全文获取类型
收费全文 | 18331篇 |
免费 | 2486篇 |
国内免费 | 659篇 |
出版年
2024年 | 84篇 |
2023年 | 640篇 |
2022年 | 591篇 |
2021年 | 1507篇 |
2020年 | 1457篇 |
2019年 | 2065篇 |
2018年 | 1334篇 |
2017年 | 884篇 |
2016年 | 790篇 |
2015年 | 1011篇 |
2014年 | 1564篇 |
2013年 | 1854篇 |
2012年 | 831篇 |
2011年 | 959篇 |
2010年 | 538篇 |
2009年 | 638篇 |
2008年 | 576篇 |
2007年 | 598篇 |
2006年 | 568篇 |
2005年 | 485篇 |
2004年 | 368篇 |
2003年 | 325篇 |
2002年 | 297篇 |
2001年 | 169篇 |
2000年 | 146篇 |
1999年 | 136篇 |
1998年 | 149篇 |
1997年 | 109篇 |
1996年 | 110篇 |
1995年 | 92篇 |
1994年 | 85篇 |
1993年 | 69篇 |
1992年 | 73篇 |
1991年 | 69篇 |
1990年 | 36篇 |
1989年 | 40篇 |
1988年 | 42篇 |
1987年 | 31篇 |
1986年 | 17篇 |
1985年 | 26篇 |
1984年 | 31篇 |
1983年 | 9篇 |
1982年 | 13篇 |
1981年 | 11篇 |
1980年 | 9篇 |
1979年 | 15篇 |
1978年 | 12篇 |
1977年 | 5篇 |
1976年 | 3篇 |
1972年 | 2篇 |
排序方式: 共有10000条查询结果,搜索用时 104 毫秒
81.
Free radical damage to cultured porcine aortic endothelial cells and lung fibroblasts: Modulation by culture conditions 总被引:5,自引:0,他引:5
Clark T. Bishop Zermeena Mirza James D. Crapo Bruce A. Freeman 《In vitro cellular & developmental biology. Plant》1985,21(4):229-236
Summary Culture conditions modulating cell damage from xanthine plus xanthine oxidase-derived partially reduced oxygen species were
studied. Porcine thoracic aorta endothelial cells and porcine lung fibroblasts were maintained in monolayer culture. Cells
were prelabeled with51Cr before xanthine plus xanthine oxidase exposure. Endothelial cells showed 30 to 100% more lysis than fibroblasts and thus
seemed more sensitive to this oxidant stress. The effect of cell culture age, as indicated by population doubling level (PDL),
was examined. Response of low PDL endothelial cells and fibroblasts subjected to oxidant stress was compared with the response
of PDL 15 cells. Both low PDL endothelial cells and fibroblasts responded differently to the lytic effect of xanthine oxidase-derived
free radicals than did higher PDL cells. Specific activities of the antioxidant enzymes catalase, managanese superoxide dismutase,
copper-zinc superoxide dismutase, glutathione peroxidase, and glucose-6-phosphate dehydrogenase were measured in both low
and high PDL fibroblasts and endothelial cells. Antioxidant enzyme specific activities could only partially explain the differences
in response to oxidant stress between fibroblasts and endothelial cells and between low and high PDL cells. Cell culture medium
composition modulated the rate of production, and relative proportions of xanthine plus xanthine oxidase-derived partially
reduced species of oxygen, i.e. superoxide, hydrogen peroxide, and hydroxyl radical. Serum content of medium was important
in modulating free radical generation; superoxide production rates decreased 32%, H2O2 became undetectable, and hydroxyl radical generation decreased 54% in the presence of 10% serum. The medium protein and iron
content also modulated free radical generation. The data suggest that cell culture media constituents, cell type, and cell
culture age greatly affect in vitro response of cells subjected to oxidant stress.
Research supported by American Lung Association Fellowship Training Grant and Research Training Grant, the R. J. Reynolds
Corporation, and National Institutes of Health Grants HL29784 and 1 HL 23805. 相似文献
82.
