首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   10583篇
  免费   821篇
  国内免费   1篇
  2023年   38篇
  2022年   37篇
  2021年   90篇
  2020年   84篇
  2019年   79篇
  2018年   192篇
  2017年   220篇
  2016年   244篇
  2015年   238篇
  2014年   291篇
  2013年   477篇
  2012年   804篇
  2011年   904篇
  2010年   447篇
  2009年   315篇
  2008年   667篇
  2007年   748篇
  2006年   651篇
  2005年   570篇
  2004年   547篇
  2003年   559篇
  2002年   484篇
  2001年   292篇
  2000年   329篇
  1999年   164篇
  1998年   114篇
  1997年   94篇
  1996年   106篇
  1995年   101篇
  1994年   93篇
  1993年   87篇
  1992年   88篇
  1991年   75篇
  1990年   73篇
  1989年   61篇
  1988年   67篇
  1987年   48篇
  1986年   62篇
  1985年   63篇
  1984年   65篇
  1983年   69篇
  1982年   66篇
  1981年   70篇
  1980年   67篇
  1979年   64篇
  1978年   65篇
  1977年   60篇
  1976年   49篇
  1974年   32篇
  1973年   34篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
31.
Summary The increased risk of nonocular cancer seen consistently in studies of survivors of retinoblastoma may be caused in part by the presence of a retinoblastoma gene that also predisposes to other cancers. It has been claimed that this gene also increases the risk for cancer among unaffected relatives of genetic retinoblastoma probands. We report here a population-based study of the risk of nonocular cancer in parents and siblings of persons notified to the Danish Cancer Registry with retinoblastoma during 1943–84. No excess was observed among first degree relatives of 61 genetic retinoblastoma probands, whereas a slight (10%) excess was seen among the parents of 115 nongenetic probands. The latter was the result of significant excesses of malignant melanoma (4 observed, 0.4 expected), multiple myeloma (2 observed, 0.2 expected) and osteogenic sarcoma (1 observed, 0.03 expected). The observed risk pattern cannot be explained by the presence of the retinoblastoma gene.  相似文献   
32.
Summary Endocytosis via the hyaluronic acid/chondroitin sulphate receptor of rat liver endothelial cells was studied ultrastructurally, by use of a probe consisting of chondroitin sulphate proteoglycan attached to 15-nm gold particles. The probe bound to the surface of the cells exclusively in coated regions of the plasma membrane. Internalization at 37° C took place in less than one minute during which time interval the bound probe was transferred to coated vesicles. Further transfer to lysosomes was delayed in association with an accumulation of probes in a prelysosomal compartment consisting of large vacuoles in which probes lined the inner aspect of the membrane. Transport to lysosomes occurred only after a lag phase of at least 40–60 min at 37° C.Abbreviations CS chondroitin sulphate - CSPG chondroitin sulphate proteoglycan - CSPG-Au CSPG-gold complex - EM electronmicroscopical or electron microscopy - HA hyaluronic acid - KC Kuppfer cells - LEC liver endothelial cells - PC parenchymal cells - RES reticuloendothelial system  相似文献   
33.
Summary Serotonin-like immunoreactivity was investigated in the pineal complex of the golden hamster by use of the indirect immunohistochemical technique. The superficial and deep portions of the pineal gland, and also the pineal stalk exhibited an intense cellular immunoreaction for serotonin. In addition, perivascular serotonin-immunoreactive nerve fibers were observed. Some serotonin-immunoreactive processes of the pinealocytes terminated on the surface of the ventricular lumen in the pineal and suprapineal recesses, indicating a receptive or secretory function of these cells. Several serotonin-immunoreactive processes connected the deep pineal with the habenular area. One week after bilateral removal of both superior cervical ganglia the serotonin immunoreaction of the entire pineal complex was greatly decreased. However, some cells in the pineal complex, of which several exhibited a neuron-like morphology, remained intensively stained after ganglionectomy. This indicates that the indoleamine content of some cells in the pineal complex of the golden hamster is independent of the sympathetic innervation.Supported by a Grant from the Italian Society for Veterinary Sciences  相似文献   
34.
35.
Summary The BCEI gene codes for a small secreted protein and is expressed in the human mammary tumour cell line MCF7 under oestrogen control and in some breast cancers. We have mapped the gene to chromosome 21 using a panel of somatic hybrid lines, and in situ hybridization has allowed a precise assignment to band 21q223. Two restriction fragment length polymorphisms (RFLP) are described that should be of use in linkage or population studies to test a possible involvement of the BCEI gene in genetic predisposition to breast cancer. This gene should also be a useful marker for the genetic and physical mapping of chromosome 21, and for a better definition of the region involved in the clinical phenotype of Downs syndrome.  相似文献   
36.
