全文获取类型
收费全文 | 11149篇 |
免费 | 503篇 |
国内免费 | 476篇 |
出版年
2023年 | 102篇 |
2022年 | 132篇 |
2021年 | 233篇 |
2020年 | 245篇 |
2019年 | 298篇 |
2018年 | 295篇 |
2017年 | 186篇 |
2016年 | 245篇 |
2015年 | 287篇 |
2014年 | 750篇 |
2013年 | 756篇 |
2012年 | 576篇 |
2011年 | 588篇 |
2010年 | 563篇 |
2009年 | 642篇 |
2008年 | 542篇 |
2007年 | 537篇 |
2006年 | 423篇 |
2005年 | 452篇 |
2004年 | 398篇 |
2003年 | 338篇 |
2002年 | 371篇 |
2001年 | 268篇 |
2000年 | 196篇 |
1999年 | 190篇 |
1998年 | 236篇 |
1997年 | 215篇 |
1996年 | 223篇 |
1995年 | 142篇 |
1994年 | 153篇 |
1993年 | 133篇 |
1992年 | 122篇 |
1991年 | 118篇 |
1990年 | 112篇 |
1989年 | 96篇 |
1988年 | 90篇 |
1987年 | 77篇 |
1986年 | 54篇 |
1985年 | 102篇 |
1984年 | 95篇 |
1983年 | 58篇 |
1982年 | 85篇 |
1981年 | 84篇 |
1980年 | 72篇 |
1979年 | 67篇 |
1978年 | 51篇 |
1977年 | 38篇 |
1976年 | 35篇 |
1975年 | 16篇 |
1973年 | 16篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
101.
Ichiro Kimura Yae Gotoh Eijiro Ozawa 《In vitro cellular & developmental biology. Plant》1989,25(3):236-242
Summary A mitogenic factor which promotes quail myoblast proliferation has been purified some 105-fold from chick embryo extract by a combination of cation-exchange chromatography and heparin-affinity chromatography. The
factor is eluted from heparin-Sepharose with 2M NaCl and is a single-chain polypeptide with an apparent molecular weight of 15000 to 17000. It is active at subnanogram level
in triggering the proliferation and thereby delaying temporarily fusion of myoblasts. It also stimulates the proliferation
of quail fibroblasts in a similar effective concentration range. For both myoblasts and fibroblasts the dose-response to the
factor is quantitatively and qualitatively comparable with that of bovine pituitary fibroblast growth factor. These observations
strongly suggest that the factor very probably corresponds to chicken fibroblast growth factor or to a closely related molecule(s)
and that it is possibly involved in the regulation of myogenesis.
This work was partly supported by a grant from the National Center of Neurology and Psychiatry (NCNP grant 86-01) of the Ministry
of Health and Welfare, Japan. 相似文献
102.
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
103.
Katrin Engelmann Peter Friedl 《In vitro cellular & developmental biology. Plant》1989,25(11):1065-1072
Summary Long-term cultivation of human corneal endothelial cells (HCEC) was optimized with respect to different components of the
culture system: 25 different nutrient media, different sera, 6 mitogens and various substrates were tested in their ability
to influence clonal growth and morphology of HCEC. F99, a 1∶1 mixture of the two media M199 and Ham’s F12, was the most effective
basal medium in promoting clonal growth of HCEC. Among various sera, human serum and fetal bovine serum showed optimal growth
promoting activities in combination with F99, whereas newborn bovine serum (NBS) was by far superior for the development of
a typically corneal endothelial morphology. Crude fibroblast growth factor (FGF), or alternatively endothelial cell growth
supplement, was absolutely essential for clonal growth of HCEC at low serum concentrations, for example 5% NBS. Formation
of a monolayer with a morphology similar to corneal endothelium in vivo was observed only on culture dishes coated with basal
membrane components such as collagen type IV, laminin, or fibronectin. The most pronounced effect on morphologic appearance
was obtained by culturing the cells on the extracellular matrix (ECM) produced by bovine corneal endothelial cells. Moreover,
ECM could substitute for crude FGF in clonal growth assays. 相似文献
104.