Edith Doucet Jacques Bourbon Michel Rieutort Lea Marin Claude Tordet 《In vitro cellular & developmental biology. Plant》1987,23(3):189-198
Summary Lung organ culture has been a widely used system for studying differentiation and maturation of alveolar epithelium through
various culture conditions. The purpose of this work was to carefully characterize in vitro lung biochemical diffeentiation
through isolation of surfactant fraction from tissue and to search for optimal culture conditions. Fetal rat lung was explanted
on the 18th gestational day for studying glycogen storage, and on the 20th gestational day for studying surfactant accretion,
and cultivated for 48 h. Morphologic differentiation was studies byelectron microscopy tissue explanted on the 17th or 18th
gestational days and cultivated for various times. Glycogen storage was greater on fluid medium, although less than occurring
in vivo. Cellular integrity and surfactant accumulation were maximal on a semisolid medium containing 0.5% agar. Use of O2-CO2 instead of air-CO2 for gassing the explants slighlty decreased phospholipid accumulation. Among media used in previous lung culture studies,
Waymouth MB 752/1 was the only one to allow net glycogen accumulation in vitro. The most favorable media for surfactant phospholipid
accretion were Waymouth MB 752/1, Eagle’s minimum essential and its Dulbeccco’s modification, CMRL 1066, and NCTC 109. They
allowed a 12- to 14-fold increase of surfactant fraction phospholipids in vitro, which is similar to the increase occurring
in vivo during the same peiod. Ham’s F10 and F12 media allowed a six fold increase. RPMI 1640 and medium 199 (M199) allowed
only a three fold increase. Phospholipid concentration in nonsurfactant fraction only doubled during culture, and differences
between various media were much less marked. DNA concentration changed little during culture. Morphologic differentiation
of epithelial cells was advanced as compared with in vivo timing in a medium allowing maximal surfactant accretion (Waymouth
MB 752/1) but not in a medium allowing low surfactant increase (RPMI 1640). The possible role of compositional differences
between media is discussed. 相似文献
83.
Host defense deficiency in newborn nonhuman primate lungs 总被引:2,自引:0,他引:2
A T Cheung G Kurland M E Miller E W Ford S A Ayin E M Walsh 《Journal of medical primatology》1986,15(1):37-47
We have investigated two major aspects of the pulmonary host defense mechanism--alveolar macrophage function as a "first line of bacterial defense" and induced neutrophil migration into the lung as a "back-up defense." Chemotactic and phagocytic/killing assays revealed a functional deficiency in the alveolar macrophages of newborn primates. Serial bronchoalveolar lavage investigations revealed diminished neutrophil migration into the newborn primate lung. The overall pulmonary host defense capability in newborn primates was deficient. The results of this investigation may have direct clinical relevance to the susceptibility of newborns to infections and pneumonia. 相似文献
84.
Biochemical significance of enhanced activity of fluorinated 1,25-dihydroxyvitamin D3 in human cultured cell lines 总被引:1,自引:0,他引:1
Several human cancer cells possess receptors for 1,25-dihydroxyvitamin D3[1,25-(OH)2D3]. In these cells 1,25-(OH)2D3 has a biphasic concentration-dependent regulatory effect on cell replication and specifically induces its own metabolism. We have studied the effects on these parameters of the native hormone together with those of two analogues fluorinated at the 24-carbon and of 1,24R,25-trihydroxyvitamin D3[1,24R,25-(OH)3D3]. The difluorinated analogue 24,24-difluoro-1,25-(OH)2D3[24,24-F2-1,25-(OH)2D3] is an approximately fivefold more potent inhibitor of cellular replication than the native hormone, while 1,24R,25-(OH)3D3 is about fivefold less potent. This enhanced potency of the fluorinated analogue parallels its enhanced potency in in vivo studies of its effects on calcium and mineral metabolism. However, although the analogue retains replication stimulatory activity, it is clearly no more potent than the native hormone in this activity: 1,24R,25-(OH)3D3 has no significant stimulatory activity. Exposure of the cells to 1,25-(OH)2D3 at 0.05 nM for 6 h increases the subsequent conversion of labelled hormone to aqueous phase soluble compounds by 6.7-fold. None of the other compounds had a similar effect at this concentration. At 10 nM all 1-hydroxylated compounds increased aqueous phase radioactivity about equally (13 to 17-fold); this effect is still specific since 25-OH D3 had no such effect even at 10 nM. Studies on the effects of the fluorinated analogues upon receptor binding of hormone in cell cytosols and uptake of hormone by intact cells clearly demonstrate that the enhanced activity of these analogues is not due to higher receptor affinity or more rapid access to intracellular receptor.(ABSTRACT TRUNCATED AT 250 WORDS) 相似文献
85.
Summary H1° histones were purified by preparative sodium dodecyl sulfate polyacrylamide gel electrophoresis from human lung carcinoma (line DMS79), human hepatoblastoma (HepG2), human adult lung and human adult and fetal liver. The purified human H1° histones were analyzed for their amino acid composition and terminal residues. The comparative analysis of the amino acid compositions of the different human H1° histones showed that: (a) all the H1° preparations have the characteristically high lysine content associated with a low arginine content, which distinguishes outer histones from core histones; (b) H1° is distinguishable from other H1 histones by the presence of methionine and histidine; (c) H1° histones from human adult, fetal and cancer cells are very similar in amino acid composition, and in cancer cells the level of the H1° histone is not inversely related with cell growth rate nor with the expression of the -fetoprotein gene. 相似文献
86.
