 | 201篇 |
1996年 | 173篇 |
1995年 | 171篇 |
1994年 | 120篇 |
1993年 | 98篇 |
1992年 | 131篇 |
1991年 | 131篇 |
1990年 | 108篇 |
1989年 | 89篇 |
1988年 | 64篇 |
1987年 | 66篇 |
1986年 | 46篇 |
1985年 | 55篇 |
1984年 | 31篇 |
1983年 | 32篇 |
1982年 | 31篇 |
1981年 | 14篇 |
1979年 | 21篇 |
1978年 | 17篇 |
1977年 | 20篇 |
1976年 | 16篇 |
1975年 | 15篇 |
1972年 | 15篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
61.
鸡胚血液中原始生殖细胞的分离及其培养的研究 总被引:2,自引:0,他引:2
从孵化48~55小时鸡胚中抽取血液,每只胚胎可获血液2~6μl左右。一步法离心分离原始生殖细胞,可使其浓度由0.1%以下提高到50%以上。将多余血细胞用微量吸管移走后,加入添加10%胎牛血清的TCM—199做为培养基,37.5℃,5%CO2,95%空气,饱和湿度下培养,原始生殖细胞可成活24小时左右。 相似文献
62.
高压静电场处理沙棘插条生根状况的初步研究 总被引:2,自引:0,他引:2
用高压静电场处理沙棘插条研究生根状况,方法简便易于处理,该装置适于进行大规模的处理,从目前研究表明:高压静电场对沙棘插条有正刺激效应,并以电场强度为3.9kv/cm,时间为40分钟处理最佳。 相似文献
63.
低能量He—Ne激光血管内照射治疗银屑病21例报告 总被引:1,自引:0,他引:1
21例寻常型银屑病患者,经用He-Ne激光血管内照射,功率3.5-5mw,每日照射一次,每次1小时,10次一疗程,同时伴用vitc2givq.d及鼻吸氧,二疗程间休息4-7天,经5-35次治疗,近期疗效:近期痊愈5例(23.81%),显效6例(28.57%),好转10例(47.62%),总有效率100%,复发1例(4.76%)。 相似文献
64.
中药固真方对一些与细胞增殖有关基因表达的影响姚明忠,顾文聪,丁卫,韩志芬,杜国光(上海中医药大学生物化学教研室,上海200032)(北京医科大学生物化学与分子生物学系,北京100083)中药固真方(VRF)具有补肾益精、延缓衰老的作用[1].能提高成... 相似文献
65.
本项研究的目的:是在一系列毒性试验的基础上进一步观察大鼠皮下给予由枯草芽孢杆菌BS224菌株制成的抑菌生后,对机体产生毒性反应的情况。结果表明:给药组大鼠饮食、活动正常。各项检测指标及病理组织学检查未发现异常,与对照组比较均无差异(P>0.05),说明抑菌生无毒性反应。 相似文献
66.
Chen J Zhang LF Han C Yu GS Ma J 《Journal of gravitational physiology : a journal of the International Society for Gravitational Physiology》1996,3(1):57-62
We have previously demonstrated that prolonged simulated microgravity (tail-suspension) leads to cardiac alterations with increased resting heart rate, myocardial degradation changes and attenuated myocardial contractility. The present study investigated the potential role of adrenoceptor mechanisms underlying them. Changes of myocardial alpha 1-adrenoceptor (alpha 1-AR) and beta 1-adrenoceptor (beta-AR) in 90-day tail-suspended rats was investigated by the method of radioligand binding assay and application of Scatchard's method. The results showed significantly decreased quantity of specific binding of 125I-BE[2-beta-(4-hydroxy-3-[125I]indophenyl)-ethylaminomethyltetralone] to alpha 1-AR present in membrane derived from ventricular myocardium of the suspended animals, despite the affinity of the alpha 1-AR to 125I-Be was unchanged. But neither the quantity nor the affinity of beta-AR binding to 125I-Pindolol was significantly altered. In addition, the spontaneously beating rate of isolated right atria from tail-suspended animals showed little change in sensitivity and reactivity to the stimulations of graded phenylephrine (alpha-agonist, measured in the presence of beta-antagonist propranolol) and isoproterenol (beta-agonist), compared with the control rats. There were also no obvious differences of the effects of the isoproterenol on the contractility of isolated left ventricular papillary muscles between the two groups. Since myocardial alpha 1-AR mediated-effects include production of cardiac hypertrophy and enhancement of myocardial glucose uptake and glycolysis, the down-regulation of the alpha 1-AR may be a contributor to the cardiac cellular accumulation and the myocardial degradation changes as found in our tail-suspended rats. The data from this study also suggest that the myocardial beta-adrenoceptors are not affected by the prolonged tail-suspension. 相似文献
67.