Jila H. Boal Scott F. Deamond Daniel E. Callahan Sarah A. Bruce Paul O. P. Ts'o L. L. Kan 《Cell biochemistry and biophysics》1989,14(3):245-256
The nuclear magnetic resonance (NMR) parameters, spin-lattice (T1), and spin-spin (T2) relaxation time, are usually longer for neoplastic cells than for normal cells of the same cell type. This has generally
been true at low NMR frequencies (≤100 MHz) when comparisons have been made between normal and neoplastic cells that have
both spent a short time in culture. We have previously demonstrated that although the T1 values of paired normal and neoplastic Syrian hamster (SH) fibroblastic cells in culture are not significantly different
when measured at 300 MHz, the 300 MHz T2 values for the neoplastic cells are smaller than those of the normal cells. (Xin et al. (1986),Cell Biophysics
8, 213.) Since treatment of normal diploid cells with polypeptide growth factors or tumor promoters frequently results in reversible
expression of neoplasia-associated phenotypes, T1 and T2 were obtained at 300 MHz for treated and untreated SH cells to see if these compounds could also produce smaller 300 MHz
T2 values. Secondary culture SH fetal fibroblast cells were treated with epidermal growth factor (EGF), fibroblast growth factor
(FGF), phorbol-12,13-didecanoate (PDD) and 4-α-phorbol-12,13-didecanoate (4αPDD). Treatment with either growth factor resulted
in smaller T2 values, but a statistically significant decrease was not observed for PDD or 4αPDD. The observed reductions in T2 values were correlated with the morphological and growth-stimulatory effects of these compounds on the cells. 相似文献
105.
Tarek Bisat Terry R. Brown Claude J. Migeon Gary D. Berkovitz 《In vitro cellular & developmental biology. Plant》1989,25(9):806-812
Summary Because the measurement of aromatase activity in cultured human genital skin fibroblasts has been proposed as a means of studying
estrogen production in men, we investigated the influence of culture conditions on aromatase activity.
Genital skin fibroblasts were seeded onto culture plates at a density of 1×106 cells/plate and aromatase activity was determined over a 1-mo. period. Enzyme activity rose slowly over the first 14 d but
then rose rapidly to a 10-fold higher plateau by Day 28. The rise in aromatase activity was similar whether activity was normalized
for protein or for DNA content. When cells were seeded at the usual density of 1×106 or at 0.25×106 cells/plate, aromatase activity was consistently lower during the first 2 wk in cells plated at lower density, but thereafter
the levels of enzyme activity in the two groups converged. In cells plated at the lower density, the lower activity observed
in the first 2 wk was associated with a lower V
max
. Preincubation of cells plated at one density with conditoned medium from cells plated at the other density did not change
the relatve levels of activity in the two groups. By contrast, dihydrotestosterone (DHT) receptor binding and 5α-reductase
activity were similar at all time points, despite differences in plating density.
In additional experiments, the culture medium was replaced daily rather than every 3rd d, and aromatase activity was assayed
on Day 7. In cells fed daily, DNA and protein content were twice that of cells fed every 3rd d. By contrast, aromatase activity
declined to 30% of the in the latter group. DHT and dexamethasone receptor binding and 5α-reductase activity were similar
in the two groups.
In summary, factors such as plating density, culture density, and frequency of media replacement dramatically influence aromatase
activity in cultured human genital skin fibroblasts. Therefore, the interpretation of aromatase activity data obtained from
cultured cells in relation to physiologic or pathologic states should be viewed with appropriate caution.
The work was supported in part by grants R01 DK 35339 and R01 DK 00180 from the National Institutes of Health, Bethesda, MD,
and by RR 00035 from CLINFO Systems at the Johns Hopkins University School of Medicine, Baltimore, MD. 相似文献
106.