Several aspects of energy metabolism (glucose utilization, lactate production,14CO2 production from labeled glucose, glutamate or pyruvate, oxygen consumption and contents of ATP and phosphocreatine) were measured in cerebellar granule cells (glutamatergic) in primary cultures and compared with corresponding data for cerebral cortical neurons (mainly GABA-ergic) and astrocytes. Cerebellar granule cells and astrocytes were metabolically more active than cerebral cortical neurons. Glutamate which is utilized as a major metabolic fuel as astrocytes and, to a lesser extent, in cerebral cortical neurons, was virtually not oxidized in cerebellar granule cells.Special Issue dedicated to Prof. Holger Hydén.  相似文献   
37.
In cultures of rat tongue epithelial cells, cell proliferation following incubation with different doses of the potent tumor promoter TPA has been studied by using a stathmokinetic method counting colchicine arrested metaphases. It was demonstrated that 24 h incubation with concentrations higher than 5 ng TPA/mL medium caused inhibition, whereas below 5 ng TPA/mL medium caused stimulation of the mitotic activity reaching a maximum around 30 h from the start of the incubation period. Based on the evidence of the anticarcinogenic effect of selenium in several animal models, experiments have been performed elucidating the influence of an atoxic dose (1/1.000.000M) of selenite on the observed TPA-induced cell proliferation. Our results indicate that addition to the culture medium of an atoxic dose of selenite, not affecting the mitotic activity of control cultures, inhibits the TPA-induced stimulation of cell proliferation.  相似文献   
38.
The uterine acetylcholinesterase and total cholinesterase (acetylcholinesterase plus butyrylcholinesterase) activities in normal and fenchlorphos treated blue fox vixens were determined during various reproductive states. AChE and Total-ChE of non-medicated vixens in oestrus were about one half of those in anoestrus. In pregnant uteri (luteal phase) the activities were 25 % and 30% compared to anoestrus. In vixens given 100 mg/kg fenchlorphos for 3 weeks during anoestrus, the remaining activity of AChE in uterus were in average 37%. Pregnant and non-pregnant vixens in the luteal phase medicated prior to mating and during time of implantation, displayed AChE activities which were only moderabely reduced (remaining activities 83% and 72% compared to medicated animals in anoestrus: remaining activity 37%). Plasma ChE-activity increased during pregnancy in the controls while enzyme activity was strongly reduced in animals given 100 mg/kg fenchlorphos daily through the whole pregnancy. It was concluded that the previous reported embryotoxic effect of fenchlorphos in the blue fox did not seem to be directed towards the moderate inhibition of the uterine cholinesterases.  相似文献   
39.
Summary A synthetic 24-mer oligodeoxynucleotide complementary to the region of proopiomelanocortin (POMC) mRNA that codes for the MSH core sequence (MSH/ACTH[4-11]), ws synthesized and labelled in the 3-end by use of terminal transferase. Probes tailed with either [3H]- or biotin-labelled nucleotides could be used for in situ hydridization studies. Biotinylated probes, hybridized to mouse and rat pituitary sections, were detected by avidinalkaline phosphatase or streptavidin-alkaline phosphatase procedures and development in 5-brome-4-chloro-3-indolyl phosphate (BCIP)-nitroblue tetrazolium (NBT). Proteinase K pretreatment of sections produced a drastic enhancement of the signal obtained, particularly in strongly fixed, paraffin-embedded material. The non-radioactive in situ hybridization technique compared favourably to radioactive in situ hybridization in terms of rapidity and precision of the localization. Controls involved deletion of the probe to prove that other components of the reaction sequence did not yield stain, digestion with RNase to prove that tissue RNA was necessary to bind the probe, prehybridazation (blocking) with unlabelled probe to prove that the biotinylated probe reacted with its anti-sense region and not its tail and Northern blotting to show that the probe reacted with only one species of pituitary RNA, having the size of mouse pituitary POMC mRNA. In addition, adrenalectomy, known to increase anterior lobe POMC, levels, resulted in both increased numbers and increased intensity of positive corticotroph-like cells. Synthetic oligodeoxynucleotides labelled with biotin appear to constitute attractive reagents for in situ hybridization studies when supported by appropriate control procedures.  相似文献   
40.
Kappel K  Sandøe P 《Bioethics》1992,6(4):297-316
... We can therefore conclude that either we should go for equality; and in that case QALYs are unfair because they haven't got enough of an ageist bias. Or we should accept consequentialism; and in that case QALYs have just the right sort of ageist bias. No plausible case can, however, be made for the claim that QALYs have an unfair bias against old people. Other things being equal we ought when distributing resources essential for survival favour the young. This ethical claim can be supported both by reference to equality (the life-time-view) and by reference to consequentialism (and the premises that resources generally will be more useful when given to young people).  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号