Laminin and Neuropeptide Y Are Increased by Synapsin Transfection in Cultured NG108-15 Neuroblastoma/Glioma Hybrid Cells 总被引:2,自引:0,他引:2
† Gabriel Fried Hui Quan Han ‡Björn Meister ‡Tomas Hökfelt Paul Greengard 《Journal of neurochemistry》1995,64(6):2674-2680
Abstract: We have investigated the presence and expression of laminin and neuropeptide Y (NPY) in several NG108-15 cell lines transfected with synapsin Ib, IIa, or IIb. The content of laminin, a basal membrane glycoprotein that promotes adhesion and induces neurite outgrowth and neuronal differentiation, was increased in all transfected cell lines examined. In cells that were chemically differentiated with prostaglandin E1 plus 3-isobutyl-1-methylxanthine, laminin levels were increased even further. The content of NPY, suggested to be a neurotransmitter/neuromodulator in peripheral sympathetic neurons as well as in central neurons, was also increased in all transfected cell lines examined. Immunohistochemical analysis combined with confocal laser microscopy showed that NPY staining was granular and very often enriched in neuritic varicosities. The distribution and the staining pattern of NPY were consistent with storage of NPY in large dense-cored vesicles. The results indicate that, in differentiated neurons, the synapsins increase the levels of a neuropeptide transmitter stored in large dense-cored vesicles and of an extracellular matrix protein associated with neuronal maturation. 相似文献
68.
A human protein selected for interference with Ras function interacts directly with Ras and competes with Raf1. 总被引:13,自引:8,他引:5 下载免费PDF全文
The overexpression of some human proteins can cause interference with the Ras signal transduction pathway in the yeast Saccharomyces cerevisiae. The functional block is located at the level of the effector itself, since these proteins do not suppress activating mutations further downstream in the same pathway. We now demonstrate, with in vivo and in vitro experiments, that the protein encoded by one human cDNA (clone 99) can interact directly with yeast Ras2p and with human H-Ras protein, and we have named this gene rin1 (Ras interaction/interference). The interaction between Ras and Rin1 is enhanced when Ras is bound to GTP. Rin1 is not able to interact with either an effector mutant or a dominant negative mutant of H-Ras. Thus, Rin1 displays a human H-Ras interaction profile that is the same as that seen for Raf1 and yeast adenylyl cyclase, two known effectors of Ras. Moreover, Raf1 directly competes with Rin1 for binding to H-Ras in vitro. Unlike Raf1, however, the Rin1 protein resides primarily at the plasma membrane, where H-Ras is localized. These data are consistent with Rin1 functioning in mammalian cells as an effector or regulator of H-Ras. 相似文献
69.
Deformation and flow of red blood cells in a synthetic lattice: evidence for an active cytoskeleton. 总被引:3,自引:0,他引:3 下载免费PDF全文
We introduce the use of microfabrication techniques to construct on a silicon wafer a synthetic capillary bed with 2.5- to 4-micron (mu)-wide channels. Establishment of a fluid pressure gradient allowed us to observe simultaneously using optical microscopy hundreds of cells flowing through the bed at physiological speeds. We find a large distribution of mobilities among red cells flowing through the structure; smaller channels provide a greater impedance to flow than larger ones, indicating that kinetic drag variations provide the origin of the distribution. The mobility of a particular cell is not correlated with the cell diameter but appears to be inversely correlated with intracellular calcium concentration of the cell, as determined by fluorescence of the calcium-binding dye fluo-3 AM. Also, we are able to use the parallel processing nature of our arrays to observe isolated events where the rigidity of the red cell seems to change suddenly over several orders of magnitude as it blocks a channel in the array. 相似文献
70.
The glucan-binding lectin (GBL) ofStreptococcus sobrinus is cell associated, enabling the bacteria to be aggregated by -1,6 glucans. Glucans, such as amylose, pullulan, laminarin and nigeran, have no affinity for the lectin. High molecular weight -1,6 glucans (dextrans) readily aggregate the bacteria, whereas low molecular weight glucans inhibit the aggregation brought about by the high molecular weight species. Methylated glucan T-2000 (an -1,6 glucan with an average molecular weight of 2 × 106 Da) aggregated the bacteria very poorly when the extent of methylation (DS, or degree of substitution) was high, and less poorly when the DS was low. Similarly, methylated low molecular weight -1,6 glucan was a poor inhibitor of aggregation induced by the high molecular weight glucan T-2000. Because the methylation occurred primarily on the hydroxyl of C-2, it is suggested that the hydroxyl is needed for formation of the lectin-glucan complex. It appears that the GBL is not only stereospecific in interaction with glucans, but also regiospecific, interacting only with the underivatized -1,6-glucan. 相似文献