Summary We describe the in vitro influence of 3,5,3′-triiodo-l-thyronine (T3),l-thyroxine (T4), a thyroid-stimulating hormone (TSH), and/or estradiol (E2: chosen as the control of the methodology) on the cell kinetics (cell distribution in the S+G2+M phases) of mouse MXT and human MCF-7 mammary cancer cells. Experiments were performed by means of a cell image processor,
analyzing MCF-7 or MXT cells that had been grown on glass cover slips and whose nuclei had been stained by the Feulgen reaction,
which is selective and quantitative (stoichiometric) with respect to DNA. We show that T3, T4, and TSH at 0.01 μM dramatically stimulate the cell kinetics of the MXT mouse and the MCF-7 human mammary cancer cell lines. Indeed, the three
hormones bring about a significant transient increase in the S+G2+M fraction as does E2. Furthermore, our data indicate that E2 and TSH are antagonistic with regards to MXT or MCF-7 cell kinetics.
This work is supported by grants awarded by the IRSIA and the Fonds de la Recherche Scientifique Médicale (FRSM, Belgium). 相似文献
87.
Metastasis is a major, life-threatening complication of cancer. The bloodstream is the most important disseminative route
for cancer cells liberated from their parent tumors. Single circulating cancer cells are arrested in the microvasculature,
where the vast majority are killed by rapid or slow processes, and the relatively few survivors grow into micrometastases.
We review the underlying causes of one type of rapid cancer cell death in the microcirculation, namely, that caused by biomechanical
interactions of cancer cells with microvessel walls, which may result in cell surface membrane expansion and lethal rupture.
These lethal interactions appear to be important rate-regulators in hematogenous metastasis, and to dictate some aspects of
metastatic patterns. Although these are not the only interactions involving cancer cells, in contrast to others involving
cellular and humoral defense mechanisms, they have received comparatively little attention. 相似文献
88.
89.
Ras interaction with the GTPase-activating protein (GAP) 总被引:18,自引:0,他引:18
M D Schaber V M Garsky D Boylan W S Hill E M Scolnick M S Marshall I S Sigal J B Gibbs 《Proteins》1989,6(3):306-315
Biologically active forms of Ras complexed to GTP can bind to the GTPase-activating protein (GAP), which has been implicated as possible target of Ras in mammalian cells. In order to study the structural features of Ras required for this interaction, we have evaluated a series of mutant ras proteins for the ability to bind GAP and a series of Ras peptides for the ability to interfere with this interaction. Point mutations in the putative effector region of Ras (residues 32-40) that inhibit biological activity also impair Ras binding to GAP. An apparent exception is the Thr to Ser substitution at residue 35; [Ser-35]Ras binds to GAP as effectively as wild-type Ras even though this mutant is biologically weak in both mammalian and S. cerevisiae cells. In vitro, [Ser-35]Ras can also efficiently stimulate the S. cerevisiae target of Ras, adenylyl cyclase, indicating that other factors may influence Ras/protein interactions in vivo. Peptides having Ras residues 17-44 and 17-32 competed with the binding of Ras to E. coli-expressed GAP with IC50 values of 2.4 and 0.9 microM, respectively, whereas Ras peptide 17-26 was without effect up to 400 microM. A related peptide from the yeast GTP-binding protein YPT1 analogous to Ras peptide 17-32 competed with an IC50 value of 19 microM even though the YPT1 protein itself is unable to bind to GAP. These results suggest that determinants within Ras peptide 17-32 may be important for Ras binding to GAP. 相似文献
90.
Teresa L. Johnson Mary Pat Moyer 《In vitro cellular & developmental biology. Plant》1990,26(11):1095-1100
Summary Normal human colon mucosa cells and cells obtained from histologically normal tissues near that cancer were fused with human
colon cancer cells. Resultant hybrid populations of normal and malignant cell fusions behaved as nonmalignant cells in culture,
were unable to grow in soft agar, did not express tumor-associated antigens, and were nontumorigenic in nude mice. Autofusion
of the cancer cell population led to a phenotype intermediate between normal and malignant cells. That is, the cultures had
a much lower plating efficiency in soft agar, and the tumors had a longer latency and slower growth rate in nude mice. This
is the first cell culture system to demonstrate that normal epithelial cells can suppress malignancy of their autologous cancer
cells, and is a prelude to more extensive studies of genetic events involved in malignant conversion of human colonic epithelium.
This study was supported by The University of Texas Health Science Center at San Antonio Center for Human Cell Biotechnology
and a graduate student stipend (T. J.) from the Department of Cellular and Structural Biology. 相似文献