陈道亮 《分子细胞生物学报》1989,(1)
本研究以爪蟾胚胎内胚层细胞间连接通讯发育的时程为指标,观察了cAMP和依赖cAMP的蛋白激酶催化亚基对这一发育的影响。将依赖cAMP的蛋白激酶催化亚基注射入爪蟾胚胎四细胞期的每一个细胞可使该胚胎内胚层细胞间连接通讯的发育明显加快,提示在正常状态下这一发育的进程是受细胞内依赖cAMP的磷酸化水平的制约。但用双丁酰cAMP和磷酸二酯酶抑制剂对胚胎进行培育,或将cAMP和磷酸二酯酶抑制剂注射入胚胎细胞,对这一发育并无影响,可能是由于这些胚胎细胞内依赖cAMP的蛋白激酶的实际有效含量较低而cAMP含量较高所致。注射这种蛋白激酶催化亚基所诱导的胚胎细胞间连接通讯在注射后约8.5小时出现。这一时间比前人在哺乳动物细胞培养中用cAMP或必需的信使RNA诱导出细胞间连接通讯所需要的时间(2—4小时)长得多,提示在胚胎中依赖cAMP的磷酸化对细胞间连接通讯发育的作用是从RNA转录的水平上开始的。 相似文献
107.
普通小麦与簇毛麦双二倍体的合成,育性及细胞遗传学研究 总被引:19,自引:3,他引:16
通过杂种幼胚无性系培养获得大量再生植株F_1,经秋水仙碱处理,合成了普通小麦与簇毛麦属间双二倍体(AABBDDVV)。其形态特征除株高、穗长、小穗数,籽粒大小和育性明显增加,生育期延长外,分别与各自的再生植株F_1相似。双二倍体的体细胞染色体数目变化范围为48—56。花粉母细胞减数分裂中期Ⅰ 2n=28Ⅱ的细胞占56.82%,每个细胞平均有27.10个二价体,1.44个单价体,0.08个三价体,0.03个四价体。经过连续两代单穗单株选择,结实率由15.91%提高到36.52%。 相似文献
108.
基因型和胚龄对小麦未成熟胚离体培养反应的影响 总被引:22,自引:0,他引:22
本文对34种基因型的小麦未成熟胚在离体培养中的反应进行了比较。结果表明,94%的供试基因型愈伤组织诱导率都可达到80%以上,若排除供体植株环境条件的不同和接种过程中的人为因素可能造成的影响,不同基因型的愈伤组织诱导率看来没有根本的差异。愈伤组织分化率因基因型的不同变动在0—60%之间,平均为32.7%。虽然同一基因型的盾片愈伤组织分化率在不同年份中有所不同,但是愈伤组织是否具有再生能力?看来是个稳定的遗传性状。因此小麦未成熟胚对愈伤组织诱导的反应和愈伤组织的再生能力可能具有不同的遗传基础。本文的结果还表明,虽然最适于培养的未成熟胚的大小为1毫米左右,伹小至0.3毫米的未成熱胚仍能以几乎100%的频率形成愈伤组织,60%左右的愈伤组织能分化出再生檀株,只是所需的时间比1毫米左右的胚较长。 相似文献
109.
110.
Platelet activating factor (PAF) production by mouse embryos in vitro and its effect on embryonic metabolism 总被引:1,自引:0,他引:1
J P Ryan N R Spinks C O'Neill A J Ammit R G Wales 《Journal of cellular biochemistry》1989,40(3):387-395
Factors affecting the production of platelet activating factor (PAF) by mouse embryos during culture in vitro were investigated. Detectable levels of embryo-derived PAF were produced within 1-4 hr with maximum PAF activity being observed after 6 hr of culture in vitro. The amount of PAF detected in media after 24 hr of culture of two-cell embryos was equivalent to 12.8 ng PAF/embryo. However, differences in activity were apparent with increased time in culture. Reduced synthesis of PAF during culture in vitro was supported by the observation that morulae stage embryos collected fresh from the reproductive tract displayed more PAF activity than morulae resulting from the 48 hr culture of two-cell embryos. In addition to determining production characteristics of PAF by embryos, we also show that the production of CO2 from carbon-1 position of lactate is positively correlated with the ability of embryos to develop during subsequent culture in vitro and therefore could be used as a measure of embryo viability. Furthermore, culture of embryos in media supplemented with PAF resulted in an increase in lactate utilization demonstrating a direct effect of PAF on the embryo. As PAF is produced by preimplantation embryos, an autocoid role of PAF in regulating embryo development is implicated. Therefore, the reduced production of PAF by embryos in vitro may explain the decreased viability of embryos commonly observed following their culture in vitro. 相似文献